iptables: NAT/masquerading functions, DNS lookups fail

cdean · 04-05-2007, 04:19 PM

I'm attempting to write my own iptables script for a little project on which I'm working. I started out with my own, hand-written one, but it wasn't working as I thought it should. So, I tried out a generator and it produced this:

Code:

#!/bin/ash -x
#
# Generated iptables firewall script for the Linux 2.4 kernel
# Script generated by Easy Firewall Generator for IPTables 1.15
# copyright 2002 Timothy Scott Morizot

###############################################################################
# 
# Local Settings
#

# sysctl location.  If set, it will use sysctl to adjust the kernel parameters.
# If this is set to the empty string (or is unset), the use of sysctl
# is disabled.

SYSCTL="/sbin/sysctl -w" 

# To echo the value directly to the /proc file instead
# SYSCTL=""

# IPTables Location - adjust if needed

IPT="/sbin/iptables"
IPTS="/sbin/iptables-save"
IPTR="/sbin/iptables-restore"

# Internet Interface
INET_IFACE="eth0"

# Local Interface Information
LOCAL_IFACE="eth1"
LOCAL_IP="10.1.1.1"
LOCAL_NET="10.1.1.1/8"
LOCAL_BCAST="10.255.255.255"

# Localhost Interface

LO_IFACE="lo"
LO_IP="127.0.0.1"

# Save and Restore arguments handled here
if [ "$1" = "save" ]
then
	echo -n "Saving firewall to /etc/firewall.d/iptables.save ... "
	$IPTS > /etc/firewall.d/iptables.save
	echo "done"
	exit 0
elif [ "$1" = "restore" ]
then
	echo -n "Restoring firewall from /etc/firewall.d/iptables.save ... "
	$IPTR < /etc/firewall.d/iptables.save
	echo "done"
	exit 0
fi

###############################################################################
#
# Kernel Parameter Configuration
#
# See http://ipsysctl-tutorial.frozentux.net/chunkyhtml/index.html
# for a detailed tutorial on sysctl and the various settings
# available.

# Required to enable IPv4 forwarding.
# Redhat users can try setting FORWARD_IPV4 in /etc/sysconfig/network to true
# Alternatively, it can be set in /etc/sysctl.conf
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/ip_forward
else
    $SYSCTL net.ipv4.ip_forward="1"
fi

# This enables dynamic address hacking.
# This may help if you have a dynamic IP address \(e.g. slip, ppp, dhcp\).
#if [ "$SYSCTL" = "" ]
#then
#    echo "1" > /proc/sys/net/ipv4/ip_dynaddr
#else
#    $SYSCTL net.ipv4.ip_dynaddr="1"
#fi

# This enables SYN flood protection.
# The SYN cookies activation allows your system to accept an unlimited
# number of TCP connections while still trying to give reasonable
# service during a denial of service attack.
#if [ "$SYSCTL" = "" ]
#then
#    echo "1" > /proc/sys/net/ipv4/tcp_syncookies
#else
#    $SYSCTL net.ipv4.tcp_syncookies="1"
#fi

# This enables source validation by reversed path according to RFC1812.
# In other words, did the response packet originate from the same interface
# through which the source packet was sent?  It's recommended for single-homed
# systems and routers on stub networks.  Since those are the configurations
# this firewall is designed to support, I turn it on by default.
# Turn it off if you use multiple NICs connected to the same network.
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/conf/all/rp_filter
else
    $SYSCTL net.ipv4.conf.all.rp_filter="1"
fi

# This option allows a subnet to be firewalled with a single IP address.
# It's used to build a DMZ.  Since that's not a focus of this firewall
# script, it's not enabled by default, but is included for reference.
# See: http://www.sjdjweis.com/linux/proxyarp/ 
#if [ "$SYSCTL" = "" ]
#then
#    echo "1" > /proc/sys/net/ipv4/conf/all/proxy_arp
#else
#    $SYSCTL net.ipv4.conf.all.proxy_arp="1"
#fi

# The following kernel settings were suggested by Alex Weeks. Thanks!

# This kernel parameter instructs the kernel to ignore all ICMP
# echo requests sent to the broadcast address.  This prevents
# a number of smurfs and similar DoS nasty attacks.
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
else
    $SYSCTL net.ipv4.icmp_echo_ignore_broadcasts="1"
fi

# This option can be used to accept or refuse source routed
# packets.  It is usually on by default, but is generally
# considered a security risk.  This option turns it off.
if [ "$SYSCTL" = "" ]
then
    echo "0" > /proc/sys/net/ipv4/conf/all/accept_source_route
else
    $SYSCTL net.ipv4.conf.all.accept_source_route="0"
fi

# This option can disable ICMP redirects.  ICMP redirects
# are generally considered a security risk and shouldn't be
# needed by most systems using this generator.
#if [ "$SYSCTL" = "" ]
#then
#    echo "0" > /proc/sys/net/ipv4/conf/all/accept_redirects
#else
#    $SYSCTL net.ipv4.conf.all.accept_redirects="0"
#fi

# However, we'll ensure the secure_redirects option is on instead.
# This option accepts only from gateways in the default gateways list.
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/conf/all/secure_redirects
else
    $SYSCTL net.ipv4.conf.all.secure_redirects="1"
fi

# This option logs packets from impossible addresses.
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/conf/all/log_martians
else
    $SYSCTL net.ipv4.conf.all.log_martians="1"
fi


###############################################################################
#
# Flush Any Existing Rules or Chains
#

echo "Flushing Tables ..."

# Reset Default Policies
$IPT -P INPUT ACCEPT
$IPT -P FORWARD ACCEPT
$IPT -P OUTPUT ACCEPT
$IPT -t nat -P PREROUTING ACCEPT
$IPT -t nat -P POSTROUTING ACCEPT
$IPT -t nat -P OUTPUT ACCEPT
$IPT -t mangle -P PREROUTING ACCEPT
$IPT -t mangle -P OUTPUT ACCEPT

# Flush all rules
$IPT -F
$IPT -t nat -F
$IPT -t mangle -F

# Erase all non-default chains
$IPT -X
$IPT -t nat -X
$IPT -t mangle -X

if [ "$1" = "stop" ]
then
	echo "Firewall completely flushed!  Now running with no firewall."
	exit 0
fi

###############################################################################
#
# Rules Configuration
#

###############################################################################
#
# Filter Table
#
###############################################################################

# Set Policies

$IPT -P INPUT DROP
$IPT -P OUTPUT DROP
$IPT -P FORWARD DROP

###############################################################################
#
# User-Specified Chains
#
# Create user chains to reduce the number of rules each packet
# must traverse.

echo "Create and populate custom rule chains ..."

# Create a chain to filter INVALID packets

$IPT -N bad_packets

# Create another chain to filter bad tcp packets

$IPT -N bad_tcp_packets

# Create separate chains for icmp, tcp (incoming and outgoing),
# and incoming udp packets.

$IPT -N icmp_packets

# Used for UDP packets inbound from the Internet
$IPT -N udp_inbound

# Used to block outbound UDP services from internal network
# Default to allow all
$IPT -N udp_outbound

# Used to allow inbound services if desired
# Default fail except for established sessions
$IPT -N tcp_inbound

# Used to block outbound services from internal network
# Default to allow all
$IPT -N tcp_outbound

###############################################################################
#
# Populate User Chains
#

# bad_packets chain
#

# Drop packets received on the external interface
# claiming a source of the local network
#$IPT -A bad_packets -p ALL -i $INET_IFACE -s $LOCAL_NET -j LOG \
#    --log-prefix "Illegal source: "

#$IPT -A bad_packets -p ALL -i $INET_IFACE -s $LOCAL_NET -j DROP

# Drop INVALID packets immediately
$IPT -A bad_packets -p ALL -m state --state INVALID -j LOG \
    --log-prefix "Invalid packet: "

$IPT -A bad_packets -p ALL -m state --state INVALID -j DROP

# Then check the tcp packets for additional problems
$IPT -A bad_packets -p tcp -j bad_tcp_packets

# All good, so return
$IPT -A bad_packets -p ALL -j RETURN

# bad_tcp_packets chain
#
# All tcp packets will traverse this chain.
# Every new connection attempt should begin with
# a syn packet.  If it doesn't, it is likely a
# port scan.  This drops packets in state
# NEW that are not flagged as syn packets.

