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[Netcat rules the net ----------]
Netcat 1.10
Netcat is a simple Unix utility which reads
and writes data across network connections,
using TCP or UDP protocol. It is designed /\_/\
to be a reliable "back-end" tool that can / 0 0 \
be used directly or easily driven by other ====v====
programs and scripts. At the same time, it \ W /
is a feature-rich network debugging and | | _
exploration tool, since it can create / ___ \ /
almost any kind of connection you would / / \ \ |
need and has several interesting built-in (((-----)))-'
capabilities. Netcat, or "nc" as the actual /
program is named, should have been supplied ( ___
long ago as another one of those cryptic \__.=|___E
but standard Unix tools. /
In the simplest usage, "nc host port" creates a TCP connection to the
given port on the given target host. Your standard input is then sent
to the host, and anything that comes back across the connection is
sent to your standard output. This continues indefinitely, until the
network side of the connection shuts down. Note that this behavior is
different from most other applications which shut everything down and
exit after an end-of-file on the standard input.
Netcat can also function as a server, by listening for inbound
connections on arbitrary ports and then doing the same reading and
writing. With minor limitations, netcat doesn't really care if it runs
in "client" or "server" mode -- it still shovels data back and forth
until there isn't any more left. In either mode, shutdown can be
forced after a configurable time of inactivity on the network side.
And it can do this via UDP too, so netcat is possibly the "udp
telnet-like" application you always wanted for testing your UDP-mode
servers. UDP, as the "U" implies, gives less reliable data
transmission than TCP connections and some systems may have trouble
sending large amounts of data that way, but it's still a useful
capability to have.
You may be asking "why not just use telnet to connect to arbitrary
ports?" Valid question, and here are some reasons. Telnet has the
"standard input EOF" problem, so one must introduce calculated delays
in driving scripts to allow network output to finish. This is the main
reason netcat stays running until the *network* side closes. Telnet
also will not transfer arbitrary binary data, because certain
characters are interpreted as telnet options and are thus removed from
the data stream. Telnet also emits some of its diagnostic messages to
standard output, where netcat keeps such things religiously separated
from its *output* and will never modify any of the real data in
transit unless you *really* want it to. And of course telnet is
incapable of listening for inbound connections, or using UDP instead.
Netcat doesn't have any of these limitations, is much smaller and
faster than telnet, and has many other advantages.
Some of netcat's major features are:
* Outbound or inbound connections, TCP or UDP, to or from any ports
* Full DNS forward/reverse checking, with appropriate warnings
* Ability to use any local source port
* Ability to use any locally-configured network source address
* Built-in port-scanning capabilities, with randomizer
* Built-in loose source-routing capability
* Can read command line arguments from standard input
* Slow-send mode, one line every N seconds
* Hex dump of transmitted and received data
* Optional ability to let another program service established
connections
* Optional telnet-options responder
Efforts have been made to have netcat "do the right thing" in all its
various modes. If you believe that it is doing the wrong thing under
whatever circumstances, please notify me and tell me how you think it
should behave. If netcat is not able to do some task you think up,
minor tweaks to the code will probably fix that. It provides a basic
and easily-modified template for writing other network applications,
and I certainly encourage people to make custom mods and send in any
improvements they make to it. This is the second release; the overall
differences from 1.00 are relatively minor and have mostly to do with
portability and bugfixes. Many people provided greatly appreciated
fixes and comments on the 1.00 release. Continued feedback from the
Internet community is always welcome!
Netcat is entirely my own creation, although plenty of other code was
used as examples. It is freely given away to the Internet community in
the hope that it will be useful, with no restrictions except giving
credit where it is due. No GPLs, Berkeley copyrights or any of that
nonsense. The author assumes NO responsibility for how anyone uses it.
If netcat makes you rich somehow and you're feeling generous, mail me
a check. If you are affiliated in any way with Microsoft Network, get
a life. Always ski in control. Comments, questions, and patches to
hobbit@avian.org.
Building
Compiling is fairly straightforward. Examine the Makefile for a
SYSTYPE that matches yours, and do "make ". The executable "nc" should
appear. If there is no relevant SYSTYPE section, try "generic". If you
create new sections for generic.h and Makefile to support another
platform, please follow the given format and mail back the diffs.
There are a couple of other settable #defines in netcat.c, which you
can include as DFLAGS="-DTHIS -DTHAT" to your "make" invocation
without having to edit the Makefile. See the following discussions for
what they are and do.
If you want to link against the resolver library on SunOS
[recommended] and you have BIND 4.9.x, you may need to change
XLIBS=-lresolv in the Makefile to XLIBS="-lresolv -l44bsd".
Linux sys/time.h does not really support presetting of FD_SETSIZE; a
harmless warning is issued.
Some systems may warn about pointer types for signal(). No problem,
though.
Exploration of features
Where to begin? Netcat is at the same time so simple and versatile,
it's like trying to describe everything you can do with your Swiss
Army knife. This will go over the basics; you should also read the
usage examples and notes later on which may give you even more ideas
about what this sort of tool is good for.
If no command arguments are given at all, netcat asks for them, reads
a line from standard input, and breaks it up into arguments
internally. This can be useful when driving netcat from certain types
of scripts, with the side effect of hiding your command line arguments
from "ps" displays.
The host argument can be a name or IP address. If -n is specified,
netcat will only accept numeric IP addresses and do no DNS lookups for
anything. If -n is not given and -v is turned on, netcat will do a
full forward and reverse name and address lookup for the host, and
warn you about the all-too-common problem of mismatched names in the
DNS. This often takes a little longer for connection setup, but is
useful to know about. There are circumstances under which this can
*save* time, such as when you want to know the name for some IP
address and also connect there. Netcat will just tell you all about
it, saving the manual steps of looking up the hostname yourself.
Normally mismatch- checking is case-insensitive per the DNS spec, but
you can define ANAL at compile time to make it case-sensitive --
sometimes useful for uncovering minor errors in your own DNS files
while poking around your networks.
A port argument is required for outbound connections, and can be
numeric or a name as listed in /etc/services. If -n is specified, only
numeric arguments are valid. Special syntax and/or more than one port
argument cause different behavior -- see details below about
port-scanning.
