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10 February 2009 @ 11:07 pm
Learning socat in terms of netcat  
In my previous post on sslrsh I wrote about a script to allow remote shell access over SSL. The script made extensive use of socat. It reminded me of how feature-complete socat is, and has motivated me to capture some socat recipes. Note that these aren't general purpose scripts; they are just snippets of functionality listed here for future reference.

I'm not the only person who has a socat tutorial, but I think this post is unique because it will attempt to describe socat by comparing it to a tool that is doubtless a major inspiration for socat, namely, netcat. Hopefully, it will clarify how to use socat, demonstrate how much more featureful socat is, but also show why you shouldn't go ahead and delete netcat outright.

This is part one of a three part series. This one compares socat with netcat. The next one will delve into UDP with socat, and the last one will get into some advanced topics.

Final comment before we start. As of this writing, socat version 2.0 has entered some beta release. socat 2.0 addresses a limitation in socat 1.x, which is that "addresses" in socat 1.x are not completely uniform, and they are not layerable. For example, there's no way to run SSL over UDP, even thought socat knows about both protocols. Similarly, there's no way to have an SSL connection be tunneled through a web PROXY, meaning you have to resort to the hack found in sslrsh. socat 2.0 addresses these limitation, but uses an enhanced syntax, which means 1) it will be even more complicated to use, and 2) this post may become obsolete rather sooner than expected.

But before we compare socat to netcat, let's compare it to their common namesake, cat.

Use cat to display a file on standard output.

jdimpson@artoo:~$ cat file.txt
This is the content of file.txt


Use socat to display a file on standard output.

jdimpson@artoo:~$ socat FILE:file.txt STDOUT
This is the content of file.txt


In general, socat takes two arguments. Both are called addresses. In the above example, FILE:... is one address, and STDOUT is the second. It's customary but not required to spell the address name in upper case. We'll see lots of address types in this post, as well as in a couple follow-on posts that I've got planned.

If you just run "cat" by itself, it will read from standard input and write to standard output, and you have to press control-D to end.

jdimpson@artoo:~$ cat
hello, world!
hello, world!


The first line after the command is typed in, the second is printed by the command.

Here's the equivalent using socat.

jdimpson@artoo:~$ socat STDIN STDOUT
hello, world!
hello, world!


Apparantly, STDIN and STDOUT are both synonyms for STDIO, and socat doesn't care if you send input to the STDOUT address, or read output from the STDIN address. "socat STDOUT STDOUT", "socat STDIN STIN", and "socat STDIO STDIO" all appear to work identically.


But even here socat can improve the situation. We can add a history, so that we can just hit up arrow to repeat what we've typed in earlier, just like bash can do. It utilizes the GNU Readline library.

jdimpson@artoo:~$ socat -u READLINE STDOUT
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!


To get this output, I first typed "hello, world!", pressed enter. socat wrote the second line. The I pressed up arrow to get the third line, and enter to get the fourth. Finally, one more up arrow for the fifth, and again enter for the sixth.

The "-u" flag tells socat to run in unidirectional mode. As we'll see later on, socat usually passes data between the first and second addresses in either direction, something cat does not do. When both addresses end up I connecting to the terminal, as is the case here, it's undertermined as to whether the line you type is being read by the first address an sent to the second, or vice versa. It took me a while to figure this out (over a month after I originally posted this!). By forcing unidirectional mode, only the first address reads what you types, and passes it to the second one.

When you quit using control-c or control-d, the terminal gets messed up, and you have to type "reset" (even though you may not be able see what you're typing) to fix it. The READLINE address has a couple of options, one of which lets you set a history file, which stores the the input history across invocations, just like your "~/.bash_history" file. This example isn't how you'd normally use READLINE, but I'm postponing further discussion on READLINE to another post.

Use cat to create a file (then again to display it)

jdimpson@artoo:~$ cat > file.txt
I like writing files using cat and control-D!!
jdimpson@artoo:~$ cat file.txt
I like writing files using cat and control-D!!


Note that, technically, the shell is actually writing the file by virtue of the redirect symbol (greater than sign).

Use socat to create a file (then use cat to display it)

jdimpson@artoo:~$ socat -u STDIN OPEN:file.txt,creat,trunc
socat needs some funny commands to write files!
jdimpson@artoo:~$ cat file.txt
socat needs some funny commands to write files!


