You'll note that
some sections are missing. This is an evolving document;
when sections are removed (because the info is moved
into other sections), I don't renumber the document.
1. What does destination port number ZZZZ mean?
- All the
traffic going through the firewall is part of a connection.
A connection consists of the pair of IP addresses
that are talking to each other, as well a pair of
port numbers. The destination port number
often indicates the type of service being connected
to. When a firewall blocks a connection, it will save
the destination port number to its logfile. This section
describes some of the meanings of these port numbers.
numbers are divided into three ranges:
In reality, machines start assigning "dynamic" ports
starting at 1024. We also see strangeness, such as
Sun starting their RPC ports at 32768.
Well Known Ports are those from 0 through 1023.
These are tightly bound to services, and usually
traffic on this port clearly indicates the protocol
for that service. For example, port 80 virtually
always indicates HTTP traffic.
Registered Ports are those from 1024 through 49151.
These are loosely bound to services, which means
that while there are numerous services "bound"
to these ports, these ports are likewise used
for many other purposes. For example, most systems
start handing out dynamic ports starting around
Dynamic and/or Private Ports are those from 49152
through 65535. In theory, no service should be
assigned to these ports.
to get a more complete list of port info:
Numbers" RFC, the official source for port assignments.
of port numbers, hyper-linked to various exploits
on those port numbers.
UNIX systems, the file
contains a list of commonly used UNIX port number
assignments. On Windows NT, this file is located
back to the protocol specifications frequently.
describing various ports.
- TLSecurity's list of Trojans. Rather than a collection
of rumors by other people, the maintainers of
this list claim to verify each and every port
Horse probes page.
1.1 What are some common incoming TCP/UDP probes
against my firewall?
section contains a list of common TCP and UDP port
scans that people see against their firewalls. Note:
there is no such thing as an ICMP port. If
you are interested in interpreting ICMP data, look
||Commonly used to help determine the operating
system. This works because on some systems,
port 0 is "invalid" and will generate a different
response when you connect to it vs. a normal
closed port. One typical scan uses a destination
IP address of 0.0.0.0 and sets the ACK bit,
with broadcast at the Ethernet layer.
||Indicates someone searching for SGI Irix machines.
Irix is the only major vendor that has implemented
tcpmux, and it is enabled by default on Irix
machines. Irix machines ship with several default
passwordless accounts, such as lp, guest, uucp,
nuucp, demos, tutor, diag, EZsetup, OutOfBox,
and 4Dgifts. Many administrators forget to close
these accounts after installation. Therefore,
hackers scan the Internet looking first for
tcpmux, then these accounts. [CA-95.15]
will see lots of these from people looking for
amplifiers sent to addresses of x.x.x.0 and
common DoS attack is an echo-loop,
where the attacker forges a UDP from one machine
and sends it to the other, then both machines
bounce packets off each other as fast as they
can (see also chargen).
common thing seen is TCP connections to this
port by DoubleClick. They use a product called
"Resonate Global Dispatch" that connects to
this port on DNS servers in order to locate
the closest one.
Harvest/squid caches will send UDP echoes from
port 3130. To quote: If the cache is configured
source_ping on, it also
bounces a HIT reply off the original host's
UDP echo port. It can generate a lot
of these packets.
is a UNIX service that will list all the running
processes on a machine and who started them.
This gives an intruder a huge amount of information
that might be used to compromise the machine,
such as indicating programs with known vulnerabilities
or user accounts. It is similar the contents
that can be displayed with the UNIX "ps" command.
ICMP doesn't have ports; if you see something
that says "ICMP port 11", you probably want
is a service that simply spits out characters.
The UDP version will respond with a packet containing
garbage characters whenever a UDP packet is
received. On a TCP connection, it spits out
a stream of garbage characters until the connection
is closed. Hackers can take advantage of IP
spoofing for denial of service attacks. Forging
UDP packets between two chargen servers, or
a chargen and echo
can overload links as the two servers attempt
to infinitely bounce the traffic back and forth.
Likewise, the "fraggle"
DoS attack broadcasts a packet destined to this
port with a forged victim address, and the victim
gets overloaded with all the responses. [CA-96.01]
most common attack you will see are hackers/crackers
looking for "open anonymous" FTP servers. These
are servers with directories that can be written
to and read from. Hackers/crackers use these
machines as way-points for transferring warez
(pirated programs) and pr0n (intentionally misspelled
word to avoid search engines classifying this
connections to this port might indicate a search
which has a few exploitable features. Many versions
using the RSAREF
library can be exploited if they are configured
in a certain fashion. (Suggestion: run ssh on
some other port).
note that the ssh package comes with
a program called make-ssh-known-hosts
that will scan a domain
for ssh hosts. You will sometimes
be scanned from innocent people running this
(rather than TCP) packets directed at this
port along with port
5632 indicate a scan for pcAnywhere. The
number 5632 is (hex) 0x1600, which byte-swapped
is 0x0016, which is 22 decimal.
intruder is looking for a remote login to UNIX.
Most of the time intruders scan for this port
simply to find out more about what operating
system is being used. In addition, if the intruder
finds passwords using some other technique,
they will try the passwords here.
||Spammers are looking for SMTP servers that allow
them to "relay" spam. Since spammers keep getting
their accounts shut down, they use dial-ups
to connect to high bandwidth e-mail servers,
and then send a single message to the relay
with multiple addresses. The relay then forwards
to all the victims. SMTP servers (esp. sendmail)
are one of the favorite ways to break into systems
because they must be exposed to the Internet
as a whole and e-mail routing is complex (complexity
+ exposure = vulnerability).
Hackers/crackers may be attempting to do zone
transfers (TCP), to spoof DNS (UDP), or even
hide other traffic since port 53 is frequently
neither filtered nor logged by firewalls.
important thing to note is that you will frequently
see port 53 used as the source UDP
port. Stateless firewalls frequently allow
such traffic on the assumption that it is
a response to a DNS query. Hackers are increasingly
exploiting this to pierce
|67 and 68
|Bootp/DHCP over UDP. Firewalls hooked to DSL
and cable-modem lines see a ton of these sent
to the broadcast address 255.255.255.255.
These machines are asking to for an address
assignment from a DHCP server. You could probably
hack into them by giving them such an assignment
and specifying yourself as the local router,
then execute a wide range of man-in-the-middle
attacks. The client requests configuration on
a broadcast to port 68 (bootps). The server
broadcasts back the response to port 67 (bootpc).
The response uses some type of broadcast because
the client doesn't yet have an IP address that
can be sent to.
UDP). Many servers support this protocol in
conjunction with BOOTP
in order to download boot code to the system.
However, they are frequently misconfigured to
provide any file from the system, such as password
files. They can also be used to write files
to the system.
are trying to:
provide easy administration of Linux boxen.
It includes a web-enabled interface at port
98 through an integrated HTTP server. It has
had a number of security issues. Some versions
root, trust the local network, create world-accessible
files in /tmp, and a buffer overflow in the
LANG environment variable. Also, because it
contains an integrated web server, it may be
vulnerable to many of the typical HTTP exploits
(buffer overruns, directory traversal using
is not nearly as popular as POP3 (see below),
but many servers support both (for backwards
compatibility). Many of the holes that can be
exploited on POP3 can also be exploited via
the POP2 port on the same server.
is used by clients accessing e-mail on their
servers. POP3 services have many well-known
vulnerabilities. At least 20 implementations
are vulnerable to a buffer overflow in the username
or password exchange (meaning that hackers can
break in at this stage before really logging
in). There are other buffer overflows that can
be executed after successfully logging in.
RPC PortMapper/RPCBIND. Access to portmapper
is the first step in scanning a system looking
for all the RPC services enabled, such as rpc.mountd,
NFS, rpc.statd, rpc.csmd, rpc.ttybd, amd, etc.
If the intruder finds the appropriate service
enabled, s/he will then run an exploit against
the port where the service is running.
that by putting a logging daemon, IDS, or
sniffer on the wire, you can find out what
programs the intruder is attempting to access
in order to figure out exactly what is going
is a protocol that runs on many machines that
identifies the user of a TCP connection. In
standard usage this reveals a LOT of information
about a machine that hackers can exploit. However,
it used by a lot of services by loggers, especially
FTP, POP, IMAP, SMTP, and IRC servers. In general,
if you have any clients accessing these services
through a firewall, you will see incoming connection
attempts on this port. Note that if you block
this port, clients will perceive slow
connections to e-mail servers on the other side
of the firewall. Many firewalls support sending
back a RST on the TCP connection as part of
the blocking procedure, which will stop these
News Transfer Protocol, carries USENET traffic.
This is the port used when you have a URL like
Attempts on this port are usually by people
hunting for open USENET servers. Most ISPs restrict
access to their news servers to only their customers.
Open news servers allow posting and reading
from anybody, and are used to access newsgroups
blocked by someone's ISP, to post anonymously,
or to post spam.
Update: @Home has started scanning their
subscribers to see if they are running USENET
servers. They are doing this in order to find
these servers and close them before spammers
can take advantage of them.
MS RPC end-point mapper
|Microsoft runs its DCE RPC end-point mapper for
its DCOM services at this port.
has much the same functionality as port
111 for UNIX systems. Services that use
DCOM and/or RPC register their location with
the end-point mapper on the machine. When
clients remotely connect to the machine, they
query the end-point mapper to find out where
the service is. Likewise, hackers can scan
the machine on this port in order to find
out such things as "is Exchange Server running
on this machine, and which version?".
port is often hit in order to scan for services
(for example, using the "epdump" utility),
but this port may also be attacked directly.
Currently, there are a few denial-of-service
attacks that can be directed at this port.
|(UDP) This is
the most common item seen by firewall administrators
and is perfectly normal. Please read the NetBIOS
section below for more details.
File and Print Sharing
|Incoming connections to this port are trying
to reach NetBIOS/SMB, the protocols used for
Windows "File and Print Sharing" as well as
SAMBA. People sharing their hard disks on this
port are probably the most common vulnerability
on the Internet.
on this port were common at the beginning
of 1999, but tapered off near the end. Now
at the start of year 2000, attempts on this
port have picked up again. Several VBS (IE5
VisualBasic Scripting) worms have appeared
that attempt to copy themselves on this port.