The -v switch controls the verbosity level of messages sent to
standard error. You will probably want to run netcat most of the time
with -v turned on, so you can see info about the connections it is
trying to make. You will probably also want to give a smallish -w
argument, which limits the time spent trying to make a connection. I
usually alias "nc" to "nc -v -w 3", which makes it function just about
the same for things I would otherwise use telnet to do. The timeout is
easily changed by a subsequent -w argument which overrides the earlier
one. Specifying -v more than once makes diagnostic output MORE
verbose. If -v is not specified at all, netcat silently does its work
unless some error happens, whereupon it describes the error and exits
with a nonzero status. Refused network connections are generally NOT
considered to be errors, unless you only asked for a single TCP port
and it was refused.
Note that -w also sets the network inactivity timeout. This does not
have any effect until standard input closes, but then if nothing
further arrives from the network in the next seconds, netcat tries to
read the net once more for good measure, and then closes and exits.
There are a lot of network services now that accept a small amount of
input and return a large amount of output, such as Gopher and Web
servers, which is the main reason netcat was written to "block" on the
network staying open rather than standard input. Handling the timeout
this way gives uniform behavior with network servers that *don't*
close by themselves until told to.
UDP connections are opened instead of TCP when -u is specified. These
aren't really "connections" per se since UDP is a connectionless
protocol, although netcat does internally use the "connected UDP
socket" mechanism that most kernels support. Although netcat claims
that an outgoing UDP connection is "open" immediately, no data is sent
until something is read from standard input. Only thereafter is it
possible to determine whether there really is a UDP server on the
other end, and often you just can't tell. Most UDP protocols use
timeouts and retries to do their thing and in many cases won't bother
answering at all, so you should specify a timeout and hope for the
best. You will get more out of UDP connections if standard input is
fed from a source of data that looks like various kinds of server
requests.
To obtain a hex dump file of the data sent either way, use "-o
logfile". The dump lines begin with "<" or ">" to respectively
indicate "from the net" or "to the net", and contain the total count
per direction, and hex and ascii representations of the traffic.
Capturing a hex dump naturally slows netcat down a bit, so don't use
it where speed is critical.
Netcat can bind to any local port, subject to privilege restrictions
and ports that are already in use. It is also possible to use a
specific local network source address if it is that of a network
interface on your machine. [Note: this does not work correctly on all
platforms.] Use "-p portarg" to grab a specific local port, and "-s
ip-addr" or "-s name" to have that be your source IP address. This is
often referred to as "anchoring the socket". Root users can grab any
unused source port including the "reserved" ones less than 1024.
Absence of -p will bind to whatever unused port the system gives you,
just like any other normal client connection, unless you use -r [see
below].
Listen mode will cause netcat to wait for an inbound connection, and
then the same data transfer happens. Thus, you can do "nc -l -p 1234 <
filename" and when someone else connects to your port 1234, the file
is sent to them whether they wanted it or not. Listen mode is
generally used along with a local port argument -- this is required
for UDP mode, while TCP mode can have the system assign one and tell
you what it is if -v is turned on. If you specify a target host and
optional port in listen mode, netcat will accept an inbound connection
only from that host and if you specify one, only from that foreign
source port. In verbose mode you'll be informed about the inbound
connection, including what address and port it came from, and since
listening on "any" applies to several possibilities, which address it
came *to* on your end. If the system supports IP socket options,
netcat will attempt to retrieve any such options from an inbound
connection and print them out in hex.
If netcat is compiled with -DGAPING_SECURITY_HOLE, the -e argument
specifies a program to exec after making or receiving a successful
connection. In the listening mode, this works similarly to "inetd" but
only for a single instance. Use with GREAT CARE. This piece of the
code is normally not enabled; if you know what you're doing, have fun.
This hack also works in UDP mode. Note that you can only supply -e
with the name of the program, but no arguments. If you want to launch
something with an argument list, write a two-line wrapper script or
just use inetd like always.
If netcat is compiled with -DTELNET, the -t argument enables it to
respond to telnet option negotiation [always in the negative, i.e.
DONT or WONT]. This allows it to connect to a telnetd and get past the
initial negotiation far enough to get a login prompt from the server.
Since this feature has the potential to modify the data stream, it is
not enabled by default. You have to understand why you might need this
and turn on the #define yourself.
Data from the network connection is always delivered to standard
output as efficiently as possible, using large 8K reads and writes.
Standard input is normally sent to the net the same way, but the -i
switch specifies an "interval time" which slows this down
considerably. Standard input is still read in large batches, but
netcat then tries to find where line breaks exist and sends one line
every interval time. Note that if standard input is a terminal, data
is already read line by line, so unless you make the -i interval
rather long, what you type will go out at a fairly normal rate. -i is
really designed for use when you want to "measure out" what is read
from files or pipes.
Port-scanning is a popular method for exploring what's out there.
Netcat accepts its commands with options first, then the target host,
and everything thereafter is interpreted as port names or numbers, or
ranges of ports in M-N syntax. CAVEAT: some port names in
/etc/services contain hyphens -- netcat currently will not correctly
parse those, so specify ranges using numbers if you can. If more than
one port is thus specified, netcat connects to *all* of them, sending
the same batch of data from standard input [up to 8K worth] to each
one that is successfully connected to. Specifying multiple ports also
suppresses diagnostic messages about refused connections, unless -v is
specified twice for "more verbosity". This way you normally get
notified only about genuinely open connections. Example: "nc -v -w 2
-z target 20-30" will try connecting to every port between 20 and 30
[inclusive] at the target, and will likely inform you about an FTP
server, telnet server, and mailer along the way. The -z switch
prevents sending any data to a TCP connection and very limited probe
data to a UDP connection, and is thus useful as a fast scanning mode
just to see what ports the target is listening on. To limit scanning
speed if desired, -i will insert a delay between each port probe.
There are some pitfalls with regard to UDP scanning, described later,
but in general it works well.
