6.4. Stronger firewall rulesets to run after initial testing

#!/bin/sh
#
# rc.firewall-2.4-stronger
FWVER=0.63s

#          An example of a stronger IPTABLES firewall with IP Masquerade 
#          support for 2.4.x kernels.  
#
# Log:
#       0.63s - Added support for the IRC module
#       0.62s - Initial version based upon the basic 2.4.x rc.firewall


echo -e "\nLoading STRONGER rc.firewall - version $FWVER..\n"


#Setting the EXTERNAL and INTERNAL interfaces for the network
#
#  Each IP Masquerade network needs to have at least one
#  external and one internal network.  The external network
#  is where the natting will occur and the internal network
#  should preferably be addressed with a RFC1918 private address
#  scheme.
#
#  For this example, "eth0" is external and "eth1" is internal"
#
#  NOTE:  If this doesnt EXACTLY fit your configuration, you must 
#         change the EXTIF or INTIF variables above. For example: 
#
#               EXTIF="ppp0" 
#
#            if you are a modem user.
#
EXTIF="eth0"
INTIF="eth1"
echo "  External Interface:  $EXTIF"
echo "  Internal Interface:  $INTIF"
echo "  ---"

# Specify your Static IP address here or let the script take care of it 
# for you.
#
#   If you prefer to use STATIC addresses in your firewalls, un-# out the
#   static example below and # out the dynamic line.  If you don't care,
#   just leave this section alone.
#
#   If you have a DYNAMIC IP address, the ruleset already takes care of
#   this for you.  Please note that the different single and double quote 
#   characters and the script MATTER.
#
#
#   DHCP users:
#   -----------
#   If you get your TCP/IP address via DHCP, **you will need ** to enable the 
#   #ed out command below underneath the PPP section AND replace the word 
#   "eth0" with the name of your EXTERNAL Internet connection (ppp0, ippp0, 
#   etc) on the lines for "ppp-ip" and "extip".  You should also note that the 
#   DHCP server can and will change IP addresses on you.  To deal with this, 
#   users should configure their DHCP client to re-run the rc.firewall ruleset 
#   everytime the DHCP lease is renewed.
#
#     NOTE #1:  Some DHCP clients like the original "pump" (the newer
#               versions have been fixed) did NOT have the ability to run 
#               scripts after a lease-renew.  Because of this, you need to 
#               replace it with something like "dhcpcd" or "dhclient".
#
#     NOTE #2:  The syntax for "dhcpcd" has changed in recent versions.
#
#               Older versions used syntax like:
#                         dhcpcd -c /etc/rc.d/rc.firewall eth0
#
#               Newer versions execute a file called /etc/dhcpc/dhcpcd-eth0.exe
#
#     NOTE #3:  For Pump users, put the following line in /etc/pump.conf:
#
#                   script /etc/rc.d/rc.firewall
#
#   PPP users:
#   ----------
#   If you aren't already aware, the /etc/ppp/ip-up script is always run when 
#   a PPP connection comes up.  Because of this, we can make the ruleset go and 
#   get the new PPP IP address and update the strong firewall ruleset.
#
#   If the /etc/ppp/ip-up file already exists, you should edit it and add a line
#   containing "/etc/rc.d/rc.firewall" near the end of the file.
#
#   If you don't already have a /etc/ppp/ip-up sccript, you need to create the 
#   following link to run the /etc/rc.d/rc.firewall script.
#
#       ln -s /etc/rc.d/rc.firewall /etc/ppp/ip-up
#
#   * You then want to enable the #ed out shell command below *
#
#
# Determine the external IP automatically:
# ----------------------------------------
#
EXTIP="`/sbin/ifconfig $EXTIF | grep 'inet addr' | awk '{print $2}' | sed -e 's/.*://'`"

# For users who with to use STATIC IP addresses:
#
#  # out the EXTIP line above and un-# out the EXTIP line below
#
#EXTIP="your.static.PPP.address"
echo "  External IP: $EXTIP"
echo "  ---"


# Assign the internal TCP/IP network and IP address
INTNET="192.168.1.0/24"
INTIP="192.168.1.1/24"
echo "  Internal Network: $INTNET"
echo "  Internal IP:      $INTIP"
echo "  ---"


# The location of various iptables and other shell programs
#
#   If your Linux distribution came with a copy of iptables, most
#   likely it is located in /sbin.  If you manually compiled 
#   iptables, the default location is in /usr/local/sbin
#
# ** Please use the "whereis iptables" command to figure out 
# ** where your copy is and change the path below to reflect 
# ** your setup
#
#IPTABLES=/sbin/iptables
IPTABLES=/usr/local/sbin/iptables
#
LSMOD=/sbin/lsmod
GREP=/bin/grep
AWK=/bin/awk