Therefore, it may be worms attempting to propagate
on this port.
security idea as POP3 above, numerous IMAP servers
have buffer overflows that allow compromise
during the login. Note that for awhile, there
was a Linux worm (admw0rm) that would spread
by compromising port 143, so a lot of scans
on this port are actually from innocent people
who have already been compromised. IMAP exploits
became popular when RedHat enabled the service
by default on its distributions. In fact, this
may have been the first widely scanned for exploit
since the Morris Worm.
port is also used for IMAP2, but that version
wasn't very popular.
people have noted attacks from port 0 to port
143, which appears to be from some attack
A very common port that intruders probe for.
SNMP allows for remote management of devices.
All the configuration and performance information
is stored in a database that can be retrieved
or set via SNMP. Many managers mistakeningly
leave this available on the Internet. Crackers
will first attempt to use the default passwords
"public" and "private" to access the system;
they may then attempt to "crack" the password
by trying all combinations.
packets may be mistakenly directed at your
network. Windows machines running HP JetDirect
remote management software uses SNMP, and
misconfigured machines are frequent. HP OBJECT
IDENTIFIERs will be seen in the packets. Newer
versions of Win98 will use SNMP for name resolution;
you will see packets broadcast on local subnets
(cable modem, DSL) looking up sysName and
||Probably a misconfiguration.
||Numerous hacks may allow access to an X-Window
console; it needs port 6000 open as well in
order to really succeed.
||Probably from UNIX machines on your DSL/cable-modem
segment broadcasting who is logged into their
servers. These people are kindly giving you
really interesting information that you can
use to hack into their systems.
If you are on a cable-modem or DSL VLAN, then
you may see broadcasts to this port. CORBA is
an object-oriented remote procedure call (RPC)
system. It is highly likely that when you see
these broadcasts, you can use the information
to hack back into the systems generating these
1524 for more info.
script kiddies feel they're contributing substantially
to the exploit programs by making a minor
change from ingreslock to pcserver
in constant text... -- Alan J. Rosenthal.
mountd bug. This is a popular bug that people
are scanning for. Most scans on this port are
UDP-based, but they are increasingly TCP-based
(mountd runs on both ports simultaneously).
Note that mountd can run at any port (for which
you must first do a portmap lookup at port 111),
it's just that Linux defaulted to port 635 in
much the same way that NFS universally runs
at port 2049.
people ask the question what this port is used
for. The answer is that this is the first port
number in the dynamic range of ports. Many applications
don't care what port they use for a network
connection, so they ask the operating system
to assign the "next freely available port".
In point of fact, they as for port 0, but are
assigned one starting with port 1024. This means
the first application on your system that requests
a dynamic port will be assigned port 1024. You
can test this fact by booting your computer,
then in one window open a Telnet session, and
in another window run "netstat -a". You will
see that the Telnet application has been assigned
port 1024 for its end of the connection. As
more applications request more and more dynamic
ports, the operating system will assign increasingly
higher port numbers. Again, you can watch this
effect with 'netstat' as your browse the Internet
with your web browser, as each web-page requires
a new connection.
protocol tunnels traffic through firewalls,
allowing many people behind the firewall access
to the Internet through a single IP address.
In theory, it should only tunnel inside traffic
out towards the Internet. However, it is frequently
misconfigured and allows hackers/crackers to
tunnel their attacks inwards, or simply bounce
through the system to other Internet machines,
masking their attacks as if they were coming
from you. WinGate, a popular Windows personal
firewall, is frequently misconfigured this way.
This is often seen when joining IRC
is rarely probed by itself, but is almost always
seen as part of the sscan
Horse (TCP). See the section on SubSeven
for more details.
attack scripts install a backdoor shell at this
port (especially those against Sun systems via
holes in sendmail and RPC services like statd,
ttdbserver, and cmsd). If you've just installed
your firewall and are seeing connection attempts
on this port, then this may be the cause. Try
telnetting to the attempted machine in order
to see if it indeed comes up with a shell. Connections
to port 600/pcserver also have this problem.
NFS program usually runs at this port. Normally,
access to portmapper
is needed to find which port this service runs
on, but since most installations run NFS on
this port, hackers/crackers can bypass portmapper
and try this port directly.
is the default port for the "squid" HTTP proxy.
An attacker scanning for this port is likely
searching for a proxy server they can use to
surf the Internet anonymously. You may see scans
for other proxies at the same time, such as
at port 8000/8001/8080/8888. Another cause of
scans at this port, for a similar reason, is
when users enter chatrooms. Others users (or
the servers themselves) will attempt to check
this port to see if the user's machines supports
proxying. See section 5.3
for more info.
may see lots of these, depending on the sort
of segment you are on. When a user opens pcAnywhere,
it scans the local Class C range looking for
potential agents. Hackers/crackers also scan
looking for open machines, so look at the source
address to see which it is. Some scans for pcAnywhere
frequently also include a UDP packet to port
22. See dialup
probes for more info.
port is used separately from the SubSeven
main port to transfer data. One example where
you might see this is when a master is controling
a slave on a dialup line, then the slave machine
hangs up. Therefore, when someone else dials-in
at that IP address, they will see a continuous
stream of connection attempts at this port.
receive incoming audio streams from servers
on UDP ports in the range 6970-7170. This is
setup by the outgoing control connection on
TCP port 7070.
"PowWow" chat program from Tribal Voice. It
allows users to open up private chat connections
with each other on this port. The program is
very aggressive at trying to establish the connection
and will "camp" on the TCP port waiting for
a response. This causes a connection attempt
at regular intervals like a heartbeat. This
can be seen by dial-up users who inherit IP
addresses from somebody who was chatting with
other people: it will appear as if many different
people are probing that port. The protocol uses
the letters "OPNG" as the first four bytes of
its connection attempt. more
||Outbound: This is seen on outbound connections.
It is caused by users inside the corporation
who have installed shareware programs using
the Conducent "adbot" wrapper. This wrapper
shows advertisements to users of the shareware.
A popular shareware program that uses this is
Bill Royds mentions that in his experience,
you can block this outbound connection with
no problem, but if you block the IP addresses
themselves, then the adbots can overload the
link trying to reach the servers by continually
connecting many times per second.
machines will attempt to resolve the DNS name
"ads.conducent.com", which resolve to the
These addresses are hosted by Exodus.
Horse (TCP). See the section on SubSeven
for more details.
Horse (TCP). This is a commonly seen scan
looking for systems compromised by this trojan.
number means "elite" in hacker/cracker spelling
(3=E, 1=L, 7=T). Lots of hacker/cracker backdoors
run at this port, but the most important is
Back Orifice. At one time, this was by far the
most popular scan on the Internet. These days,
it's popularity is waning and other remote access
trojans are becoming popular.
traffic on this port is currently being seen
due to the "Hack-a-tack" RAT (Remote Access
Trojan). This trojan includes a built-in scanner
that scans from port 31790, so any packets FROM
31789 TO 317890 indicate a possible intrusion.
(Port 31789 is the control connection; port
31790 is the file transfer connection).
|32770 ~ 32900
Solaris puts most of its RPC services in this
range. In particular, older versions of Solaris
(pre-2.5.1) put a portmapper
in this range, allowing hackers access to this
even when low ports are blocked by a firewall.
Probes in this range might either be for this
portmapper, or for known RPC
services that can be exploited.
|33434 - 33600
you see a series of UDP packets within this
port range (and only within thisrange), then
it is probably indicative of traceroute. See
for more info.
||Inoculan on UDP. Older versions of Inoculan apparently
generate huge quantities of UDP traffic directed
at subnets in order to discover each other.
More info can be found at http://www.circlemud.org/~jelson/software/udpsend.html
Thanks to Jerry Leslie, NeoNET < leslie at
clio dot rice dot edu>
1.2 What do the following source ports mean?
1-1024 are for reserved services, and almost never
appear as the source. There are some exceptions,
such as when connections come from NAT machines.
1.9 for some more details.
closely after 1024 (i.e. 1024-5000) are the ones
most commonly seen. These are the "dynamic" range
that are assigned to applications that don't care
what port they use for their connection.
1-5 are indicative of a script called 'sscan'
servers usually transfer files from this port.
servers will send UDP responses from this port.
You may also see TCP connections with source/destination
ports of 53.
(Simple) Network Time Protocol (S/NTP) servers
run at this port. They will also send broadcasts
to this port.
(and Quake-derived games) usually run servers
at these ports. Therefore, UDP packet from this
range (and to this range) will usually be games.
above 61000 might come from machines behind
a Linux NAT server called "IP Masquerade".
1.3 I'm seeing attempts on the same set of ports
from widely varying sources all over the Internet.
is due to a "decoy" scan, such as in 'nmap'. One
of them is the attacker; the others are not.
and protocol analysis can be used to track down
who this is. For example, if you ping each of the
systems, you can match up the TTL fields in those
responses with the connection attempts. This will
at least point a finger at a decoy scan. (The TTLs
should match; if not, then they are being spoofed).
[Newer versions of scanner now randomize the attackers
own TTL, making it harder to weed them out].
can also attempt to go back further in your logs,
looking for all the decoy addresses or people from
the same subnets. You will often see that the attacker
has actually connected to you recently, while the
decoyed addresses haven't.
first stage of a Trojan Horse attack is to get the
program on a user's machine. Typical techniques
the program to newsgroups claiming to be some
mailing lists with the attached program
program to websites
via instant messenger programs and chat systems
(ICQ, AIM, IRC, etc.)
e-mail from the ISP (like AOL) with a hoax message
asking somebody to run a program (such as a software
to startup folder via "File and Print Sharing".
next stage of the attack is to scan the Internet
looking for machines that might be compromised.
The problem is that most of the techniques outlined
above don't tell the cracker/hacker where their
victim machine is. Therefore, the cracker/hacker
must scan the Internet looking for the machines
they might have compromised.
leads the condition where owners of firewalls (including
personal firewalls) regularly see "probes" directed
at their machines from crackers/hackers looking
for these machines. However, if the machine hasn't
been compromised, then these probes are not a problem.
The probes cannot compromise the machine by themselves.
Administrators can usually ignore these "attacks".
ports used by these probes are listed below. In
order to tell if your machine might be running one
of these trojans, run the program "netstat -an"
on your machine. Look for the ports that might be
"listening" for incoming connections.
has become the most popular remote access trojan.