For each range of ports specified, scanning is normally done downward
within that range. If the -r switch is used, scanning hops randomly
around within that range and reports open ports as it finds them. [If
you want them listed in order regardless, pipe standard error through
"sort"...] In addition, if random mode is in effect, the local source
ports are also randomized. This prevents netcat from exhibiting any
kind of regular pattern in its scanning. You can exert fairly fine
control over your scan by judicious use of -r and selected port ranges
to cover. If you use -r for a single connection, the source port will
have a random value above 8192, rather than the next one the kernel
would have assigned you. Note that selecting a specific local port
with -p overrides any local-port randomization.
Many people are interested in testing network connectivity using IP
source routing, even if it's only to make sure their own firewalls are
blocking source-routed packets. On systems that support it, the -g
switch can be used multiple times [up to 8] to construct a
loose-source-routed path for your connection, and the -G argument
positions the "hop pointer" within the list. If your network allows
source-routed traffic in and out, you can test connectivity to your
own services via remote points in the internet. Note that although
newer BSD-flavor telnets also have source-routing capability, it isn't
clearly documented and the command syntax is somewhat clumsy. Netcat's
handling of "-g" is modeled after "traceroute".
Netcat tries its best to behave just like "cat". It currently does
nothing to terminal input modes, and does no end-of-line conversion.
Standard input from a terminal is read line by line with normal
editing characters in effect. You can freely suspend out of an
interactive connection and resume. ^C or whatever your interrupt
character is will make netcat close the network connection and exit. A
switch to place the terminal in raw mode has been considered, but so
far has not been necessary. You can send raw binary data by reading it
out of a file or piping from another program, so more meaningful
effort would be spent writing an appropriate front-end driver.
Netcat is not an "arbitrary packet generator", but the ability to talk
to raw sockets and/or nit/bpf/dlpi may appear at some point. Such
things are clearly useful; I refer you to Darren Reed's excellent
ip_filter package, which now includes a tool to construct and send raw
packets with any contents you want.
Example uses -- the light side
Again, this is a very partial list of possibilities, but it may get
you to think up more applications for netcat. Driving netcat with
simple shell or expect scripts is an easy and flexible way to do
fairly complex tasks, especially if you're not into coding network
tools in C. My coding isn't particularly strong either [although
undoubtedly better after writing this thing!], so I tend to construct
bare-metal tools like this that I can trivially plug into other
applications. Netcat doubles as a teaching tool -- one can learn a
great deal about more complex network protocols by trying to simulate
them through raw connections!
An example of netcat as a backend for something else is the
shell-script Web browser, which simply asks for the relevant parts of
a URL and pipes "GET /what/ever" into a netcat connection to the
server. I used to do this with telnet, and had to use calculated sleep
times and other stupidity to kludge around telnet's limitations.
Netcat guarantees that I get the whole page, and since it transfers
all the data unmodified, I can even pull down binary image files and
display them elsewhere later. Some folks may find the idea of a
shell-script web browser silly and strange, but it starts up and gets
me my info a hell of a lot faster than a GUI browser and doesn't hide
any contents of links and forms and such. This is included, as
scripts/web, along with several other web-related examples.
Netcat is an obvious replacement for telnet as a tool for talking to
daemons. For example, it is easier to type "nc host 25", talk to
someone's mailer, and just ^C out than having to type ^]c or QUIT as
telnet would require you to do. You can quickly catalog the services
on your network by telling netcat to connect to well-known services
and collect greetings, or at least scan for open ports. You'll
probably want to collect netcat's diagnostic messages in your output
files, so be sure to include standard error in the output using `>&
file' in *csh or `> file 2>&1' in bourne shell.
A scanning example: "echo QUIT | nc -v -w 5 target 20-250 500-600
5990-7000" will inform you about a target's various well-known TCP
servers, including r-services, X, IRC, and maybe a few you didn't
expect. Sending in QUIT and using the timeout will almost guarantee
that you see some kind of greeting or error from each service, which
usually indicates what it is and what version. [Beware of the
"chargen" port, though...] SATAN uses exactly this technique to
collect host information, and indeed some of the ideas herein were
taken from the SATAN backend tools. If you script this up to try every
host in your subnet space and just let it run, you will not only see
all the services, you'll find out about hosts that aren't correctly
listed in your DNS. Then you can compare new snapshots against old
snapshots to see changes. For going after particular services, a more
intrusive example is in scripts/probe.
Netcat can be used as a simple data transfer agent, and it doesn't
really matter which end is the listener and which end is the client --
input at one side arrives at the other side as output. It is helpful
to start the listener at the receiving side with no timeout specified,
and then give the sending side a small timeout. That way the listener
stays listening until you contact it, and after data stops flowing the
client will time out, shut down, and take the listener with it. Unless
the intervening network is fraught with problems, this should be
completely reliable, and you can always increase the timeout. A
typical example of something "rsh" is often used for: on one side,
nc -l -p 1234 | uncompress -c | tar xvfp -
and then on the other side
tar cfp - /some/dir | compress -c | nc -w 3 othermachine 1234
will transfer the contents of a directory from one machine to another,
without having to worry about .rhosts files, user accounts, or inetd
configurations at either end. Again, it matters not which is the
listener or receiver; the "tarring" machine could just as easily be
running the listener instead. One could conceivably use a scheme like
this for backups, by having cron-jobs fire up listeners and backup
handlers [which can be restricted to specific addresses and ports
between each other] and pipe "dump" or "tar" on one machine to "dd
of=/dev/tapedrive" on another as usual. Since netcat returns a nonzero
exit status for a denied listener connection, scripts to handle such
tasks could easily log and reject connect attempts from third parties,
and then retry.
Another simple data-transfer example: shipping things to a PC that
doesn't have any network applications yet except a TCP stack and a web
browser. Point the browser at an arbitrary port on a Unix server by
telling it to download something like http://unixbox:4444/foo, and
have a listener on the Unix side ready to ship out a file when the
connect comes in. The browser may pervert binary data when told to
save the URL, but you can dig the raw data out of the on-disk cache.