# Setting a few other local variables
#
UNIVERSE="0.0.0.0/0"

#======================================================================
#== No editing beyond this line is required for initial MASQ testing ==

# Need to verify that all modules have all required dependencies
#
echo "  - Verifying that all kernel modules are ok"
/sbin/depmod -a

echo -en "    Loading kernel modules: "

# With the new IPTABLES code, the core MASQ functionality is now either
# modular or compiled into the kernel.  This HOWTO shows ALL IPTABLES
# options as MODULES.  If your kernel is compiled correctly, there is
# NO need to load the kernel modules manually.  
#
#  NOTE: The following items are listed ONLY for informational reasons.
#        There is no reason to manual load these modules unless your
#        kernel is either mis-configured or you intentionally disabled
#        the kernel module autoloader.
#

# Upon the commands of starting up IP Masq on the server, the
# following kernel modules will be automatically loaded:
#
# NOTE:  Only load the IP MASQ modules you need.  All current IP MASQ 
#        modules are shown below but are commented out from loading.
# ===============================================================

#Load the main body of the IPTABLES module - "iptable"
#  - Loaded automatically when the "iptables" command is invoked
#
#  - Loaded manually to clean up kernel auto-loading timing issues
#
echo -en "ip_tables, "
#
#Verify the module isn't loaded.  If it is, skip it
#
if [ -z "` $LSMOD | $GREP ip_tables | $AWK {'print $1'} `" ]; then
   /sbin/insmod ip_tables
fi


#Load the IPTABLES filtering module - "iptable_filter" 
#
#  - Loaded automatically when filter policies are activated


#Load the stateful connection tracking framework - "ip_conntrack"
#
# The conntrack  module in itself does nothing without other specific 
# conntrack modules being loaded afterwards such as the "ip_conntrack_ftp"
# module
#
#  - This module is loaded automatically when MASQ functionality is 
#    enabled 
#
#  - Loaded manually to clean up kernel auto-loading timing issues
#
echo -en "ip_conntrack, "
#
#Verify the module isn't loaded.  If it is, skip it
#
if [ -z "` $LSMOD | $GREP ip_conntrack | $AWK {'print $1'} `" ]; then
   /sbin/insmod ip_conntrack
fi


#Load the FTP tracking mechanism for full FTP tracking
#
# Enabled by default -- insert a "#" on the next line to deactivate
#
echo -e "ip_conntrack_ftp, "
#
#Verify the module isn't loaded.  If it is, skip it
#
if [ -z "` $LSMOD | $GREP ip_conntrack_ftp | $AWK {'print $1'} `" ]; then
   /sbin/insmod ip_conntrack_ftp
fi


#Load the IRC tracking mechanism for full IRC tracking
#
# Enabled by default -- insert a "#" on the next line to deactivate
#
echo -en "                             ip_conntrack_irc, "
#
#Verify the module isn't loaded.  If it is, skip it
#
if [ -z "` $LSMOD | $GREP ip_conntrack_irc | $AWK {'print $1'} `" ]; then
   /sbin/insmod ip_conntrack_irc
fi


#Load the general IPTABLES NAT code - "iptable_nat"
#  - Loaded automatically when MASQ functionality is turned on
# 
#  - Loaded manually to clean up kernel auto-loading timing issues
#
echo -en "iptable_nat, "
#
#Verify the module isn't loaded.  If it is, skip it
#
if [ -z "` $LSMOD | $GREP iptable_nat | $AWK {'print $1'} `" ]; then
   /sbin/insmod iptable_nat
fi


#Loads the FTP NAT functionality into the core IPTABLES code
# Required to support non-PASV FTP.
#
# Enabled by default -- insert a "#" on the next line to deactivate
#
echo -e "ip_nat_ftp"
#
#Verify the module isn't loaded.  If it is, skip it
#
if [ -z "` $LSMOD | $GREP ip_nat_ftp | $AWK {'print $1'} `" ]; then
   /sbin/insmod ip_nat_ftp
fi

echo "  ---"

