Comments on: Netcat – a couple of useful examples

By: Robbert Hamburg

Robbert Hamburg — Fri, 02 Mar 2007 11:47:25 +0000

Hmm that comment of gary was very usefull to me !

Thanks !

By: Gary

Gary — Fri, 05 Jan 2007 15:42:05 +0000

The main drawback of netcat is that it needs to be started on both sides of the wire to make a transfer work. What I usually do is this:

tar cz /etc | ssh user@host -c ‘cd /bak ; cat > etc.tgz’

On the localhost, this will create an archive of /etc, which is piped into an ssh connection. On the remote host, the snippet enclosed in ” is executed: Change to the backup directory, and pipe the content of the connection into a file. If I wanted to extract the files again on the receiving side, I could do:

tar cz /etc | ssh user@host -c ‘cd /bak ; cat tar xz’

Note that the ‘-f -’ given in the article is redundant. If tar is not given a file, it uses STDIN/OUT. The advantage over recursive scp is that this method preserves device nodes and file permissions. The advantage over rsync is that it is using familiar tar semantics on both sides (of course, if you want diffing, you need rsync, which works just fine over SSH).

Of course, the other direction (pulling files from a host) is equally possible:

ssh user@host -c ‘tar cz /etc’ > /bak/etc.tgz

By: Cucirca

Cucirca — Thu, 04 Jan 2007 16:44:20 +0000

Thanks for this useful commands ;)

By: GNot

GNot — Mon, 27 Nov 2006 09:21:53 +0000

Thanks. There are indeed many ways to use netcat effectively. Very interesting.

By: Traveller

Traveller — Sat, 25 Nov 2006 10:38:52 +0000

One handy use of netcat I didn’t see mentioned is a fast way to get an bit-for-bit image copy of a working machine’s root partition onto another machine.

Boot source and destination off live CD’s, make sure source and destination partitions are similar in size with cfdisk and then pipe directly from the source machine’s partition to the destination machine’s partition via netcat. When copy is complete mount the destination partition and tweak IP address, hostname and /etc/iftab. Reboot. You should now have a duplicate working machine.

Booting off the live CD’s insures that there are no open files in source and destination partition and provides tools for adjusting partition sizes etc. In addition since the partition contents are copied directly no intermediate storage is required.

By: anonymous

anonymous — Thu, 16 Nov 2006 18:51:46 +0000

Here is a fine tutorial on using netcat to pipe audio over a lan.

By: GNot

GNot — Fri, 10 Nov 2006 07:35:29 +0000

Thanks for providing an example. CLI option syntax may vary from distribution to distribution. It seems that this syntax difference when setting the listening port is one of the major differences.

By: DP

DP — Thu, 09 Nov 2006 17:23:18 +0000

This article is excellent.

When I tried to listen, I faced some problem. Because in my system, command syntax is different. I have to use netcat -l -p 3333 in my system.

By: GNot

GNot — Wed, 08 Nov 2006 06:05:54 +0000

Exactly. The same functionality can be achieved with other tools and protocols like FTP, SSH, RSync etc. As I have written above, netcat is not a necessary tool in the everyday use, but its primitive functionality could be useful in a particular occasion.

Using scp is my method of choice in order to transfer files between networked computers. I just wanted to show how to transfer files using netcat through an SSH tunnel. That’s all :)

Thanks for your comment.

By: anjan bacchu

anjan bacchu — Tue, 07 Nov 2006 21:33:50 +0000

hi there,

nice article.

question : if one already has left the SSH port open, we can as well use scp to get the file. If so, why would we want to use netcat on top of that ?

BR,
~A