If you build netcat with GAPING_SECURITY_HOLE defined, you can use it
as an "inetd" substitute to test experimental network servers that
would otherwise run under "inetd". A script or program will have its
input and output hooked to the network the same way, perhaps sans some
fancier signal handling. Given that most network services do not bind
to a particular local address, whether they are under "inetd" or not,
it is possible for netcat avoid the "address already in use" error by
binding to a specific address. This lets you [as root, for low ports]
place netcat "in the way" of a standard service, since inbound
connections are generally sent to such specifically-bound listeners
first and fall back to the ones bound to "any". This allows for a
one-off experimental simulation of some service, without having to
screw around with inetd.conf. Running with -v turned on and collecting
a connection log from standard error is recommended.
Netcat as well can make an outbound connection and then run a program
or script on the originating end, with input and output connected to
the same network port. This "inverse inetd" capability could enhance
the backup-server concept described above or help facilitate things
such as a "network dialback" concept. The possibilities are many and
varied here; if such things are intended as security mechanisms, it
may be best to modify netcat specifically for the purpose instead of
wrapping such functions in scripts. Speaking of inetd, netcat will
function perfectly well *under* inetd as a TCP connection redirector
for inbound services, like a "plug-gw" without the authentication
step. This is very useful for doing stuff like redirecting traffic
through your firewall out to other places like web servers and mail
hubs, while posing no risk to the firewall machine itself. Put netcat
behind inetd and tcp_wrappers, perhaps thusly:
www stream tcp nowait nobody /etc/tcpd /bin/nc -w 3 realwww 80
and you have a simple and effective "application relay" with access
control and logging. Note use of the wait time as a "safety" in case
realwww isn't reachable or the calling user aborts the connection --
otherwise the relay may hang there forever.
You can use netcat to generate huge amounts of useless network data
for various performance testing. For example, doing
yes AAAAAAAAAAAAAAAAAAAAAA | nc -v -v -l -p 2222 > /dev/null
on one side and then hitting it with
yes BBBBBBBBBBBBBBBBBBBBBB | nc othermachine 2222 > /dev/null
from another host will saturate your wires with A's and B's. The "very
verbose" switch usage will tell you how many of each were sent and
received after you interrupt either side. Using UDP mode produces
tremendously MORE trash per unit time in the form of fragmented 8
Kbyte mobygrams -- enough to stress-test kernels and network
interfaces. Firing random binary data into various network servers may
help expose bugs in their input handling, which nowadays is a popular
thing to explore. A simple example data-generator is given in
data/data.c included in this package, along with a small collection of
canned input files to generate various packet contents. This program
is documented in its beginning comments, but of interest here is using
"%r" to generate random bytes at well-chosen points in a data stream.
If you can crash your daemon, you likely have a security problem.
The hex dump feature may be useful for debugging odd network
protocols, especially if you don't have any network monitoring
equipment handy or aren't root where you'd need to run "tcpdump" or
something. Bind a listening netcat to a local port, and have it run a
script which in turn runs another netcat to the real service and
captures the hex dump to a log file. This sets up a transparent relay
between your local port and wherever the real service is. Be sure that
the script-run netcat does *not* use -v, or the extra info it sends to
standard error may confuse the protocol. Note also that you cannot
have the "listen/exec" netcat do the data capture, since once the
connection arrives it is no longer netcat that is running.
Binding to an arbitrary local port allows you to simulate things like
r-service clients, if you are root locally. For example, feeding
"^@root^@joe^@pwd^@" [where ^@ is a null, and root/joe could be any
other local/remote username pair] into a "rsh" or "rlogin" server,
FROM your port 1023 for example, duplicates what the server expects to
receive. Thus, you can test for insecure .rhosts files around your
network without having to create new user accounts on your client
machine. The program data/rservice.c can aid this process by
constructing the "rcmd" protocol bytes. Doing this also prevents
"rshd" from trying to create that separate standard-error socket and
still gives you an input path, as opposed to the usual action of "rsh
-n". Using netcat for things like this can be really useful sometimes,
because rsh and rlogin generally want a host *name* as an argument and
won't accept IP addresses. If your client-end DNS is hosed, as may be
true when you're trying to extract backup sets on to a dumb client,
"netcat -n" wins where normal rsh/rlogin is useless.
If you are unsure that a remote syslogger is working, test it with
netcat. Make a UDP connection to port 514 and type in "<0>message",
which should correspond to "kern.emerg" and cause syslogd to scream
into every file it has open [and possibly all over users' terminals].
You can tame this down by using a different number and use netcat
inside routine scripts to send syslog messages to places that aren't
configured in syslog.conf. For example, "echo '<38>message' | nc -w 1
-u loggerhost 514" should send to auth.notice on loggerhost. The exact
number may vary; check against your syslog.h first.
Netcat provides several ways for you to test your own packet filters.
If you bind to a port normally protected against outside access and
make a connection to somewhere outside your own network, the return
traffic will be coming to your chosen port from the "outside" and
should be blocked. TCP may get through if your filter passes all "ack
syn", but it shouldn't be even doing that to low ports on your
network. Remember to test with UDP traffic as well! If your filter
passes at least outbound source-routed IP packets, bouncing a
connection back to yourself via some gateway outside your network will
create "incoming" traffic with your source address, which should get
dropped by a correctly configured anti-spoofing filter. This is a
"non-test" if you're also dropping source-routing, but it's good to be
able to test for that too. Any packet filter worth its salt will be
blocking source-routed packets in both directions, but you never know
what interesting quirks you might turn up by playing around with
source ports and addresses and watching the wires with a network
monitor.
You can use netcat to protect your own workstation's X server against
outside access. X is stupid enough to listen for connections on "any"
and never tell you when new connections arrive, which is one reason it
is so vulnerable. Once you have all your various X windows up and
running you can use netcat to bind just to your ethernet address and
listen to port 6000. Any new connections from outside the machine will
hit netcat instead your X server, and you get a log of who's trying.