# Just to be complete, here is a list of the remaining kernel modules 
# and their function.  Please note that several modules should be only
# loaded by the correct master kernel module for proper operation.
# --------------------------------------------------------------------
#
#    ipt_mark       - this target marks a given packet for future action.
#                     This automatically loads the ipt_MARK module
#
#    ipt_tcpmss     - this target allows to manipulate the TCP MSS
#                     option for braindead remote firewalls.
#                     This automatically loads the ipt_TCPMSS module
#
#    ipt_limit      - this target allows for packets to be limited to
#                     to many hits per sec/min/hr
#
#    ipt_multiport  - this match allows for targets within a range
#                     of port numbers vs. listing each port individually
#
#    ipt_state      - this match allows to catch packets with various
#                     IP and TCP flags set/unset
#
#    ipt_unclean    - this match allows to catch packets that have invalid
#                     IP/TCP flags set
#
#    iptable_filter - this module allows for packets to be DROPped, 
#                     REJECTed, or LOGged.  This module automatically 
#                     loads the following modules:
#
#                     ipt_LOG - this target allows for packets to be 
#                               logged
#
#                     ipt_REJECT - this target DROPs the packet and returns 
#                                  a configurable ICMP packet back to the 
#                                  sender.
# 
#    iptable_mangle - this target allows for packets to be manipulated
#                     for things like the TCPMSS option, etc.


#CRITICAL:  Enable IP forwarding since it is disabled by default since
#
#           Redhat Users:  you may try changing the options in
#                          /etc/sysconfig/network from:
#
#                       FORWARD_IPV4=false
#                             to
#                       FORWARD_IPV4=true
#
echo "  Enabling forwarding.."
echo "1" > /proc/sys/net/ipv4/ip_forward


# Dynamic IP users:
#
#   If you get your IP address dynamically from SLIP, PPP, or DHCP, 
#   enable this following option.  This enables dynamic-address hacking
#   which makes the life with Diald and similar programs much easier.
#
echo "  Enabling DynamicAddr.."
echo "1" > /proc/sys/net/ipv4/ip_dynaddr

echo "  ---"

#############################################################################
#
# Enable Stronger IP forwarding and Masquerading
#
#  NOTE:  In IPTABLES speak, IP Masquerading is a form of SourceNAT or SNAT.
#
#  NOTE #2:  The following is an example for an internal LAN address in the
#            192.168.0.x network with a 255.255.255.0 or a "24" bit subnet 
#            mask connecting to the Internet on external interface "eth0".  
#            This example will MASQ internal traffic out to the Internet 
#            not not allow non-initiated traffic into your internal network.
#
#            
#         ** Please change the above network numbers, subnet mask, and your 
#         *** Internet connection interface name to match your setup
#         

#Clearing any previous configuration
#
#  Unless specified, the defaults for INPUT, OUTPUT, and FORWARD to DROP.
#
#    You CANNOT change this to REJECT as it isn't a vaild setting for a
#    policy.  If you want REJECT, you must explictly REJECT at the end
#    of a giving INPUT, OUTPUT, or FORWARD chain
#
echo "  Clearing any existing rules and setting default policy to DROP.."
$IPTABLES -P INPUT DROP  
$IPTABLES -F INPUT 
$IPTABLES -P OUTPUT DROP  
$IPTABLES -F OUTPUT 
$IPTABLES -P FORWARD DROP  
$IPTABLES -F FORWARD 
$IPTABLES -F -t nat
#Not needed and it will only load the unneeded kernel module
#$IPTABLES -F -t mangle
#
# Flush the user chain.. if it exists
if [ -n "`$IPTABLES -L | $GREP drop-and-log-it`" ]; then
   $IPTABLES -F drop-and-log-it
fi
#
# Delete all User-specified chains
$IPTABLES -X
#
# Reset all IPTABLES counters
$IPTABLES -Z


#Configuring specific CHAINS for later use in the ruleset
#
#  NOTE:  Some users prefer to have their firewall silently
#         "DROP" packets while others prefer to use "REJECT"
#         to send ICMP error messages back to the remote 
#         machine.  The default is "REJECT" but feel free to
#         change this below.
#
# NOTE: Without the --log-level set to "info", every single
#       firewall hit will goto ALL vtys.  This is a very big
#       pain.
#
echo "  Creating a DROP chain.."
$IPTABLES -N drop-and-log-it
$IPTABLES -A drop-and-log-it -j LOG --log-level info 
$IPTABLES -A drop-and-log-it -j DROP

echo -e "\n   - Loading INPUT rulesets"

#######################################################################
# INPUT: Incoming traffic from various internfaces.  All rulesets are 
#        already flushed and set to a default policy of DROP. 
#