At this time, it is the easiest-to-use and most powerful
trojan. The reasons for this are:
is actively maintained/updated. Most other Trojans
were created once then development stopped except
for a couple of bug fixes.
program not only includes a scanner, but also
can tell a slave machine to scan as well.
creator has a contest for cracked sites using
"port redirection", so that any attack can be
funneled through a victim's machines.
extensive tricks to play with ICQ, AOL IM, MSN
Messenger, and Yahoo messenger, including password
sniffing, posting messages, and other features.
UI tricks, such as flipping the screen, talking
through the victim's speaker, and spying on the
it not only is an excellent hacking tool, the little
"magic" tricks are designed to scare the <bleep>
out of victims.
is written by a hacker who calls himself "Mobman".
His site can be reached at http://subseven.slak.org/.
might use the following ports:
default connection port for older versions.
not sure what this port is for, but it has been
claimed that this can serve as a "backdoor" in
some versions. (Yes, a backdoor program with a
backdoor to avoid password prompts).
for the "matrix" chat program
default port appearing in v2.0
1.9 DNS packets from low numbered ports
I've seen many DNS requests from many low port numbers
below 1024. Aren't they supposed to be reserved? Aren't
they supposed to use 1024-65535 range?
A: These are coming from machines behind NAT firewalls.
A NAT doesn't necessarily have the concept of reserved
port numbers. thanks to Ryan Russell Ryan.Russell
at sybase dot com
My filters reject incoming packets with source ports
below 1024, so the DNS lookups are failing.
A: Don't filter that way. Lots of firewalls have
similar rules, but this is somewhat "misguided"
since hackers/crackers can forge whatever ports
Are these NAT firewalls doing it incorrectly?
A: Not in theory, but in practice it will result
in failures. The "correct" way would be more strictly
control DNS traffic in any case (such as essentially
"proxying" DNS and forcing out through port 53).
I thought DNS lookup was supposed to use a random
source port above 1024?
A: In practice, your average DNS client will use
a non-reserved port. However, a lot of implementations
use a source port of 53. In any case, the NAT issue
is completely separate because it completely changes
the entire 'socket' (IP address + port combo).
1.10 Immediately upon dialing up to my ISP, my personal firewall
starts alarming me about probes against port X.
is very common. The cause is that somebody hung
up just before you dialed in and your ISP assigned
you the same IP address. You are now seeing the
remnants of communication with the previous person.
example is chat
programs. If someone simply hangs up, then everyone
who was chatting with that person will attempt to
still send traffic to them. Some programs take a
long time to timeout. Typical programs that show
this behavior are PowWow and ICQ.
example is on-line, multiple games. You might see
such traffic from gaming providers like MPlayer,
or maybe from unknown servers (Quake servers litter
the Internet). These games are typically UDP based,
so there is no concept of a connection that can
be dropped. They also are quite aggressive at maintaining
connections, in order to make a good user experience.
Some game ports that you might see are:
example is multimedia audio/visual. For example,
uses UDP ports in the range of 6970-7170
for clients to receive audio streams.
sure that you carefully figure out the correct side
of the connection. For example, an ICQ server runs
on port 4000, and the client chooses a random high-numbered
port. That means you will see UDP packets from port
4000 going to the random port. In other words, don't
go looking in a port database trying to figure what
that random, high-numbered port means. The significant
port is the source.
trojan has a similar problem. It uses separate TCP
connections for different services. If the slave
agent goes away, it will continue to create connection
attempts to the slave ports, especially at port
1.11 IRC servers are
of the most popular applications is "chat", like
IRC. One feature of chat programs is that they reveal
the IP address of the people you are chatting with.
One problem with chatrooms is that people enter
the rooms "anonymously" and play around, either
by disrupting conversations with offtopic comments
and flamebait, or by "flooding" the servers or other
clients in an attempt to kicked them off.
both servers and clients are implementing measures
to stop "anonymous" use of chatrooms. In particular,
they check people entering chatrooms in order to
see if they are "proxying" through some other connection.
The most popular of such probes is SOCKS. The assumption
is that if the IP address of where you are coming
from supports SOCKS, then it is possible that you
have a completely separate machine and are only
going through the indicated machine in order to
hide your true identity. Undernet's policy on this
can be found at http://help.undernet.org/proxyscan.
same time, crackers/hackers will scan people's machines
in order to determine if they are running some sort
of server that can be bounced through. Again, by
checking for SOCKS, the attacker hopes to find somebody
that has left SOCKS open, such as a home user implementing
connection sharing using SOCKS, but accidentally
configured it so that anybody on the Internet has
access to it.
1.12 What are "remapped" ports?
- A common
technique is to remap ports to some other address.
For example, whereas the default port for HTTP is
80, many people remap it to another port, such as
8080 (hence, this document could reside at http://www.robertgraham.com:8080/pubs/firewall-seen.html
if I were to remap the port).
is done under the theory that making the port harder
to find will make it more difficult for a hacker
to exploit. Instead of simply exploiting a well-known
service at a well-known port, the hacker will have
to port scan the machine.
port remapping is done at some variation of the
original port. Therefore, most HTTP ports are based
upon a variation of the theme "80":
81, 88, 8000, 8080, 8888, and so forth. POP, which
is originally at port 110
can often be found at port 1100.
are other statistically significant chosen numbers,
like 12345, 23456, 34567, etc. Many people also
choose numbers that are well known for other reasons;
42, 69, 666, 31337, and so on. The recent proliferation
of Remote Access Trojans (RATs) has resulted in
hackers/crackers choosing the same defaults for
their programs. For example, NetBus defaults to
R. Swopes points out that remapping is also done
because on UNIX machines, your server needs root
privileges to listen on ports below 1024. If you
don't have root level access and want to run a web
service, you will need to install it on a high-numbered
port. Likewise, some ISPs might firewall low-numbered
ports, forcing you to remap even when you own the
1.13 I still can't figure out what somebody
is trying to connect to a port, what can I do?
- Use netcat
in order to setup a listening process. For port '1234',
netcat -L -p 1234
A lot of
protocols will send data as the first part of the
connection. By setting up netcat listening on the
port, you might be able to figure out what protocol
that are using. If you are lucky, the protocol in
question will be HTTP, which will give you a wealth
of information that you can use to track down what
"-L" option means to listen continuously. Normally,
netcat would accept a single connection, dump the
contents, then exit. By adding this option, it will
remain running for multiple connections.
TCP and UDP carry data, ICMP contains purely control
messages. Therefore, ICMP messages cannot really
be used to break into your machine. Hackers use ICMP
messages to attempt to scan networks, DoS
machines, or redirect traffic.
firewalls incorrectly label ICMP fields as "ports".
ICMP has no ports like TCP or UDP, but it does have
two fields called "type" and "code". While these
fields serve completely unrelated purposes, the
fact that there are two of them have led to firewalls
mislabeling them. For more on ICMP, please read
my Infosec Lexicon entry on ICMP
official reference for what ICMP Type/Code fields
mean is found at http://www.isi.edu/in-notes/iana/assignments/icmp-parameters.
While that document describes the official meanings,
this section describes what hackers are trying to
do. This section contains a brief summary at top,
then more details descriptions down below.
response to a ping. |
indication back from a host or router that some
packet did not reach its destination. |
configuration problem or incorrectly specified
IP address. |
means that the router one hop before the desired
host could not ARP
server tells the client that nobody is listening
at the port the client attempted to contact.
||Fragmentation Needed but DF set
||Important: If you are seeing these in
your firewall reject logs, then you've misconfigured
your firewall. You should allow this packet
to pass through, otherwise your clients will
see their TCP connections mysteriously hang.
||Congestion on the Internet. |
||Somebody is trying to redirect your default router.
This could be from a hacker trying to execute
against you by causing you to route through
their own machine.
is exists a hack against Win9x and Solaris such
that a hacker can DoS you by redirecting your
default router. A neighboring hacker can also
do a man-in-the-middle
attack by directing you through his/her router.
Exceeded In Transit
means that a packet never reached its target
because something timed out.
dropped the packet either because of a routing
loop or maybe because of a traceroute. |
||Fragment reassembly timeout
host dropped the packet because it didn't receive
all the fragments. |
||Something unusual is going on, and probably indicates
2.0 Type = 0 (Echo Reply)
- The sender
is responding to a ping from your address. This could
ping that person
behind the firewall is sending pings to the target.
of applications use pings for various purposes,
such as to see if their communication partner
is alive, or to measure the response time. A big
cause of this is VitalSign's Net.Medic, which
sends pings of various sizes in order to measure
is using your IP address as a decoy in a ping
sweep, so you are seeing the responses.
- Covert-channel communications
network managers block incoming pings (type=8),
but allow ping responses (type=0). Therefore,
hackers have begun using ping replies as ways
of bypassing firewalls. For example, in the massive
DDoS attacks against Internet sites, commands
could be imbedded in ping responses, and floods
of responses were directed against the sites in
order to clog their Internet connections.
2.3 Type = 3 (Destination Unreachable)
- The exact
code is important in the Unreachable packet.
that Unreachables sometimes play a part in defeating
SYN floods. This means that if a host you are talking
to is under SYN flood attack, you will not be able
to reach them if you block incoming Unreachables.
cases, you will receive destination unreachable
packets from hosts you have never heard of. The
most common cause of this is a "decoy scan". An
attacker is sending spoofed packets a target using
possibly hundreds of source addresses, including
one that is the real address. The hacker's theory
is that the victim won't wade through all the decoys
in order to pin them down.
best way to solve this is to examine the actual
packets as described below.
Try to discover is the pattern looks like what one
would see in a decoy scan. For example, look for
alternating port numbers in TCP or UDP headers contained
within the ICMP portion of the packet.
2.3.0 Type = 3, Code = 0 (Destination Net
route to host A router tells the client that
it does not know how to route to anything at all
in the network range that includes the host the
client is talking to. This indicates either the
client chose the wrong IP address, or that routing
tables are misconfigured somewhere. Note that
sometimes you see the message "No route to host"
on your own UNIX machine when your own routing
tables are messed up, which is especially common
when configuring point-to-point links.
2.3.3 Type = 3, Code = 3 (Destination Port
packet is sent by a SERVER when a CLIENT tries
to connect to a UDP port that isn't running.