You can either tell netcat to drop the connection, or perhaps run
another copy of itself to relay to your actual X server on
"localhost". This may not work for dedicated X terminals, but it may
be possible to authorize your X terminal only for its boot server, and
run a relay netcat over on the server that will in turn talk to your X
terminal. Since netcat only handles one listening connection per run,
make sure that whatever way you rig it causes another one to run and
listen on 6000 soon afterward, or your real X server will be reachable
once again. A very minimal script just to protect yourself could be
while true ; do
nc -v -l -s -p 6000 localhost 2
done
which causes netcat to accept and then close any inbound connection to
your workstation's normal ethernet address, and another copy is
immediately run by the script. Send standard error to a file for a log
of connection attempts. If your system can't do the "specific bind"
thing all is not lost; run your X server on display ":1" or port 6001,
and netcat can still function as a probe alarm by listening on 6000.
Does your shell-account provider allow personal Web pages, but not CGI
scripts? You can have netcat listen on a particular port to execute a
program or script of your choosing, and then just point to the port
with a URL in your homepage. The listener could even exist on a
completely different machine, avoiding the potential ire of the
homepage-host administrators. Since the script will get the raw
browser query as input it won't look like a typical CGI script, and
since it's running under your UID you need to write it carefully. You
may want to write a netcat-based script as a wrapper that reads a
query and sets up environment variables for a regular CGI script. The
possibilities for using netcat and scripts to handle Web stuff are
almost endless. Again, see the examples under scripts/.
Example uses -- the dark side
Equal time is deserved here, since a versatile tool like this can be
useful to any Shade of Hat. I could use my Victorinox to either fix
your car or disassemble it, right? You can clearly use something like
netcat to attack or defend -- I don't try to govern anyone's social
outlook, I just build tools. Regardless of your intentions, you should
still be aware of these threats to your own systems.
The first obvious thing is scanning someone *else's* network for
vulnerable services. Files containing preconstructed data, be it
exploratory or exploitive, can be fed in as standard input, including
command-line arguments to netcat itself to keep "ps" ignorant of your
doings. The more random the scanning, the less likelihood of detection
by humans, scan-detectors, or dynamic filtering, and with -i you'll
wait longer but avoid loading down the target's network. Some examples
for crafting various standard UDP probes are given in data/*.d.
Some configurations of packet filters attempt to solve the FTP-data
problem by just allowing such connections from the outside. These come
FROM port 20, TO high TCP ports inside -- if you locally bind to port
20, you may find yourself able to bypass filtering in some cases.
Maybe not to low ports "inside", but perhaps to TCP NFS servers, X
servers, Prospero, ciscos that listen on 200x and 400x... Similar
bypassing may be possible for UDP [and maybe TCP too] if a connection
comes from port 53; a filter may assume it's a nameserver response.
Using -e in conjunction with binding to a specific address can enable
"server takeover" by getting in ahead of the real ones, whereupon you
can snarf data sent in and feed your own back out. At the very least
you can log a hex dump of someone else's session. If you are root, you
can certainly use -s and -e to run various hacked daemons without
having to touch inetd.conf or the real daemons themselves. You may not
always have the root access to deal with low ports, but what if you
are on a machine that also happens to be an NFS server? You might be
able to collect some interesting things from port 2049, including
local file handles. There are several other servers that run on high
ports that are likely candidates for takeover, including many of the
RPC services on some platforms [yppasswdd, anyone?]. Kerberos tickets,
X cookies, and IRC traffic also come to mind. RADIUS-based terminal
servers connect incoming users to shell-account machines on a high
port, usually 1642 or thereabouts. SOCKS servers run on 1080. Do
"netstat -a" and get creative.
There are some daemons that are well-written enough to bind separately
to all the local interfaces, possibly with an eye toward heading off
this sort of problem. Named from recent BIND releases, and NTP, are
two that come to mind. Netstat will show these listening on address.53
instead of *.53. You won't be able to get in front of these on any of
the real interface addresses, which of course is especially
interesting in the case of named, but these servers sometimes forget
about things like "alias" interface addresses or interfaces that
appear later on such as dynamic PPP links. There are some hacked web
servers and versions of "inetd" floating around that specifically bind
as well, based on a configuration file -- these generally *are* bound
to alias addresses to offer several different address-based services
from one machine.
Using -e to start a remote backdoor shell is another obvious sort of
thing, easier than constructing a file for inetd to listen on
"ingreslock" or something, and you can access-control it against other
people by specifying a client host and port. Experience with this
truly demonstrates how fragile the barrier between being "logged in"
or not really is, and is further expressed by scripts/bsh. If you're
already behind a firewall, it may be easier to make an *outbound*
connection and then run a shell; a small wrapper script can
periodically try connecting to a known place and port, you can later
listen there until the inbound connection arrives, and there's your
shell. Running a shell via UDP has several interesting features,
although be aware that once "connected", the UDP stub sockets tend to
show up in "netstat" just like TCP connections and may not be quite as
subtle as you wanted. Packets may also be lost, so use TCP if you need
reliable connections. But since UDP is connectionless, a hookup of
this sort will stick around almost forever, even if you ^C out of
netcat or do a reboot on your side, and you only need to remember the
ports you used on both ends to reestablish. And outbound UDP-plus-exec
connection creates the connected socket and starts the program
immediately. On a listening UDP connection, the socket is created once
a first packet is received. In either case, though, such a
"connection" has the interesting side effect that only your
client-side IP address and [chosen?] source port will thereafter be
able to talk to it. Instant access control! A non-local third party
would have to do ALL of the following to take over such a session:
* forge UDP with your source address [trivial to do; see below]
* guess the port numbers of BOTH ends, or sniff the wire for them
* arrange to block ICMP or UDP return traffic between it and your
real
* source, so the session doesn't die with a network write error.
The companion program data/rservice.c is helpful in scripting up any
sort of r-service username or password guessing attack. The arguments
to "rservice" are simply the strings that get null-terminated and
passed over an "rcmd"-style connection, with the assumption that the
client does not need a separate standard-error port. Brute-force
password banging is best done via "rexec" if it is available since it
is less likely to log failed attempts. Thus, doing "rservice joe
joespass pwd | nc target exec" should return joe's home dir if the
password is right, or "Permission denied." Plug in a dictionary and go
to town. If you're attacking rsh/rlogin, remember to be root and bind
to a port between 512 and 1023 on your end, and pipe in "rservice joe
joe pwd" and such.