Again. a huge difference between socat and cat is that socat, by default, is bidirectional. So both addresses are read from and written to. cat is always unidirectional. And, in socat, when either one of the addresses sends an EOF (End of File), it waits some amount of time and then exits. And again, the "-u" flag tells socat to be unidirectional. Without it, the above socat invocation will read from the file, get EOF, and exit. Or, if the file doesn't exist, it will quit with an error. There would be no time to type anything in. If instead you pipe something in to socat, like this echo foo | socat STDIN OPEN:file.txt,creat,trunc, the -u isn't needed. Presumably, when invoked within a shell pipe, socat realizes that the fact and know that pipes are always unidirectional, and will behave as if the -u flag were given.

Note the options used, creat and trunc. You could also use append, and lots of other options available to the open() system call. Also, without the trunc option, socat will write bytes into the file in-place. Omitting trunc and using the seek option, you can change arbitrary bytes in the file. There's rdonly and wronly options (read-only and write-only, respectively). I had thought that if I used wronly option, I wouldn't need the -u flag. That didn't work because socat still tried to read from the file, got an error, and exited. Probably the determination of uni- or bi-directionality is done without input from address-specific options. It does work as expected if you pipe input into socat. socat also has a CREATE address based on the creat(), but this is equivalent to OPEN with the creat option.

That covers the major forms of cat, and how socat emulates them, and in some cases enhances them. I don't suggest ever using socat to do what cat can do, but you should have a better sense for how to invoke socat. Now let's compare socat with netcat.

In netcat, connect to TCP port 80 on localhost, as a poor man's web browser.

jdimpson@artoo:~$ nc localhost 80
HEAD / HTTP/1.0
User-agent: netcat, baby!

HTTP/1.1 200 OK
Date: Wed, 28 Jan 2009 13:06:43 GMT
Server: Apache/2.2.8 (Ubuntu) PHP/5.2.4-2ubuntu5.4 with Suhosin-Patch mod_ssl/2.2.8 OpenSSL/0.9.8g mod_perl/2.0.3 Perl/v5.8.8
Last-Modified: Sat, 10 Jan 2009 22:01:08 GMT
ETag: "24008-369-4602802c0f100"
Accept-Ranges: bytes
Content-Length: 873
Connection: close
Content-Type: text/html



I typed in the first three lines (third one is an empty line). The rest is output from the server.

In socat, connect to TCP port 80 on localhost, as a poor man's web browser.

jdimpson@artoo:~$ socat - TCP:localhost:80
HEAD / HTTP/1.0
User-agent: socat, natch!

HTTP/1.1 200 OK
Date: Wed, 28 Jan 2009 13:07:51 GMT
Server: Apache/2.2.8 (Ubuntu) PHP/5.2.4-2ubuntu5.4 with Suhosin-Patch mod_ssl/2.2.8 OpenSSL/0.9.8g mod_perl/2.0.3 Perl/v5.8.8
Last-Modified: Sat, 10 Jan 2009 22:01:08 GMT
ETag: "24008-369-4602802c0f100"
Accept-Ranges: bytes
Content-Length: 873
Connection: close
Content-Type: text/html



Note the "-". That's a shortcut for writing "STDIO", so the above command is equivalent to "socat STDIO TCP:localhost:80".

netcat as a server, listening on TCP port 11111.

nc -l -p 11111


Use "nc localhost 11111" from another window to connect to it. You can type in both windows, and should see each others input in both

There are a couple versions of netcat out there, and some versions (like the OpenBSD one) have had the command flag syntax changed. If you get an error running netcat as decribed above, try it without the -p flag, like this: "nc -l 11111". Some people really don't get the ideas of compatibility and portability--if they want to change the way a program works (presumably because they think they are improving it), fine. But the should also change the name of the program so that hundreds of scripts don't break, and that future script writers don't have to test for each version. Anyways...

socat as a server, listening on TCP port 11111.

socat STDIO TCP-LISTEN:11111,reuseaddr


Use "nc localhost 11111" from another window to connect to it. You can type in both windows, and should see each others input in both.

Note that because socat is bidirectional, it doesn't matter which order you put the addresses. The above is equivalent to "socat TCP-LISTEN:11111,reuseaddr STDIO".