# loopback interfaces are valid.
#
$IPTABLES -A INPUT -i lo -s $UNIVERSE -d $UNIVERSE -j ACCEPT


# local interface, local machines, going anywhere is valid
#
$IPTABLES -A INPUT -i $INTIF -s $INTNET -d $UNIVERSE -j ACCEPT


# remote interface, claiming to be local machines, IP spoofing, get lost
#
$IPTABLES -A INPUT -i $EXTIF -s $INTNET -d $UNIVERSE -j drop-and-log-it


# external interface, from any source, for ICMP traffic is valid
#
#  If you would like your machine to "ping" from the Internet, 
#  enable this next line
#
#$IPTABLES -A INPUT -i $EXTIF -p ICMP -s $UNIVERSE -d $EXTIP -j ACCEPT


# remote interface, any source, going to permanent PPP address is valid
#
#$IPTABLES -A INPUT -i $EXTIF -s $UNIVERSE -d $EXTIP -j ACCEPT


# Allow any related traffic coming back to the MASQ serer in
#
$IPTABLES -A INPUT -i $EXTIF -s $UNIVERSE -d $EXTIP -m state --state \
ESTABLISHED,RELATED -j ACCEPT


# Catch all rule, all other incoming is denied and logged. 
#
$IPTABLES -A INPUT -s $UNIVERSE -d $UNIVERSE -j drop-and-log-it


echo -e "   - Loading OUTPUT rulesets"

#######################################################################
# OUTPUT: Outgoing traffic from various internfaces.  All rulesets are 
#         already flushed and set to a default policy of DROP. 
#

# loopback interface is valid.
#
$IPTABLES -A OUTPUT -o lo -s $UNIVERSE -d $UNIVERSE -j ACCEPT


# local interfaces, any source going to local net is valid
#
$IPTABLES -A OUTPUT -o $INTIF -s $EXTIP -d $INTNET -j ACCEPT


# local interface, any source going to local net is valid
#
$IPTABLES -A OUTPUT -o $INTIF -s $INTIP -d $INTNET -j ACCEPT


# outgoing to local net on remote interface, stuffed routing, deny
#
$IPTABLES -A OUTPUT -o $EXTIF -s $UNIVERSE -d $INTNET -j drop-and-log-it


# anything else outgoing on remote interface is valid
#
$IPTABLES -A OUTPUT -o $EXTIF -s $EXTIP -d $UNIVERSE -j ACCEPT


# Catch all rule, all other outgoing is denied and logged. 
#
$IPTABLES -A OUTPUT -s $UNIVERSE -d $UNIVERSE -j drop-and-log-it


echo -e "   - Loading FORWARD rulesets"

#######################################################################
# FORWARD: Enable Forwarding and thus IPMASQ
#

echo "     - FWD: Allow all connections OUT and only existing/related IN"
$IPTABLES -A FORWARD -i $EXTIF -o $INTIF -m state --state ESTABLISHED,RELATED \
-j ACCEPT
$IPTABLES -A FORWARD -i $INTIF -o $EXTIF -j ACCEPT
$IPTABLES -A FORWARD -j drop-and-log-it


echo "     - NAT: Enabling SNAT (MASQUERADE) functionality on $EXTIF"
#
#More liberal form
#$IPTABLES -t nat -A POSTROUTING -o $EXTIF -j MASQUERADE
#
#Stricter form
$IPTABLES -t nat -A POSTROUTING -o $EXTIF -j SNAT --to $EXTIP


#######################################################################
echo -e "\nStronger rc.firewall-2.4 $FWVER done.\n"

#!/bin/sh
#
# /etc/rc.d/rc.firewall: An example of a Semi-Strong IPCHAINS firewall ruleset. 
#

PATH=/sbin:/bin:/usr/sbin:/usr/bin

# Load all required IP MASQ modules
#
#   NOTE:  Only load the IP MASQ modules you need.  All current IP MASQ modules
#          are shown below but are commented from loading.