# Return to the calling chain if the bad packets originate
# from the local interface. This maintains the approach
# throughout this firewall of a largely trusted internal
# network.
$IPT -A bad_tcp_packets -p tcp -i $LOCAL_IFACE -j RETURN

# However, I originally did apply this filter to the forward chain
# for packets originating from the internal network.  While I have
# not conclusively determined its effect, it appears to have the
# interesting side effect of blocking some of the ad systems.
# Apparently some ad systems have the browser initiate a NEW
# connection that is not flagged as a syn packet to retrieve
# the ad image.  If you wish to experiment further comment the
# rule above. If you try it, you may also wish to uncomment the
# rule below.  It will keep those packets from being logged.
# There are a lot of them.
# $IPT -A bad_tcp_packets -p tcp -i $LOCAL_IFACE ! --syn -m state \
#     --state NEW -j DROP

$IPT -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j LOG \
    --log-prefix "New not syn: "
$IPT -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL NONE -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL NONE -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL ALL -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL ALL -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL FIN,URG,PSH -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL FIN,URG,PSH -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,RST SYN,RST -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,RST SYN,RST -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,FIN SYN,FIN -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP

# All good, so return
$IPT -A bad_tcp_packets -p tcp -j RETURN

# icmp_packets chain
#
# This chain is for inbound (from the Internet) icmp packets only.
# Type 8 (Echo Request) is not accepted by default
# Enable it if you want remote hosts to be able to reach you.
# 11 (Time Exceeded) is the only one accepted
# that would not already be covered by the established
# connection rule.  Applied to INPUT on the external interface.
# 
# See: http://www.ee.siue.edu/~rwalden/networking/icmp.html
# for more info on ICMP types.
#
# Note that the stateful settings allow replies to ICMP packets.
# These rules allow new packets of the specified types.

# ICMP packets should fit in a Layer 2 frame, thus they should
# never be fragmented.  Fragmented ICMP packets are a typical sign
# of a denial of service attack.
$IPT -A icmp_packets --fragment -p ICMP -j LOG \
    --log-prefix "ICMP Fragment: "
$IPT -A icmp_packets --fragment -p ICMP -j DROP

# Echo - uncomment to allow your system to be pinged.
# Uncomment the LOG command if you also want to log PING attempts
# 
# $IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j LOG \
#    --log-prefix "Ping detected: "
# $IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j ACCEPT

# By default, however, drop pings without logging. Blaster
# and other worms have infected systems blasting pings.
# Comment the line below if you want pings logged, but it
# will likely fill your logs.
$IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j DROP

# Time Exceeded
$IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 11 -j ACCEPT

# Not matched, so return so it will be logged
$IPT -A icmp_packets -p ICMP -j RETURN

# TCP & UDP
# Identify ports at:
#    http://www.chebucto.ns.ca/~rakerman/port-table.html
#    http://www.iana.org/assignments/port-numbers

# udp_inbound chain
#
# This chain describes the inbound UDP packets it will accept.
# It's applied to INPUT on the external or Internet interface.
# Note that the stateful settings allow replies.
# These rules are for new requests.
# It drops netbios packets (windows) immediately without logging.

# Drop netbios calls
# Please note that these rules do not really change the way the firewall
# treats netbios connections.  Connections from the localhost and
# internal interface (if one exists) are accepted by default.
# Responses from the Internet to requests initiated by or through
# the firewall are also accepted by default.  To get here, the
# packets would have to be part of a new request received by the
# Internet interface.  You would have to manually add rules to
# accept these.  I added these rules because some network connections,
# such as those via cable modems, tend to be filled with noise from
# unprotected Windows machines.  These rules drop those packets
# quickly and without logging them.  This prevents them from traversing
# the whole chain and keeps the log from getting cluttered with
# chatter from Windows systems.
$IPT -A udp_inbound -p UDP -s 0/0 --destination-port 137 -j DROP
$IPT -A udp_inbound -p UDP -s 0/0 --destination-port 138 -j DROP

# Dynamic Address
# If DHCP, the initial request is a broadcast. The response
# doesn't exactly match the outbound packet.  This explicitly
# allow the DHCP ports to alleviate this problem.
# If you receive your dynamic address by a different means, you
# can probably comment this line.
$IPT -A udp_inbound -p UDP -s 0/0 --source-port 67 --destination-port 68 \
     -j ACCEPT


# Not matched, so return for logging
$IPT -A udp_inbound -p UDP -j RETURN

# udp_outbound chain
#
# This chain is used with a private network to prevent forwarding for
# UDP requests on specific protocols.  Applied to the FORWARD rule from
# the internal network.  Ends with an ACCEPT

# ICQ uses UDP 4000 - Instant messaging blocked
#$IPT -A udp_outbound -p UDP -s 0/0 --destination-port 4000 -j REJECT


# No match, so ACCEPT
$IPT -A udp_outbound -p UDP -s 0/0 -j ACCEPT

# tcp_inbound chain
#
# This chain is used to allow inbound connections to the
# system/gateway.  Use with care.  It defaults to none.
# It's applied on INPUT from the external or Internet interface.


# Not matched, so return so it will be logged
$IPT -A tcp_inbound -p TCP -j RETURN

# tcp_outbound chain
#
# This chain is used with a private network to prevent forwarding for
# requests on specific protocols.  Applied to the FORWARD rule from
# the internal network.  Ends with an ACCEPT

/etc/firewall.d/blocks.sh

# No match, so ACCEPT
$IPT -A tcp_outbound -p TCP -s 0/0 -j ACCEPT

###############################################################################
#
# INPUT Chain
#

echo "Process INPUT chain ..."

# Allow all on localhost interface
$IPT -A INPUT -p ALL -i $LO_IFACE -j ACCEPT

# Drop bad packets
$IPT -A INPUT -p ALL -j bad_packets

# DOCSIS compliant cable modems
# Some DOCSIS compliant cable modems send IGMP multicasts to find
# connected PCs.  The multicast packets have the destination address
# 224.0.0.1.  You can accept them.  If you choose to do so,
# Uncomment the rule to ACCEPT them and comment the rule to DROP
# them  The firewall will drop them here by default to avoid
# cluttering the log.  The firewall will drop all multicasts
# to the entire subnet (224.0.0.1) by default.  To only affect
# IGMP multicasts, change '-p ALL' to '-p 2'.  Of course,
# if they aren't accepted elsewhere, it will only ensure that
# multicasts on other protocols are logged.
# Drop them without logging.
$IPT -A INPUT -p ALL -d 224.0.0.1 -j DROP
# The rule to accept the packets.
# $IPT -A INPUT -p ALL -d 224.0.0.1 -j ACCEPT

# Rules for the private network (accessing gateway system itself)
$IPT -A INPUT -p ALL -i $LOCAL_IFACE -s $LOCAL_NET -j ACCEPT
$IPT -A INPUT -p ALL -i $LOCAL_IFACE -d $LOCAL_BCAST -j ACCEPT

# Allow DHCP client request packets inbound from internal network
$IPT -A INPUT -p UDP -i $LOCAL_IFACE --source-port 68 --destination-port 67 \
     -j ACCEPT
# Allow DNS client request packets inbound from internet network
$IPT -A INPUT -p UDP -i $LOCAL_IFACE --destination-port 53 -j ACCEPT

# Inbound Internet Packet Rules

# Accept Established Connections
$IPT -A INPUT -p ALL -i $INET_IFACE -m state --state ESTABLISHED,RELATED \
     -j ACCEPT

# Route the rest to the appropriate user chain
$IPT -A INPUT -p TCP -i $INET_IFACE -j tcp_inbound
$IPT -A INPUT -p UDP -i $INET_IFACE -j udp_inbound
$IPT -A INPUT -p ICMP -i $INET_IFACE -j icmp_packets

# Drop without logging broadcasts that get this far.
# Cuts down on log clutter.
# Comment this line if testing new rules that impact
# broadcast protocols.
$IPT -A INPUT -m pkttype --pkt-type broadcast -j DROP

# Log packets that still don't match
$IPT -A INPUT -m limit --limit 3/minute --limit-burst 3 -j LOG \
    --log-prefix "INPUT packet died: "

###############################################################################
#
# FORWARD Chain
#

echo "Process FORWARD chain ..."

