Subject: Re: kern/10134: ipfilter with careless ICMP rules will send ICMP in response to ICMP
To: None <>
From: Wolfgang Rupprecht <>
List: netbsd-bugs
Date: 05/16/2000 17:15:24
> 	I have a lot of sympathy with just saying "dont do that".
> 	But I think the most-correct (vis-a-vis rfc1222)  action
> 	is to modify IPfilter's  "block return-icmp" to check if the
> 	blocked packet is an ICMP, and if it is, to silently
> 	drop the ICMP packet.

Although it would be beneficial if some ICMP's such as
icmp-echo-request and icmp-echo-reply were allowed to generate ICMP
error msgs.  It is only sending ICMP error messages in reply to ICMP
error messages that cause the infinite-ping-ponging error message

Personally I'd like to see as little policy in the code as possible
and just put a big fat note in the ipfilter documentation that says
always precede a return-icmp with a test for the error-icmp's and
silently drop them.

(Sort of like having the block for sending network packets
off-host done via a user-level "route add -net
-reject" call instead of hardwired in kernel code.)

       Wolfgang Rupprecht <>
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