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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><H1
><A
NAME="NETWORKPROGRAMMING"
></A
>Chapter 30. Network Programming</H1
><TABLE
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><TD
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>&nbsp;</TD
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><I
><P
><I
>The Net's a cross between an elephant and a white
	      elephant sale: it never forgets, and it's always crap.</I
></P
><P
><I
>--Nemo</I
></P
></I
></TD
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><P
>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.</P
><P
><A
NAME="CGISCRIPT"
></A
>Here is a simple CGI
      script that demonstrates connecting to a remote server.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="TESTCGI"
></A
><P
><B
>Example 30-1. Print the server environment</B
></P
><TABLE
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# test-cgi.sh
   3&nbsp;# by Michael Zick
   4&nbsp;# Used with permission
   5&nbsp;
   6&nbsp;# May have to change the location for your site.
   7&nbsp;# (At the ISP's servers, Bash may not be in the usual place.)
   8&nbsp;# Other places: /usr/bin or /usr/local/bin
   9&nbsp;# Might even try it without any path in sha-bang.
  10&nbsp;
  11&nbsp;# Disable filename globbing.
  12&nbsp;set -f
  13&nbsp;
  14&nbsp;# Header tells browser what to expect.
  15&nbsp;echo Content-type: text/plain
  16&nbsp;echo
  17&nbsp;
  18&nbsp;echo CGI/1.0 test script report:
  19&nbsp;echo
  20&nbsp;
  21&nbsp;echo environment settings:
  22&nbsp;set
  23&nbsp;echo
  24&nbsp;
  25&nbsp;echo whereis bash?
  26&nbsp;whereis bash
  27&nbsp;echo
  28&nbsp;
  29&nbsp;
  30&nbsp;echo who are we?
  31&nbsp;echo ${BASH_VERSINFO[*]}
  32&nbsp;echo
  33&nbsp;
  34&nbsp;echo argc is $#. argv is "$*".
  35&nbsp;echo
  36&nbsp;
  37&nbsp;# CGI/1.0 expected environment variables.
  38&nbsp;
  39&nbsp;echo SERVER_SOFTWARE = $SERVER_SOFTWARE
  40&nbsp;echo SERVER_NAME = $SERVER_NAME
  41&nbsp;echo GATEWAY_INTERFACE = $GATEWAY_INTERFACE
  42&nbsp;echo SERVER_PROTOCOL = $SERVER_PROTOCOL
  43&nbsp;echo SERVER_PORT = $SERVER_PORT
  44&nbsp;echo REQUEST_METHOD = $REQUEST_METHOD
  45&nbsp;echo HTTP_ACCEPT = "$HTTP_ACCEPT"
  46&nbsp;echo PATH_INFO = "$PATH_INFO"
  47&nbsp;echo PATH_TRANSLATED = "$PATH_TRANSLATED"
  48&nbsp;echo SCRIPT_NAME = "$SCRIPT_NAME"
  49&nbsp;echo QUERY_STRING = "$QUERY_STRING"
  50&nbsp;echo REMOTE_HOST = $REMOTE_HOST
  51&nbsp;echo REMOTE_ADDR = $REMOTE_ADDR
  52&nbsp;echo REMOTE_USER = $REMOTE_USER
  53&nbsp;echo AUTH_TYPE = $AUTH_TYPE
  54&nbsp;echo CONTENT_TYPE = $CONTENT_TYPE
  55&nbsp;echo CONTENT_LENGTH = $CONTENT_LENGTH
  56&nbsp;
  57&nbsp;exit 0
  58&nbsp;
  59&nbsp;# Here document to give short instructions.
  60&nbsp;:&#60;&#60;-'_test_CGI_'
  61&nbsp;
  62&nbsp;1) Drop this in your http://domain.name/cgi-bin directory.
  63&nbsp;2) Then, open http://domain.name/cgi-bin/test-cgi.sh.