# Needed to initially load modules
#
/sbin/depmod -a

# Supports the proper masquerading of FTP file transfers using the PORT method
#
/sbin/modprobe ip_masq_ftp

# Supports the masquerading of RealAudio over UDP.  Without this module,
#       RealAudio WILL function but in TCP mode.  This can cause a reduction
#       in sound quality
#
/sbin/modprobe ip_masq_raudio

# Supports the masquerading of IRC DCC file transfers
#
#/sbin/modprobe ip_masq_irc


# Supports the masquerading of Quake and QuakeWorld by default.  These modules are
#   for multiple users behind the Linux MASQ server.  If you are going to 
#   play Quake I, II, and III, use the second example.
#
#   NOTE:  If you get ERRORs loading the QUAKE module, you are running an old
#   -----  kernel that has bugs in it.  Please upgrade to the newest kernel.
#
#Quake I / QuakeWorld (ports 26000 and 27000)
#/sbin/modprobe ip_masq_quake
#
#Quake I/II/III / QuakeWorld (ports 26000, 27000, 27910, 27960)
#/sbin/modprobe ip_masq_quake 26000,27000,27910,27960


# Supports the masquerading of the CuSeeme video conferencing software
#
#/sbin/modprobe ip_masq_cuseeme

#Supports the masquerading of the VDO-live video conferencing software
#
#/sbin/modprobe ip_masq_vdolive


#CRITICAL:  Enable IP forwarding since it is disabled by default
#
#           Redhat Users:  you may try changing the options in 
#                          /etc/sysconfig/network from:
#
#                       FORWARD_IPV4=false
#                             to
#                       FORWARD_IPV4=true
#
echo "1" > /proc/sys/net/ipv4/ip_forward


#CRITICAL:  Enable automatic IP defragmentation since it is disabled by default 
#           in 2.2.x kernels 
#
#           This used as a compile-time option but the behavior was changed 
#           in 2.2.12.  It should also be noted that some distributions have
#           removed this option from the /proc table.  If this entry isn't
#           present in your /proc, don't worry about it.
#
echo "1" > /proc/sys/net/ipv4/ip_always_defrag


# Dynamic IP users:
#
#   If you get your IP address dynamically from SLIP, PPP, or DHCP, enable this 
#   following option.  This enables dynamic-ip address hacking in IP MASQ, 
#   making life with Diald and similar programs much easier.
#
#echo "1" > /proc/sys/net/ipv4/ip_dynaddr


# Enable the LooseUDP patch which some Internet-based games require
#
#  If you are trying to get an Internet game to work through your IP MASQ box,
#  and you configured it to the best of your ability without it working, try
#  enabling this option (delete the "#" character).  This option is disabled
#  by default due to possible internal machine UDP port scanning
#  vunerabilities.
#
#echo "1" > /proc/sys/net/ipv4/ip_masq_udp_dloose


# Specify your Static IP address here.
#
#   If you have a DYNAMIC IP address, you need to make this ruleset recognize 
#   your IP address everytime you get a new IP.  To do this, enable the 
#   following one-line script.  (Please note that the different single and 
#   double quote characters MATTER).
#
#
#   DHCP users:
#   -----------
#   If you get your TCP/IP address via DHCP, **you will need ** to enable the 
#   #ed out command below underneath the PPP section AND replace the word 
#   "ppp0" with the name of your EXTERNAL Internet connection (eth0, eth1, etc) 
#   on the lines for "ppp-ip" and "extip".  You should note that the 
#   DHCP server can change IP addresses on you.  To fix this, users should 
#   configure their DHCP client to re-run the firewall ruleset everytime the 
#   DHCP lease is renewed.
#
#     NOTE #1:  Some DHCP clients like the original "pump" (the newer
#               versions have been fixed) did NOT have the ability to run 
#               scripts after a lease-renew.  Because of this, you need to 
#               replace it with something like "dhcpcd" or "dhclient".
#
#     NOTE #2:  The syntax for "dhcpcd" has changed in recent versions.
#
#               Older versions used syntax like:
#                         dhcpcd -c /etc/rc.d/rc.firewall eth0
#
#               Newer versions use syntax like:
#                         dhcpcd eth0 /etc/rc.d/rc.firewall
#
#     NOTE #3:  For Pump users, put the following line in /etc/pump.conf:
#
#                   script /etc/rc.d/rc.firewall
#
#   PPP users:
#   ----------
#   If you aren't already aware, the /etc/ppp/ip-up script is always run when 
#   a PPP connection comes up.  Because of this, we can make the ruleset go and 
#   get the new PPP IP address and update the strong firewall ruleset.
#
#   If the /etc/ppp/ip-up file already exists, you should edit it and add a line
#   containing "/etc/rc.d/rc.firewall" near the end of the file.
#
#   If you don't already have a /etc/ppp/ip-up sccript, you need to create the 
#   following link to run the /etc/rc.d/rc.firewall script.
#
#       ln -s /etc/rc.d/rc.firewall /etc/ppp/ip-up
#
#   * You then want to enable the #ed out shell command below *
#
#
# PPP and DHCP Users:
# -------------------
# Remove the # on the line below and place a # in front of the line after that.
#
#extip="`/sbin/ifconfig ppp0 | grep 'inet addr' | awk '{print $2}' | sed -e 's/.*://'`"

