Iptables setting script

# Http://www.cs.princeton.edu /~ Jns/security/iptables/index.html # PreparedbyJamesC. Stephen # (jns@gfdl.noaa.gov )#! /Bin/bash # Loadappropriatemodules. modprobeip_table

# http://www.cs.princeton.edu/~jns/security/iptables -Fiptables -Xiptables -Z# Set up a default DROP policy for the built-in chains.# If we modify and re-run the script mid-session then (because we have a default DROP# policy), what happens is that there is a small time period when packets are denied until# the new rules are back in place. There is no period, however small, when packets we# don't want are allowed.iptables -P INPUT DROPiptables -P FORWARD DROPiptables -P OUTPUT DROP## ===========================================================## Some definitions:NAMESERVER_1="x.x.x.x"NAMESERVER_2="x.x.x.x"BROADCAST="x.x.x.255"LOOPBACK="127.0.0.0/8"CLASS_A="10.0.0.0/8"CLASS_B="172.16.0.0/12"CLASS_C="192.168.0.0/16"CLASS_D_MULTICAST="224.0.0.0/4"CLASS_E_RESERVED_NET="240.0.0.0/5"P_PORTS="0:1023"UP_PORTS="1024:65535"TR_SRC_PORTS="32769:65535"TR_DEST_PORTS="33434:33523"## ============================================================## Kernel flags# To dynamically change kernel parameters and variables on the fly you need# CONFIG_SYSCTL defined in your kernel. I would advise the following:# Disable response to ping./bin/echo "1" > /proc/sys/net/ipv4/icmp_echo_ignore_all# Disable response to broadcasts.# You don't want yourself becoming a Smurf amplifier./bin/echo "1" > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts# Don't accept source routed packets. Attackers can use source routing to generate# traffic pretending to be from inside your network, but which is routed back along# the path from which it came, namely outside, so attackers can compromise your# network. Source routing is rarely used for legitimate purposes./bin/echo "0" > /proc/sys/net/ipv4/conf/all/accept_source_route# Disable ICMP redirect acceptance. ICMP redirects can be used to alter your routing# tables, possibly to a bad end./bin/echo "0" > /proc/sys/net/ipv4/conf/all/accept_redirects# Enable bad error message protection./bin/echo "1" > /proc/sys/net/ipv4/icmp_ignore_bogus_error_responses# Turn on reverse path filtering. This helps make sure that packets use# legitimate source addresses, by automatically rejecting incoming packets# if the routing table entry for their source address doesn't match the network# interface they're arriving on. This has security advantages because it prevents# so-called IP spoofing, however it can pose problems if you use asymmetric routing# (packets from you to a host take a different path than packets from that host to you)# or if you operate a non-routing host which has several IP addresses on different# interfaces. (Note - If you turn on IP forwarding, you will also get this).for interface in /proc/sys/net/ipv4/conf/*/rp_filter; do/bin/echo "1" > ${interface}done# Log spoofed packets, source routed packets, redirect packets./bin/echo "1" > /proc/sys/net/ipv4/conf/all/log_martians# Make sure that IP forwarding is turned off. We only want this for a multi-homed host./bin/echo "0" > /proc/sys/net/ipv4/ip_forward# Note: With connection tracking, all fragments are reassembled before being# passed to the packet-filtering code so there is no ip_always_defrag switch as there# was in the 2.2 kernel.## ============================================================# RULES## LOOPBACK# Allow unlimited traffic on the loopback interface.iptables -A INPUT -i lo -j ACCEPTiptables -A OUTPUT -o lo -j ACCEPT## SYN-FLOODING PROTECTION# This rule maximises the rate of incoming connections. In order to do this we divert tcp# packets with the SYN bit set off to a user-defined chain. Up to limit-burst connections# can arrive in 1/limit seconds ..... in this case 4 connections in one second. After this, one# of the burst is regained every second and connections are allowed again. The default limit# is 3/hour. The default limit burst is 5.