  64&nbsp;
  65&nbsp;_test_CGI_</PRE
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><HR></DIV
><P
>For security purposes, it may be helpful to identify the IP
      addresses a computer is accessing.</P
><P
><A
NAME="IPTABLES02"
></A
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="IPADDRESSES"
></A
><P
><B
>Example 30-2. IP addresses</B
></P
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# ip-addresses.sh
   3&nbsp;# List the IP addresses your computer is connected to.
   4&nbsp;
   5&nbsp;#  Inspired by Greg Bledsoe's ddos.sh script,
   6&nbsp;#  Linux Journal, 09 March 2011.
   7&nbsp;#    URL:
   8&nbsp;#  http://www.linuxjournal.com/content/back-dead-simple-bash-complex-ddos
   9&nbsp;#  Greg licensed his script under the GPL2,
  10&nbsp;#+ and as a derivative, this script is likewise GPL2.
  11&nbsp;
  12&nbsp;connection_type=TCP      # Also try UDP.
  13&nbsp;field=2           # Which field of the output we're interested in.
  14&nbsp;no_match=LISTEN   # Filter out records containing this. Why?
  15&nbsp;lsof_args=-ni     # -i lists Internet-associated files.
  16&nbsp;                  # -n preserves numerical IP addresses.
  17&nbsp;		  # What happens without the -n option? Try it.
  18&nbsp;router="[0-9][0-9][0-9][0-9][0-9]-&#62;"
  19&nbsp;#       Delete the router info.
  20&nbsp;
  21&nbsp;lsof "$lsof_args" | grep $connection_type | grep -v "$no_match" |
  22&nbsp;      awk '{print $9}' | cut -d : -f $field | sort | uniq |
  23&nbsp;      sed s/"^$router"//
  24&nbsp;
  25&nbsp;#  Bledsoe's script assigns the output of a filtered IP list,
  26&nbsp;#  (similar to lines 19-22, above) to a variable.
  27&nbsp;#  He checks for multiple connections to a single IP address,
  28&nbsp;#  then uses:
  29&nbsp;#
  30&nbsp;#    iptables -I INPUT -s $ip -p tcp -j REJECT --reject-with tcp-reset
  31&nbsp;#
  32&nbsp;#  ... within a 60-second delay loop to bounce packets from DDOS attacks.
  33&nbsp;
  34&nbsp;
  35&nbsp;#  Exercise:
  36&nbsp;#  --------
  37&nbsp;#  Use the 'iptables' command to extend this script
  38&nbsp;#+ to reject connection attempts from well-known spammer IP domains.</PRE
></TD
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><HR></DIV
><P
>More examples of network programming:
       <OL
TYPE="1"
><LI
><P
><A
HREF="devproc.html#NPREF"
>Getting the time from
           <I
CLASS="FIRSTTERM"
>nist.gov</I
></A
></P
></LI
><LI
><P
><A
HREF="devproc.html#NW001"
>Downloading a
           URL</A
></P
></LI
><LI
><P
><A
HREF="system.html#IPSCRIPT0"
>A GRE
	   tunnel</A
></P
></LI
><LI
><P
><A
HREF="communications.html#PING0"
>Checking
	   if an Internet server is up</A
></P
></LI
><LI
><P
><A
HREF="communications.html#ISSPAMMER"
>Example 16-41</A
></P
></LI
><LI
><P
><A
HREF="contributed-scripts.html#ISSPAMMER2"
>Example A-28</A
></P
></LI
><LI
><P
><A
HREF="contributed-scripts.html#WHX"
>Example A-29</A
></P
></LI
><LI
><P
><A
HREF="devproc.html#DEVTCP"
>Example 29-1</A
></P
></LI
></OL
>
     </P
><P
>See also the <A
HREF="system.html#NETWORKSYS1"
>networking commands</A
>
       in the <A
HREF="system.html"
>System and
       Administrative Commands</A
> chapter and the <A
HREF="communications.html"
>communications commands</A
> in
       the <A
HREF="external.html"
>External Filters, Programs and
       Commands</A
> chapter.</P
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