Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting
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# Chapter 30. Network Programming

| The Net's a cross between an elephant and a white elephant sale: it never forgets, and it's always crap. --Nemo

A Linux system has quite a number of tools for accessing, manipulating, and troubleshooting network connections. We can incorporate some of these tools into scripts -- scripts that expand our knowledge of networking, useful scripts that can facilitate the administration of a network.

Here is a simple CGI script that demonstrates connecting to a remote server.

Example 30-1. Print the server environment

       1 #!/bin/bash
       2 # test-cgi.sh
       3 # by Michael Zick
       4 # Used with permission
       5
       6 # May have to change the location for your site.
       7 # (At the ISP's servers, Bash may not be in the usual place.)
       8 # Other places: /usr/bin or /usr/local/bin
       9 # Might even try it without any path in sha-bang.
      10
      11 # Disable filename globbing.
      12 set -f
      13
      14 # Header tells browser what to expect.
      15 echo Content-type: text/plain
      16 echo
      17
      18 echo CGI/1.0 test script report:
      19 echo
      20
      21 echo environment settings:
      22 set
      23 echo
      24
      25 echo whereis bash?
      26 whereis bash
      27 echo
      28
      29
      30 echo who are we?
      31 echo ${BASH_VERSINFO[*]}
      32 echo
      33
      34 echo argc is $#. argv is "$*".
      35 echo
      36
      37 # CGI/1.0 expected environment variables.
      38
      39 echo SERVER_SOFTWARE = $SERVER_SOFTWARE
      40 echo SERVER_NAME = $SERVER_NAME
      41 echo GATEWAY_INTERFACE = $GATEWAY_INTERFACE
      42 echo SERVER_PROTOCOL = $SERVER_PROTOCOL
      43 echo SERVER_PORT = $SERVER_PORT
      44 echo REQUEST_METHOD = $REQUEST_METHOD
      45 echo HTTP_ACCEPT = "$HTTP_ACCEPT"
      46 echo PATH_INFO = "$PATH_INFO"
      47 echo PATH_TRANSLATED = "$PATH_TRANSLATED"
      48 echo SCRIPT_NAME = "$SCRIPT_NAME"
      49 echo QUERY_STRING = "$QUERY_STRING"
      50 echo REMOTE_HOST = $REMOTE_HOST
      51 echo REMOTE_ADDR = $REMOTE_ADDR
      52 echo REMOTE_USER = $REMOTE_USER
      53 echo AUTH_TYPE = $AUTH_TYPE
      54 echo CONTENT_TYPE = $CONTENT_TYPE
      55 echo CONTENT_LENGTH = $CONTENT_LENGTH
      56
      57 exit 0
      58
      59 # Here document to give short instructions.
      60 :<<-'_test_CGI_'
      61
      62 1) Drop this in your http://domain.name/cgi-bin directory.
      63 2) Then, open http://domain.name/cgi-bin/test-cgi.sh.
      64
      65 _test_CGI_

For security purposes, it may be helpful to identify the IP addresses a computer is accessing.

Example 30-2. IP addresses

       1 #!/bin/bash
       2 # ip-addresses.sh
       3 # List the IP addresses your computer is connected to.
       4
       5 #  Inspired by Greg Bledsoe's ddos.sh script,
       6 #  Linux Journal, 09 March 2011.
       7 #    URL:
       8 #  http://www.linuxjournal.com/content/back-dead-simple-bash-complex-ddos
       9 #  Greg licensed his script under the GPL2,
      10 #+ and as a derivative, this script is likewise GPL2.
      11
      12 connection_type=TCP      # Also try UDP.
      13 field=2           # Which field of the output we're interested in.
      14 no_match=LISTEN   # Filter out records containing this. Why?
      15 lsof_args=-ni     # -i lists Internet-associated files.
      16                   # -n preserves numerical IP addresses.
      17 		  # What happens without the -n option? Try it.
      18 router="[0-9][0-9][0-9][0-9][0-9]->"
      19 #       Delete the router info.
      20
      21 lsof "$lsof_args" | grep $connection_type | grep -v "$no_match" |
      22       awk '{print $9}' | cut -d : -f $field | sort | uniq |
      23       sed s/"^$router"//
      24
      25 #  Bledsoe's script assigns the output of a filtered IP list,
      26 #  (similar to lines 19-22, above) to a variable.
      27 #  He checks for multiple connections to a single IP address,
      28 #  then uses:
      29 #
      30 #    iptables -I INPUT -s $ip -p tcp -j REJECT --reject-with tcp-reset
      31 #
      32 #  ... within a 60-second delay loop to bounce packets from DDOS attacks.
      33
      34
      35 #  Exercise:
      36 #  --------
      37 #  Use the 'iptables' command to extend this script
      38 #+ to reject connection attempts from well-known spammer IP domains.

More examples of network programming:

1. Getting the time from nist.gov

2. Downloading a URL

3. A GRE tunnel

4. Checking if an Internet server is up

5. Example 16-41

6. Example A-28

7. Example A-29

8. Example 29-1

See also the networking commands in the System and Administrative Commands chapter and the communications commands in the External Filters, Programs and Commands chapter.

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