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Date:      Mon, 20 Mar 2000 15:11:04 -0500 (EST)
From:      Garrett Wollman <wollman@khavrinen.lcs.mit.edu>
To:        Archie Cobbs <archie@whistle.com>
Cc:        wollman@khavrinen.lcs.mit.edu (Garrett Wollman), freebsd-current@FreeBSD.ORG
Subject:   Re: ICMP socket weirdness
Message-ID:  <200003202011.PAA88925@khavrinen.lcs.mit.edu>
In-Reply-To: <200003201903.LAA05714@bubba.whistle.com>
References:  <200003181954.OAA77677@khavrinen.lcs.mit.edu> <200003201903.LAA05714@bubba.whistle.com>

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<<On Mon, 20 Mar 2000 11:03:07 -0800 (PST), Archie Cobbs <archie@whistle.com> said:

[Quoting my original description of icmp_input()'s behavior:]

>> The ICMP never passes certain packets up to raw listeners.  These
>> include ECHO REQUEST, TIMESTAMP REQUEST, and SUBNET MASK REQUEST
>> packets -- but not the corresponding replies!  So, when you ping the
>> local machine, you will see the ECHO REPLY packets on all raw
>> listners, but not the initial ECHO REQUESTs.  When you ping from a
>> remote machine, you never see the ECHO REQUEST packets because the
>> kernel takes care of them, and you never see the ECHO REPLY packets
>> because they are addressed to the other machine.

> Is this a FreeBSD-specific thing, or to other UNIX's have this
> same peculiar behavior?

It was the same in 4.3.  I don't have 4.2 sources handy so I can't
check there -- but in any case, the answers to your questions are
``no'' and ``yes''.  The raw ICMP socket is defined to only see the
traffic which the kernel is unable to handle itself.

-GAWollman



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