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  2. MIDRANGE-L
  3. May 2006

RE: Ping to a specific port



Just curious:
I'm pretty sure that there is a way to setup a server/router to not
reply to ping.  How can this be done if PING is a internal function of
ip?
TCP/IP is a network protoocl. Like all network protocol the data is organized into packets.
A packet is a small piece of data, usually about 1500 bytes long. In that 1500 bytes are addresses that tell where the packet is headed for, and where it came from. There are also identifying numbers that specify the protocol that it's speaking, and lots of data that's specific to that protocol.
Indeed, an iSeries user who is familiar with databases might think of a datagram as being very much like a "record". It's small chunk of data with fields that identify things.
The first 20 bytes of every datagram contain IP information. One of the fields is a 1-byte integer (RPG data type 3U 0) that identifies the protocol that's carried inside the IP packet. That protocol might be TCP, UDP or ICMP. (In the case of PING, it'd be ICMP)
After those 20 bytes will be the protocol-specific data. In the case of ICMP, there's 28 bytes of data specific to the ICMP protocol. A bunch of fiedls that tell ICMP how to operate. One of those fields is the icmp_type field. This is a 1-byte integer that identifies the type of ICMP packet being sent. In the case of PING, it'll either contain an ICMP_ECHO (8) or ICMP_ECHOREPLY (0) value.
For a firewall to block PING requests, all it needs to do is check every packet. If the protocol is ICMP, and the ICMP type field is 8 or 0, it can discard the packet, thus blocking PING.

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