Filtering ICMP (Was Re: SMURF amplifier block list)

Alex P. Rudnev alex at
Tue Apr 21 09:06:22 UTC 1998

Sorry, I do not know who wrote original message, but it should be 
corrected as::

 deny icmp any aaa.bbb.ccc.ddd www.ccc.nnn.mmm echo-request log

to prevent smurf originating, or

 deny icmp any aaa.bbb.ccc.ddd www.ccc.nnn.mmm echo-reply

to prevent smurf flooding into your network.

No important ICMP are affected this case.

On Mon, 20 Apr 1998, Mark Whitis wrote:

> Date: Mon, 20 Apr 1998 16:02:52 -0400 (EDT)
> From: Mark Whitis <whitis at>
> To: jlixfeld at
> Cc: nanog at
> Subject: Filtering ICMP (Was Re: SMURF amplifier block list)
> On Sun, 19 Apr 1998 jlixfeld at wrote:
> > You could always "deny icmp any aaa.bbb.ccc.ddd www.ccc.nnn.mmm log" on
> Using "deny icmp" as anything other than an extremely temporary measure
> while you (or your customers) are actively under DoS attack and focused
> only on the affected hosts/nets (the systems under attack and/or the smurf
> amplifiers if it is a smurf attack) is downright irresponsible.  Even
> then, it will cause problems but they may be less than those caused by the
> DoS attack.  Most ICMP traffic is necessary to the correct operation of
> the net.
> Filtering ICMP not only breaks ping, it breaks path mtu discovery which
> can cause much grief which is hard for the people affected to diagnose.
> Breaking PMTU discovery has the effect that connections between any two
> hosts that have an MTU greater than that of the smallest MTU hop on the
> path will not be able to communicate.  Packets will be dropped,
> consistently enough to prevent communications even though some small part
> of each flow will frequently get through (generally the same part).  This
> is a pattern (packet size) sensitive packet loss not a random one so TCP
> retransmission does not recover.  For SMTP, the HELO, MAIL, RCPT dialog
> will happen and then the connection will hang on the message DATA (unless
> the message is very short) tying up the servers at both ends until they
> timeout.  Normally all attempts to resend the message will also fail. 
> For HTTP, the browser will probably be able to do a "HEAD" (short
> response) but a "GET" will fail.  The symptom to the user is that
> they are consistently unable to get through to certain web sites
> with the connection stalling at the same place each time.  On very rare
> ocassions, I have seen a connection actually succeed to one of these
> unreachable servers when it was sufficiently loaded that it transmitted
> the data in smaller chunks.
> If this happens to you, a workaround is to set the Max MTU to 576 on each
> of your clients (to fix outbound connections) and servers (to fix inbound
> ones).  Setting the Mtu on your router does not seem to work (it might
> help in one direction (of data flow), by sending ICMP too bigs to your
> inside hosts at a threshold lower than the lost ICMP too bigs, but not the
> other direction in which both sets of "too bigs" get dropped.  This does
> put a higher packet load on the backbone (more smaller packets have to be
> routed).  The cause is usually because a router somewhere drops packets
> which are too large but have the DF (don't fragment) bit set without
> generating the required ICMP too big message (there is some defective
> hardware out there) or, more likely, that some cluelesss network operator
> filtered all ICMP traffic, usually as a naive attempt to protect against
> real or anticipated DoS attacks.
> This is made much worse by filtering software which does not allow
> you to filter specific ICMP types and (unfragmented) packet sizes.
> A much better way to handle most of the DoS attacks (except smurf)
> is to force fragment reassasembly on a router which is not sucseptible
> before forwarding the packets; this puts a significant load on the
> router so it is best done on a router/firewall close to the
> systems being protected (which is also desireable because it
> gives more protection).
> As an aside on the original topic, filtering on mask
> is also irresponsible and never should have been suggested here.
> The lame arguments that anyone who has a host in that range is
> asking for trouble are specious; just because they may be adversely
> affected by some clueless individual somewhere does not justify
> your being clueless as well.  Yes, personally, I would avoid putting
> a (externally accessable) host at .255 because of the general clue
> deficit.
> ---------------------------------------------------------------------------
> ---  Mark Whitis <whitis at>     WWW: ---
> ---  428-B Moseley Drive; Charlottesville, VA 22903        804-962-4268 ---
> ---------------------------------------------------------------------------

Aleksei Roudnev, Network Operations Center, Relcom, Moscow
(+7 095) 194-19-95 (Network Operations Center Hot Line),(+7 095) 239-10-10, N 13729 (pager)
(+7 095) 196-72-12 (Support), (+7 095) 194-33-28 (Fax)

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