For example, if you try to send an SNMP packet
to port 161, but the machine doesn't support
the SNMP service, you will get back an ICMP
Destination Port Unreachable packet.
first thing to debug this problem is to check
the port numbers within the packet. You probably
need to use a sniffing
utility as firewalls tend not to log the information.
This technique relies upon the fact that ICMP
messages include the IP and UDP headers of the
original packet. Here is a hex dump of an ICMP
00 00 BA 5E BA 11 00 60 97 07 C0 FF 08 00 45 00
00 38 6F DF 00 00 80 01 B4 12 0A 00 01 0B 0A 00
01 C9 03 03 C2 D2 00 00 00 00 45 00 00 47 07 F0
00 00 80 11 1B E3 0A 00 01 C9 0A 00 01 0B 08 A7
79 19 00 33 B8 36
the bytes 03
03 are the type/code for the ICMP packet.
The last 8 bytes of the packet are the original
UDP header, which decodes as:
Source Port = 2215
May be dynamically allocated, so no always
Destination Port = 31001
This is very important, it meant the person
was originally attempting to contact a service
on port 31001.
Length = 51
The length of the original UDP data might
Checksum = 0xB836
The checksum may or may not be important
are some reasons why you may be seeing this:
may be scanning the person who sent you the
ICMP packet. They are forging the source as
one of your IP addresses. They will in reality
forge lots of different source addresses so
that they victim can't be sure who it really
is. If you receive large numbers of these
packets from the same source in a short time
frame, then this is a likely bet. Check the
UDP Destination Port field. If it is
constantly changing, then this is a very likely
client may send a DNS request to your server,
which takes a long time to resolve. By the
time your DNS server responds, the client
has already forgotten about you and closed
the UDP port assigned to receive your response.
Check the UDP Source Port field to
see if it equals 53. If so, then this is a
likely occurrence. Why does this happen? The
server may be resolving a recursive query,
but its own query packet was lost, so it had
to time out and try again. By the time it
gets back to the client, it has timed out.
Many client applications (especially on Windows)
do their own DNS resolution, meaning that
they must create their own socket to do so.
If they passed the request onto the OS, it
is likely the OS would simply have left the
- Multi-response DNS
variation is when the client receives multiple
responses to the same request. It receives
the first response, then closes the socket.
Subsequent responses will be dropped. There
other variations on this problem. A Sun machine
connected with multiple NICs on the same Ethernet
will assign both NICs the same MAC address,
causing it to receive two copies of every
frame, then send multiple responses. Likewise,
a poorly written client program (it has been
claimed that some DNS resolvers are multi-threaded,
but not thread safe) sometimes send out multiple
requests, then close the socket on the first
response. However, there may be an attempt
at DNS spoofing, where a hacker is
attempting to corrupt the resolver's cache
by sending both a recursive query and a response.
- NetBIOS Resolution
the receiver of the ICMP packets is a Windows
machine, look to see if the UDP Destination
Port is 137. In this case, the cause of
this is the Windows system trying to execute
the 'gethostbyaddr()' function, which attempts
to resolve the IP address into a name using
both DNS and NetBIOS. The DNS request gets
sent to a DNS server somewhere (and not sent
to the target), but the NetBIOS request gets
sent directly to the target. If the target
doesn't support NetBIOS, then it will send
back an ICMP unreachable.
traceroute programs (with the exception of
Windows tracert.exe) send UDP packets to closed
ports. This causes a sequence of back-to-back
ICMP Port Unreachable packets to be sent back
to the machine doing the traceroute. Thus,
if you are seeing these ICMP packets on your
firewall, then somebody inside might be doing
a traceroute. You may also see TTL exceeded
2.3.4 Type = 3, Code = 4 (Fragmentation Needed
and Don't Fragment was Set)
are sent by routers attempting to forward IP datagrams
that are marked "DF" (Don't Fragment).
Both IP and TCP fragment data, but in different
ways. TCP is vastly more efficient at fragmentation
than IP. Therefore, stacks attempt to find the
"Path MTU (Maximum Transmission Unit)". This
ICMP message is sent during that process.
consider ALICE talking to BOB. Both are on Ethernets
(max frame size = 1500 bytes), but some intervening
link limits the maximum IP packet size to 600
bytes. This means all IP packets sent will be
fragmented by the routers on that link into
3 fragments. Since it is much more efficient
to fragment at the TCP layer, the TCP stack
will attempt to discover the MTU. It does this
by setting the "DF" (Don't Fragment) bit in
all its packets. As soon as it hits a router
than cannot forward a packet that large, the
router will send back this ICMP error message.
From that, the TCP stack will know how to fragment
should probably let these packets through the
firewall. Otherwise, the intended recipient
will have a hung connection as small packets
get through to set up the connection, but the
large packets are mysteriously dropped. A common
result from this are people who see web pages
that are only halfway returned.
MTU Discovery is becoming more and more integrated
into communication. For example, IPsec needs
2.4 Type = 4 (Source Quench)
packets are supposed to be transmitted by routers/destination
when traffic level exceeds a certain threshold. Many
systems today, however, do not generate them. The
reason is that we now believe that simple packet loss
is the best indication of congestions (since the only
reason packets are dropped, in practice, is congestion).
the rules for source quenches are now (RFC 1122):
SHOULD NOT generate them.
MAY generate them.
SHOULD honor them.
SHOULD discard them.
hosts still react to Source Quenches by slowing
communication, so they can be used as a denial of
service. Firewalls should filter these out. If a
DoS is suspected, the source address of the packets
will be meaningless, because the IP addresses are
quenches have been known to be sent by some SMTP
are ping request packets. They are used all over
the place; it may indicate hostile intent of someone
trying to scan your computer, but it may be part
of the normal network functionality. See Type =
0 (Echo Response) above for more info.
of network management "scanners" will precede a
scan using a special ping packet. These include
ISS scanner, WhatsUp monitor, and others. This will
be visible in the payload of the scanner. Most firewalls
don't log this payload, so you may need to use some
sort of sniffer
to capture them or some time of Intrusion
Detection System to flag them.
that blocking incoming PINGs does not mean a hacker
can't scan the network. There are many other ways
of doing this. For example, TCP ACK scanning becoming
popular -- they usually get through the firewall,
and they illicit a response from the target system.
sent to broadcast IP addresses like x.x.x.0
or x.x.x.255 are probably attempts to use
your network as a smurf
2.11 Type = 11 (Time Exceeded In Transit)
probably doesn't indicate an attack from a hacker/cracker.
2.11.0 Type = 11, Code = 0 (TTL Exceeded
can be caused by a number of things. If somebody
from your site is doing traceroutes out to the
Internet, you will see lots of TTL exceeded responses
from routers. This is how traceroute works: forces
the routers to generate TTL exceeded messages
in order to find them.
common reason firewall administrators see this
is due to routing loops developing in the Internet.
Route flapping (constant route changes) is a
common problem, and will often briefly result
in a loop. This means that while a IP packet
is heading towards it destination, the packet
gets misrouted to a router that it previously
visited it. The packet then gets routed in a
circle infinitely -- or it would be, if the
routers didn't decrement the TTL field each
time and discard the packet once that value
cause of this is distance. Many machines start
with a default TTL of 127 (Windows) or even
lower. Routers will often decrement the TTL
more than by one in order to reflect slow lines
like dialups or transcontinental links. Therefore,
a site might not be reachable with a low initial
TTL. In addition, some hackers/crackers like
to make their site unreachable through this
2.11.1 Type = 11, Code = 1 (Fragment Reassembly
sending fragmented IP datagrams, the sender of
this message never received all the fragments.
Normally, most TCP/IP traffic shouldn't even be
fragmented. You will only see this if the traffic
is both fragmented AND there congestion somewhere
between you and the target.
2.12 Type = 12 (Parameter Problem)
probably indicates an attack. There are a number of
techniques that will generate these packets.
3.1 What are source routed packets?
route is an option in the IP header that allows the
sender to override some or all of the routing decisions.
Normally, routers between the source and destination
decide how to route the packet.
are a couple of network management uses of this
packet, such as testing to see if two computers
can talk to each other. A network manager at point
A may send a packet to B through point C. This tells
A if B & C can talk to each other.
same technique can be used to evade firewalls, subvert
trust relationships, and communicate with machines
using "private" address (10.x.x.x, 192.168.x.x,
say you are a hacker/cracker on the Internet and
you want to talk to some machines behind a firewall
who use 10.x.x.x as their IP addresses. Since the
routers on the Internet do not know where this subnet
is located, they will drop your packets. However,
you put a loose source route option in the IP packet
and tell all the Internet routers to first forward
to the firewall. Since the firewall straddles both
the Internet and the private network, it will know
how to forward the packet appropriately. Thus, you
can carry on a conversation with the victim by bouncing
all packets through the firewall.
can be used with IP spoofing. You pretend to be
a router (like the firewall mentioned above) and
pretend that somebody else is bouncing packets through
you. Thus, pick some random machine on the Internet
(ALICE) as the spoofee, then send packets from ALICE
to your victim BOB. BOB will think the packets are
coming from ALICE, but in reality they are coming
from you. This masks the real source of the attack.
is even better if you know that BOB trusts ALICE.
IP addresses are often used as part of authentication.
Let's say the firewall has a rule allowing all traffic
from ALICE into the network. By forging all IP packets
to be from ALICE (but being source routed through
your own machine), then you get free access to the
and more core Internet routers are disabling source
routed packets. They slow down routing anyway, but
they are a huge security risk. There is also no
real need for them. Managers should do the same
and disable source routing everywhere: on firewalls,
on routers, and even on end-nodes so that they won't
even accept incoming source routed packets.
Microsoft Knowledge Base article Q217336 for setting
the "DisableIPSourceRouting" on WinNT SP5 systems
3.2 I'm seeing the IP address 255.255.255.255 in my reject log
is happening a lot these days as more and more people
use DSL or cable-modem connections. The reason is
that unlike point-to-point connections (like T-1,
frame relay, etc.), these new high-speed technologies
drop you onto an ATM VLAN, which is a single broadcast
domains. In fact, many cable-modem users are seeing
multiple megabytes of traffic per day simply from
must remember that such packets MUST be local. Routers
(generally) refuse to forward packets with the IP
address of 255.255.255.255. This address is known
as a "local broadcast" for this reason: it never
travels past the local segment (or these days, the
local "virtual" segment).
are these packets for?
the list of ports at the top of this document. If
it is not listed there, then the only way to figure
this out is to capture them with a sniffer
and view their contents.
example, a common service that runs with a random
port number is CORBA IIOP packets. Many services
run at port
535, but it is frequently reconfigured to broadcast
on other ports. If you look at the hex dump in the
sniffer, you will see the letters "IIOP" somewhere
in the contents.
case, this is rarely something to be concerned about.
