Hey, I am running slackware 8.1 with kernel 2.4.18. My linux computer has two network cards, one gets an IP from my DSL provider via DHCP and the other network card acts as a gateway so my Windows XP computer has internet access etc. I am currently using IPtables for masquerading to do this.

I am looking for a simple firewall script that will keep my computer secure but also allow my windows xp computer to have access to everything on the internet. I just want stuff coming in to be blocked. Is there pre-made firewall scripts out there that will do what I am looking for without having to change much in the configurations? I am new to all of this and a lot of it doesn't make sense to me just yet. I want to at least get a basic firewall up and running and then work on it from there.

personally i just use firestarter to build scripts for me. not bombproof, but i've not learnt to do better by hand...

Hi,

try few of these links out

http://linux-firewall-tools.com/linu...all/index.html

http://projectfiles.com/firewall/

The first link will create a firewall script online with your setting.

Chamkila

shrug, something like

# they what I'm blocking a feature. personally I call it a hole.
iptables -t iusedefaults -p TCP ! --syn -m state --state NEW -j DROP

# accept all pre-established connections
iptables -t iusedefaults -m state --state ESTABLISHED,RELATED -j ACCEPT

# accept connections from your internal address
iptables -t iusedefaults -i <y0 internal interface> -d 0/0 -j ACCEPT

iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT DROP

iptables -A INPUT -j iusedefaults
iptables -A FORWARD -j iusedefaults

iptables -A OUTPUT -s <internal IP> -j ACCEPT
iptables -A OUTPUT -s <external IP> -j ACCEPT

iptables -t nat -A POSTROUTING -i <y0 internal interface> -o <y0 external interface> -j SNAT --to-source <y0 external IP>

I dunno, that was from memory, so it probably doesn't work. anyhow iptables is like, easy n' stuff, though I'm gonna have egg on my face since that script above is gonna choke on something stupid

Sorry RijilV,

there's a lot missing from that script bitty. No udp rules for dns forwarding, and his external ip is dynamic, so he will need to Masquerade rather than SNAT etc...

Probably better to start with Firestarter as acid_kewpie says and then learn what it has done.

My favourite iptables tutorial is at http://www.netfilter.org/documentati...ials/blueflux/

Regards,
Peter

I posted this one out of the an iptables howto (i think it was that anyway..) a few days ago. It does exactly what you need and is very useful for learning purposes.

Mux

#!/bin/sh
#
# rc.firewall-2.4-stronger
FWVER=0.73s
# An example of a stronger IPTABLES firewall with IP Masquerade
# support for 2.4.x kernels.
#
# Log:
# 0.73s - Added comments in the output section that DHCPd is optional
# and changed the default settings to disabled
# 0.72s - Changed the filter from the INTNET to the INTIP to be
# stateful; moved the command VARs to the top and made the
# rest of the script to use them
# 0.70s - Added a disabled examples for allowing internal DHCP
# and external WWW access to the server
# 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" # 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
#
LSMOD=/sbin/lsmod
DEPMOD=/sbin/depmod
INSMOD=/sbin/insmod
GREP=/bin/grep
AWK=/usr/bin/awk
SED=/usr/bin/sed
IFCONFIG=/sbin/ifconfig

#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="`$IFCONFIG $EXTIF | $GREP 'inet addr' | $AWK '{print $2}' | \
$SED -e 's/.*://'`"

# For users who wish 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="192.168.0.0/24"
INTIP="192.168.0.1/24"
echo " Internal Network: $INTNET"
echo " Internal IP: $INTIP"
echo " ---"

# 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 -en " - Verifying kernel modules.."
$DEPMOD -a
######echo -en " Loading kernel modules: "
echo -en ".."
# 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 - "ip_tables"
# - Loaded automatically when the "iptables" command is invoked
#
# - Loaded manually to clean up kernel auto-loading timing issues
#
#####echo -en "ip_tables, "
echo -en ".."
#
#Verify the module isn't loaded. If it is, skip it
#
if [ -z "` $LSMOD | $GREP ip_tables | $AWK {'print $1'} `" ]; then
$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, "
echo -en ".."
#
#Verify the module isn't loaded. If it is, skip it
#
if [ -z "` $LSMOD | $GREP ip_conntrack | $AWK {'print $1'} `" ]; then
$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, "
echo -en ".."
#
#Verify the module isn't loaded. If it is, skip it
#
if [ -z "` $LSMOD | $GREP ip_conntrack_ftp | $AWK {'print $1'} `" ]; then
$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
# $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, "
echo -en ".."
#
#Verify the module isn't loaded. If it is, skip it
#
if [ -z "` $LSMOD | $GREP iptable_nat | $AWK {'print $1'} `" ]; then
$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"
echo -e "..OK"
#
#Verify the module isn't loaded. If it is, skip it
#
if [ -z "` $LSMOD | $GREP ip_nat_ftp | $AWK {'print $1'} `" ]; then
$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 the 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
# 192.168.1.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
# but 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 interfaces. 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 server in
#
$IPTABLES -A INPUT -i $EXTIF -s $UNIVERSE -d $EXTIP -m state --state \
ESTABLISHED,RELATED -j ACCEPT

# ----- Begin OPTIONAL Section -----

# # DHCPd - Enable the following lines if you run an INTERNAL DHCPd server
#
#$IPTABLES -A INPUT -i $INTIF -p tcp --sport 68 --dport 67 -j ACCEPT
#$IPTABLES -A INPUT -i $INTIF -p udp --sport 68 --dport 67 -j ACCEPT

# HTTPd - Enable the following lines if you run an EXTERNAL WWW server
#
#echo -e " - Allowing EXTERNAL access to the WWW server"
#$IPTABLES -A INPUT -i $EXTIF -m state --state NEW,ESTABLISHED,RELATED \
#-p tcp -s $UNIVERSE -d $EXTIP --dport 80 -j ACCEPT
#

#
# ----- End OPTIONAL Section -----


# 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 interfaces. 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

# ----- Begin OPTIONAL Section -----

# # DHCPd - Enable the following lines if you run an INTERNAL DHCPd server
#
#$IPTABLES -A OUTPUT -o $INTIF -p tcp -s $INTIP --sport 67 \ -d 255.255.255.255 --dport 68 -j ACCEPT
#$IPTABLES -A OUTPUT -o $INTIF -p udp -s $INTIP --sport 67 \ -d 255.255.255.255 --dport 68 -j ACCEPT
#
# ----- End OPTIONAL Section -----

# 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

# Catch all rule, all other forwarding is denied and logged.
#
$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"

There aren't any rules for dns udp port 53 in that script...

Did you add some yourself later?

Regards,
Peter