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Old 02-04-2009, 05:38 AM   #1
Registered: Jan 2006
Posts: 172

Rep: Reputation: 15
Iptables Issue

Iam using Redhat 9 proxy server with 3 diffreent networks here i am giving you the complete details


Here i am giving the firewall file which i taken from linux sites for my ftp, irc and port forwarding

# Initial SIMPLE IP Masquerade test for 2.4.x kernels
# using IPTABLES.
# Once IP Masquerading has been tested, with this simple
# ruleset, it is highly recommended to use a stronger
# IPTABLES ruleset either given later in this HOWTO or
# from another reputable resource.
# Log:
# 0.75 - Added more kernel modules to the comments section
# 0.74 - the ruleset now uses modprobe vs. insmod
# 0.73 - REJECT is not a legal policy yet; back to DROP
# 0.72 - Changed the default block behavior to REJECT not DROP
# 0.71 - Added clarification that PPPoE users need to use
# "ppp0" instead of "eth0" for their external interface
# 0.70 - Added commented option for IRC nat module
# - Added additional use of environment variables
# - Added additional formatting
# 0.63 - Added support for the IRC IPTABLES module
# 0.62 - Fixed a typo on the MASQ enable line that used eth0
# instead of $EXTIF
# 0.61 - Changed the firewall to use variables for the internal
# and external interfaces.
# 0.60 - 0.50 had a mistake where the ruleset had a rule to DROP
# all forwarded packets but it didn't have a rule to ACCEPT
# any packets to be forwarded either
# - Load the ip_nat_ftp and ip_conntrack_ftp modules by default
# 0.50 - Initial draft

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

# The location of the iptables and kernel module programs
# If your Linux distribution came with a copy of iptables,
# most likely all the programs will be located in /sbin. If
# you manually compiled iptables, the default location will
# be 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

#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:
# If you are a PPPoE or analog modem user:
# EXTIF="ppp0"
echo " External Interface: $EXTIF"
echo " Internal Interface: $INTIF"

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

echo -en " loading modules: "

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

# 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.
# ===============================================================

echo "----------------------------------------------------------------------"

#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, "
$MODPROBE ip_tables

#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, "
$MODPROBE ip_conntrack

#Load the FTP tracking mechanism for full FTP tracking
# Enabled by default -- insert a "#" on the next line to deactivate
echo -en "ip_conntrack_ftp, "
$MODPROBE ip_conntrack_ftp

#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, "
$MODPROBE ip_conntrack_irc

#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, "
$MODPROBE iptable_nat

#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 -en "ip_nat_ftp, "
$MODPROBE ip_nat_ftp

#Loads the IRC NAT functionality into the core IPTABLES code
# Required to support NAT of IRC DCC requests
# Disabled by default -- remove the "#" on the next line to activate
#echo -e "ip_nat_irc"
#$MODPROBE ip_nat_irc

echo "----------------------------------------------------------------------"

# Just to be complete, here is a partial list of some of the other
# IPTABLES kernel modules and their function. Please note that most
# of these modules (the ipt ones) are automatically loaded by the
# master kernel module for proper operation and don't need to be
# manually loaded.
# --------------------------------------------------------------------
# ip_nat_snmp_basic - this module allows for proper NATing of some
# SNMP traffic
# iptable_mangle - this target allows for packets to be
# manipulated for things like the TCPMSS
# option, etc.
# --
# 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.

echo -e " Done loading modules.\n"

#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
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

# Enable simple 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 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 and OUTPUT is ACCEPT
# The default for FORWARD is DROP (REJECT is not a valid policy)
echo " Clearing any existing rules and setting default policy.."
$IPTABLES -t nat -F

echo " FWD: Allow all connections OUT and only existing and related ones IN"

echo " Enabling SNAT (MASQUERADE) functionality on $EXTIF"

echo -e "\nrc.firewall-2.4 v$FWVER done.\n"

Now here i am doing another network to work with eth4 lan card with following rules

route add -net netmask gw
route add -net netmask gw
iptables -I INPUT -p tcp --dport 1100 -j ACCEPT
iptables -I INPUT -p udp --dport 1100 -j ACCEPT
iptables -I INPUT -p udp --dport 2449 -j ACCEPT
iptables -I INPUT -p udp --dport 2449 -j ACCEPT
echo "1" > /proc/sys/net/ipv4/ip_forward
iptables -I INPUT -p tcp --dport 21 -j ACCEPT
iptables -I INPUT -p udp --dport 21 -j ACCEPT
iptables -I INPUT -p tcp --dport 1550 -j ACCEPT
iptables -I INPUT -p udp --dport 1550 -j ACCEPT
iptables -A FORWARD -i eth1 -o eth3 -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -i eth3 -o eth1 -j ACCEPT
iptables -A FORWARD -j LOG
iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE
iptables -t filter -P FORWARD DROP
iptables -A OUTPUT -p 47 -j ACCEPT
iptables -A INPUT -p 47 -j ACCEPT
iptables -A FORWARD -s -d -j ACCEPT
iptables -A INPUT -p tcp -m tcp --dport 1723 -j ACCEPT
iptables -A INPUT -s -d -j ACCEPT
iptables -A FORWARD -p tcp -s 0/0 -d 0/0 --dport 21 -j ACCEPT

My route -n look's like this

[root@localhost root]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface UG 0 0 0 eth3 U 0 0 0 eth2 U 0 0 0 eth3 U 0 0 0 eth0 UG 0 0 0 eth3 UG 0 0 0 eth3 U 0 0 0 eth1 U 0 0 0 eth3 U 0 0 0 lo UG 0 0 0 eth1

Now the problem is in the following command

iptables -t filter -P FORWARD DROP

when i ping to i get request timed out but when i change the above command to following one

iptables -t filter -P FORWARD ACCEPT

i get ping and my application also works, i want to know the above command what does for drop?


Old 02-04-2009, 05:51 AM   #2
Registered: Jun 2007
Posts: 359

Rep: Reputation: 34
hi satish,

iptables -t filter -P FORWARD DROP

when i ping to i get request timed out but when i change the above command to following one
from which rule - that -t filter chain derived?
i dont see it anywhere from above script
if you dont specify any ACCEPT connection rule prior to DROP policy then the result is predictable.



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