# For PPP users with STATIC IP addresses:
#
extip="your.static.PPP.address"

# ALL PPP and DHCP users must set this for the correct EXTERNAL interface name
extint="ppp0"

# Assign the internal IP
intint="eth0"
intnet="192.168.0.0/24"


# MASQ timeouts
#
#   2 hrs timeout for TCP session timeouts
#  10 sec timeout for traffic after the TCP/IP "FIN" packet is received
#  60 sec timeout for UDP traffic (MASQ'ed ICQ users must enable a 30sec 
#     firewall timeout in ICQ itself)
#
ipchains -M -S 7200 10 60

#############################################################################
# Incoming, flush and set default policy of reject. Actually the default policy
# is irrelevant because there is a catch all rule with deny and log.
#
ipchains -F input
ipchains -P input REJECT

# local interface, local machines, going anywhere is valid
#
ipchains -A input -i $intint -s $intnet -d 0.0.0.0/0 -j ACCEPT

# remote interface, claiming to be local machines, IP spoofing, get lost
#
ipchains -A input -i $extint -s $intnet -d 0.0.0.0/0 -l -j REJECT

# remote interface, any source, going to permanent PPP address is valid
#
ipchains -A input -i $extint -s 0.0.0.0/0 -d $extip/32 -j ACCEPT

# loopback interface is valid.
#
ipchains -A input -i lo -s 0.0.0.0/0 -d 0.0.0.0/0 -j ACCEPT

# catch all rule, all other incoming is denied and logged. pity there is no
# log option on the policy but this does the job instead.
#
ipchains -A input -s 0.0.0.0/0 -d 0.0.0.0/0 -l -j REJECT

#############################################################################
# Outgoing, flush and set default policy of reject. Actually the default policy
# is irrelevant because there is a catch all rule with deny and log.
#
ipchains -F output
ipchains -P output REJECT

# local interface, any source going to local net is valid
#
ipchains -A output -i $intint -s 0.0.0.0/0 -d $intnet -j ACCEPT

# outgoing to local net on remote interface, stuffed routing, deny
#
ipchains -A output -i $extint -s 0.0.0.0/0 -d $intnet -l -j REJECT

# outgoing from local net on remote interface, stuffed masquerading, deny
#
ipchains -A output -i $extint -s $intnet -d 0.0.0.0/0 -l -j REJECT

# anything else outgoing on remote interface is valid
#
ipchains -A output -i $extint -s $extip/32 -d 0.0.0.0/0 -j ACCEPT

# loopback interface is valid.
#
ipchains -A output -i lo -s 0.0.0.0/0 -d 0.0.0.0/0 -j ACCEPT

# catch all rule, all other outgoing is denied and logged. pity there is no
# log option on the policy but this does the job instead.
#
ipchains -A output -s 0.0.0.0/0 -d 0.0.0.0/0 -l -j REJECT

#############################################################################
# Forwarding, flush and set default policy of deny. Actually the default policy
# is irrelevant because there is a catch all rule with deny and log.
#
ipchains -F forward
ipchains -P forward DENY

# Masquerade from local net on local interface to anywhere.
#
ipchains -A forward -i $extint -s $intnet -d 0.0.0.0/0 -j MASQ
#
# catch all rule, all other forwarding is denied and logged. pity there is no
# log option on the policy but this does the job instead.
#
ipchains -A forward -s 0.0.0.0/0 -d 0.0.0.0/0 -l -j REJECT

#End of file.

In the /etc/rc.d/rc.firewall ruleset:
... start of "input" rules ...

# reject and log local interface, local machines going to 204.50.10.13
#
ipchains -A input -s 192.168.0.0/24 -d 204.50.10.13/32 -l -j REJECT


# local interface, local machines, going anywhere is valid
#
ipchains -A input -s 192.168.0.0/24 -d 0.0.0.0/0 -l -j ACCEPT


... end of "input" rules ...

#!/bin/sh
#
# /etc/rc.d/rc.firewall: An example of a semi-STRONG IPFWADM firewall ruleset
#

PATH=/sbin:/bin:/usr/sbin:/usr/bin

# testing, wait a bit then clear all firewall rules.
# uncomment the following lines if you want the firewall to automatically
# disable after 10 minutes.
# (sleep 600; \
# ipfwadm -I -f; \
# ipfwadm -I -p accept; \
# ipfwadm -O -f; \
# ipfwadm -O -p accept; \
# ipfwadm -F -f; \
# ipfwadm -F -p accept; \
# ) &

# Load all required IP MASQ modules
#
#   NOTE:  Only load the IP MASQ modules you need.  All current IP MASQ modules
#          are shown below but are commented from loading.

