<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML ><HEAD ><TITLE >Bitmore indepth version</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK REL="HOME" TITLE="Masquerading Made Simple HOWTO" HREF="index.html"><LINK REL="PREVIOUS" TITLE="Summary: (I like doing summaries first)" HREF="summary.html"><LINK REL="NEXT" TITLE="Post-install Instructions" HREF="post-install.html"></HEAD ><BODY CLASS="SECT1" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >Masquerading Made Simple HOWTO</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="summary.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="post-install.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="INDEPTH" ></A >3. Bitmore indepth version</H1 ><P > Compiling the kernel: (Use a 2.4.x kernel or greater) </P ><P > You need the following support in the kernel: </P ><P ></P ><UL ><LI ><P > Under Networking Options </P ><P ></P ><UL ><LI ><P > Network packet filtering (CONFIG_NETFILTER) </P ></LI ></UL ></LI ><LI ><P > Under Networking Options->Netfilter Configuration </P ><P ></P ><UL ><LI ><P > Connection tracking (CONFIG_IP_NF_CONNTRACK) </P ></LI ><LI ><P > FTP Protocol support (CONFIG_IP_NF_FTP) </P ></LI ><LI ><P > IP tables support (CONFIG_IP_NF_IPTABLES) </P ></LI ><LI ><P > Connection state match support (CONFIG_IP_NF_MATCH_STATE) </P ></LI ><LI ><P > Packet filtering (CONFIG_IP_NF_FILTER) </P ><P ></P ><UL ><LI ><P > REJECT target support (CONFIG_IP_NF_TARGET_REJECT) </P ></LI ></UL ></LI ><LI ><P > Full NAT (CONFIG_IP_NF_NAT) </P ><P ></P ><UL ><LI ><P > MASQUERADE target support (CONFIG_IP_NF_TARGET_MASQUERADE) </P ></LI ><LI ><P > REDIRECT target support (CONFIG_IP_NF_TARGET_REDIRECT) </P ></LI ></UL ></LI ><LI ><P > Packet mangling (CONFIG_IP_NF_MANGLE) </P ></LI ><LI ><P > LOG target support (CONFIG_IP_NF_TARGET_LOG) </P ></LI ></UL ></LI ></UL ><P > First, if the iptable and masq modules are not compiled into the kernel and not installed, but do exist as modules, we need to install them. If you insmod ipt_MASQUERADE it will load ip_tables, ip_conntrack and iptable_nat. </P ><TABLE BORDER="1" BGCOLOR="#E0E0E0" WIDTH="100%" ><TR ><TD ><FONT COLOR="#000000" ><PRE CLASS="SCREEN" ><TT CLASS="PROMPT" >$></TT > <B CLASS="COMMAND" >modprobe ipt_MASQERADE</B ></PRE ></FONT ></TD ></TR ></TABLE ><P > Now either your Intranet is large, or you're just trying to get two or three machines to work on the internet - it doesn't make much difference either way. </P ><P > Okay, I'm assuming that you have no other rules, so do: </P ><TABLE BORDER="1" BGCOLOR="#E0E0E0" WIDTH="100%" ><TR ><TD ><FONT COLOR="#000000" ><PRE CLASS="SCREEN" ><TT CLASS="PROMPT" >$></TT > <B CLASS="COMMAND" >iptables -F; iptables -t nat -F; iptables -t mangle -F</B ></PRE ></FONT ></TD ></TR ></TABLE ><P > If you get an error saying can't find iptables, go find it and install it. If it says no such table 'nat', recompile the kernel with nat support. If it says no such table as 'mangle', don't worry about it, it's not necessary for MASQ'ing. If it says iptables is incompatible with your kernel, go get > 2.4 and compile that with iptables support. </P ><P > Then if you have a static ip do (e.g. network card not using DHCP): </P ><TABLE BORDER="1" BGCOLOR="#E0E0E0" WIDTH="100%" ><TR ><TD ><FONT COLOR="#000000" ><PRE CLASS="SCREEN" ><TT CLASS="PROMPT" >$></TT > <B CLASS="COMMAND" >iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 123.12.23.43</B ></PRE ></FONT ></TD ></TR ></TABLE ><P > or for dynamic (e.g. a modem - you have to call a number first): </P ><TABLE BORDER="1" BGCOLOR="#E0E0E0" WIDTH="100%" ><TR ><TD ><FONT COLOR="#000000" ><PRE CLASS="SCREEN" ><TT CLASS="PROMPT" >$></TT > <B CLASS="COMMAND" >iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE</B ></PRE ></FONT ></TD ></TR ></TABLE ><P > Then finally to tell the kernel yes, you really do want to start forwarding packets: (This only needs to be done once per reboot - but dosen't hurt to do it lots) </P ><TABLE BORDER="1" BGCOLOR="#E0E0E0" WIDTH="100%" ><TR ><TD ><FONT COLOR="#000000" ><PRE CLASS="SCREEN" ><TT CLASS="PROMPT" >$></TT > <B CLASS="COMMAND" >echo 1 > /proc/sys/net/ipv4/ip_forward</B ></PRE ></FONT ></TD ></TR ></TABLE ><P > Once you have checked this all works (See under Post-install) only allow masquerading from the internal network - you don't want to allow people on the internet to use it after all :) </P ><P > First, allow any existing connections, or anything related (e.g. ftp server connecting back to you) </P ><TABLE BORDER="1" BGCOLOR="#E0E0E0" WIDTH="100%" ><TR ><TD ><FONT COLOR="#000000" ><PRE CLASS="SCREEN" ><TT CLASS="PROMPT" >$></TT > <B CLASS="COMMAND" >iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT</B ></PRE ></FONT ></TD ></TR ></TABLE ><P > If this gives an error, then you most likely don't have state tracking in the kernel - go recompile. Then allow new connections only from our intranet (local/internal network). Replace the ppp0 with eth0 or whatever your <EM >external</EM > device is. (The ! means anything but) </P ><TABLE BORDER="1" BGCOLOR="#E0E0E0" WIDTH="100%" ><TR ><TD ><FONT COLOR="#000000" ><PRE CLASS="SCREEN" ><TT CLASS="PROMPT" >$></TT > <B CLASS="COMMAND" >iptables -A INPUT -m state --state NEW -i ! ppp0 -j ACCEPT</B ></PRE ></FONT ></TD ></TR ></TABLE ><P > And now deny everything else: </P ><TABLE BORDER="1" BGCOLOR="#E0E0E0" WIDTH="100%" ><TR ><TD ><FONT COLOR="#000000" ><PRE CLASS="SCREEN" ><TT CLASS="PROMPT" >$></TT > <B CLASS="COMMAND" >iptables -P INPUT DROP</B > #only if the first two are succesful</PRE ></FONT ></TD ></TR ></TABLE ><P > If either of the first two rules failed, then this last rule with prevent the masquerading from working at all. To undo this rule do "<B CLASS="COMMAND" >iptables -P INPUT ACCEPT</B >". </P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="summary.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="post-install.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Summary: (I like doing summaries first)</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" > </TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Post-install Instructions</TD ></TR ></TABLE ></DIV ></BODY ></HTML >