CentOS / Redhat Iptables Firewall Configuration Tutorial
Written by vaheeD on January 4, 2013
Iptables Config File
The default config files for RHEL / CentOS / Fedora Linux are:
- /etc/sysconfig/iptables – The system scripts that activate the firewall by reading this file.
Task: Display Default Rules
Type the following command:
iptables --line-numbers -n -L
Sample outputs:
Chain INPUT (policy ACCEPT) num target prot opt source destination 1 RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0 Chain FORWARD (policy ACCEPT) num target prot opt source destination 1 RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0 Chain OUTPUT (policy ACCEPT) num target prot opt source destination Chain RH-Firewall-1-INPUT (2 references) num target prot opt source destination 1 ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 2 ACCEPT icmp -- 0.0.0.0/0 0.0.0.0/0 icmp type 255 3 ACCEPT udp -- 0.0.0.0/0 224.0.0.251 udp dpt:5353 4 ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 udp dpt:53 5 ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED 6 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:22 7 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:53 8 REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited
Task: Turn On Firewall
Type the following two commands to turn on firewall:
chkconfig iptables on service iptables start # restart the firewall service iptables restart # stop the firewall service iptables stop
Understanding Firewall
There are total 4 chains:
- INPUT – The default chain is used for packets addressed to the system. Use this to open or close incoming ports (such as 80,25, and 110 etc) and ip addresses / subnet (such as 202.54.1.20/29).
- OUTPUT – The default chain is used when packets are generating from the system. Use this open or close outgoing ports and ip addresses / subnets.
- FORWARD – The default chains is used when packets send through another interface. Usually used when you setup Linux as router. For example, eth0 connected to ADSL/Cable modem and eth1 is connected to local LAN. Use FORWARD chain to send and receive traffic from LAN to the Internet.
- RH-Firewall-1-INPUT – This is a user-defined custom chain. It is used by the INPUT, OUTPUT and FORWARD chains.
Packet Matching Rules
- Each packet starts at the first rule in the chain .
- A packet proceeds until it matches a rule.
- If a match found, then control will jump to the specified target (such as REJECT, ACCEPT, DROP).
Target Meanings
- The target ACCEPT means allow packet.
- The target REJECT means to drop the packet and send an error message to remote host.
- The target DROP means drop the packet and do not send an error message to remote host or sending host.
/etc/sysconfig/iptables
Edit /etc/sysconfig/iptables, enter:
# vi /etc/sysconfig/iptables
You will see default rules as follows:
*filter :INPUT ACCEPT [0:0] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [0:0] :RH-Firewall-1-INPUT - [0:0] -A INPUT -j RH-Firewall-1-INPUT -A FORWARD -j RH-Firewall-1-INPUT -A RH-Firewall-1-INPUT -i lo -j ACCEPT -A RH-Firewall-1-INPUT -p icmp --icmp-type any -j ACCEPT -A RH-Firewall-1-INPUT -p udp --dport 5353 -d 224.0.0.251 -j ACCEPT -A RH-Firewall-1-INPUT -p udp -m udp --dport 53 -j ACCEPT -A RH-Firewall-1-INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT -A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT -A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 53 -j ACCEPT -A RH-Firewall-1-INPUT -j REJECT --reject-with icmp-host-prohibited COMMIT
Drop All Traffic
Find lines:
*filter :INPUT ACCEPT [0:0] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [0:0]
Update as follows to change the default policy to DROP from ACCEPT for the INPUT and FORWARD built-in chains:
:INPUT DROP [0:0] :FORWARD DROP [0:0]
Log and Drop Spoofing Source Addresses
Append the following lines before final COMMIT line:
-A INPUT -i eth0 -s 10.0.0.0/8 -j LOG --log-prefix "IP DROP SPOOF " -A INPUT -i eth0 -s 172.16.0.0/12 -j LOG --log-prefix "IP DROP SPOOF " -A INPUT -i eth0 -s 192.168.0.0/16 -j LOG --log-prefix "IP DROP SPOOF " -A INPUT -i eth0 -s 224.0.0.0/4 -j LOG --log-prefix "IP DROP MULTICAST " -A INPUT -i eth0 -s 240.0.0.0/5 -j LOG --log-prefix "IP DROP SPOOF " -A INPUT -i eth0 -d 127.0.0.0/8 -j LOG --log-prefix "IP DROP LOOPBACK " -A INPUT -i eth0 -s 169.254.0.0/16 -j LOG --log-prefix "IP DROP MULTICAST " -A INPUT -i eth0 -s 0.0.0.0/8 -j LOG --log-prefix "IP DROP " -A INPUT -i eth0 -s 240.0.0.0/4 -j LOG --log-prefix "IP DROP " -A INPUT -i eth0 -s 255.255.255.255/32 -j LOG --log-prefix "IP DROP " -A INPUT -i eth0 -s 168.254.0.0/16 -j LOG --log-prefix "IP DROP " -A INPUT -i eth0 -s 248.0.0.0/5 -j LOG --log-prefix "IP DROP "
Log And Drop All Traffic
Find the lines:
-A RH-Firewall-1-INPUT -j REJECT --reject-with icmp-host-prohibited COMMIT
Update it as follows:
-A RH-Firewall-1-INPUT -j LOG -A RH-Firewall-1-INPUT -j DROP COMMIT
Open Port
To open port 80 (Http server) add the following before COMMIT line:
-A RH-Firewall-1-INPUT -m tcp -p tcp --dport 80 -j ACCEPT
To open port 53 (DNS Server) add the following before COMMIT line:
-A RH-Firewall-1-INPUT -m tcp -p tcp --dport 53 -j ACCEPT -A RH-Firewall-1-INPUT -m udp -p tcp --dport 53 -j ACCEPT
To open port 443 (Https server) add the following before COMMIT line:
-A RH-Firewall-1-INPUT -m tcp -p tcp --dport 443 -j ACCEPT
To open port 25 (smtp server) add the following before COMMIT line:
-A RH-Firewall-1-INPUT -m tcp -p tcp --dport 25 -j ACCEPT
Only allow SSH traffic From 192.168.1.0/24
-A RH-Firewall-1-INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 22 -j ACCEPT
Enable Printing Access For 192.168.1.0/24
-A RH-Firewall-1-INPUT -s 192.168.1.0/24 -p udp -m udp --dport 631 -j ACCEPT -A RH-Firewall-1-INPUT -s 192.168.1.0/24 -p tcp -m tcp --dport 631 -j ACCEPT
Allow Legitimate NTP Clients to Access the Server
-A RH-Firewall-1-INPUT -s 192.168.1.0/24 -m state --state NEW -p udp --dport 123 -j ACCEPT
Open FTP Port 21 (FTP)
-A RH-Firewall-1-INPUT -m state --state NEW -p tcp --dport 21 -j ACCEPT
Save and close the file. Edit /etc/sysconfig/iptables-config, enter:
# vi /etc/sysconfig/iptables-config
Make sure ftp module is loaded with the space-separated list of modules:
IPTABLES_MODULES="ip_conntrack_ftp"
To restart firewall, type the following commands:
# service iptables restart
# iptables -vnL --line-numbers
Edit /etc/sysctl.conf For DoS and Syn Protection
Edit /etc/sysctl.conf to defend against certain types of attacks and append / update as follows:
net.ipv4.conf.all.log_martians = 1 net.ipv4.conf.default.accept_source_route = 0 net.ipv4.conf.default.accept_redirects = 0 net.ipv4.conf.default.secure_redirects = 0 net.ipv4.icmp_echo_ignore_broadcasts = 1 #net.ipv4.icmp_ignore_bogus_error_messages = 1 net.ipv4.tcp_syncookies = 1 net.ipv4.conf.all.rp_filter = 1 net.ipv4.conf.default.rp_filter = 1
See previous FAQ, “Linux Kernel /etc/sysctl.conf Security Hardening” for more details.
Alternate Configuration Option
You can skip /etc/sysconfig/iptables file and create a shell script from scratch as follows:
#!/bin/bash
# A sample firewall shell script
IPT="/sbin/iptables"
SPAMLIST="blockedip"
SPAMDROPMSG="BLOCKED IP DROP"
SYSCTL="/sbin/sysctl"
BLOCKEDIPS="/root/scripts/blocked.ips.txt"
# Stop certain attacks
echo "Setting sysctl IPv4 settings..."
$SYSCTL net.ipv4.ip_forward=0
$SYSCTL net.ipv4.conf.all.send_redirects=0
$SYSCTL net.ipv4.conf.default.send_redirects=0
$SYSCTL net.ipv4.conf.all.accept_source_route=0
$SYSCTL net.ipv4.conf.all.accept_redirects=0
$SYSCTL net.ipv4.conf.all.secure_redirects=0
$SYSCTL net.ipv4.conf.all.log_martians=1
$SYSCTL net.ipv4.conf.default.accept_source_route=0
$SYSCTL net.ipv4.conf.default.accept_redirects=0
$SYSCTL net.ipv4.conf.default.secure_redirects=0
$SYSCTL net.ipv4.icmp_echo_ignore_broadcasts=1
#$SYSCTL net.ipv4.icmp_ignore_bogus_error_messages=1
$SYSCTL net.ipv4.tcp_syncookies=1
$SYSCTL net.ipv4.conf.all.rp_filter=1
$SYSCTL net.ipv4.conf.default.rp_filter=1
$SYSCTL kernel.exec-shield=1
$SYSCTL kernel.randomize_va_space=1
echo "Starting IPv4 Firewall..."