# Needed to initially load modules
#
/sbin/depmod -a

# Supports the proper masquerading of FTP file transfers using the PORT method
#
/sbin/modprobe ip_masq_ftp

# Supports the masquerading of RealAudio over UDP.  Without this module,
#       RealAudio WILL function but in TCP mode.  This can cause a reduction
#       in sound quality
#
#/sbin/modprobe ip_masq_raudio

# Supports the masquerading of IRC DCC file transfers
#
#/sbin/modprobe ip_masq_irc


# Supports the masquerading of Quake and QuakeWorld by default.  This modules is
#   for multiple users behind the Linux MASQ server.  If you are going to 
#   play Quake I, II, and III, use the second example.
#
#   NOTE:  If you get ERRORs loading the QUAKE module, you are running an old
#   -----  kernel that has bugs in it.  Please upgrade to the newest kernel.
#
#Quake I / QuakeWorld (ports 26000 and 27000)
#/sbin/modprobe ip_masq_quake
#
#Quake I/II/III / QuakeWorld (ports 26000, 27000, 27910, 27960)
#/sbin/modprobe ip_masq_quake 26000,27000,27910,27960


# Supports the masquerading of the CuSeeme video conferencing software
#
#/sbin/modprobe ip_masq_cuseeme

#Supports the masquerading of the VDO-live video conferencing software
#
#/sbin/modprobe ip_masq_vdolive


#CRITICAL:  Enable IP forwarding, since it is disabled by default
#
#           Redhat Users:  you may try changing the options in /etc/sysconfig/network 
#                          from:
#
#                       FORWARD_IPV4=false
#                             to
#                       FORWARD_IPV4=true
#
echo "1" > /proc/sys/net/ipv4/ip_forward


#CRITICAL:  Enable automatic IP defragmenting since it is disabled by default 
#           in 2.2.x kernels
#
#           This used to be a compile-time option but the behavior was changed 
#           in 2.2.12
#
echo "1" > /proc/sys/net/ipv4/ip_always_defrag


# Dynamic IP users:
#
#   If you get your IP address dynamically from SLIP, PPP, or DHCP, enable this 
#   following option.  This allows dynamic-ip address hacking in IP MASQ, 
#   making the life with Diald and similar programs much easier.
#
#echo "1" > /proc/sys/net/ipv4/ip_dynaddr


# Specify your Static IP address here.  
#
#   If you have a DYNAMIC IP address, you need to make this ruleset understand 
#   your IP address everytime you get a new IP.  To do this, enable the 
#   following one-line script.  (Please note that the different single and 
#   double quote characters MATTER).  
#
#
#   DHCP users:  
#   -----------
#   If you get your TCP/IP address via DHCP, **you will need ** to enable the 
#   #ed out command below underneath the PPP section AND replace the word 
#   "ppp0" with the name of your EXTERNAL Internet connection (eth0, eth1, 
#   etc).  It should be also noted that the DHCP server can change IP 
#   addresses on you.  To fix this, users should configure their DHCP client 
#   to re-run the firewall ruleset everytime the DHCP lease is renewed.  
#
#     NOTE #1:  Some DHCP clients like the older version of "pump" (the newer
#               versions have been fixed) did NOT have the ability to run 
#               scripts after a lease-renew.  Because of this, you need to 
#               replace it with something like "dhcpcd" or "dhclient".
#
#     NOTE #2:  The syntax for "dhcpcd" has changed in recent versions.
#
#               Older versions used syntax like:
#                         dhcpcd -c /etc/rc.d/rc.firewall eth0
#
#               Newer versions use syntax like:
#                         dhcpcd eth0 /etc/rc.d/rc.firewall
#
#     NOTE #3:  For Pump users, put the following line in /etc/pump.conf:
#
#                   script /etc/rc.d/rc.firewall
#
#   PPP users:  
#   ----------
#   If you aren't already aware, the /etc/ppp/ip-up script is always run when 
#   a PPP connection comes up.  Because of this, we can make the ruleset go 
#   and get the new PPP IP address and update the strong firewall ruleset.
#
#   If the /etc/ppp/ip-up file already exists, you should edit it and add a line
#   containing "/etc/rc.d/rc.firewall" near the end of the file.
#
#   If you don't already have a /etc/ppp/ip-up sccript, you need to create the 
#   following link to run the /etc/rc.d/rc.firewall script.
#
#       ln -s /etc/rc.d/rc.firewall /etc/ppp/ip-up
#
#   * You then want to enable the #ed out shell command below *
#
#  
# PPP and DHCP Users: 
# -------------------
# Remove the # on the line below and place a # in front of the line after that.
#
#ppp_ip="`/sbin/ifconfig ppp0 | grep 'inet addr' | awk '{print $2}' | sed -e 's/.*://'`"
#
ppp_ip="your.static.PPP.address"


