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Re: ICMP pokes holes in firewalls...
From: Darren Reed <avalon () caligula anu edu au>
Date: Sat, 27 Sep 2003 19:21:36 +1000 (Australia/ACT)

In some mail from Daniel Hartmeier, sie said:

On Fri, Sep 26, 2003 at 10:13:56AM +1000, Darren Reed wrote:

There's also a general problem here, that needs attention and that
is you really shouldn't allow more ICMP error packets through than
you see normal connection packets.  ie. one UDP packet out should
not allow more than one ICMP error message back in.

Technically, a single packet may cause multiple legitimate ICMP errors.
As per RFC 792, an ICMP redirect does not imply that the packet was dropped
(quite the contrary) and ICMP source quench may be sent without dropping
the packet. Hence, further hops may send further ICMP errors for the
same packet.

Only if a sending host is misbehaving will you ever get more than 1
redirect per packet flow as routers beyond the first hop should not
be sending redirects that make it back to the origin.

So what if a source quench gets dropped ? In situations where it's
likely to be sent, it getting dropped is more likely than normal.

And since you're quoting RFC's...

I have to wonder whether or not you read the OpenBSD source code before
saying this or maybe the OpenBSD source code is missing this comment
that I can see in NetBSD's IP code:

                 * a router should not generate ICMP_SOURCEQUENCH as
                 * required in RFC1812 Requirements for IP Version 4 Routers.
                 * source quench could be a big problem under DoS attacks,
                 * or if the underlying interface is rate-limited.

RFC 1812, section 4.3.3.3 (page 57) discusses this.

Rate limiting the ICMP errors with a strict 1:1 ratio would break
traceroute through a gateway that forwards back to the same network, or
one operating near its capacity limit, for instance.

It won't break any version of traceroute that I'm aware of.

So, what do we gain by rate limiting the ICMP errors?

If I send 1 UDP packet out, how many ICMP errors should I ever
receive that match it ?  1 ?  10 ?  100 ?  Up to the ttl value
of the packet as it passed through ?  What does your experience
tell you?  What do you consider "normal" vs what should be
considered "acceptable" ?

On the other end of this, have *you* had any experience with rate
limited ICMP error message generation ?  If you did, did traceroute
not work because it was present ?

Darren


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