Netcat can prevent inadvertently sending extra information over a
telnet connection. Use "nc -t" in place of telnet, and daemons that
try to ask for things like USER and TERM environment variables will
get no useful answers, as they otherwise would from a more recent
telnet program. Some telnetds actually try to collect this stuff and
then plug the USER variable into "login" so that the caller is then
just asked for a password! This mechanism could cause a login attempt
as YOUR real username to be logged over there if you use a
Borman-based telnet instead of "nc -t".
Got an unused network interface configured in your kernel [e.g. SLIP],
or support for alias addresses? Ifconfig one to be any address you
like, and bind to it with -s to enable all sorts of shenanigans with
bogus source addresses. The interface probably has to be UP before
this works; some SLIP versions need a far-end address before this is
true. Hammering on UDP services is then a no-brainer. What you can do
to an unfiltered syslog daemon should be fairly obvious; trimming the
conf file can help protect against it. Many routers out there still
blindly believe what they receive via RIP and other routing protocols.
Although most UDP echo and chargen servers check if an incoming packet
was sent from *another* "internal" UDP server, there are many that
still do not, any two of which [or many, for that matter] could keep
each other entertained for hours at the expense of bandwidth. And you
can always make someone wonder why she's being probed by nsa.gov.
Your TCP spoofing possibilities are mostly limited to destinations you
can source-route to while locally bound to your phony address. Many
sites block source-routed packets these days for precisely this
reason. If your kernel does oddball things when sending source-routed
packets, try moving the pointer around with -G. You may also have to
fiddle with the routing on your own machine before you start receiving
packets back. Warning: some machines still send out traffic using the
source address of the outbound interface, regardless of your binding,
especially in the case of localhost. Check first. If you can open a
connection but then get no data back from it, the target host is
probably killing the IP options on its end [this is an option inside
TCP wrappers and several other packages], which happens after the
3-way handshake is completed. If you send some data and observe the
"send-q" side of "netstat" for that connection increasing but never
getting sent, that's another symptom. Beware: if Sendmail 8.7.x
detects a source-routed SMTP connection, it extracts the hop list and
sticks it in the Received: header!
SYN bombing [sometimes called "hosing"] can disable many TCP servers,
and if you hit one often enough, you can keep it unreachable for days.
As is true of many other denial-of-service attacks, there is currently
no defense against it except maybe at the human level. Making kernel
SOMAXCONN considerably larger than the default and the half-open
timeout smaller can help, and indeed some people running large
high-performance web servers have *had* to do that just to handle
normal traffic. Taking out mailers and web servers is sociopathic, but
on the other hand it is sometimes useful to be able to, say, disable a
site's identd daemon for a few minutes. If someone realizes what is
going on, backtracing will still be difficult since the packets have a
phony source address, but calls to enough ISP NOCs might eventually
pinpoint the source. It is also trivial for a clueful ISP to watch for
or even block outgoing packets with obviously fake source addresses,
but as we know many of them are not clueful or willing to get involved
in such hassles. Besides, outbound packets with an [otherwise
unreachable] source address in one of their net blocks would look
fairly legitimate.
Notes
A discussion of various caveats, subtleties, and the design of the
innards.
As of version 1.07 you can construct a single file containing command
arguments and then some data to transfer. Netcat is now smart enough
to pick out the first line and build the argument list, and send any
remaining data across the net to one or multiple ports. The first
release of netcat had trouble with this -- it called fgets() for the
command line argument, which behind the scenes does a large read()
from standard input, perhaps 4096 bytes or so, and feeds that out to
the fgets() library routine. By the time netcat 1.00 started directly
read()ing stdin for more data, 4096 bytes of it were gone. It now uses
raw read() everywhere and does the right thing whether reading from
files, pipes, or ttys. If you use this for multiple-port connections,
the single block of data will now be a maximum of 8K minus the first
line. Improvements have been made to the logic in sending the saved
chunk to each new port. Note that any command-line arguments hidden
using this mechanism could still be extracted from a core dump.
When netcat receives an inbound UDP connection, it creates a
"connected socket" back to the source of the connection so that it can
also send out data using normal write(). Using this mechanism instead
of recvfrom/sendto has several advantages -- the read/write select
loop is simplified, and ICMP errors can in effect be received by
non-root users. However, it has the subtle side effect that if further
UDP packets arrive from the caller but from different source ports,
the listener will not receive them. UDP listen mode on a multihomed
machine may have similar quirks unless you specifically bind to one of
its addresses. It is not clear that kernel support for UDP connected
sockets and/or my understanding of it is entirely complete here, so
experiment...
You should be aware of some subtleties concerning UDP scanning. If -z
is on, netcat attempts to send a single null byte to the target port,
twice, with a small time in between. You can either use the -w
timeout, or netcat will try to make a "sideline" TCP connection to the
target to introduce a small time delay equal to the round-trip time
between you and the target. Note that if you have a -w timeout and -i
timeout set, BOTH take effect and you wait twice as long. The TCP
connection is to a normally refused port to minimize traffic, but if
you notice a UDP fast-scan taking somewhat longer than it should, it
could be that the target is actually listening on the TCP port. Either
way, any ICMP port-unreachable messages from the target should have
arrived in the meantime. The second single-byte UDP probe is then
sent. Under BSD kernels, the ICMP error is delivered to the "connected
socket" and the second write returns an error, which tells netcat that
there is NOT a UDP service there. While Linux seems to be a fortunate
exception, under many SYSV derived kernels the ICMP is not delivered,
and netcat starts reporting that *all* the ports are "open" -- clearly
wrong. [Some systems may not even *have* the "udp connected socket"
concept, and netcat in its current form will not work for UDP at all.]
If -z is specified and only one UDP port is probed, netcat's exit
status reflects whether the connection was "open" or "refused" as with
TCP.
