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From: Alexander Motin <mav@alkar.net>
To: freebsd-ipfw@freebsd.org, freebsd-net@freebsd.org
Date: Fri, 21 Nov 2003 12:52:50 +0200
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Subject: Re: dummynet & IP fragmentation bug
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I successfully reproduced this on few different 4.8 routers.
Does anybody knows what is this? How to fix or workaround this problem?

Alexander Motin wrote:
> I have one strange problem with dummynet & IP fragmentation.
> 
> I have FreeBSD 4.8-RELEASE router with few interfaces:
> em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
>         options=3<rxcsum,txcsum>
>         inet 195.248.191.172 netmask 0xffffffc0 broadcast 195.248.191.191
>         ether 00:30:48:20:8e:7e
>         media: Ethernet autoselect (1000baseTX <full-duplex>)
>         status: active
> ng4: flags=88d1<UP,POINTOPOINT,RUNNING,NOARP,SIMPLEX,MULTICAST> mtu 1492
>         inet 195.248.191.172 --> 212.86.231.58 netmask 0xffffffff
> 
> Interface ng4 have MTU 1492 because it is PPPoE link.
> When I do not use dummynet on router and somebody send a big
> (>1492bytes) packet to 212.86.231.58 with DontFragment flag set router
> generates ICMP reply message (Fragmentation Needed). This is correct.
> 
> But when I use dummynet on that interface:
> 10170 pipe 10009 ip from any to any out xmit ng4
> 10175 allow ip from any to any via ng4
> 
> 10009: 128.000 Kbit/s    0 ms   50 sl. 1 queues (1 buckets) droptail
>     mask: 0x00 0x00000000/0x0000 -> 0x00000000/0x0000
> BKT Prot ___Source IP/port____ ____Dest. IP/port____ Tot_pkt/bytes
> Pkt/Byte Drp
>   0 udp   195.248.191.65/53      212.86.231.58/1118  50965 28380582  0
>    0 143
> 
> router stops sending that ICMP messages. Pipe is not overflowed at that
> tme, it is empty.

-- 
Alexander Motin