[CentOS] Why is iptables configured to accept packets on ports 50 and 51?

[Michel van Deventer]

Hi,

On Tue, 2011-05-17 at 11:13 -0700, Aleksey Tsalolikhin wrote:
> [root at hwdltsaloli ~]# cat /etc/sysconfig/iptables
> # 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 -m state --state ESTABLISHED,RELATED -j ACCEPT
> -A RH-Firewall-1-INPUT -j REJECT --reject-with icmp-host-prohibited
> COMMIT
> [root at hwdltsaloli ~]# lsof -i:50
> [root at hwdltsaloli ~]# lsof -i:51
> [root at hwdltsaloli ~]#
> 
> 
> /etc/services says:
> 
> re-mail-ck      50/tcp                          # Remote Mail Checking Protocol
> re-mail-ck      50/udp                          # Remote Mail Checking Protocol
> 
> la-maint        51/tcp                          #
> la-maint        51/udp                          # IMP Logical Address
> Maintenance
> 
> Google turns up RMCP is a simple lightweight DP protocol for checking
> if you have mail on a server
> 
> A quick Google search failed to turn up what is "IMP Logical Address
> Maintenance"
The -p you are referring to is NOT a port, but a protocol (number), 50
and 51 stand for IPSEC protocols (AH and ESP).

[michel at deltaflyer ~]$ cat /etc/protocols | grep 51
ah	51	AH		# Authentication Header
ipv6-auth	51	IPv6-Auth		# Authentication Header for IPv6 (not in
official list)
[michel at deltaflyer ~]$ cat /etc/protocols | grep 50
esp	50	ESP		# Encap Security Payload
ipv6-crypt	50	IPv6-Crypt		# Encryption Header for IPv6 (not in official
list)


Please read 'man iptables' :)

	Regards,

	Michel