It may also be that UDP packets are being blocked by filters with no
ICMP error returns, in which case everything will time out and return
"open". This all sounds backwards, but that's how UDP works. If you're
not sure, try "echo w00gumz | nc -u -w 2 target 7" to see if you can
reach its UDP echo port at all. You should have no trouble using a
BSD-flavor system to scan for UDP around your own network, although
flooding a target with the high activity that -z generates will cause
it to occasionally drop packets and indicate false "opens". A more
"correct" way to do this is collect and analyze the ICMP errors, as
does SATAN's "udp_scan" backend, but then again there's no guarantee
that the ICMP gets back to you either. Udp_scan also does the
zero-byte probes but is excruciatingly careful to calculate its own
round-trip timing average and dynamically set its own response
timeouts along with decoding any ICMP received. Netcat uses a much
sleazier method which is nonetheless quite effective. Cisco routers
are known to have a "dead time" in between ICMP responses about
unreachable UDP ports, so a fast scan of a cisco will show almost
everything "open". If you are looking for a specific UDP service, you
can construct a file containing the right bytes to trigger a response
from the other end and send that as standard input. Netcat will read
up to 8K of the file and send the same data to every UDP port given.
Note that you must use a timeout in this case [as would any other UDP
client application] since the two-write probe only happens if -z is
specified.
Many telnet servers insist on a specific set of option negotiations
before presenting a login banner. On a raw connection you will see
this as small amount of binary gook. My attempts to create fixed input
bytes to make a telnetd happy worked some places but failed against
newer BSD-flavor ones, possibly due to timing problems, but there are
a couple of much better workarounds. First, compile with -DTELNET and
use -t if you just want to get past the option negotiation and talk to
something on a telnet port. You will still see the binary gook -- in
fact you'll see a lot more of it as the options are responded to
behind the scenes. The telnet responder does NOT update the total byte
count, or show up in the hex dump -- it just responds negatively to
any options read from the incoming data stream. If you want to use a
normal full-blown telnet to get to something but also want some of
netcat's features involved like settable ports or timeouts, construct
a tiny "foo" script:
#! /bin/sh
exec nc -otheroptions targethost 23
and then do
nc -l -p someport -e foo localhost &
telnet localhost someport
and your telnet should connect transparently through the exec'ed
netcat to the target, using whatever options you supplied in the "foo"
script. Don't use -t inside the script, or you'll wind up sending
*two* option responses.
I've observed inconsistent behavior under some Linuxes [perhaps just
older ones?] when binding in listen mode. Sometimes netcat binds only
to "localhost" if invoked with no address or port arguments, and
sometimes it is unable to bind to a specific address for listening if
something else is already listening on "any". The former problem can
be worked around by specifying "-s 0.0.0.0", which will do the right
thing despite netcat claiming that it's listening on [127.0.0.1]. This
is a known problem -- for example, there's a mention of it in the
makefile for SOCKS. On the flip side, binding to localhost and sending
packets to some other machine doesn't work as you'd expect -- they go
out with the source address of the sending interface instead. The
Linux kernel contains a specific check to ensure that packets from
127.0.0.1 are never sent to the wire; other kernels may contain
similar code. Linux, of course, *still* doesn't support
source-routing, but they claim that it and many other network
improvements are at least breathing hard.
There are several possible errors associated with making TCP
connections, but to specifically see anything other than "refused",
one must wait the full kernel-defined timeout for a connection to
fail. Netcat's mechanism of wrapping an alarm timer around the connect
prevents the *real* network error from being returned -- "errno" at
that point indicates "interrupted system call" since the connect
attempt was interrupted. Some old 4.3 BSD kernels would actually
return things like "host unreachable" immediately if that was the
case, but most newer kernels seem to wait the full timeout and *then*
pass back the real error. Go figure. In this case, I'd argue that the
old way was better, despite those same kernels generally being the
ones that tear down *established* TCP connections when ICMP-bombed.
Incoming socket options are passed to applications by the kernel in
the kernel's own internal format. The socket-options structure for
source-routing contains the "first-hop" IP address first, followed by
the rest of the real options list. The kernel uses this as is when
sending reply packets -- the structure is therefore designed to be
more useful to the kernel than to humans, but the hex dump of it that
netcat produces is still useful to have.
Kernels treat source-routing options somewhat oddly, but it sort of
makes sense once one understands what's going on internally. The
options list of addresses must contain hop1, hop2, ..., destination.
When a source-routed packet is sent by the kernel [at least BSD], the
actual destination address becomes irrelevant because it is replaced
with "hop1", "hop1" is removed from the options list, and all the
other addresses in the list are shifted up to fill the hole. Thus the
outbound packet is sent from your chosen source address to the first
*gateway*, and the options list now contains hop2, ..., destination.
During all this address shuffling, the kernel does NOT change the
pointer value, which is why it is useful to be able to set the pointer
yourself -- you can construct some really bizarre return paths, and
send your traffic fairly directly to the target but around some larger
loop on the way back. Some Sun kernels seem to never flip the
source-route around if it contains less than three hops, never reset
the pointer anyway, and tries to send the packet [with options
containing a "completed" source route!!] directly back to the source.
This is way broken, of course. [Maybe ipforwarding has to be on? I
haven't had an opportunity to beat on it thoroughly yet.]
"Credits" section: The original idea for netcat fell out of a
long-standing desire and fruitless search for a tool resembling it and
having the same features. After reading some other network code and
realizing just how many cool things about sockets could be controlled
by the calling user, I started on the basics and the rest fell
together pretty quickly. Some port-scanning ideas were taken from
Venema/Farmer's SATAN tool kit, and Pluvius' "pscan" utility. Healthy
amounts of BSD kernel source were perused in an attempt to dope out
socket options and source-route handling; additional help was obtained
from Dave Borman's telnet sources. The select loop is loosely based on
fairly well-known code from "rsh" and Richard Stevens' "sock" program
[which itself is sort of a "netcat" with more obscure features], with
some more paranoid sanity-checking thrown in to guard against the
distinct likelihood that there are subtleties about such things I
still don't understand. I found the argument-hiding method cleanly
implemented in Barrett's "deslogin"; reading the line as input allows
greater versatility and is much less prone to cause bizarre problems
than the more common trick of overwriting the argv array. After the
first release, several people contributed portability fixes; they are
credited in generic.h and the Makefile. Lauren Burka inspired the
ascii art for this revised document. Dean Gaudet at Wired supplied a
precursor to the hex-dump code, and mudge@l0pht.com originally
experimented with and supplied code for the telnet-options responder.