# Used if forwarding for a private network

# Drop bad packets
$IPT -A FORWARD -p ALL -j bad_packets

# Accept TCP packets we want to forward from internal sources
$IPT -A FORWARD -p tcp -i $LOCAL_IFACE -j tcp_outbound

# Accept UDP packets we want to forward from internal sources
$IPT -A FORWARD -p udp -i $LOCAL_IFACE -j udp_outbound

# If not blocked, accept any other packets from the internal interface
$IPT -A FORWARD -p ALL -i $LOCAL_IFACE -j ACCEPT

# Deal with responses from the internet
$IPT -A FORWARD -i $INET_IFACE -m state --state ESTABLISHED,RELATED \
     -j ACCEPT

# Log packets that still don't match
$IPT -A FORWARD -m limit --limit 3/minute --limit-burst 3 -j LOG \
    --log-prefix "FORWARD packet died: "

###############################################################################
#
# OUTPUT Chain
#

echo "Process OUTPUT chain ..."

# Generally trust the firewall on output

# However, invalid icmp packets need to be dropped
# to prevent a possible exploit.
$IPT -A OUTPUT -m state -p icmp --state INVALID -j DROP

# Localhost
$IPT -A OUTPUT -p ALL -s $LO_IP -j ACCEPT
$IPT -A OUTPUT -p ALL -o $LO_IFACE -j ACCEPT

# To internal network
$IPT -A OUTPUT -p ALL -s $LOCAL_IP -j ACCEPT
$IPT -A OUTPUT -p ALL -o $LOCAL_IFACE -j ACCEPT

# To internet
$IPT -A OUTPUT -p ALL -o $INET_IFACE -j ACCEPT

# Log packets that still don't match
$IPT -A OUTPUT -m limit --limit 3/minute --limit-burst 3 -j LOG \
    --log-prefix "OUTPUT packet died: "

###############################################################################
#
# nat table
#
###############################################################################

# The nat table is where network address translation occurs if there
# is a private network.  If the gateway is connected to the Internet
# with a static IP, snat is used.  If the gateway has a dynamic address,
# masquerade must be used instead.  There is more overhead associated
# with masquerade, so snat is better when it can be used.
# The nat table has a builtin chain, PREROUTING, for dnat and redirects.
# Another, POSTROUTING, handles snat and masquerade.

echo "Load rules for nat table ..."

###############################################################################
#
# PREROUTING chain
#

#$IPT -t nat -A PREROUTING -i $LOCAL_IFACE -j NFQUEUE

###############################################################################
#
# POSTROUTING chain
#


$IPT -t nat -A POSTROUTING -o $INET_IFACE -j MASQUERADE

###############################################################################
#
# mangle table
#
###############################################################################

# The mangle table is used to alter packets.  It can alter or mangle them in
# several ways.  For the purposes of this generator, we only use its ability
# to alter the TTL in packets.  However, it can be used to set netfilter
# mark values on specific packets.  Those marks could then be used in another
# table like filter, to limit activities associated with a specific host, for
# instance.  The TOS target can be used to set the Type of Service field in
# the IP header.  Note that the TTL target might not be included in the
# distribution on your system.  If it is not and you require it, you will
# have to add it.  That may require that you build from source.

echo "Load rules for mangle table ..."

I put in the DNS line in the INPUT section, but it apparently doesn't work, as clients on the LAN cannot access the DNS server running on the box on which this firewall is running.

FYI: eth0 is WAN, eth1 is LAN. The LAN address space is 10.1.1.1/8. Hosts on the LAN should have full access to services running on the firewalled host (router, really), and the WAN shouldn't be able to get through except for DHCP traffic.

wykthorr · 04-06-2007, 02:05 AM

Your iptables script calls an external file called "/etc/firewall.d/blocks.sh". Please post that one too. The problem might be there.

There's another way to go around this problem but I'd like to see the blocks.sh file first.

cdean · 04-06-2007, 02:14 PM

It's full of specifically-blocked ports, but all lines are commented out.

Code:

#!/bin/ash
# Block IRC
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 194 -j REJECT

# Block Outbound Telneti
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 23 -j REJECT

# Block SSH
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 22 -j REJECT

# Block Usenet Access
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 119 -j REJECT

# Block Web Access

# HTTP
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 80 -j REJECT

# HTTPS (Secure Web)
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 443 -j REJECT

# Block FTP Access

# Control
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 21 -j REJECT

# Data
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 20 -j REJECT

# Block Instant Messaging

# AIM
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 5190 -j REJECT

# AIM Images
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 4443 -j REJECT

# MSN Messenger
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 1863 -j REJECT

# Block Email Access to external servers

# SMTP
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 25 -j REJECT

# POP3
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 110 -j REJECT

# IMAP4
#$IPT -A OUTPUT -p TCP -s 0/0 --destination-port 143 -j REJECT

wykthorr · 04-06-2007, 02:53 PM

I think I've found where the error was. The values for the LOCAL_NET and LOCAL_BCAST did not seem ok. Try this changed version. Let me know what the result was. If it doesen't work I'll try something else.

Code:

#!/bin/ash -x
#
# Generated iptables firewall script for the Linux 2.4 kernel
# Script generated by Easy Firewall Generator for IPTables 1.15
# copyright 2002 Timothy Scott Morizot

###############################################################################
# 
# Local Settings
#

# sysctl location.  If set, it will use sysctl to adjust the kernel parameters.
# If this is set to the empty string (or is unset), the use of sysctl
# is disabled.

SYSCTL="/sbin/sysctl -w" 

# To echo the value directly to the /proc file instead
# SYSCTL=""

# IPTables Location - adjust if needed

IPT="/sbin/iptables"
IPTS="/sbin/iptables-save"
IPTR="/sbin/iptables-restore"

# Internet Interface
INET_IFACE="eth0"

# Local Interface Information
LOCAL_IFACE="eth1"
LOCAL_IP="10.1.1.1"
LOCAL_NET="10.1.1.0/24"
LOCAL_BCAST="10.1.1.255"

# Localhost Interface

LO_IFACE="lo"
LO_IP="127.0.0.1"

# Save and Restore arguments handled here
if [ "$1" = "save" ]
then
	echo -n "Saving firewall to /etc/firewall.d/iptables.save ... "
	$IPTS > /etc/firewall.d/iptables.save
	echo "done"
	exit 0
elif [ "$1" = "restore" ]
then
	echo -n "Restoring firewall from /etc/firewall.d/iptables.save ... "
	$IPTR < /etc/firewall.d/iptables.save
	echo "done"
	exit 0
fi

###############################################################################
#
# Kernel Parameter Configuration
#
# See http://ipsysctl-tutorial.frozentux.net/chunkyhtml/index.html
# for a detailed tutorial on sysctl and the various settings
# available.