In fact, it advertises something about the person
sending the traffic that can be used to hack them.
Hackers rarely attack their own neighborhoods (because
it is easy to detect), so it probably is accidental,
be noted that with today's ATM networks, the source
of the broadcast may not even be in the same state
as you are; they may be hundreds of miles away.
The word "local" means in terms of the network topology,
3.3 How do I track down the owner of an IP address?
that IP addresses can be spoofed, so that the "owner"
of an IP address may be innocent. Increasingly,
attacks are coming from compromised machines. The
owner of the IP may actually be grateful! Both of
these statements come to the same conclusion: be
polite and professional.
companies have established the e-mail address "firstname.lastname@example.org"
(replace "example" with the proper company). This
e-mail role is for both e-mail abuse (such as spam)
as well as for network abuse. When you find the
owner of the IP address, you should probably compose
a message including the evidence of the attack.
past, all the IP address owners were kept by the
Internic. A database built from that information
is at http://ipindex.dragonstar.net/. There are
now 3 official registrars for North America, Asia,
and Europe. Unfortunately, you will have to query
each individual database. However, if you start
with the North America registrar, it will tell you
if the address belongs to one of the other three.
Warning: The returned information is fragile;
so don't send flames to these people because you
have only about 30% chance of reaching the right
traceroute will often find at least the ISP who
is hosting the IP address. A reverse DNS lookup
on the actual IP address is easy to spoof, but the
route to the machine will reveal who is hosting
the possible intruder.
attacks are now coming from cable-modem subscribers
in the 24.x.x.x range. These are probably from machines
who have been compromised by a Remote Access Trojan
(RAT). (While hackers/crackers frequently use dial-up
lines because they don't care if their account gets
canceled, few users want to have their cable-modem
important range is the "private address" ranges
of 10.x.x.x, 192.168.x.x, and 172.16.x.x-172.31.x.x.
like 127.x.x.x indicate "localhost" and should never
be seen on the Internet.
address range 192.0.2.x has been designated for
"examples", like "example.com".
3.4 I'm seeing packets from "private" addresses
(10.x.x.x et al.) on the Internet side of my firewall
"private address" ranges are 10.x.x.x, 192.168.x.x,
been seeing these in three cases:
routers on the Internet are increasingly being
assigned IP address in this range. There is really
no need for a router to be reached from the Internet.
The "forwarding" function really is independent
from "sending/receiving" packets. When a router
drops a packet and sends back a "ICMP TTL Exceeded"
message, it will use the private address. Note
that some routers are multi-homed with both private
and non-private addresses. Other routers have
only private addresses.
- cable-modem, DSL
cable-modem and DSL connections are on virtual
LANs over ATM. You will often see broadcast packets
from neighboring machines with these private addresses.
rarely, you may see an address from a hackers
who are spoofing addresses in this range.
3.5 What kind of scans should I expect to see
from quasi-legitimate sources?
- You will
often see scans from somewhat legitimate sources.
What I mean by this is that people will scan you without
the intention of actually hacking you. For example,
search engines will index your site, but it isn't
to people in order to redirect them to a nearer
server for their advertising.
websites looking for illegal activities, such
as copyrighted items.
3.6 I'm seeing source IP address of 0.0.0.0?
- If the
port is also 0, then this is probably an attempt to
3.7 What are directed broadcasts and what do
indicate people scanning your subnet
looking for smurf amplifiers
3.8 I'm seeing strange addresses like 169.254.x.x?
document on auto-configuration of IP addresses when
Once a DHCP Client has determined it must auto-configure an IP
address, it chooses an address. The algorithm for choosing an
address is implementation dependant. The address range to use MUST
be "169.254/16", which is registered with the IANA as the LINKLOCAL
only happens when the normal DHCP process fails.
new technique was introduced with Microsoft Win98
and Apple MacOS 8.5.
4. Stuff doesn't work
4.1 Installing a firewall causes slow connections to POP and SMTP services
is because the POP and SMTP servers are trying to
establish an identd/AUTH
connection back to the client. These reverse-connections
are blocked, and it takes a while before the servers
timeout and continue.
identd/AUTH service identifies the user of the TCP
connection (user name, process id, etc.). When the
e-mail server accepts the incoming TCP connection,
before sending the greetings, it will first attempt
to gather information via the identd protocol. This
consists of a TCP connection in the reverse direction.
In other words, when I connect to my e-mail server,
my e-mail server attempts to connect back to me
on port 113, the identd port. My e-mail connection
just sits there until the e-mail server resolves
the identd information.
problem comes about because the firewall silently
drops the SYN packet. The e-mail server is expecting
an immediate SYN-ACK (identd supported) or RST (identd
not supported), but when the firewall drops the
packet it keeps trying until the connection times
that the e-mail server doesn't care if I don't support
identd, and indeed most people don't on their clients.
It just wants an immediate response one way or the
other. The firewall blocks that. This is why some
personal firewalls for Windows (like BlackICE Defender
from my company) contain default rules that allow
identd/AUTH to pass through. Windows doesn't reveal
the information that UNIX does, and opening it up
gives the immediate response these servers are looking
the e-mail server to stop querying identd info
the firewall to RST all those connections
the firewall to allow this protocol, but this
would be a BAD IDEA because identd/AUTH
reveals a HUGE amount of information about your
that this means you should be seeing lots of dropped
incoming connection attempts at port
113 in your log files because of this.
5. What are some typical signatures of well-known
program "traceroute" is based upon a very intelligent
hack by Van Jacobson (also famous for other nifty
kludges). Every IP packet has a time-to-live
(TTL) field that indicates how many hops the
packet can travel before being dropped. This field
is needed because routers sometimes get misconfigured
and will forward packets in a continuous: i.e. Alice
forwards the packet to Bob who forwards it to Charlene
who mistakenly forwards it back to Alice.
each router decrements (subtracts 1) from the TTL
field. When each reaches zero, the router who currently
has the packet will simply "drop" it (not forward
it on). When a router drops a packet, it sends a
message back to the sender informing for this. This
message is called an ICMP "TLL Exceeded in Transit".
nifty thing about this is that the router uses its
own IP address as the source address of the ICMP
message. Therefore, if you send a packet to a target
but with a TTL of only 1, the first router will
receive the packet, decrement the field to 0, drop
it, then send back the ICMP notification. This informs
you of the first router along the route (which you
probably knew anyway).
same goes for an initial TTL of 2. The first router
gets it, decrements to 1, then forwards to the second
router along the route. This router then decrements
to 0, drops the packet, and sends back and error
this process, you eventually end up with the list
of routers between yourself and the target.
are various versions of the traceroute program.
In particular, the Windows program "tracert.exe"
uses pings as the packet it sends to the target.
Therefore, you might see ICMP Echoes on your firewall.
most popular "traceroute" program for UNIX programs
sends UDP datagrams to port 33434
for the first packet sent, then increases this port
number by one for each successive packet. This means
that you will never see port 33434 on your firewall,
but you will start to see successive ones starting
at higher port numbers. Traceroute programs typically
send 3 packets for each hop (in case some get dropped).
Therefore, if somebody is 10 hops away, the first
port you will see is 33434 + 3*10 = 33464.
administrators should learn the symptoms of traceroute
scans in 33434-33600
brief sequential "port scan" in this range usually
indicates a traceroute for a UNIX machine, as
explained in this section.
someone inside the network is attempting a traceroute,
then you'll see these incoming packets. Many admins
allow these through the firewall.
indicates that somebody is tracerouting you. This
doesn't necessarily indicate hostile activity,
but somebody is scanning you. These should be
blocked by the firewall.
ICMP port unreachable
a traceroute successfully hits a target, it will
generate back-to-back "ICMP port unreachable"
messages (probably 3 in a row).
traceroutes are designed to bypass firewalls. See
for more information.
'sscan' tool has become a popular scanning tool
on the Internet. It not only "port scans" but attempts
to discover some common vulnerabilities. There are
several versions of sscan, and it is very configurable,
so matching an exact signature to this program may
be difficult. The 'sscan' program is derived from
the older 'mscan' tool.
goes through several phases:
program will attempt to see if the host is reachable
by scanning for the most common services, namely
This phase is easily detected because both the
source and destination port are the same.
attempts are made to several services in order
to see if they are available. This is highly configurable.
Typically configured probes are those above, as
well as 111/rpc,
contains a basic OS fingerprinting technique,
easily detected because it uses source ports 1-5.
The fingerprinting is not as complete as the techniques
used by Queso or nmap.
- vulnerability assessment
then looks at the ports that are open and checks
the banners that might indicate a vulnerable version
of one of the services. It also scans for a range
of known vulnerable CGI scripts.
upon what it finds, it can further launch configured
scripts against the system.
following is a record pulled from an intrusion detection
22 23 25 53 79 110 111 143 1114 2766 6000 31337
the system consolidates alerts, discards duplicates,
and keeps the port numbers in sort order. In a real
scan, several of the ports would have duplicate
connection attempts, and port 1/tcpmux would be
one of the last probes, not one of the first.
5.3 Proxy scanners
- One of
the most common scans on the Internet looks for HTTP
proxy servers. Normally, the hackers aren't looking
to compromise systems, they simply want the ability
to "anonymize" their connections. For example, most
anonymous e-mail services (HotMail, Yahoo mail, etc.) will store
the IP address in the e-mail headers, making them
not so anonymous (many people have been caught this
way). By bouncing HTTP traffic through a proxy server,
the hacker can complete erase his/her tracks.
summer of 1999, probes for ports 80/8080/3128 were
particularly noticed. These came from all over the
Internet and were fairly disjoint. These came from
a Trojan Horse called "Ring0" (RingZero). It
would infect PCs, then scan random IP addresses
for proxy servers. The SANS Institute (a security
training/conference organization) coordinated an
effort to track down exactly what was happening
from reports from many of their customers. A common
symptom of this Trojan is 3 probes spaced within
a minute from the same IP address from this Trojan.
