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From: Archie Cobbs <archie@whistle.com>
Message-Id: <200003201903.LAA05714@bubba.whistle.com>
Subject: Re: ICMP socket weirdness
In-Reply-To: <200003181954.OAA77677@khavrinen.lcs.mit.edu> from Garrett Wollman at "Mar 18, 2000 02:54:26 pm"
To: wollman@khavrinen.lcs.mit.edu (Garrett Wollman)
Date: Mon, 20 Mar 2000 11:03:07 -0800 (PST)
Cc: freebsd-current@FreeBSD.ORG
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Garrett Wollman writes:
> > When the program is run, if you ping the IP address from the
> > local machine, it sees packets.  However, if you ping it from
> > a remote machine, it doesn't see packets.
> 
> 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 would be possible to pass all ICMP packets to the raw listeners,
> but it would require rewriting parts of icmp_input() (which would not
> be a bad idea) either to avoid modifying the packet in-place or to
> keep a copy of the original before responding -- either of which would
> slow down `ping' processing.

The existence of m_dup() makes the latter option a lot easier..

-Archie

___________________________________________________________________________
Archie Cobbs   *   Whistle Communications, Inc.  *   http://www.whistle.com


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