bash Cookbook

Using bash Net-Redirection

Problem

You need to send or receive very simple network traffic but you do not have a tool such as netcat installed.

Solution

If you have bash version 2.04+ compiled with --enable-net-redirections (it isn’t compiled this way in Debian and derivatives), you can use bash itself. The following example is also used in Finding My IP Address:

$ exec 3<> /dev/tcp/www.ippages.com/80
$ echo -e "GET /simple/?se=1 HTTP/1.0\n" >&3
$ cat <&3
HTTP/1.1 200 OK
Date: Tue, 28 Nov 2006 08:13:08 GMT
Server: Apache/2.0.52 (Red Hat)
X-Powered-By: PHP/4.3.9
Set-Cookie: smipcomID=6670614; expires=Sun, 27-Nov-2011 08:13:09 GMT; path=/
Pragma: no-cache
Cache-Control: no-cache, must-revalidate
Content-Length: 125
Connection: close
Content-Type: text/plain; charset=ISO-8859-1

72.NN.NN.225 (US-United States) http://www..com Tue, 28 Nov 2006 08:13:09 UTC/GMT
flagged User Agent - reduced functionality

Warning

As noted, this recipe will probably not work under Debian and derivatives such as Ubuntu since they expressly do not compile bash with --enable-net-redirections.

Discussion

As noted in Redirecting Output for the Life of a Script, it is possible to use exec to permanently redirect file handles within the current shell session, so the first command sets up input and output on file handle 3. The second line sends a trivial command to a path on the web server defined in the first command. Note that the user agent will appear as “-” on the web server side, which is what is causing the “flagged ...