RE: Ping to a specific port

["Lim Hock-Chai" <Lim.Hock-Chai@xxxxxxxxxxxxxxx>]

Thanks for the detail info.  Another dumb question:
>From what you described below, is it safe to assume that most OS doesn't
have an option to block PING?  PING blocking is usually block by a
firewall and if the server is outside of the firewall, there is no way
to block that?



-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx
[mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of Scott Klement
Sent: Wednesday, May 03, 2006 10:15 AM
To: Midrange Systems Technical Discussion
Subject: 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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