$IPT -F
$IPT -X
$IPT -t nat -F
$IPT -t nat -X
$IPT -t mangle -F
$IPT -t mangle -X
# load modules
modprobe ip_conntrack
[ -f "$BLOCKEDIPS" ] && BADIPS=$(egrep -v -E "^#|^$" "${BLOCKEDIPS}")
# interface connected to the Internet
PUB_IF="eth0"
#Unlimited traffic for loopback
$IPT -A INPUT -i lo -j ACCEPT
$IPT -A OUTPUT -o lo -j ACCEPT
# DROP all incomming traffic
$IPT -P INPUT DROP
$IPT -P OUTPUT DROP
$IPT -P FORWARD DROP
if [ -f "${BLOCKEDIPS}" ];
then
# create a new iptables list
$IPT -N $SPAMLIST
for ipblock in $BADIPS
do
$IPT -A $SPAMLIST -s $ipblock -j LOG --log-prefix "$SPAMDROPMSG "
$IPT -A $SPAMLIST -s $ipblock -j DROP
done
$IPT -I INPUT -j $SPAMLIST
$IPT -I OUTPUT -j $SPAMLIST
$IPT -I FORWARD -j $SPAMLIST
fi
# Block sync
$IPT -A INPUT -i ${PUB_IF} -p tcp ! --syn -m state --state NEW -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "Drop Sync"
$IPT -A INPUT -i ${PUB_IF} -p tcp ! --syn -m state --state NEW -j DROP
# Block Fragments
$IPT -A INPUT -i ${PUB_IF} -f -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "Fragments Packets"
$IPT -A INPUT -i ${PUB_IF} -f -j DROP
# Block bad stuff
$IPT -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL FIN,URG,PSH -j DROP
$IPT -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL ALL -j DROP
$IPT -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL NONE -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "NULL Packets"
$IPT -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL NONE -j DROP # NULL packets
$IPT -A INPUT -i ${PUB_IF} -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
$IPT -A INPUT -i ${PUB_IF} -p tcp --tcp-flags SYN,FIN SYN,FIN -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "XMAS Packets"
$IPT -A INPUT -i ${PUB_IF} -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP #XMAS
$IPT -A INPUT -i ${PUB_IF} -p tcp --tcp-flags FIN,ACK FIN -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "Fin Packets Scan"
$IPT -A INPUT -i ${PUB_IF} -p tcp --tcp-flags FIN,ACK FIN -j DROP # FIN packet scans
$IPT -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP
# Allow full outgoing connection but no incomming stuff
$IPT -A INPUT -i ${PUB_IF} -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPT -A OUTPUT -o ${PUB_IF} -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
# Allow ssh
$IPT -A INPUT -i ${PUB_IF} -p tcp --destination-port 22 -j ACCEPT
# Allow http / https (open port 80 / 443)
$IPT -A INPUT -i ${PUB_IF} -p tcp --destination-port 80 -j ACCEPT
#$IPT -A INPUT -o ${PUB_IF} -p tcp --destination-port 443 -j ACCEPT
# allow incomming ICMP ping pong stuff
$IPT -A INPUT -i ${PUB_IF} -p icmp --icmp-type 8 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
#$IPT -A OUTPUT -o ${PUB_IF} -p icmp --icmp-type 0 -m state --state ESTABLISHED,RELATED -j ACCEPT
# Allow port 53 tcp/udp (DNS Server)
$IPT -A INPUT -i ${PUB_IF} -p udp --dport 53 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
#$IPT -A OUTPUT -o ${PUB_IF} -p udp --sport 53 -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPT -A INPUT -i ${PUB_IF} -p tcp --destination-port 53 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
#$IPT -A OUTPUT -o ${PUB_IF} -p tcp --sport 53 -m state --state ESTABLISHED,RELATED -j ACCEPT
# Open port 110 (pop3) / 143
$IPT -A INPUT -i ${PUB_IF} -p tcp --destination-port 110 -j ACCEPT
$IPT -A INPUT -i ${PUB_IF} -p tcp --destination-port 143 -j ACCEPT
##### Add your rules below ######
#
#
##### END your rules ############
# Do not log smb/windows sharing packets - too much logging
$IPT -A INPUT -p tcp -i ${PUB_IF} --dport 137:139 -j REJECT
$IPT -A INPUT -p udp -i ${PUB_IF} --dport 137:139 -j REJECT
# log everything else and drop
$IPT -A INPUT -j LOG
$IPT -A FORWARD -j LOG
$IPT -A INPUT -j DROP
exit 0