iptablesEdit

See

• Official site: http://www.netfilter.org/projects/iptables/index.html
• Official RHEL 5 documentation on IPTables: http://www.redhat.com/docs/manuals/enterprise/RHEL-5-manual/Deployment_Guide-en-US/ch-iptables.html
• Tutorials:
  • "Simple Firewall Configuration Using NetFilter/iptables": http://www.novell.com/coolsolutions/feature/18139.html
  • "How To Set Up A Debian Linux Firewall": http://www.aboutdebian.com/firewall.htm
  • http://www.cyberciti.biz/faq/howto-rhel-linux-open-port-using-iptables/
• Example configs:
  • http://www.avforums.com/forums/linux/733858-iptables-common-port-settings.html
  • http://forums.devshed.com/security-and-cryptography-17/iptables-block-mysql-port-to-everybody-but-just-one-ip-182005.html
• Some random firewall article found on the web: http://www.linuxtopia.org/online_books/rhel5/rhel5_administration/rhel5_ch-fw.html

Default RHEL ruleset

Explained here and here:

# Firewall configuration written by system-config-securitylevel
# Manual customization of this file is not recommended.
*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 50 -j ACCEPT
-A RH-Firewall-1-INPUT -p 51 -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 631 -j ACCEPT
-A RH-Firewall-1-INPUT -p tcp -m tcp --dport 631 -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 -j REJECT --reject-with icmp-host-prohibited
COMMIT

In short:

• all inbound packets on the loopback interface are accepted
• all ICMP (ping) traffic is accepted
• ports 50 and 51 are related to IPSec and are accepted:

esp     50      IPSEC-ESP       # Encap Security Payload [RFC2406]
ah      51      IPSEC-AH        # Authentication Header [RFC2402]

• port 5353 is Apple’s Digital Audio Access Protocol (DAAP) and is accepted
• port 631 is for CUPS, the "Common UNIX Print System":

ipp             631/tcp                         # Internet Printing Protocol
ipp             631/udp                         # Internet Printing Protocol

• packets pertaining or related to established connections are accepted
• inbound connections to port 22 (SSH) are accepted
• everything else is rejected

Related articles in this wiki

• Using at to safely try out iptables configuration changes