<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> <HTML> <HEAD> <META NAME="GENERATOR" CONTENT="LinuxDoc-Tools 0.9.73"> <TITLE>Linux 2.4 Packet Filtering HOWTO: Rusty's Really Quick Guide To Packet Filtering</TITLE> <LINK HREF="packet-filtering-HOWTO-6.html" REL=next> <LINK HREF="packet-filtering-HOWTO-4.html" REL=previous> <LINK HREF="packet-filtering-HOWTO.html#toc5" REL=contents> </HEAD> <BODY> <A HREF="packet-filtering-HOWTO-6.html">Next</A> <A HREF="packet-filtering-HOWTO-4.html">Previous</A> <A HREF="packet-filtering-HOWTO.html#toc5">Contents</A> <HR> <H2><A NAME="s5">5.</A> <A HREF="packet-filtering-HOWTO.html#toc5">Rusty's Really Quick Guide To Packet Filtering</A></H2> <P>Most people just have a single PPP connection to the Internet, and don't want anyone coming back into their network, or the firewall:</P> <P> <BLOCKQUOTE><CODE> <PRE> ## Insert connection-tracking modules (not needed if built into kernel). # insmod ip_conntrack # insmod ip_conntrack_ftp ## Create chain which blocks new connections, except if coming from inside. # iptables -N block # iptables -A block -m state --state ESTABLISHED,RELATED -j ACCEPT # iptables -A block -m state --state NEW -i ! ppp0 -j ACCEPT # iptables -A block -j DROP ## Jump to that chain from INPUT and FORWARD chains. # iptables -A INPUT -j block # iptables -A FORWARD -j block </PRE> </CODE></BLOCKQUOTE> </P> <HR> <A HREF="packet-filtering-HOWTO-6.html">Next</A> <A HREF="packet-filtering-HOWTO-4.html">Previous</A> <A HREF="packet-filtering-HOWTO.html#toc5">Contents</A> </BODY> </HTML>