# MASQ timeouts 
#
#   2 hrs timeout for TCP session timeouts
#  10 sec timeout for traffic after the TCP/IP "FIN" packet is received
#  60 sec timeout for UDP traffic (MASQ'ed ICQ users must enable a 30sec 
#     firewall timeout in ICQ itself) 
#
/sbin/ipfwadm -M -s 7200 10 60


#############################################################################
# Incoming, flush and set default policy of reject. Actually the default policy
# is irrelevant because there is a catch all rule with deny and log.
#
/sbin/ipfwadm -I -f
/sbin/ipfwadm -I -p reject

# local interface, local machines, going anywhere is valid
#
/sbin/ipfwadm -I -a accept -V 192.168.0.1 -S 192.168.0.0/24 -D 0.0.0.0/0

# remote interface, claiming to be local machines, IP spoofing, get lost
#
/sbin/ipfwadm -I -a reject -V $ppp_ip -S 192.168.0.0/24 -D 0.0.0.0/0 -o

# remote interface, any source, going to permanent PPP address is valid
#
/sbin/ipfwadm -I -a accept -V $ppp_ip -S 0.0.0.0/0 -D $ppp_ip/32

# loopback interface is valid.
#
/sbin/ipfwadm -I -a accept -V 127.0.0.1 -S 0.0.0.0/0 -D 0.0.0.0/0

# catch all rule, all other incoming is denied and logged. pity there is no
# log option on the policy but this does the job instead.
#
/sbin/ipfwadm -I -a reject -S 0.0.0.0/0 -D 0.0.0.0/0 -o


#############################################################################
# Outgoing, flush and set default policy of reject. Actually the default policy
# is irrelevant because there is a catch all rule with deny and log.
#
/sbin/ipfwadm -O -f
/sbin/ipfwadm -O -p reject

# local interface, any source going to local net is valid
#
/sbin/ipfwadm -O -a accept -V 192.168.0.1 -S 0.0.0.0/0 -D 192.168.0.0/24

# outgoing to local net on remote interface, stuffed routing, deny
#
/sbin/ipfwadm -O -a reject -V $ppp_ip -S 0.0.0.0/0 -D 192.168.0.0/24 -o

# outgoing from local net on remote interface, stuffed masquerading, deny
#
/sbin/ipfwadm -O -a reject -V $ppp_ip -S 192.168.0.0/24 -D 0.0.0.0/0 -o

# outgoing from local net on remote interface, stuffed masquerading, deny
#
/sbin/ipfwadm -O -a reject -V $ppp_ip -S 0.0.0.0/0 -D 192.168.0.0/24 -o

# anything else outgoing on remote interface is valid
#
/sbin/ipfwadm -O -a accept -V $ppp_ip -S $ppp_ip/32 -D 0.0.0.0/0

# loopback interface is valid.
#
/sbin/ipfwadm -O -a accept -V 127.0.0.1 -S 0.0.0.0/0 -D 0.0.0.0/0

# catch all rule, all other outgoing is denied and logged. pity there is no
# log option on the policy but this does the job instead.
#
/sbin/ipfwadm -O -a reject -S 0.0.0.0/0 -D 0.0.0.0/0 -o


#############################################################################
# Forwarding, flush and set default policy of deny. Actually the default policy
# is irrelevant because there is a catch all rule with deny and log.
#
/sbin/ipfwadm -F -f
/sbin/ipfwadm -F -p deny

# Masquerade from local net on local interface to anywhere.
#
/sbin/ipfwadm -F -a masquerade -W ppp0 -S 192.168.0.0/24 -D 0.0.0.0/0
#
# catch all rule, all other forwarding is denied and logged.  Pity there is no
# log option on the policy but this does the job instead.
#
/sbin/ipfwadm -F -a reject -S 0.0.0.0/0 -D 0.0.0.0/0 -o

#End of file.