Outbound "-e " resulted from a need to quietly bypass a firewall
installation. Other suggestions and patches have rolled in for which I
am always grateful, but there are only 26 hours per day and a
discussion of feature creep near the end of this document.
Netcat was written with the Russian railroad in mind -- conservatively
built and solid, but it *will* get you there. While the coding style
is fairly "tight", I have attempted to present it cleanly [keeping
*my* lines under 80 characters, dammit] and put in plenty of comments
as to why certain things are done. Items I know to be questionable are
clearly marked with "XXX". Source code was made to be modified, but
determining where to start is difficult with some of the tangles of
spaghetti code that are out there. Here are some of the major points I
feel are worth mentioning about netcat's internal design, whether or
not you agree with my approach.
Except for generic.h, which changes to adapt more platforms, netcat is
a single source file. This has the distinct advantage of only having
to include headers once and not having to re-declare all my functions
in a billion different places. I have attempted to contain all the
gross who's-got-what-.h-file things in one small dumping ground.
Functions are placed "dependencies-first", such that when the compiler
runs into the calls later, it already knows the type and arguments and
won't complain. No function prototyping -- not even the __P(()) crock
-- is used, since it is more portable and a file of this size is easy
enough to check manually. Each function has a standard-format comment
ahead of it, which is easily found using the regexp " :$". I freely
use gotos. Loops and if-clauses are made as small and non-nested as
possible, and the ends of same *marked* for clarity [I wish everyone
would do this!!].
Large structures and buffers are all malloc()ed up on the fly,
slightly larger than the size asked for and zeroed out. This reduces
the chances of damage from those "end of the buffer" fencepost errors
or runaway pointers escaping off the end. These things are permanent
per run, so nothing needs to be freed until the program exits.
File descriptor zero is always expected to be standard input, even if
it is closed. If a new network descriptor winds up being zero, a
different one is asked for which will be nonzero, and fd zero is
simply left kicking around for the rest of the run. Why? Because
everything else assumes that stdin is always zero and "netfd" is
always positive. This may seem silly, but it was a lot easier to code.
The new fd is obtained directly as a new socket, because trying to
simply dup() a new fd broke subsequent socket-style use of the new fd
under Solaris' stupid streams handling in the socket library.
The catch-all message and error handlers are implemented with an ample
list of phoney arguments to get around various problems with varargs.
Varargs seems like deliberate obfuscation in the first place, and
using it would also require use of vfprintf() which not all platforms
support. The trailing sleep in bail() is to allow output to flush,
which is sometimes needed if netcat is already on the other end of a
network connection.
The reader may notice that the section that does DNS lookups seems
much gnarlier and more confusing than other parts. This is NOT MY
FAULT. The sockaddr and hostent abstractions are an abortion that
forces the coder to deal with it. Then again, a lot of BSD kernel code
looks like similar struct-pointer hell. I try to straighten it out
somewhat by defining my own HINF structure, containing names,
ascii-format IP addresses, and binary IP addresses. I fill this
structure exactly once per host argument, and squirrel everything
safely away and handy for whatever wants to reference it later.
Where many other network apps use the FIONBIO ioctl to set
non-blocking I/O on network sockets, netcat uses straightforward
blocking I/O everywhere. This makes everything very lock-step, relying
on the network and filesystem layers to feed in data when needed. Data
read in is completely written out before any more is fetched. This may
not be quite the right thing to do under some OSes that don't do timed
select() right, but this remains to be seen.
The hexdump routine is written to be as fast as possible, which is why
it does so much work itself instead of just sprintf()ing everything
together. Each dump line is built into a single buffer and atomically
written out using the lowest level I/O calls. Further improvements
could undoubtedly be made by using writev() and eliminating all
sprintf()s, but it seems to fly right along as is. If both exec-a-prog
mode and a hexdump file is asked for, the hexdump flag is deliberately
turned off to avoid creating random zero-length files. Files are
opened in "truncate" mode; if you want "append" mode instead, change
the open flags in main().
main() may look a bit hairy, but that's only because it has to go down
the argv list and handle multiple ports, random mode, and exit status.
Efforts have been made to place a minimum of code inside the getopt()
loop. Any real work is sent off to functions in what is hopefully a
straightforward way.
Obligatory vendor-bash: If "nc" had become a standard utility years
ago, the commercial vendors would have likely packaged it setuid root
and with -DGAPING_SECURITY_HOLE turned on but not documented. It is
hoped that netcat will aid people in finding and fixing the no-brainer
holes of this sort that keep appearing, by allowing easier
experimentation with the "bare metal" of the network layer.
It could be argued that netcat already has too many features. I have
tried to avoid "feature creep" by limiting netcat's base functionality
only to those things which are truly relevant to making network
connections and the everyday associated DNS lossage we're used to.
Option switches already have slightly overloaded functionality. Random
port mode is sort of pushing it. The hex-dump feature went in later
because it *is* genuinely useful. The telnet-responder code *almost*
verges on the gratuitous, especially since it mucks with the data
stream, and is left as an optional piece. Many people have asked for
example "how 'bout adding encryption?" and my response is that such
things should be separate entities that could pipe their data
*through* netcat instead of having their own networking code. I am
therefore not completely enthusiastic about adding any more features
to this thing, although you are still free to send along any mods you
think are useful.
Nonetheless, at this point I think of netcat as my tcp/ip swiss army
knife, and the numerous companion programs and scripts to go with it
as duct tape. Duct tape of course has a light side and a dark side and
binds the universe together, and if I wrap enough of it around what
I'm trying to accomplish, it *will* work. Alternatively, if netcat is
a large hammer, there are many network protocols that are increasingly
looking like nails by now...
_H* 960320 v1.10 RELEASE -- happy spring!
[Netcat rules the net ----------]