# Required to enable IPv4 forwarding.
# Redhat users can try setting FORWARD_IPV4 in /etc/sysconfig/network to true
# Alternatively, it can be set in /etc/sysctl.conf
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/ip_forward
else
    $SYSCTL net.ipv4.ip_forward="1"
fi

# This enables dynamic address hacking.
# This may help if you have a dynamic IP address \(e.g. slip, ppp, dhcp\).
#if [ "$SYSCTL" = "" ]
#then
#    echo "1" > /proc/sys/net/ipv4/ip_dynaddr
#else
#    $SYSCTL net.ipv4.ip_dynaddr="1"
#fi

# This enables SYN flood protection.
# The SYN cookies activation allows your system to accept an unlimited
# number of TCP connections while still trying to give reasonable
# service during a denial of service attack.
#if [ "$SYSCTL" = "" ]
#then
#    echo "1" > /proc/sys/net/ipv4/tcp_syncookies
#else
#    $SYSCTL net.ipv4.tcp_syncookies="1"
#fi

# This enables source validation by reversed path according to RFC1812.
# In other words, did the response packet originate from the same interface
# through which the source packet was sent?  It's recommended for single-homed
# systems and routers on stub networks.  Since those are the configurations
# this firewall is designed to support, I turn it on by default.
# Turn it off if you use multiple NICs connected to the same network.
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/conf/all/rp_filter
else
    $SYSCTL net.ipv4.conf.all.rp_filter="1"
fi

# This option allows a subnet to be firewalled with a single IP address.
# It's used to build a DMZ.  Since that's not a focus of this firewall
# script, it's not enabled by default, but is included for reference.
# See: http://www.sjdjweis.com/linux/proxyarp/ 
#if [ "$SYSCTL" = "" ]
#then
#    echo "1" > /proc/sys/net/ipv4/conf/all/proxy_arp
#else
#    $SYSCTL net.ipv4.conf.all.proxy_arp="1"
#fi

# The following kernel settings were suggested by Alex Weeks. Thanks!

# This kernel parameter instructs the kernel to ignore all ICMP
# echo requests sent to the broadcast address.  This prevents
# a number of smurfs and similar DoS nasty attacks.
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
else
    $SYSCTL net.ipv4.icmp_echo_ignore_broadcasts="1"
fi

# This option can be used to accept or refuse source routed
# packets.  It is usually on by default, but is generally
# considered a security risk.  This option turns it off.
if [ "$SYSCTL" = "" ]
then
    echo "0" > /proc/sys/net/ipv4/conf/all/accept_source_route
else
    $SYSCTL net.ipv4.conf.all.accept_source_route="0"
fi

# This option can disable ICMP redirects.  ICMP redirects
# are generally considered a security risk and shouldn't be
# needed by most systems using this generator.
#if [ "$SYSCTL" = "" ]
#then
#    echo "0" > /proc/sys/net/ipv4/conf/all/accept_redirects
#else
#    $SYSCTL net.ipv4.conf.all.accept_redirects="0"
#fi

# However, we'll ensure the secure_redirects option is on instead.
# This option accepts only from gateways in the default gateways list.
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/conf/all/secure_redirects
else
    $SYSCTL net.ipv4.conf.all.secure_redirects="1"
fi

# This option logs packets from impossible addresses.
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/conf/all/log_martians
else
    $SYSCTL net.ipv4.conf.all.log_martians="1"
fi


###############################################################################
#
# Flush Any Existing Rules or Chains
#

echo "Flushing Tables ..."

# Reset Default Policies
$IPT -P INPUT ACCEPT
$IPT -P FORWARD ACCEPT
$IPT -P OUTPUT ACCEPT
$IPT -t nat -P PREROUTING ACCEPT
$IPT -t nat -P POSTROUTING ACCEPT
$IPT -t nat -P OUTPUT ACCEPT
$IPT -t mangle -P PREROUTING ACCEPT
$IPT -t mangle -P OUTPUT ACCEPT

# Flush all rules
$IPT -F
$IPT -t nat -F
$IPT -t mangle -F

# Erase all non-default chains
$IPT -X
$IPT -t nat -X
$IPT -t mangle -X

if [ "$1" = "stop" ]
then
	echo "Firewall completely flushed!  Now running with no firewall."
	exit 0
fi

###############################################################################
#
# Rules Configuration
#

###############################################################################
#
# Filter Table
#
###############################################################################

# Set Policies

$IPT -P INPUT DROP
$IPT -P OUTPUT DROP
$IPT -P FORWARD DROP

###############################################################################
#
# User-Specified Chains
#
# Create user chains to reduce the number of rules each packet
# must traverse.

echo "Create and populate custom rule chains ..."

# Create a chain to filter INVALID packets

$IPT -N bad_packets

# Create another chain to filter bad tcp packets

$IPT -N bad_tcp_packets

# Create separate chains for icmp, tcp (incoming and outgoing),
# and incoming udp packets.

$IPT -N icmp_packets

# Used for UDP packets inbound from the Internet
$IPT -N udp_inbound

# Used to block outbound UDP services from internal network
# Default to allow all
$IPT -N udp_outbound

# Used to allow inbound services if desired
# Default fail except for established sessions
$IPT -N tcp_inbound

# Used to block outbound services from internal network
# Default to allow all
$IPT -N tcp_outbound

###############################################################################
#
# Populate User Chains
#

# bad_packets chain
#

# Drop packets received on the external interface
# claiming a source of the local network
#$IPT -A bad_packets -p ALL -i $INET_IFACE -s $LOCAL_NET -j LOG \
#    --log-prefix "Illegal source: "

#$IPT -A bad_packets -p ALL -i $INET_IFACE -s $LOCAL_NET -j DROP

# Drop INVALID packets immediately
$IPT -A bad_packets -p ALL -m state --state INVALID -j LOG \
    --log-prefix "Invalid packet: "

$IPT -A bad_packets -p ALL -m state --state INVALID -j DROP

# Then check the tcp packets for additional problems
$IPT -A bad_packets -p tcp -j bad_tcp_packets

# All good, so return
$IPT -A bad_packets -p ALL -j RETURN

# bad_tcp_packets chain
#
# All tcp packets will traverse this chain.
# Every new connection attempt should begin with
# a syn packet.  If it doesn't, it is likely a
# port scan.  This drops packets in state
# NEW that are not flagged as syn packets.

# Return to the calling chain if the bad packets originate
# from the local interface. This maintains the approach
# throughout this firewall of a largely trusted internal
# network.
$IPT -A bad_tcp_packets -p tcp -i $LOCAL_IFACE -j RETURN

# However, I originally did apply this filter to the forward chain
# for packets originating from the internal network.  While I have
# not conclusively determined its effect, it appears to have the
# interesting side effect of blocking some of the ad systems.
# Apparently some ad systems have the browser initiate a NEW
# connection that is not flagged as a syn packet to retrieve
# the ad image.  If you wish to experiment further comment the
# rule above. If you try it, you may also wish to uncomment the
# rule below.  It will keep those packets from being logged.
# There are a lot of them.
# $IPT -A bad_tcp_packets -p tcp -i $LOCAL_IFACE ! --syn -m state \
#     --state NEW -j DROP

$IPT -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j LOG \
    --log-prefix "New not syn: "
$IPT -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL NONE -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL NONE -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL ALL -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL ALL -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL FIN,URG,PSH -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL FIN,URG,PSH -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,RST SYN,RST -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,RST SYN,RST -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,FIN SYN,FIN -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP

# All good, so return
$IPT -A bad_tcp_packets -p tcp -j RETURN

# icmp_packets chain
#
# This chain is for inbound (from the Internet) icmp packets only.
# Type 8 (Echo Request) is not accepted by default
# Enable it if you want remote hosts to be able to reach you.
# 11 (Time Exceeded) is the only one accepted
# that would not already be covered by the established
# connection rule.  Applied to INPUT on the external interface.
# 
# See: http://www.ee.siue.edu/~rwalden/networking/icmp.html
# for more info on ICMP types.
#
# Note that the stateful settings allow replies to ICMP packets.
# These rules allow new packets of the specified types.

