How to log iptables messages in a different log file

Generally all the info to error level messages are logged inside /var/log/messages but it makes hard to differentiate between long list of logs inside messages.

To separate the iptables related logs in different file follow the following procedure
NOTE: For RHEL 6 file name will be rsyslog.conf

To log all the iptables related messages

# vi /etc/syslog.conf
# Log all kernel messages to firewall.log.     
kern.*                                                          /var/log/frewall.log
# Log anything (except mail) of level info or higher.
# Don't log private authentication messages!
# Don't log kernel related any messages
*.info;mail.none;authpriv.none;cron.none;kern.none                /var/log/messages

To log only warning level iptables messages

# Log all kernel messages to firewall.log.
kern.warn                                                 /var/log/frewall-warn.log

To log all the messages for info level to error level

kern.info;kern.!crit          /var/log/firewall.log

To log the messages as per their severity level in different files

kern.*                       /var/log/iptables.log
kern.crit                    /var/log/iptables-crit.log
kern.info                    /var/log/iptables-info.log

For more information on severity levels of syslog follow the below link
SYSLOG Tutorial

Once done restart the syslog services
For RHEL 6

# service rsyslog restart

For RHEL 5 or older

# service syslog restart

For example
Q. Create a rule to monitor the ping logs from 192.168.1.10 to the localhost

# iptables -I INPUT -s 192.168.1.10 -p icmp -j LOG --log-prefix "PING TEST "

Now let us verify it in our log file

Try to ping from 192.168.1.10 and observe the log file

# cat /var/log/iptables.log
Mar  6 11:22:36 test1 kernel: PING TEST IN=eth3 OUT= MAC=00:0c:29:51:aa:e1:00:0c:29:a3:f5:fa:08:00 SRC=192.168.1.10 DST=192.168.1.6 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=1037 SEQ=2
Mar  6 11:23:03 test1 kernel: PING TEST IN=eth3 OUT= MAC=00:0c:29:51:aa:e1:00:0c:29:a3:f5:fa:08:00 SRC=192.168.1.10 DST=192.168.1.6 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=3341 SEQ=1
Mar  6 11:23:04 test1 kernel: PING TEST IN=eth3 OUT= MAC=00:0c:29:51:aa:e1:00:0c:29:a3:f5:fa:08:00 SRC=192.168.1.10 DST=192.168.1.6 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=3341 SEQ=2