Q.Can you explain the meaning of following two firewall rules present in my /etc/sysconfig/iptables rules under CentOS Enterprise Linux version 5.2?

-A RH-Firewall-1-INPUT -p 50 -j ACCEPT
-A RH-Firewall-1-INPUT -p 51 -j ACCEPT

A. This is related to Internet Protocol Security (IPsec) is a suite of protocols for securing Internet Protocol (IP) communications by authenticating and/or encrypting each IP packet in a data stream.

The -p option is used to specify protocol name such as tcp, udp, icmp or it can be a numeric value, representing one of these protocols or a different one. A protocol name from /etc/protocols is allowed. In short

• 50 is Encap Security Payload (esp / IPSEC-ESP ) protocol
• 51 is IPSEC-AH – Authentication Header protocol

Above two rules allowing IPsec communication i.e. IPSEC packets passed via your firewall. If you would like to block IPsec, change rules as follows:
-A RH-Firewall-1-INPUT -p 50 -j REJECT
-A RH-Firewall-1-INPUT -p 51 -j REJECT
Reload firewall, enter:
# service iptables restart
For further information refer to iptables man page and /etc/protocols:
man iptables

Sample /etc/protocols file

$ cat /etc/protocols
Output:

# Internet (IP) protocols
#
# Updated from http://www.iana.org/assignments/protocol-numbers and other
# sources.
# New protocols will be added on request if they have been officially
# assigned by IANA and are not historical.
# If you need a huge list of used numbers please install the nmap package.

ip      0       IP              # internet protocol, pseudo protocol number
#hopopt 0       HOPOPT          # IPv6 Hop-by-Hop Option [RFC1883]
icmp    1       ICMP            # internet control message protocol
igmp    2       IGMP            # Internet Group Management
ggp     3       GGP             # gateway-gateway protocol
ipencap 4       IP-ENCAP        # IP encapsulated in IP (officially ``IP'')
st      5       ST              # ST datagram mode
tcp     6       TCP             # transmission control protocol
egp     8       EGP             # exterior gateway protocol
igp     9       IGP             # any private interior gateway (Cisco)
pup     12      PUP             # PARC universal packet protocol
udp     17      UDP             # user datagram protocol
hmp     20      HMP             # host monitoring protocol
xns-idp 22      XNS-IDP         # Xerox NS IDP
rdp     27      RDP             # "reliable datagram" protocol
iso-tp4 29      ISO-TP4         # ISO Transport Protocol class 4 [RFC905]
xtp     36      XTP             # Xpress Transfer Protocol
ddp     37      DDP             # Datagram Delivery Protocol
idpr-cmtp 38    IDPR-CMTP       # IDPR Control Message Transport
ipv6    41      IPv6            # Internet Protocol, version 6
ipv6-route 43   IPv6-Route      # Routing Header for IPv6
ipv6-frag 44    IPv6-Frag       # Fragment Header for IPv6
idrp    45      IDRP            # Inter-Domain Routing Protocol
rsvp    46      RSVP            # Reservation Protocol
gre     47      GRE             # General Routing Encapsulation
esp     50      IPSEC-ESP       # Encap Security Payload [RFC2406]
ah      51      IPSEC-AH        # Authentication Header [RFC2402]
skip    57      SKIP            # SKIP
ipv6-icmp 58    IPv6-ICMP       # ICMP for IPv6
ipv6-nonxt 59   IPv6-NoNxt      # No Next Header for IPv6
ipv6-opts 60    IPv6-Opts       # Destination Options for IPv6
rspf    73      RSPF CPHB       # Radio Shortest Path First (officially CPHB)
vmtp    81      VMTP            # Versatile Message Transport
eigrp   88      EIGRP           # Enhanced Interior Routing Protocol (Cisco)
ospf    89      OSPFIGP         # Open Shortest Path First IGP
ax.25   93      AX.25           # AX.25 frames
ipip    94      IPIP            # IP-within-IP Encapsulation Protocol
etherip 97      ETHERIP         # Ethernet-within-IP Encapsulation [RFC3378]
encap   98      ENCAP           # Yet Another IP encapsulation [RFC1241]
#       99                      # any private encryption scheme
pim     103     PIM             # Protocol Independent Multicast
ipcomp  108     IPCOMP          # IP Payload Compression Protocol
vrrp    112     VRRP            # Virtual Router Redundancy Protocol
l2tp    115     L2TP            # Layer Two Tunneling Protocol [RFC2661]
isis    124     ISIS            # IS-IS over IPv4
sctp    132     SCTP            # Stream Control Transmission Protocol
fc      133     FC              # Fibre Channel