# ICMP packets should fit in a Layer 2 frame, thus they should
# never be fragmented.  Fragmented ICMP packets are a typical sign
# of a denial of service attack.
$IPT -A icmp_packets --fragment -p ICMP -j LOG \
    --log-prefix "ICMP Fragment: "
$IPT -A icmp_packets --fragment -p ICMP -j DROP

# Echo - uncomment to allow your system to be pinged.
# Uncomment the LOG command if you also want to log PING attempts
# 
# $IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j LOG \
#    --log-prefix "Ping detected: "
# $IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j ACCEPT

# By default, however, drop pings without logging. Blaster
# and other worms have infected systems blasting pings.
# Comment the line below if you want pings logged, but it
# will likely fill your logs.
$IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j DROP

# Time Exceeded
$IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 11 -j ACCEPT

# Not matched, so return so it will be logged
$IPT -A icmp_packets -p ICMP -j RETURN

# TCP & UDP
# Identify ports at:
#    http://www.chebucto.ns.ca/~rakerman/port-table.html
#    http://www.iana.org/assignments/port-numbers

# udp_inbound chain
#
# This chain describes the inbound UDP packets it will accept.
# It's applied to INPUT on the external or Internet interface.
# Note that the stateful settings allow replies.
# These rules are for new requests.
# It drops netbios packets (windows) immediately without logging.

# Drop netbios calls
# Please note that these rules do not really change the way the firewall
# treats netbios connections.  Connections from the localhost and
# internal interface (if one exists) are accepted by default.
# Responses from the Internet to requests initiated by or through
# the firewall are also accepted by default.  To get here, the
# packets would have to be part of a new request received by the
# Internet interface.  You would have to manually add rules to
# accept these.  I added these rules because some network connections,
# such as those via cable modems, tend to be filled with noise from
# unprotected Windows machines.  These rules drop those packets
# quickly and without logging them.  This prevents them from traversing
# the whole chain and keeps the log from getting cluttered with
# chatter from Windows systems.


$IPT -A udp_inbound -p UDP -s 0/0 --destination-port 137 -j DROP
$IPT -A udp_inbound -p UDP -s 0/0 --destination-port 138 -j DROP

# Dynamic Address
# If DHCP, the initial request is a broadcast. The response
# doesn't exactly match the outbound packet.  This explicitly
# allow the DHCP ports to alleviate this problem.
# If you receive your dynamic address by a different means, you
# can probably comment this line.
$IPT -A udp_inbound -p UDP -s 0/0 --source-port 67 --destination-port 68 \
     -j ACCEPT


# Not matched, so return for logging
$IPT -A udp_inbound -p UDP -j RETURN

# udp_outbound chain
#
# This chain is used with a private network to prevent forwarding for
# UDP requests on specific protocols.  Applied to the FORWARD rule from
# the internal network.  Ends with an ACCEPT

# ICQ uses UDP 4000 - Instant messaging blocked
#$IPT -A udp_outbound -p UDP -s 0/0 --destination-port 4000 -j REJECT


# No match, so ACCEPT
$IPT -A udp_outbound -p UDP -s 0/0 -j ACCEPT

# tcp_inbound chain
#
# This chain is used to allow inbound connections to the
# system/gateway.  Use with care.  It defaults to none.
# It's applied on INPUT from the external or Internet interface.


# Not matched, so return so it will be logged
$IPT -A tcp_inbound -p TCP -j RETURN

# tcp_outbound chain
#
# This chain is used with a private network to prevent forwarding for
# requests on specific protocols.  Applied to the FORWARD rule from
# the internal network.  Ends with an ACCEPT


/etc/firewall.d/blocks.sh

# No match, so ACCEPT
$IPT -A tcp_outbound -p TCP -s 0/0 -j ACCEPT

###############################################################################
#
# INPUT Chain
#

echo "Process INPUT chain ..."

# Allow all on localhost interface
$IPT -A INPUT -p ALL -i $LO_IFACE -j ACCEPT

# Drop bad packets
$IPT -A INPUT -p ALL -j bad_packets

# DOCSIS compliant cable modems
# Some DOCSIS compliant cable modems send IGMP multicasts to find
# connected PCs.  The multicast packets have the destination address
# 224.0.0.1.  You can accept them.  If you choose to do so,
# Uncomment the rule to ACCEPT them and comment the rule to DROP
# them  The firewall will drop them here by default to avoid
# cluttering the log.  The firewall will drop all multicasts
# to the entire subnet (224.0.0.1) by default.  To only affect
# IGMP multicasts, change '-p ALL' to '-p 2'.  Of course,
# if they aren't accepted elsewhere, it will only ensure that
# multicasts on other protocols are logged.
# Drop them without logging.
$IPT -A INPUT -p ALL -d 224.0.0.1 -j DROP
# The rule to accept the packets.
# $IPT -A INPUT -p ALL -d 224.0.0.1 -j ACCEPT

# Rules for the private network (accessing gateway system itself)
$IPT -A INPUT -p ALL -i $LOCAL_IFACE -s $LOCAL_NET -j ACCEPT
$IPT -A INPUT -p ALL -i $LOCAL_IFACE -d $LOCAL_BCAST -j ACCEPT

# Allow DHCP client request packets inbound from internal network
$IPT -A INPUT -p UDP -i $LOCAL_IFACE --source-port 68 --destination-port 67 \
     -j ACCEPT
# Allow DNS client request packets inbound from internet network
$IPT -A INPUT -p UDP -i $LOCAL_IFACE --destination-port 53 -j ACCEPT

# Inbound Internet Packet Rules

# Accept Established Connections
$IPT -A INPUT -p ALL -i $INET_IFACE -m state --state ESTABLISHED,RELATED \
     -j ACCEPT

# Route the rest to the appropriate user chain
$IPT -A INPUT -p TCP -i $INET_IFACE -j tcp_inbound
$IPT -A INPUT -p UDP -i $INET_IFACE -j udp_inbound
$IPT -A INPUT -p ICMP -i $INET_IFACE -j icmp_packets

# Drop without logging broadcasts that get this far.
# Cuts down on log clutter.
# Comment this line if testing new rules that impact
# broadcast protocols.
$IPT -A INPUT -m pkttype --pkt-type broadcast -j DROP

# Log packets that still don't match
$IPT -A INPUT -m limit --limit 3/minute --limit-burst 3 -j LOG \
    --log-prefix "INPUT packet died: "

###############################################################################
#
# FORWARD Chain
#

echo "Process FORWARD chain ..."

# Used if forwarding for a private network

# Drop bad packets
$IPT -A FORWARD -p ALL -j bad_packets

# Accept TCP packets we want to forward from internal sources
$IPT -A FORWARD -p tcp -i $LOCAL_IFACE -j tcp_outbound

# Accept UDP packets we want to forward from internal sources
$IPT -A FORWARD -p udp -i $LOCAL_IFACE -j udp_outbound

# If not blocked, accept any other packets from the internal interface
$IPT -A FORWARD -p ALL -i $LOCAL_IFACE -j ACCEPT

# Deal with responses from the internet
$IPT -A FORWARD -i $INET_IFACE -m state --state ESTABLISHED,RELATED \
     -j ACCEPT

# Log packets that still don't match
$IPT -A FORWARD -m limit --limit 3/minute --limit-burst 3 -j LOG \
    --log-prefix "FORWARD packet died: "

###############################################################################
#
# OUTPUT Chain
#

echo "Process OUTPUT chain ..."