More information can be found at: http://www.sans.org/newlook/resources/ringzero.htm.
A news article by CMP can be found at: http://www.techweb.com/wire/story/TWB19991013S0018
of open proxies can be found at: http://freebooks.hypermart.net/proxy/proxies.htm
with variations of the "80" them (81, 88, 8000,
8080, 8888, etc) are most commonly used for proxies.
In addition, a popular free proxy server called
"squid" runs at port 3128.
programs send packets to broadcast addresses with
source address of the victim. Everybody on that subnet
then sends responses back to that address, flooding
is a ping (ICMP
Echo Request) whereas a fraggle is a
port 7/echo. These are named after the programs/scripts
that first implemented them.
packets are sent to broadcast addresses.
In IP, a directed broadcast has all the "host"
bits set to either one or zero. This means an address
that looks something like 192.0.2.0 or 192.0.2.255
is likely a broadcast. The key thing to remember
is that such addresses are only broadcasts if the
router on that subnet chooses to interpret it as
a broadcast. If that router has this configured
as a broadcast in its routing tables, it will forward
the single IP packet as broadcast on that (Ethernet)
segment, causing all systems on that (Ethernet)
segment to receive the packet.
there are two configuration problems:
Both can be fixed.
forwarding directed broadcasts.
responding to broadcasts.
saw the following incident with millions of incoming
packets. Below are some examples of these packets:
Some questions that have
been asked about this are:
Why are these only aimed at strategic points like
A: Because if a single packet is sent to
a broadcast, then it generates lots of responses
to the spoofed address of the victim.
I monitor multiple networks. Why is only this network
being attacked this way?
A: Your network isn't being attacked; instead
it is the third party in a fraggle attack. Your
network is being used to attack somebody else (the
source address of the packets, which is spoofed).
Either your other networks aren't nearly as effective
as fraggle amplifiers, or they have been registered
in smurf/fraggle registries yet. Hackers rarely
look for their own amplifiers, but instead simply
look up good amplifiers in such directories. If
you get registered, then multiple hackers will use/abuse
Why port UDP 7
A: There are a number of reasons. The first
is that script-kiddies aren't too bright. If they
only scripts available use port 7, then that is
all they can use. Secondly, the service has to respond
to broadcast requests. Therefore, you cannot use
TCP (which will only respond to directed queries).
Many other UDP services only respond to directed
queries. Finally, when fraggle was first developed,
many firewalls allowed Echos to pass through (because
they were used for performance monitoring). More
dangerous protocols like NetBIOS (port 137) are
already blocked by firewalls.
7. What do these other logs mean?
- The following
information helps interpret the meaning of events
generated by logging systems, not necessarily from
a firewall. They might come from the service itself,
detection systems, or really smart firewalls.
7.1 What do the following DNS errors mean?
- Response from unexpected source
DNS server might report this when it receives
an incoming response with a different IP address
than the corresponding request. There are several
causes of this.
that DNS servers will "recursively" send out
queries when resolving names on behalf of clients.
Each outgoing request is given a unique transaction
identifier; incoming responses contain the
same transaction identifier.
if a server sends request #45689 to server 192.0.2.131,
but gets response #45689 back from server 192.0.2.3,
then it triggers this alert.
most common cause of this is due to proxying,
caching, and dual-homed hosts. For example,
the DNS server might have two IP addresses:
[192.0.2.131] and [192.0.2.3]. The typical way
of writing a DNS server is to not bind the sockets
to individual IP addresses. What this means
is that the DNS server does not know which IP
address the request was received on, nor does
it tell the underlying TCP/IP stack which IP
address to use when sending the response. Therefore,
when the DNS server sends the response, the
underlying stack uses one of the IP addresses
at random (which can be the wrong one).
- Various errors with 127.0.0.1
servers are misconfigured to map this address.
On the other hand, it is also a hacker technique
to cause names within the hacker domain to resolve
to addresses within a company (including localhost/127.0.0.1).
hacker is attempting to list all the DNS names
within a domain. This is an attempt to "map" your
network. Managers should consider using split
DNS aka shadow domains, whereby the
public DNS contains only those records that must
be accessed publicly, but use a separate (and
distinct) DNS server for internal machines. Note
that some people are fairly benign. If the transfers
are coming from the IP addresses 22.214.171.124
and 126.96.36.199, you might want to let them through.
are scouring the Internet querying the version
of BIND (Berkeley Internet Name Daemon). It is
a TXT record of CHAOS class that contains the
version string for your server. If you have a
server on the Internet, you will see such queries.
You might see something like:
May 12 04:33:01 ns1 named: unapproved query from [192.0.2.71].35687 for "VERSION.BIND"
7.2 What do the following URL's mean in weblogs?
- A lot
of these pop up in logs as "404 Not Found" errors:
MSIE5 (Microsoft Internet Explorer v.50), when
a user adds a link to his/her "Favorites" (Bookmarks)
or drags the link to the desktop, the browser
attempts to retrieve an icon for it. It first
searches in the same directory as the file being
linked to, then walks up the directory structure
until it hits the root. A lot of sites (example:
Yahoo!) now supply icons for their sites.
a search engine (like AltaVista, Infoseek, Excite,
etc.) attempts to index your site, it will first
get the file "/robots.txt". If you don't want
parts of your website indexed, you can put rules
here. On the other hand, hackers will sometimes
grab this file as well on the assumption that
if you tell a search engine not to index some
directories, they might be something interesting
to look at. Indeed, network managers do believe
that putting directories in "robots.txt" hides
them, when in reality it exposes them more.
beginning with http://
occasionally see the following type of line in
their webserver log:
14:03:00 192.0.2.243 GET /index.html - 200 Mozilla/4.0 - -
14:03:03 192.0.2.243 GET http://www.example.com/ - 200 - - -
The first is a normal line, but what is that complete
URL starting with "HTTP"? This is an attempt to
see if the machine supports proxying. This is
how pretty much all HTTP proxies work -- they
receive a complete URL, then fetch that URL for
for more info.
7.3 What do the following mean in my RPC portmapper logs?
lookup an RPC program in portmapper/rpcbind
in order to find out which port number the service
runs on. A hacker will either dump all the
listings (using rpcinfo -p <host>)
or lookup the mapping (using getport) for the
particular RPC he/she wants to exploits.
these attempts are usually from scans against thousands/millions
of machines rather than against you in particular.
Every few months, a new exploit script is published
for Linux or Solaris services, and script kiddies
start scanning the Internet for that service. Most
of the vulnerabilities in the services listed are
that on Sun Solaris machines, these services usually
have port numbers in the range starting at port
32770. Many other times, RPC services will have
ports below 1024, on the assumption that it provides
a little better security because
info on RPC can be found in RFC1833.txt.
7.3.1 What do the following RPC portmapper
- The portmapper
service has six commands (numbered 0-5).
is a "ping" style command -- it just verifies
that the service is running. You see these almost
you see this go across the wire, then it is
an intrusion attempt. This should be used only
internally as RPC-based programs register themselves
you see this go across the wire, then it is
an intrusion attempt. This should be used only
internally as RPC-based programs unregister
themselves with portmapper. It is sometimes
used as a DoS
attack in order to kill your services. Such
attacks are frequently spoofed.
is the normal use of portmapper that you should
see 99.9% of the time going across the wire.
An external client looks up the corresponding
port number for the desired service. When reviewing
logs, if you see requests to strange services,
you can lookup the program number in the table
dumps all the mappings in the portmapper database.
The UNIX command "rpcinfo -p" carries
out this command. This is a common reconnaissance
technique for hackers.
may be an attempt to compromise the system.
The callit feature was created for RPC
broadcasts. Because a desired service runs on
different ports on different systems, one cannot
simply broadcast to it. Therefore, portmapper
will accept incoming broadcasts on port 111,
then forward them to the appropriate program.
However, some even protocols that don't support
broadcasts can be compromised by sending the
requests through this service.
7.3.2 What do the following RPC program numbers
- An RPC
program number is assigned by Sun (email@example.com).
put an astrisk * next to the ones that have been
seen to use the callit feature.
CPU, network traffic, and disk statistics to
be remotely monitored. Hackers may use this
as part of recon.
the users on a machine, which reveals lots of
info to hackers.
late 1998, the RedHat Linux distribution contained
a buffer overflow bug in the mountd
service running at port 635.
The popularity of RedHat and the fact that the
service ran at a common port number resulting
in popularity among hackers. Not only did hackers
scour the Internet for such machines, but a
was created to spread via this service. [CA-98.12]
program walld, which sends messages
to users from the system administrator (such
as notifying them the system is about to be
rebooted, so they had better save their work).
Messages are frequently sent via callit
Calender Messaging Service
the middle of 1999, a buffer-overflow was
found in this service. Immediately after this
discovering, hackers started doing extensive
scans for this service, resulting in thousands
of hacks against web-sites using Solaris.
||ToolTalk (rpc.ttdbserverd) [CA-98.11]
Solstice Adminsuite, installed by default
on Solaris systems 2.5 and above (2.4 and below
installed a similar service called rpc.admind).
late 1999, a buffer overflow bug was found
in the logging service. While any code based
upon the original BSD sources is vulnerable,
hackers are probably scanning for the Linux
implementation includes in many distros. [CA-99-12]
not sure what this service is, but UnixWare
sends callit broadcasts across this
number has been assigned to FrameMaker for UNIX.
You can download an evaluation copy of this
program at: http://www.adobe.com/support/downloads/fmunix.htm.
Apparently, the license manager supports callit
broadcasts. This license manager supports a
"roving" license whereby many people can have
it installed, but only a few can use the product.
|Legato NetWorker Server Remote Status.
This is a backup service (also OEMed as Solstice
Backup). Status updates are broadcast via callit.
of NeXTstep replacement for YP. When a child
netinfod process starts up, it searches
for a parent by broadcasting a NIBIND_BIND procedure
(function=8) on the local subnet.
7.4 What do the following mean in my SMTP (e-mail) logs?
not your classic packet filtering firewall, SMTP (e-mail)
are important gateways between the outside world and
your internal network. They should be considered along
the same lines as your firewall.
7.4.1 What is this message about "relay" attempts?
- A relay
is where somebody sends your e-mail server not destined
for anybody who you serve e-mail for. For example,
I might connect to your e-mail server and attempt
to send mail to "firstname.lastname@example.org". Your e-mail server
should not accept the e-mail ("relay not allowed").