TCP-L can be used as a shortcut for TCP-LISTEN. The reuseaddr option lets you quit socat and run it again immediately. netcat does that by default.

netcat as a server, listening on TCP port 11111, handling multiple connections. This one is untested, and created from memory.

nc -L -p 11111


There used to be some versions of netcat that could handle more than one incoming connection when given the "-L" flag, but I can't find a copy of netcat that works that way, nor even any documentation for it. (Maybe I've imagined it!) It's almost equivalent to this shell script snippet: "while true; do nc -l -p 11111; done", except that this snippet only handles one connection at a time, not multiple ones. The OpenBSD variant of netcat has a -k option which works just like the shell snippet, but still doesn't handle multiple simultaneous connections.

socat as a server, listening on TCP port 11111, handling multiple connections.

socat STDIO TCP-L:11111,reuseaddr,fork


Now open two more windows and run "nc localhost 11111" in each. These are clients to your socat server. What you type in each client window gets displayed in the server window. But what you type in the server window only goes to one of the clients. Each line alternates between each client.  The fork option to TCP-L tells socat to fork a new process for each received connection on port 11111.  Each new process then reads and writes on the standard input/output.

TCP: will use IPv4 or IPv6 depending on which type of address you provide. TCP-LISTEN: will listen on all local addresses (IPv4 and IPv6) unless limited by the bind option. There exist TCP4, TCP6, TCP4-LISTEN, and TCP6-LISTEN variations, as well.

netcat as a UDP server on port 11111.

nc -u -l -p 11111

and then as a UDP client.

nc localhost 11111


socat as a UDP server on port 11111
.

socat - UDP-LISTEN:11111

and then as a UDP client.

socat - UDP:localhost:11111


Again, UDP-L can be used instead of UDP-LISTEN. UDP will use IPv4 or IPv6 depending on which type of address you provide. UDP-LISTEN will listen on all local addresses (IPv4 and IPv6) unless limited by the bind option. There exist UDP4, UDP6, UDP4-LISTEN, and UDP6-LISTEN variations, as well.

socat has other UDP-based addresses that implement other communication patterns beyond what netcat can do. I started to enumerate them here, but the UDP subject ended up dominating this article, so I've pulled it out and link to it here, so this one can remain focused on comparison with netcat.

The coolest, and most dangerous, netcat option is -e, which causes netcat to execute a command when it connects out or receives a connection. A simple remote access server looks like this:


nc -l -p 2323 -e /bin/bash


The strict equivalent simple remote access server in socat is:

socat TCP-LISTEN:2323,reuseaddr EXEC:/bin/bash


However, you can improve on this in several ways. First, the argument to -e in netcat has to be the name of an executable program, found somewhere on the disk. It can't be multiple commands, and can't rely on shell behaviours, like variable handling or wildcard expansion. Not a major impedance, because you can always write out your commands into a shell script, but sometimes doing that is inconvenient. But socat has the SYSTEM address, which uses the system() call rather than a call to exec(), which is what -e in netcat and EXEC in socat do. It enables something like this:


socat TCP-LISTEN:2323,reuseaddr SYSTEM:'echo $HOME; ls -la'


As always whenever the system() call is involved, be aware when writing scripts to not allow unchecked input to be invoked by the system() call. If you try the above in netcat ("nc -l -p 2323 -e 'echo $HOME; ls -la'"), you'll get an error like this: "exec echo $HOME; ls -la failed : No such file or directory", because netcat tried to execute a program called, literally, "echo $HOME; ls -la", spaces and all. Some versions of netcat have a "-c" option, which uses system() instead of exec(), which would allow multiple commands and shell behaviours to work. But again, it depends on which version you have.

netcat is often employed as a data forwarder, aka a simple proxy, listening for incoming connections only to redirect data to another destination port and/or address. It does so by going in to listen mode with -l, then using -e to invoke itself as a client. Because of the use of exec() instead of system(), you have to put the client call into a shell script. First, the client script, "nc-cli", looks like this:

#!/bin/sh
nc localhost 22


Then the call to netcat looks like this:

nc -l -p 2323 -e "./nc-cli"


This redirects incoming connections to port 2323 around to port 22.

