Filtering ICMP (Was Re: SMURF amplifier block list)

Mark Whitis whitis at
Mon Apr 20 20:02:52 UTC 1998

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

---  Mark Whitis <whitis at>     WWW: ---
---  428-B Moseley Drive; Charlottesville, VA 22903        804-962-4268 ---

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