Your e-mail server should only accept incoming e-mail
to your users (or outgoing e-mail from your users).
problem is that many administrators simply install
servers without taking these simple precautions.
Spammers take advantage of this fact. They give
a single e-mail to the mail server and a recipient
list containing hundreds of unrelated recipients.
This allows them to send huge quantities of e-mail
using a slow dialup connection. This is important
because once the ISPs get enough complaints, they
will terminate the user's account, so they must
continual get new dialup connections. It also has
the effect of partially hiding the true source of
get error messages about relaying, that is a good
thing: you've configured your server correctly.
If you don't get such messages, this is a bad thing.
This means that you are probably not rejecting relayed
messages. Has your server seemed slow lately?
only do spammers hunt for open relays, anti-spam
organizations do the same in an attempt to "blacklist"
open relays. Some of the good guys are:
Internet Mail Consortium reports that in 1999,
roughly 17% of e-mail systems had open relays.
- MAPS RBL
MAPS RBL (Realtime Blackhole List) allows you
to configure your e-mail server to blackball known
open relays that send out bulk spam. It is used
by a huge percentage of e-mail servers on the
the Internet looking for open relays. ORBS uses
relay tests from New Zealand (e.g. manawatu.co.nz).
only do you receive relay attempts from spammers,
you also get attempts from anti-spam organizations.
There are several organizations that regularly scan
the Internet looking for open relays. The most common
is from "manawatu.co.nz"; don't get too upset --
7.4.2 What are these messages about rejected
EXPN and VRFY attempts?
- The "expand"
and "verify commands will expand mailing lists or
verify user names (respectively).
do the command "VRFY root", you might be able to
find out the postmaster's e-mail address. This is
good reconnaissance technique.
a "VRFY decode" or "VRFY uudecode", you might be
able to find out some security holes in the system
related to these subsystems. Other commonly scanned
user names are "bbs", "lp", "demo", "guest", and
systems have buffer overflows in this command, either
in the command itself or in the logging system behind
the command. You might see entries for very long
strings like "xxxxxxxxxxxxxxxxxxxxxxxx".
see a bunch of these in a row, you are probably
being scanned by a vulnerability scanner (ISS/CyberCop/Nessus).
They will generate a bunch of other junk in your
logs as well.
7.5 What are these identd/auth messages?
- The UNIX
identd service identifies which of the logged
on users owns a particular TCP connection.
7.5.1 What does No Ident response
IRC servers spit this out. It means that the ident
service at port 113
isn't available. Either the firewall is blocking it
or it isn't running. Most IRC clients come with an
8. How do I configure filters?
of the logged packets on your firewall result from
incorrect configuration. This section doesn't describe
how to configure your firewall, but instead helps
describe some common configuration steps you might
want to take when you see rejects pop up in your firewall
8.1 What ICMP traffic should I deny?
- The "correct"
configuration of ICMP filters in a firewall is hotly
debated. The problem is that ICMP are the "control
messages" for TCP/IP. If you block some incoming ICMP,
then you will break communication.
absolute minimum ICMP traffic to allow is the packets
dealing with TCP path MTU discovery. Fragmenting
a stream is more efficient at the TCP layer rather
than the IP layer, so the TCP layer will try to
discover when IP packets are being inadvertently
fragmented. They do this by setting the "DF"
(Don't Fragment) on all outgoing packets. When a
router cannot forward the packet because it is too
big, rather than fragmenting it, it sends back a
"fragmentation needed" ICMP packet (type=3/code=4).
The TCP stack then starts sending smaller IP packets,
segmenting the data at the TCP layer rather than
allow routers to fragment at the IP layer. Therefore,
firewalls must be configured to allow incoming ICMP
type=3, code=4 packets.
issue is Host unreachable and Destination
Unreachable packets. Allowing these to come
in through your firewall will allow connections
to timeout faster, but they can also be used as
a denial of service attack (by disconnecting clients
will constantly ask for the ability to ping and
traceroute machines on the Internet. Most firewall
adminsitrators will eventually give into these demands.
Nobody really needs to ping/traceroute, but they
really want to. It should be remembered, however,
that ICMP ping responses are often used as a covert-cahnnel.
(The massive DDoS attacks against Internet portals
used this as a covert channel).
more information on this, you may want to consult
"Protect and Survive Using IBM Firewall 3.1 for
AIX", IBM publication SG24-2577-02. See http://www.redbooks.ibm.com/ for more info.
I disagree with it, though.
good document is http://www.worldgate.com/~marcs/mtu/.
8.2 split DNS
a separate primary DNS server for internal use vs.
external use. An external DNS server should only have
entries for publicly available servers, such as webservers,
FTP servers, e-mail servers, and so forth.
9. Packet Zen
- You can
deduce a lot of information by examining fields within
the TCP/IP haeders. What seems like random or meaningless
numbers to most people can in fact reveal a lot of
9.1 How do I interpret the IP identification
fields? (IP ID Zen)
- The IP
identification (ID) field is a two-byte
field contained within the packet. Its sole purpose
in life is allow IP packets to be fragmented: all
fragments should contain the same ID as the original
packet so that they can be pasted back together again.
Most systems use the concept of a monotonically
increasing ID: for each packet sent, the field
is increased by one.
is a little twist to this scenario. A little-endian
machine (like Intel processors) stores numbers in
reverse byte-order than how numbers are represented
on the wire. This means that a monotonically increasing
integer from a Wintel box will increment the high-order
byte first, whereas a Sun SPARC box will increment
the low-order byte first. Therefore, lets say that
you are being pinged steadily from both a Sun SPARC
and a Wintel, you will see the following sort of
progression in the IP ID field:
above numbers are shown in hexadecimal,
which shows the byte-order problem. However, many
firewall logs (stupidly) show these numbers in decimal.
If a firewall system assumes the number is big-endian
but the incoming packets are little endian, then
the progression of the numbers is hidden. For example:
entire issue is complicated by the fact that a firewall
running on a platform doesn't have to base its decimal
calculation of the IP ID field on the underlying
CPU. What I mean by this is that the C code that
interprets the IP ID could be written in two ways;
/* ID field is a 2-byte number at offset 4 within the IP header */
int ipid_cpu = *(unsigned short*)(iphdr+4);
int ipid_be = iphdr * 256 + iphdr;
first example is CPU dependent: x86 CPUs will pull
it out as a little-endian number, but SPARC CPUs
will pull it out as a big-endian number. The second
form is CPU independent: it tells all CPUs
to interpret the field as a big-endian number. Note:
ntohs(*(unsigned short*)(iphdr+4)) will
crash a SPARC CPU and is not a good solution
if you are running a Linux-based x86 firewall that
interprets the IP ID field as a little-endian number,
then a string of packets from a Wintel box will
demonstrate a monotonically increasing number. However,
a stream from a SPARC box will show skipping numbers.
Conversely, if the Linux-based firewall uses the
(correct) field parsing method, you'll see the reverse.
of the story: Find out the byte order interpretation
of the IP ID field used within your firewall. Also
send your firewall vendor flames telling them to
get with the program and represent the field in
hex in the first place.
9.2 How do I interpret the TTL fields? (TTL Zen)
- The Time-to-Live
(TTL) field is decremented by one every time
a router forwards a packet. When it reaches zero,
the router discards the packet. Routing loops
are a frequent occurrence on the Internet as routers
get confused as to the proper direction in which to
forward packets. The TTL mechanism assures that packet
eventually "die" when and don't get routed in loops
means that you can tell how far away a person is
from the TTL field, and sometimes what kind of platform
they are running. Most Windows machines send packets
with a starting TTL of 128. This means that if your
firewall log shows a TTL=112, then you can make
the guess that the sender is 16 hops away, and that
they are using a Windows machine.
UNIX machines typically choose 64 as the starting
TTL, so a packet when the TTL is 51, then it probably
isn't Windows, but it is probably 13 hops away.
technique was once used to find the source of nmap
decoy scans. The decoys where given random TTLs,
but the real scans were give normal TTLs. This allowed
the astute observer the ability to sift through
the incoming decoys and find the real scan. The
nmap program was soon fixed to randomize
the TTL of the real scan as well.
9.x Other resources
of people is a common topic when sniffing packets.
Some articles that describe this are:
Vision's "Passive Host Fingerprinting"
Spitzner's "Passive Fingerprinting"
10. What's the deal with NetBIOS (UDP port 137)?
requests to UDP port 137 are the most common item
you will see in your firewall reject logs. This comes
about from a feature in Microsoft's Windows:
when a program resolves an IP address into
a name, it may send a NetBIOS query
to IP address. This is part of the background radiation
of the Internet, and is nothing to be concerned about.
discussion of these NetBIOS packets crops up over
and over again on firewall/incident mailing lists.
In this section, I've tried to come up with the
"definitive" answer to this question.
that you will see NetBIOS scans, such as from hackers
running the Legion NetBIOS scanner or other
scanners. In this case, you'll likely see a scan
of your entire address range. The important thing
to remember is that few NetBIOS packets are from
10.1 What does it mean to resolve an IP address
to a name?
- You are
familiar with the normal DNS resolution. You
type into your web browser an address like http://www.robertgraham.com/, but it looks
up the web sites name with DNS in order to find IP
address. Underneath, it is really IP addresses that
are used for communication.
DNS a directory service, where the word directory
has the same meaning as in phone networks. In the
U.S., we can dial directory assistance at
411 rather than looking up a name in the phone book.
Either way, the goal is to lookup a name,
and receive a number.
similar manner, sometimes you have a number, and
you want to find the name. Let's say that you have
caller ID and somebody calls you with the phone
number (212) 555-1038. This doesn't tell you who
this is, so you want to do the reverse lookup and
discover the person's name.
the same fashion, the Internet provides a number
of capabilities to resolve an IP address into a
10.2 Where do the NetBIOS packets come from?
Why does Windows send them?
