Advanced Bash-Scripting Guide:
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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

    #!/bin/bash
    # test-cgi.sh
    # by Michael Zick
    # Used with permission

    # May have to change the location for your site.
    # (At the ISP's servers, Bash may not be in the usual place.)
    # Other places: /usr/bin or /usr/local/bin
    # Might even try it without any path in sha-bang.

    # Disable filename globbing.
    set -f

    # Header tells browser what to expect.
    echo Content-type: text/plain
    echo

    echo CGI/1.0 test script report:
    echo

    echo environment settings:
    set
    echo

    echo whereis bash?
    whereis bash
    echo

    echo who are we?
    echo ${BASH_VERSINFO[*]}
    echo

    echo argc is $#. argv is "$*".
    echo

    # CGI/1.0 expected environment variables.

    echo SERVER_SOFTWARE = $SERVER_SOFTWARE
    echo SERVER_NAME = $SERVER_NAME
    echo GATEWAY_INTERFACE = $GATEWAY_INTERFACE
    echo SERVER_PROTOCOL = $SERVER_PROTOCOL
    echo SERVER_PORT = $SERVER_PORT
    echo REQUEST_METHOD = $REQUEST_METHOD
    echo HTTP_ACCEPT = "$HTTP_ACCEPT"
    echo PATH_INFO = "$PATH_INFO"
    echo PATH_TRANSLATED = "$PATH_TRANSLATED"
    echo SCRIPT_NAME = "$SCRIPT_NAME"
    echo QUERY_STRING = "$QUERY_STRING"
    echo REMOTE_HOST = $REMOTE_HOST
    echo REMOTE_ADDR = $REMOTE_ADDR
    echo REMOTE_USER = $REMOTE_USER
    echo AUTH_TYPE = $AUTH_TYPE
    echo CONTENT_TYPE = $CONTENT_TYPE
    echo CONTENT_LENGTH = $CONTENT_LENGTH

    exit 0

    # Here document to give short instructions.
    :<<-'_test_CGI_'

    1) Drop this in your http://domain.name/cgi-bin directory.
    2) Then, open http://domain.name/cgi-bin/test-cgi.sh.

    _test_CGI_

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

Example 30-2. IP addresses

    #!/bin/bash
    # ip-addresses.sh
    # List the IP addresses your computer is connected to.

    #  Inspired by Greg Bledsoe's ddos.sh script,
    #  Linux Journal, 09 March 2011.
    #    URL:
    #  http://www.linuxjournal.com/content/back-dead-simple-bash-complex-ddos
    #  Greg licensed his script under the GPL2,
    #+ and as a derivative, this script is likewise GPL2.

    connection_type=TCP      # Also try UDP.
    field=2           # Which field of the output we're interested in.
    no_match=LISTEN   # Filter out records containing this. Why?
    lsof_args=-ni     # -i lists Internet-associated files.
                      # -n preserves numerical IP addresses.
    		  # What happens without the -n option? Try it.
    router="[0-9][0-9][0-9][0-9][0-9]->"
    #       Delete the router info.

    lsof "$lsof_args" | grep $connection_type | grep -v "$no_match" |
          awk '{print $9}' | cut -d : -f $field | sort | uniq |
          sed s/"^$router"//

    #  Bledsoe's script assigns the output of a filtered IP list,
    #  (similar to lines 19-22, above) to a variable.
    #  He checks for multiple connections to a single IP address,
    #  then uses:
    #
    #    iptables -I INPUT -s $ip -p tcp -j REJECT --reject-with tcp-reset
    #
    #  ... within a 60-second delay loop to bounce packets from DDOS attacks.

    #  Exercise:
    #  --------
    #  Use the 'iptables' command to extend this script
    #+ 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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