#iptables -N syn-floodiptables -A INPUT -i $IFACE -p tcp --syn -j syn-floodiptables -A syn-flood -m limit --limit 1/s --limit-burst 4 -j RETURNiptables -A syn-flood -j DROP## Make sure NEW tcp connections are SYN packetsiptables -A INPUT -i $IFACE -p tcp ! --syn -m state --state NEW -j DROP## FRAGMENTS# I have to say that fragments scare me more than anything.# Sending lots of non-first fragments was what allowed Jolt2 to effectively "drown"# Firewall-1. Fragments can be overlapped, and the subsequent interpretation of such# fragments is very OS-dependent (see this paper for details).# I am not going to trust any fragments.# Log fragments just to see if we get any, and deny them too.iptables -A INPUT -i $IFACE -f -j LOG --log-prefix "IPTABLES FRAGMENTS: "iptables -A INPUT -i $IFACE -f -j DROP## SPOOFING# Most of this anti-spoofing stuff is theoretically not really necessary with the flags we# have set in the kernel above ........... but you never know there isn't a bug somewhere in# your IP stack.## Refuse spoofed packets pretending to be from your IP address.iptables -A INPUT -i $IFACE -s $IPADDR -j DROP# Refuse packets claiming to be from a Class A private network.iptables -A INPUT -i $IFACE -s $CLASS_A -j DROP# Refuse packets claiming to be from a Class B private network.iptables -A INPUT -i $IFACE -s $CLASS_B -j DROP# Refuse packets claiming to be from a Class C private network.iptables -A INPUT -i $IFACE -s $CLASS_C -j DROP# Refuse Class D multicast addresses. Multicast is illegal as a source address.iptables -A INPUT -i $IFACE -s $CLASS_D_MULTICAST -j DROP# Refuse Class E reserved IP addresses.iptables -A INPUT -i $IFACE -s $CLASS_E_RESERVED_NET -j DROP# Refuse packets claiming to be to the loopback interface.# Refusing packets claiming to be to the loopback interface protects against# source quench, whereby a machine can be told to slow itself down by an icmp source# quench to the loopback.iptables -A INPUT -i $IFACE -d $LOOPBACK -j DROP# Refuse broadcast address packets.iptables -A INPUT -i $IFACE -d $BROADCAST -j DROP## DNS# NOTE: DNS uses tcp for zone transfers, for transfers greater than 512 bytes (possible, but unusual), and on certain# platforms like AIX (I am told), so you might have to add a copy of this rule for tcp if you need it# Allow UDP packets in for DNS client from nameservers.iptables -A INPUT -i $IFACE -p udp -s $NAMESERVER_1 --sport 53 -m state --state ESTABLISHED -j ACCEPTiptables -A INPUT -i $IFACE -p udp -s $NAMESERVER_2 --sport 53 -m state --state ESTABLISHED -j ACCEPT# Allow UDP packets to DNS servers from client.iptables -A OUTPUT -o $IFACE -p udp -d $NAMESERVER_1 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPTiptables -A OUTPUT -o $IFACE -p udp -d $NAMESERVER_2 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT## SSH# Allow ssh outbound.iptables -A INPUT -i $IFACE -p tcp --sport 22 -m state --state ESTABLISHED -j ACCEPTiptables -A OUTPUT -o $IFACE -p tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT## WWW# Allow www outbound to 80.iptables -A INPUT -i $IFACE -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPTiptables -A OUTPUT -o $IFACE -p tcp --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT# Allow www outbound to 443.iptables -A INPUT -i $IFACE -p tcp --sport 443 -m state --state ESTABLISHED -j ACCEPTiptables -A OUTPUT -o $IFACE -p tcp --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT## TELNET# Allow telnet outbound.iptables -A INPUT -i $IFACE -p tcp --sport 23 -m state --state ESTABLISHED -j ACCEPTiptables -A OUTPUT -o $IFACE -p tcp --dport 23 -m state --state NEW,ESTABLISHED -j ACCEPT## FTP# Allow ftp outbound.iptables -A INPUT -i $IFACE -p tcp --sport 21 -m state --state ESTABLISHED -j ACCEPTiptables -A OUTPUT -o $IFACE -p tcp --dport 21 -m state --state NEW,ESTABLISHED -j ACCEPT# Now for the connection tracking part of ftp. This is discussed more completely in my section# on connection tracking to be found here.