- On virtually
all systems (UNIX, Macintosh, Windows), programs call
the function 'gethostbyaddr()' with the desired
address. This function will then do the appropriate
lookup, and return the name. This function is part
of the sockets
key thing to remember about gethostbyaddr()
is that it is virtual. It doesn't specify
how it resolves an address into a name. In
practice, it will use all available mechanisms.
look at UNIX, Windows, and Macintosh systems, we
see the following techniques:
in-addr.arpa PTR queries sent to the DNS server
NodeStatus queries sent to the IP address
in the /etc/hosts
over IP name query sent to the IP address
query sent to the UNIX NIS server
lookup sent to the WINS server
systems do the /etc/hosts,
DNS, WINS, and NodeStatus techniques.
excruciating detail, Microsoft has a generic system
component called a naming service. All the
protocol stacks in the system (NetBIOS, TCP/IP,
Novel IPX, AppleTalk, Banyan, etc.) register the
kinds of name resolutions they can perform. Some
RPC products will likewise register an NIS naming
service. When a program requests to resolve an address,
this address gets passed onto the generic naming
service. Windows will try each registered name resolution
subsystem sequentially until it gets an answer.
(Side note: User's sometimes complained that
accessing Windows servers is slow. This is caused
by installing unneeded protocol stacks that must
timeout first before the real protocol stack is
queried for the server name.).
order in which it performs these resolution steps
for IP addresses can be configured under the Windows
registry key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\ServiceProvider.
Of course, that doesn't help you the firewall admin.
10.3 But my network doesn't run any Windows
machines. Why am I being sent these packets?
- It has
nothing to do whether you run Windows, NetBIOS, or
Samba on your machines.
process is simply that a program requests the name
for an IP address, and sends this request to all
the protocol stacks. If the NetBIOS stack receives
such a request, it always sends a NetBIOS query
to the IP address. It doesn't matter if you have
(or haven't) an existing NetBIOS connection to the
words, the only requirement necessary in order to
receive such packets is that you have an IP address.
10.4 Why are reverse resolutions so common?
- One would
think that a reverse query would be rare. They are
instead very common. Here are some reasons why programs
might do reverse lookups.
the user executes a ping -a 192.0.2.168,
then Windows will attempt to find the name for
that address. This doesn't happen so often.
traceroute program finds all the hops between
the client and the server. Users sometimes do
this from the command-line. The most common source
of this is from programs that automatically traceroute
the servers the user visits. Note that if they
are tracing through several hops, you will get
separate queries for each hop.
IIS web server
webserver has the option to log the machine name
of the client accessing the web site. Each time
one of your users behind your firewall browses
an IIS-based server, you'll get a query for the
name of the user's machine.
if name resolution is disabled on the webserver,
the site administrator may run the webserver logfiles
through a reporting tool like Webtrends. Most
of these tools have the ability to resolve IP
addresses to names. At this stage, you will see
a flury of port 137 packets from the address the
tool is run from (which may be different from
the original webserver). This is especially a
problem because they request such a huge amount
of DNS PTR queries that they overwhelm the DNS
server. Thus, even though DNS queries would normally
be resolved, they might fail during analysis of
a log file, thereby generating NetBIOS queries.
Since these logfiles analyzers are often run on
a scheduled (i.e. nightly) basis, you may see
such activity from the same host during the same
period of the day.
web browsing, reverse IP name resolution is a
fixture in many Windows client apps like IRC,
ICQ, and so forth.
firewalls will attempt reverse resolution of the
IP addresses. The "auto-learning" personal firewalls
that prompt the user for each outgoing connection
can be particular offenders in this regard. If
BlackICE Defender sees an intrusions attempt,
it may also do its own NetBIOS lookups independently
from the underlying Windows system.
that starting in late 1999, desktop security tools
like personal firewalls have exploded. This means
that the number of NetBIOS queries have dramatically
see section 10.6
for an explanation of how a simple configuration
error in DNS can cause you to be suddenly flooded
with such requests.
10.5 What is the exact signature I can expect
machines use both a source port of 137 as well as
a destination port of 137. In contrast, if UNIX machines
attempt to resolve NetBIOS names (via SAMBA), they
will use dynamic ports above 1024.
Windows box is trying to find the name for the IP
address 192.0.2.21, it will do the following steps:
the DNS "PTR" record for 188.8.131.52.in-addr.arpa;
this request is sent to the local DNS server,
which recursively forwards the query to the appropriate
DNS server as required.
the DNS answer comes back, it won't query
NetBIOS. If a negative response comes back, it
will immediately query NetBIOS. If the DNS server
times-out, it will wait 14-seconds, then query
resolving with NetBIOS, it will send out a "NodeStatus"
query that is sent to the 192.0.2.12:137 from
x.x.x.x:137. (I.e. the query is sent to the IP
address being resolved to its port 137, and is
sent from the Windows machine port 137).
NetBIOS request is a "NodeStatus" query that looks
up the name "*". It is 50 bytes worth of data
(58 including the UDP header, 78 including the
IP header, 92 including an Ethernet header).
NetBIOS queries are sent with a 1.5 second timeout.
personal firewall BlackICE Defender will
may do its own NetBIOS queries separate from the
underlying Windows OS. These will look like UNIX
queries from dynamic ports, and have longer, progressive
timeouts of 15-seconds, 30-seconds, and 1-minute.
packet looks something like the example below. For
more information about interpreting this, please
see my sniffing FAQ at http://www.robertgraham.com/pubs/sniffing-faq.html.
ETHERNET: ETYPE = 0x0800 : Protocol = IP: DOD Internet Protocol
IP: ID = 0x3E16; Proto = UDP; Len: 78
UDP: Src Port: NETBIOS (137); Dst Port: NETBIOS (137); Length = 58
NBT: NS: Query req. for *<00...(15)>
NBT: Transaction ID = 57032 (0xDEC8)
+ NBT: Flags Summary = 0x0000 - Req.; Query; Success
NBT: Question Count = 1 (0x1)
NBT: Answer Count = 0 (0x0)
NBT: Name Service Count = 0 (0x0)
NBT: Additional Record Count = 0 (0x0)
NBT: Question Name = *<00...(15)>
NBT: Question Type = Node Status Request
NBT: Question Class = Internet Class
00000: 00 E0 18 E0 0C E7 00 40 05 A4 79 32 08 00 45 00 .......@..y2..E.
00010: 00 4E 3E 16 00 00 80 11 2F CE 0A 0A 00 09 C0 00 .N>...../.......
00020: 02 A8 00 89 00 89 00 3A 14 AB DE C8 00 00 00 01 .......:........
00030: 00 00 00 00 00 00 20 43 4B 41 41 41 41 41 41 41 ...... CKAAAAAAA
00040: 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 AAAAAAAAAAAAAAAA
00050: 41 41 41 41 41 41 41 00 00 21 00 01 AAAAAAA..!..
10.6 How do I reduce this traffic so I don't
get so many?
will not send a NetBIOS query if the initial DNS query
comes back in a timely manner. Let me repeat: Windows
only sends NetBIOS queries when the DNS lookup fails.
Therefore, the proximate cause of NetBIOS queries
is a fault in the DNS system. The first thing you
should hunt down is the DNS fault causing the DNS
PTR queries to fail.
are seeing a lot of these requests, it probably
means you have one of the following DNS issues.
DNS servers are slow; the Windows machine needs
a response within 14 seconds.
link is unreliable/congested, causing the DNS
queries to be dropped.
haven't configured the PTR entries within your
ISP doesn't forward the PTR queries to your DNS
client's ISP cannot handle CNAME -> PTR indirection
for CIDR addresses.
that in this day/age with CIDR and address blocks
smaller than 255 members, a lot of ISPs don't know
how to forward DNS PTR requests to your server.
what you do, you will still get requests because
of configuration errors on the client's ISP. However,
making sure the issues above are resolved on your
own DNS servers will be an important first step.
10.6.1 What is a DNS PTR query?
- For reasons
of historical irrelevance, a normal DNS query is called
an A record. A reverse query is called a PTR
(pointer) query. The names A and PTR don't really
mean anything; remember that a lot of such things
come about because some engineer created "temporary"
names from the top of his head, meaning to change
them later, but they sort of just stick around.
thing to remember is that A and PTR queries are
you register your domain name (example.com)
you go to the owner of .com (Network Solutions)
and purchase the address. As part of your registration,
you tell Network Solutions something to effect "Please
pass any DNS queries for the domain example.com
to my DNS server ns1.example.com which
is located at the IP address 192.0.2.168".
when resolving www.example.com, you first
ask .com for the DNS server for example.com,
which is ns1.example.com/192.0.2.168. You
then ask that server for the A record for
going the reverse direction is a bit tougher. When
trying to figure out who owns the IP address 192.0.2.3,
you've got a problem. What is the first step? The
solution was to query for a PTR record with the
pseudo-name that looks something like "184.108.40.206.in-addr.arpa".
Like the .com domain, the .arpa
domain is run by a special company. It forwards
the requests to the backbone ISPs, which then forward
the request to the smaller ISPs and customers.
forwarding mechanism is easily broken due to CIDR
addresses. An ISP may assign 192.0.2.[0.127] to
one customer, and 192.0.2.[128-255] to another customer.
In order to fix this issue, the ISP must support
special CNAME records that delegate lookups. For
the network 192.0.2.128/25, then the CNAME record
would look like 128/220.127.116.11.IN-ADDR.ARPA.
This is kinda complex, easy to get wrong, and the
administrators at ISPs often don't know how to do
see CNAME -> PTR indirection described in RFC
2317 for more details on this. Also see http://www.dns.net/dnsrd/ for extensive
upshot is that you probably have a CIDR allocation
that breaks PTR queries causing NetBIOS queries.
Harangue your ISP until they fix this.
10.7 What attacks can people go through NetBIOS/137?
are numerous tools that scan for open shares.
The first popular tool for this was called "Legion"
from Rhino9. Since then, numerous other tools
have been created. Some of these tools will do
a lookup on port 137 before connecting to TCP
in 1999, numerous NetBIOS worms have been seen.
These include ExploreZip virus/worm, Network.VBS
VisualBasic script, and the 911 worm (which also
calls 911 out your modem). All of these worms
will attempt connection to you machine.
people just scan the Internet looking for people's
names. Since most people leave port 137 open,
it is pretty fun.
Intruder Detection Checklist. If you believe
you've been compromised, this document describes
how to go through your UNIX system and find signs
of this intrusion.
you have evidence of a cybercrime that you believe
warrants the attention of the FBI, this is a good
place to start. Note that you can't simply hand
it off to them and say "you take care of it".
They are only willing to take part of you are
willing to spend the necessary time in gathering
evidence. For example, you may have to ship your
compromised machine to them.