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List:       busybox
Subject:    Re: ping: large pings don't work
From:       walter harms <wharms () bfs ! de>
Date:       2008-08-26 20:18:51
Message-ID: 48B4652B.8040302 () bfs ! de
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Doug Graham schrieb:
> On Tue, Aug 26, 2008 at 02:40:37AM +0200, Denys Vlasenko wrote:
>> On Tuesday 26 August 2008 02:00, Doug Graham wrote:
>>> However, in my tests between these two processors, there's a datasize
>>> range from 65494 through 65507 where the ping fails quietly (gets no
>> What happens with sizes > 65507?
> 
> I get this:
> 
>  % ./busybox ping -s 65508 192.168.0.2
>  PING 192.168.0.2 (192.168.0.2): 65508 data bytes
>  ping: sendto: Message too long
> 
> I haven't done the math, but I take this to mean that the maximum
> size of an IP datagram has been exceeded.  I get a similar error
> on my rh desktop using its installed (iputils) ping:
> 
>  % ping -s 65508 47.128.20.74
>  WARNING: packet size 65508 is too large. Maximum is 65507
>  PING 47.128.20.74 (47.128.20.74) 65508(65536) bytes of data.
>  ping: local error: Message too long, mtu=1500
>  ping: local error: Message too long, mtu=1500
> 
> I think the math does work out.  65508 + 8 byte ICMP header + 20 byte
> IP header = 65536 which is larger than the maximum IP datagram size of
> 64k - 1.
> 
>>> response) without issuing any error message.  I was going to suggest
>>> a further patch to limit the maxium packet size to MAXPACKET (already
>>> defined in ping.c but never used for anything), but then I realized that
>>> this symptom probably really does demonstrate a quirk somewhere in the
>>> networking stack on the destination machine. 
>> Are you saying that you verified that there genuinely were no reply packets
>> (say, with tcpdump), or do you just guess?
> 
> Yes, I did verify this with tcpdump.  Also verified that the "netstat -s"
> stats on the destination machine look quite odd.  The destination machine
> got and reassembled the packet without error, but didn't recognize it
> as an ICMP packet and didn't send a reply.  This destination machine is
> running a 3rd party protocol stack, so I think we've found a bug to report
> to the vendor.  Busybox's ping works fine with sizes in this range when
> the destination is a standard desktop machine, so whatever is going on,
> it's not a busybox problem.
> 


i guess the LKML would like to hear about the problem.

re,
  wh

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