# Generally trust the firewall on output

# However, invalid icmp packets need to be dropped
# to prevent a possible exploit.
$IPT -A OUTPUT -m state -p icmp --state INVALID -j DROP

# Localhost
$IPT -A OUTPUT -p ALL -s $LO_IP -j ACCEPT
$IPT -A OUTPUT -p ALL -o $LO_IFACE -j ACCEPT

# To internal network
$IPT -A OUTPUT -p ALL -s $LOCAL_IP -j ACCEPT
$IPT -A OUTPUT -p ALL -o $LOCAL_IFACE -j ACCEPT

# To internet
$IPT -A OUTPUT -p ALL -o $INET_IFACE -j ACCEPT

# Log packets that still don't match
$IPT -A OUTPUT -m limit --limit 3/minute --limit-burst 3 -j LOG \
    --log-prefix "OUTPUT packet died: "

###############################################################################
#
# nat table
#
###############################################################################

# The nat table is where network address translation occurs if there
# is a private network.  If the gateway is connected to the Internet
# with a static IP, snat is used.  If the gateway has a dynamic address,
# masquerade must be used instead.  There is more overhead associated
# with masquerade, so snat is better when it can be used.
# The nat table has a builtin chain, PREROUTING, for dnat and redirects.
# Another, POSTROUTING, handles snat and masquerade.

echo "Load rules for nat table ..."

###############################################################################
#
# PREROUTING chain
#

#$IPT -t nat -A PREROUTING -i $LOCAL_IFACE -j NFQUEUE

###############################################################################
#
# POSTROUTING chain
#


$IPT -t nat -A POSTROUTING -o $INET_IFACE -j MASQUERADE

###############################################################################
#
# mangle table
#
###############################################################################

# The mangle table is used to alter packets.  It can alter or mangle them in
# several ways.  For the purposes of this generator, we only use its ability
# to alter the TTL in packets.  However, it can be used to set netfilter
# mark values on specific packets.  Those marks could then be used in another
# table like filter, to limit activities associated with a specific host, for
# instance.  The TOS target can be used to set the Type of Service field in
# the IP header.  Note that the TTL target might not be included in the
# distribution on your system.  If it is not and you require it, you will
# have to add it.  That may require that you build from source.

echo "Load rules for mangle table ..."

cdean · 04-06-2007, 04:21 PM

You changed them from 10.1.1.1/8 to 10.1.1.0/24, right?

I'm intending to use the entire 10.* address space, but I'll see if it makes a difference.

cdean · 04-06-2007, 06:22 PM

Nope, no difference.

wykthorr · 04-07-2007, 01:49 AM

Ok this is the last shot. I have deleted the address restrictions from the intranet input chain. Now all traffic on the localnet interface is allowed. If this does not work either you might have mixed the interface connections up. Check if the cables are plugged in the right adapter.

Code:

#!/bin/ash -x
#
# Generated iptables firewall script for the Linux 2.4 kernel
# Script generated by Easy Firewall Generator for IPTables 1.15
# copyright 2002 Timothy Scott Morizot

###############################################################################
# 
# Local Settings
#

# sysctl location.  If set, it will use sysctl to adjust the kernel parameters.
# If this is set to the empty string (or is unset), the use of sysctl
# is disabled.

SYSCTL="/sbin/sysctl -w" 

# To echo the value directly to the /proc file instead
# SYSCTL=""

# IPTables Location - adjust if needed

IPT="/sbin/iptables"
IPTS="/sbin/iptables-save"
IPTR="/sbin/iptables-restore"

# Internet Interface
INET_IFACE="eth0"

# Local Interface Information
LOCAL_IFACE="eth1"
LOCAL_IP="10.1.1.1"
LOCAL_NET="10.1.1.0/24"
LOCAL_BCAST="10.1.1.255"

# Localhost Interface

LO_IFACE="lo"
LO_IP="127.0.0.1"

# Save and Restore arguments handled here
if [ "$1" = "save" ]
then
	echo -n "Saving firewall to /etc/firewall.d/iptables.save ... "
	$IPTS > /etc/firewall.d/iptables.save
	echo "done"
	exit 0
elif [ "$1" = "restore" ]
then
	echo -n "Restoring firewall from /etc/firewall.d/iptables.save ... "
	$IPTR < /etc/firewall.d/iptables.save
	echo "done"
	exit 0
fi

###############################################################################
#
# Kernel Parameter Configuration
#
# See http://ipsysctl-tutorial.frozentux.net/chunkyhtml/index.html
# for a detailed tutorial on sysctl and the various settings
# available.

# Required to enable IPv4 forwarding.
# Redhat users can try setting FORWARD_IPV4 in /etc/sysconfig/network to true
# Alternatively, it can be set in /etc/sysctl.conf
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/ip_forward
else
    $SYSCTL net.ipv4.ip_forward="1"
fi

# This enables dynamic address hacking.
# This may help if you have a dynamic IP address \(e.g. slip, ppp, dhcp\).
#if [ "$SYSCTL" = "" ]
#then
#    echo "1" > /proc/sys/net/ipv4/ip_dynaddr
#else
#    $SYSCTL net.ipv4.ip_dynaddr="1"
#fi

# This enables SYN flood protection.
# The SYN cookies activation allows your system to accept an unlimited
# number of TCP connections while still trying to give reasonable
# service during a denial of service attack.
#if [ "$SYSCTL" = "" ]
#then
#    echo "1" > /proc/sys/net/ipv4/tcp_syncookies
#else
#    $SYSCTL net.ipv4.tcp_syncookies="1"
#fi

# This enables source validation by reversed path according to RFC1812.
# In other words, did the response packet originate from the same interface
# through which the source packet was sent?  It's recommended for single-homed
# systems and routers on stub networks.  Since those are the configurations
# this firewall is designed to support, I turn it on by default.
# Turn it off if you use multiple NICs connected to the same network.
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/conf/all/rp_filter
else
    $SYSCTL net.ipv4.conf.all.rp_filter="1"
fi

# This option allows a subnet to be firewalled with a single IP address.
# It's used to build a DMZ.  Since that's not a focus of this firewall
# script, it's not enabled by default, but is included for reference.
# See: http://www.sjdjweis.com/linux/proxyarp/ 
#if [ "$SYSCTL" = "" ]
#then
#    echo "1" > /proc/sys/net/ipv4/conf/all/proxy_arp
#else
#    $SYSCTL net.ipv4.conf.all.proxy_arp="1"
#fi

# The following kernel settings were suggested by Alex Weeks. Thanks!

# This kernel parameter instructs the kernel to ignore all ICMP
# echo requests sent to the broadcast address.  This prevents
# a number of smurfs and similar DoS nasty attacks.
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
else
    $SYSCTL net.ipv4.icmp_echo_ignore_broadcasts="1"
fi

# This option can be used to accept or refuse source routed
# packets.  It is usually on by default, but is generally
# considered a security risk.  This option turns it off.
if [ "$SYSCTL" = "" ]
then
    echo "0" > /proc/sys/net/ipv4/conf/all/accept_source_route
else
    $SYSCTL net.ipv4.conf.all.accept_source_route="0"
fi

# This option can disable ICMP redirects.  ICMP redirects
# are generally considered a security risk and shouldn't be
# needed by most systems using this generator.
#if [ "$SYSCTL" = "" ]
#then
#    echo "0" > /proc/sys/net/ipv4/conf/all/accept_redirects
#else
#    $SYSCTL net.ipv4.conf.all.accept_redirects="0"
#fi

# However, we'll ensure the secure_redirects option is on instead.
# This option accepts only from gateways in the default gateways list.
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/conf/all/secure_redirects
else
    $SYSCTL net.ipv4.conf.all.secure_redirects="1"
fi

# This option logs packets from impossible addresses.
if [ "$SYSCTL" = "" ]
then
    echo "1" > /proc/sys/net/ipv4/conf/all/log_martians
else
    $SYSCTL net.ipv4.conf.all.log_martians="1"
fi


###############################################################################
#
# Flush Any Existing Rules or Chains
#

echo "Flushing Tables ..."

