From owner-freebsd-hackers  Fri Jan 26 10:43:22 2001
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From: Archie Cobbs <archie@dellroad.org>
Message-Id: <200101261843.KAA09789@curve.dellroad.org>
Subject: Re: Divert Sockets & Fragmentation revisited
In-Reply-To: <Pine.SOL.4.21.0101252258280.9067-100000@gradient.cis.upenn.edu>
 "from Alwyn Goodloe at Jan 25, 2001 10:59:27 pm"
To: Alwyn Goodloe <agoodloe@gradient.cis.upenn.edu>
Date: Fri, 26 Jan 2001 10:43:00 -0800 (PST)
Cc: hackers@FreeBSD.ORG
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Alwyn Goodloe writes:
>   Guys still having problems with divert sockets and fragmentation. 
> 
> As I said in a previous post the divert operations and corresponding program
> work fine when the datagram  sent have size < MTU (1500) but when the
> datagram has size > MTU and hence get fragmented the recfrom just
> waits never receiving anything.  I am attaching the relevent code
> fragments below.
> 
>   tcpdump tells me that the packets arrive on the interface.
> Hence I know the fragments arrive.
> 
>   Now my ipfw commands are:
> 
> ipfw add 60000 divert 4422 udp from any  to any 3322  in
> ipfw add 65000 allow ip from any to any 

I think the problem is that the first fragment is matching your
rule, but not subsequent fragments (because the port number is
only contained in the first fragment..)

>   Now I thought that that maybe the divert being so specific was
> a problem so I tried flushing ipfw and using the command:
>   ipfw add 60000 divert  4422 ip from any to any 
> 
> thus diverting any ip packets and still nothing.

That doesn't make sense.. 

Try adding the "log" keyword to your ipfw commands, or checking the
stats with "ipfw show" so you can see exactly what's being diverted.

> Now according to the man page on divert:
> 
>   Incomming packets which get diverted are fully reassembled before
>   delivery of any one fragment. Diversion of any one packet causes
>   the entire packet to get diverted. I different fragments get 
>   diverted to different ports, then which port ultimately gets
>   diverted is unpredictable.
>   
> I was under the impression that the packets wern't reassemblembed before
> diversion. Am I wrong here?

Yes... but all fragments must match.

-Archie

__________________________________________________________________________
Archie Cobbs     *     Packet Design     *     http://www.packetdesign.com


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