# 1) Active ftp.# This involves a connection INbound from port 20 on the remote machine, to a local port# passed over the ftp channel via a PORT command. The ip_conntrack_ftp module recognizes# the connection as RELATED to the original outgoing connection to port 21 so we don't# need NEW as a state match.iptables -A INPUT -i $IFACE -p tcp --sport 20 -m state --state ESTABLISHED,RELATED -j ACCEPTiptables -A OUTPUT -o $IFACE -p tcp --dport 20 -m state --state ESTABLISHED -j ACCEPT# 2) Passive ftp.# This involves a connection outbound from a port >1023 on the local machine, to a port >1023# on the remote machine previously passed over the ftp channel via a PORT command. The# ip_conntrack_ftp module recognizes the connection as RELATED to the original outgoing# connection to port 21 so we don't need NEW as a state match.iptables -A INPUT -i $IFACE -p tcp --sport $UP_PORTS --dport $UP_PORTS-m state --state ESTABLISHED -j ACCEPTiptables -A OUTPUT -o $IFACE -p tcp --sport $UP_PORTS --dport $UP_PORTS-m state --state ESTABLISHED,RELATED -j ACCEPT## SMTP# Allow smtp outbound.iptables -A INPUT -i $IFACE -p tcp --sport 25 -m state --state ESTABLISHED -j ACCEPTiptables -A OUTPUT -o $IFACE -p tcp --dport 25 -m state --state NEW,ESTABLISHED -j ACCEPT## AUTH server# Reject ident probes with a tcp reset.# I need to do this for a broken mailhost that won't accept my mail if I just drop its ident probe.iptables -A INPUT -i $IFACE -p tcp --dport 113 -j REJECT --reject-with tcp-reset## TRACEROUTE# Outgoing traceroute anywhere.# The reply to a traceroute is an icmp time-exceeded which is dealt with by the next rule.iptables -A OUTPUT -o $IFACE -p udp --sport $TR_SRC_PORTS --dport $TR_DEST_PORTS-m state --state NEW -j ACCEPT# ICMP# We accept icmp in if it is "related" to other connections (e.g a time exceeded (11)# from a traceroute) or it is part of an "established" connection (e.g. an echo reply (0)# from an echo-request (8)).iptables -A INPUT -i $IFACE -p icmp -m state --state ESTABLISHED,RELATED -j ACCEPT# We always allow icmp out.iptables -A OUTPUT -o $IFACE -p icmp -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT## LOGGING# You don't have to split up your logging like I do below, but I prefer to do it this way# because I can then grep for things in the logs more easily. One thing you probably want# to do is rate-limit the logging. I didn't do that here because it is probably best not too# when you first set things up ................. you actually really want to see everything going to# the logs to work out what isn't working and why. You cam implement logging with# "-m limit --limit 6/h --limit-burst 5" (or similar) before the -j LOG in each case.## Any udp not already allowed is logged and then dropped.iptables -A INPUT -i $IFACE -p udp -j LOG --log-prefix "IPTABLES UDP-IN: "iptables -A INPUT -i $IFACE -p udp -j DROPiptables -A OUTPUT -o $IFACE -p udp -j LOG --log-prefix "IPTABLES UDP-OUT: "iptables -A OUTPUT -o $IFACE -p udp -j DROP# Any icmp not already allowed is logged and then dropped.iptables -A INPUT -i $IFACE -p icmp -j LOG --log-prefix "IPTABLES ICMP-IN: "iptables -A INPUT -i $IFACE -p icmp -j DROPiptables -A OUTPUT -o $IFACE -p icmp -j LOG --log-prefix "IPTABLES ICMP-OUT: "iptables -A OUTPUT -o $IFACE -p icmp -j DROP# Any tcp not already allowed is logged and then dropped.iptables -A INPUT -i $IFACE -p tcp -j LOG --log-prefix "IPTABLES TCP-IN: "iptables -A INPUT -i $IFACE -p tcp -j DROPiptables -A OUTPUT -o $IFACE -p tcp -j LOG --log-prefix "IPTABLES TCP-OUT: "iptables -A OUTPUT -o $IFACE -p tcp -j DROP# Anything else not already allowed is logged and then dropped.# It will be dropped by the default policy anyway ........ but let's be paranoid.iptables -A INPUT -i $IFACE -j LOG --log-prefix "IPTABLES PROTOCOL-X-IN: "iptables -A INPUT -i $IFACE -j DROPiptables -A OUTPUT -o $IFACE -j LOG --log-prefix "IPTABLES PROTOCOL-X-OUT: "iptables -A OUTPUT -o $IFACE -j DROP