# Reset Default Policies
$IPT -P INPUT ACCEPT
$IPT -P FORWARD ACCEPT
$IPT -P OUTPUT ACCEPT
$IPT -t nat -P PREROUTING ACCEPT
$IPT -t nat -P POSTROUTING ACCEPT
$IPT -t nat -P OUTPUT ACCEPT
$IPT -t mangle -P PREROUTING ACCEPT
$IPT -t mangle -P OUTPUT ACCEPT

# Flush all rules
$IPT -F
$IPT -t nat -F
$IPT -t mangle -F

# Erase all non-default chains
$IPT -X
$IPT -t nat -X
$IPT -t mangle -X

if [ "$1" = "stop" ]
then
	echo "Firewall completely flushed!  Now running with no firewall."
	exit 0
fi

###############################################################################
#
# Rules Configuration
#

###############################################################################
#
# Filter Table
#
###############################################################################

# Set Policies

$IPT -P INPUT DROP
$IPT -P OUTPUT DROP
$IPT -P FORWARD DROP

###############################################################################
#
# User-Specified Chains
#
# Create user chains to reduce the number of rules each packet
# must traverse.

echo "Create and populate custom rule chains ..."

# Create a chain to filter INVALID packets

$IPT -N bad_packets

# Create another chain to filter bad tcp packets

$IPT -N bad_tcp_packets

# Create separate chains for icmp, tcp (incoming and outgoing),
# and incoming udp packets.

$IPT -N icmp_packets

# Used for UDP packets inbound from the Internet
$IPT -N udp_inbound

# Used to block outbound UDP services from internal network
# Default to allow all
$IPT -N udp_outbound

# Used to allow inbound services if desired
# Default fail except for established sessions
$IPT -N tcp_inbound

# Used to block outbound services from internal network
# Default to allow all
$IPT -N tcp_outbound

###############################################################################
#
# Populate User Chains
#

# bad_packets chain
#

# Drop packets received on the external interface
# claiming a source of the local network
#$IPT -A bad_packets -p ALL -i $INET_IFACE -s $LOCAL_NET -j LOG \
#    --log-prefix "Illegal source: "

#$IPT -A bad_packets -p ALL -i $INET_IFACE -s $LOCAL_NET -j DROP

# Drop INVALID packets immediately
$IPT -A bad_packets -p ALL -m state --state INVALID -j LOG \
    --log-prefix "Invalid packet: "

$IPT -A bad_packets -p ALL -m state --state INVALID -j DROP

# Then check the tcp packets for additional problems
$IPT -A bad_packets -p tcp -j bad_tcp_packets

# All good, so return
$IPT -A bad_packets -p ALL -j RETURN

# bad_tcp_packets chain
#
# All tcp packets will traverse this chain.
# Every new connection attempt should begin with
# a syn packet.  If it doesn't, it is likely a
# port scan.  This drops packets in state
# NEW that are not flagged as syn packets.

# Return to the calling chain if the bad packets originate
# from the local interface. This maintains the approach
# throughout this firewall of a largely trusted internal
# network.
$IPT -A bad_tcp_packets -p tcp -i $LOCAL_IFACE -j RETURN

# However, I originally did apply this filter to the forward chain
# for packets originating from the internal network.  While I have
# not conclusively determined its effect, it appears to have the
# interesting side effect of blocking some of the ad systems.
# Apparently some ad systems have the browser initiate a NEW
# connection that is not flagged as a syn packet to retrieve
# the ad image.  If you wish to experiment further comment the
# rule above. If you try it, you may also wish to uncomment the
# rule below.  It will keep those packets from being logged.
# There are a lot of them.
# $IPT -A bad_tcp_packets -p tcp -i $LOCAL_IFACE ! --syn -m state \
#     --state NEW -j DROP

$IPT -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j LOG \
    --log-prefix "New not syn: "
$IPT -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL NONE -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL NONE -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL ALL -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL ALL -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL FIN,URG,PSH -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL FIN,URG,PSH -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,RST SYN,RST -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,RST SYN,RST -j DROP

$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,FIN SYN,FIN -j LOG \
    --log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP

# All good, so return
$IPT -A bad_tcp_packets -p tcp -j RETURN

# icmp_packets chain
#
# This chain is for inbound (from the Internet) icmp packets only.
# Type 8 (Echo Request) is not accepted by default
# Enable it if you want remote hosts to be able to reach you.
# 11 (Time Exceeded) is the only one accepted
# that would not already be covered by the established
# connection rule.  Applied to INPUT on the external interface.
# 
# See: http://www.ee.siue.edu/~rwalden/networking/icmp.html
# for more info on ICMP types.
#
# Note that the stateful settings allow replies to ICMP packets.
# These rules allow new packets of the specified types.

# ICMP packets should fit in a Layer 2 frame, thus they should
# never be fragmented.  Fragmented ICMP packets are a typical sign
# of a denial of service attack.
$IPT -A icmp_packets --fragment -p ICMP -j LOG \
    --log-prefix "ICMP Fragment: "
$IPT -A icmp_packets --fragment -p ICMP -j DROP

# Echo - uncomment to allow your system to be pinged.
# Uncomment the LOG command if you also want to log PING attempts
# 
# $IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j LOG \
#    --log-prefix "Ping detected: "
# $IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j ACCEPT

# By default, however, drop pings without logging. Blaster
# and other worms have infected systems blasting pings.
# Comment the line below if you want pings logged, but it
# will likely fill your logs.
$IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j DROP

# Time Exceeded
$IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 11 -j ACCEPT

# Not matched, so return so it will be logged
$IPT -A icmp_packets -p ICMP -j RETURN

# TCP & UDP
# Identify ports at:
#    http://www.chebucto.ns.ca/~rakerman/port-table.html
#    http://www.iana.org/assignments/port-numbers

# udp_inbound chain
#
# This chain describes the inbound UDP packets it will accept.
# It's applied to INPUT on the external or Internet interface.
# Note that the stateful settings allow replies.
# These rules are for new requests.
# It drops netbios packets (windows) immediately without logging.

# Drop netbios calls
# Please note that these rules do not really change the way the firewall
# treats netbios connections.  Connections from the localhost and
# internal interface (if one exists) are accepted by default.
# Responses from the Internet to requests initiated by or through
# the firewall are also accepted by default.  To get here, the
# packets would have to be part of a new request received by the
# Internet interface.  You would have to manually add rules to
# accept these.  I added these rules because some network connections,
# such as those via cable modems, tend to be filled with noise from
# unprotected Windows machines.  These rules drop those packets
# quickly and without logging them.  This prevents them from traversing
# the whole chain and keeps the log from getting cluttered with
# chatter from Windows systems.