(Sometimes you see inetd or xinetd configured to use netcat to do redirecting.)

Of course, you can implement the exact netcat behaviour with "socat STDIO EXEC:nc-cli", or even "socat TCP-L:2323 SYSTEM:'socat STDIO TCP:localhost:22'". However, there's a better way to do data forwarding with socat, which doesn't need a client shell script or even a recursive call to socat. By now you should have enough information about socat to figure it out yourself, so I'll put the example beneath a cut.


socat TCP-L:2323 TCP:localhost:22



And of course, you can replace either address with any other socat address we've already talked about (UDP, UDP-L), or ones we'll talk about in another post (e.g. SSL).

socat can also handle common forwarding requests that netcat doesn't handle. While netcat can bridge between TCP and UDP (insert the -u flag in the above netcat example as appropriate), it can only handle UDP data that is essentially connection-oriented. With socat, any other communication patterns for which UDP is commonly used are also do-able. Just replace the STDIO address in any of the examples in the socat UDP article with TCP or TCP-L addresses as appropriate.

socat can even behave as a
socket gender changer! This part might be a bit confusing to understand; there used to be a file called "TCP-IP_GenderChanger_CSNC_V1.0.pdf" that described the problem, but it seems to be absent from its original location. So I shall try to describe it. The "gender" of a socket is, in this analogy, whether it is a client or a server socket. So a socket gender changer allows two client sockets to connect to each other, or two server sockets to connect to each other. In either case, the gender changer must be running on a host reachable by both clients or both servers. It can run on the same host as either pair, or on a third host. netcat can do this, but with some limitations.

Why would you need this? Off-hand, I can't think of any network protocols that would allow two clients or two servers to just start communicating. So it's not a capability in demand as often as an audio cable gender changer is. But there is one case where it may be useful. Say you have a host running a service that's hidden behind a firewall. No one can connect to the service because the firewall prevents incoming connections. It will allow outgoing connections. Now imagine you can run software on a system outside of the firewall. If you run a server-server gender changer on the external host, and a client-client gender change on the internal host (with one client connecting to the internal service, and the other to one of the server ports on the external host) you have in effect fooled the firewall into allowing access to the internal service despite its access control rules forbidding incoming connections. The above-referenced URL has the specifics of how to use socat to do this. Notice that socat has all sort of retry and timing options to get the desired behaviour. netcat doesn't have all these options, although you may be able to compensate for their absence with a shell script.

That brings to an end the direct comparison of socat and netcat functionality. There's a lot more that socat can do, which I'll address in another article (one on UDP and multicast, the other on everything else). There are some things netcat can do that I didn't discuss, like how it can do telnet negotiation or port scanning. I really consider those out of place in netcat, because they're too application focused. I tried to point out all the netcat options that are only available in some versions of netcat where appropriate. I didn't talk about the source routing ability of (again, some versions of) netcat. socat can do this too, using the ipoptions option, but it's difficult to use. Mostly, though, I don't know enough about source routing to compare the two; something to add to my list of things to figure out. Don't forget, here's the socat & UDP article.
 
 
 
simonsharry on May 17th, 2012 04:53 am (UTC)
An excellent article!
Thank you so much for not just writing up such an excellent article but for also identifying the very need in the first place for such an article. I am a networking/tcp-ip newbie, and boy, have you made a world of a difference to my understanding. The man page of socat is so intimidating otherwise.

Please write more articles on such system-level tools. I'm trying to learn about openssl, stunnel, snort, etc. It will be wonderful if you could write on these. There seems to be no RSS feed mechanism on LiveJournal, so not sure how to stay informed about your posts. Please comment back, if you get a chance.
simonsharry on June 27th, 2012 12:38 pm (UTC)
Here's possibly the dead link to the file.
> there used to be a file called
> "TCP-IP_GenderChanger_CSNC_V1.0.pdf" that
> described the problem...

I certainly couldn't locate the above file. However, the file (or, probably its equivalent) can be accessed from:
http://www.bandwidthco.com/whitepapers/netforensics/tcpip/TCP-IP%20Gender%20Changer.pdf

jdimpsonjdimpson on July 3rd, 2012 06:53 pm (UTC)
Re: Here's possibly the dead link to the file.
Yes, that's the paper. Your google skills surpass my own. Thanks!