[Snort-users] Is this a valid traffic?

Joe Matusiewicz joem at ...692...
Wed Apr 3 11:13:29 EST 2002


At 12:39 PM 4/3/02, Skip Carter wrote:


>         -- the packet size is 1500 bytes.   There is never any reason for an
>ICMP packet
>            to be larger than 128 bytes.


I see these all the time and have commented this rule out.  This looks like 
the MTU discovery that comes with some of the newer *nix versions.  A ping 
packet is sent out with
a "don't fragment" bit set, 1500 bytes in size, and if a remote
router can't forward the packet on a smaller MTU channel (say some
old serial link), it will return a "can't fragment" ICMP pkt.
to the web server, which will then lower the packet size for
that host.  In today's internet, those who can't handle
a 1500 byte packet should be < .1%, so all that traffic is extraneous.

One of our groups shut this off in their AIX web server a year ago because 
folks were complaining that they were getting ping scanned from it.  They 
upgraded the OS on Monday and guess what -- it was turned back on by the 
upgrade.  They shut it back off.  I've also seen this behavior in Solaris 
and HP-UX.

-- Joe





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