$IPT -A udp_inbound -p UDP -s 0/0 --destination-port 137 -j DROP
$IPT -A udp_inbound -p UDP -s 0/0 --destination-port 138 -j DROP

# Dynamic Address
# If DHCP, the initial request is a broadcast. The response
# doesn't exactly match the outbound packet.  This explicitly
# allow the DHCP ports to alleviate this problem.
# If you receive your dynamic address by a different means, you
# can probably comment this line.
$IPT -A udp_inbound -p UDP -s 0/0 --source-port 67 --destination-port 68 \
     -j ACCEPT


# Not matched, so return for logging
$IPT -A udp_inbound -p UDP -j RETURN

# udp_outbound chain
#
# This chain is used with a private network to prevent forwarding for
# UDP requests on specific protocols.  Applied to the FORWARD rule from
# the internal network.  Ends with an ACCEPT

# ICQ uses UDP 4000 - Instant messaging blocked
#$IPT -A udp_outbound -p UDP -s 0/0 --destination-port 4000 -j REJECT


# No match, so ACCEPT
$IPT -A udp_outbound -p UDP -s 0/0 -j ACCEPT

# tcp_inbound chain
#
# This chain is used to allow inbound connections to the
# system/gateway.  Use with care.  It defaults to none.
# It's applied on INPUT from the external or Internet interface.


# Not matched, so return so it will be logged
$IPT -A tcp_inbound -p TCP -j RETURN

# tcp_outbound chain
#
# This chain is used with a private network to prevent forwarding for
# requests on specific protocols.  Applied to the FORWARD rule from
# the internal network.  Ends with an ACCEPT


/etc/firewall.d/blocks.sh

# No match, so ACCEPT
$IPT -A tcp_outbound -p TCP -s 0/0 -j ACCEPT

###############################################################################
#
# INPUT Chain
#

echo "Process INPUT chain ..."

# Allow all on localhost interface
$IPT -A INPUT -p ALL -i $LO_IFACE -j ACCEPT

# Rules for the private network (accessing gateway system itself)
$IPT -A INPUT -p ALL -i $LOCAL_IFACE  -j ACCEPT
$IPT -A INPUT -p ALL -i $LOCAL_IFACE  -j ACCEPT

# Drop bad packets
$IPT -A INPUT -p ALL -j bad_packets

# DOCSIS compliant cable modems
# Some DOCSIS compliant cable modems send IGMP multicasts to find
# connected PCs.  The multicast packets have the destination address
# 224.0.0.1.  You can accept them.  If you choose to do so,
# Uncomment the rule to ACCEPT them and comment the rule to DROP
# them  The firewall will drop them here by default to avoid
# cluttering the log.  The firewall will drop all multicasts
# to the entire subnet (224.0.0.1) by default.  To only affect
# IGMP multicasts, change '-p ALL' to '-p 2'.  Of course,
# if they aren't accepted elsewhere, it will only ensure that
# multicasts on other protocols are logged.
# Drop them without logging.
$IPT -A INPUT -p ALL -d 224.0.0.1 -j DROP
# The rule to accept the packets.
# $IPT -A INPUT -p ALL -d 224.0.0.1 -j ACCEPT



# Allow DHCP client request packets inbound from internal network
$IPT -A INPUT -p UDP -i $LOCAL_IFACE --source-port 68 --destination-port 67 \
     -j ACCEPT
# Allow DNS client request packets inbound from internet network
$IPT -A INPUT -p UDP -i $LOCAL_IFACE --destination-port 53 -j ACCEPT

# Inbound Internet Packet Rules

# Accept Established Connections
$IPT -A INPUT -p ALL -i $INET_IFACE -m state --state ESTABLISHED,RELATED \
     -j ACCEPT

# Route the rest to the appropriate user chain
$IPT -A INPUT -p TCP -i $INET_IFACE -j tcp_inbound
$IPT -A INPUT -p UDP -i $INET_IFACE -j udp_inbound
$IPT -A INPUT -p ICMP -i $INET_IFACE -j icmp_packets

# Drop without logging broadcasts that get this far.
# Cuts down on log clutter.
# Comment this line if testing new rules that impact
# broadcast protocols.
$IPT -A INPUT -m pkttype --pkt-type broadcast -j DROP

# Log packets that still don't match
$IPT -A INPUT -m limit --limit 3/minute --limit-burst 3 -j LOG \
    --log-prefix "INPUT packet died: "

###############################################################################
#
# FORWARD Chain
#

echo "Process FORWARD chain ..."

# Used if forwarding for a private network

# Drop bad packets
$IPT -A FORWARD -p ALL -j bad_packets

# Accept TCP packets we want to forward from internal sources
$IPT -A FORWARD -p tcp -i $LOCAL_IFACE -j tcp_outbound

# Accept UDP packets we want to forward from internal sources
$IPT -A FORWARD -p udp -i $LOCAL_IFACE -j udp_outbound

# If not blocked, accept any other packets from the internal interface
$IPT -A FORWARD -p ALL -i $LOCAL_IFACE -j ACCEPT

# Deal with responses from the internet
$IPT -A FORWARD -i $INET_IFACE -m state --state ESTABLISHED,RELATED \
     -j ACCEPT

# Log packets that still don't match
$IPT -A FORWARD -m limit --limit 3/minute --limit-burst 3 -j LOG \
    --log-prefix "FORWARD packet died: "

###############################################################################
#
# OUTPUT Chain
#

echo "Process OUTPUT chain ..."

# Generally trust the firewall on output

# However, invalid icmp packets need to be dropped
# to prevent a possible exploit.
$IPT -A OUTPUT -m state -p icmp --state INVALID -j DROP

# Localhost
$IPT -A OUTPUT -p ALL -s $LO_IP -j ACCEPT
$IPT -A OUTPUT -p ALL -o $LO_IFACE -j ACCEPT

# To internal network
$IPT -A OUTPUT -p ALL -s $LOCAL_IP -j ACCEPT
$IPT -A OUTPUT -p ALL -o $LOCAL_IFACE -j ACCEPT

# To internet
$IPT -A OUTPUT -p ALL -o $INET_IFACE -j ACCEPT

# Log packets that still don't match
$IPT -A OUTPUT -m limit --limit 3/minute --limit-burst 3 -j LOG \
    --log-prefix "OUTPUT packet died: "

###############################################################################
#
# nat table
#
###############################################################################

# The nat table is where network address translation occurs if there
# is a private network.  If the gateway is connected to the Internet
# with a static IP, snat is used.  If the gateway has a dynamic address,
# masquerade must be used instead.  There is more overhead associated
# with masquerade, so snat is better when it can be used.
# The nat table has a builtin chain, PREROUTING, for dnat and redirects.
# Another, POSTROUTING, handles snat and masquerade.

echo "Load rules for nat table ..."

###############################################################################
#
# PREROUTING chain
#

#$IPT -t nat -A PREROUTING -i $LOCAL_IFACE -j NFQUEUE

###############################################################################
#
# POSTROUTING chain
#


$IPT -t nat -A POSTROUTING -o $INET_IFACE -j MASQUERADE

###############################################################################
#
# mangle table
#
###############################################################################

# The mangle table is used to alter packets.  It can alter or mangle them in
# several ways.  For the purposes of this generator, we only use its ability
# to alter the TTL in packets.  However, it can be used to set netfilter
# mark values on specific packets.  Those marks could then be used in another
# table like filter, to limit activities associated with a specific host, for
# instance.  The TOS target can be used to set the Type of Service field in
# the IP header.  Note that the TTL target might not be included in the
# distribution on your system.  If it is not and you require it, you will
# have to add it.  That may require that you build from source.

echo "Load rules for mangle table ..."

cdean · 04-07-2007, 04:55 PM

It's completely not the firewall. I tracked down the counts with

Code:

iptables -nvL

and every was getting accepted.

I then tried flushing all the rules and setting the policy to drop. It still wouldn't resolve DNS.

I messed around with some dnsmasq configs, and here I'd mistaken the command to add an extra file from which to get hosts entries for the command to specify a location for nameserver information other than /etc/resolv.conf.

It works now. Thanks for your help, wykthorr.

wykthorr · 04-08-2007, 03:24 AM

With pleasure. Glad I could help