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 be used directly or easily driven by other programs and scripts. At the same time, it 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 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:
* 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!
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 you are affiliated in any way with Microsoft Network, get a life. Always ski in control.
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:
* 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.