[uml-devel] question about udp fragmentation under UML

["Yari Adán Petralanda" <mustang@yadan.org>]

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Hi,

I'm developing a network block device, which uses both TCP and UDP sockets to 
transmit it's data. When I try to send UDP frames bigger than the MTU, they 
are dropped before they hit the wire. Trying to debug the problem, I ran 
netcat using udp under a UML. It uses writes of 8192 bytes over a UDP socket. 
The scenario that made me post here is this: 
running UML on tracing thread mode, host kernel is 2.6.10, uml kernel is 
2.6.8.1-1um, if I set the MTU of the localhost device (same problem on 
tun/tap) to 1500 bytes, netcat doesn't receive any frames sent to the 
loopback interface. path_mtu_no_discover is 0. The same scenario, on my host 
computer, runs fine. 

Seems to me that IP is refusing to fragment the udp datagrams, and drops them. 
I don't know if it's a UML problem, but when I saw that not only my code, but 
netcat also drops frames, it stranged me.

Thank you very much

-- 
   .-.    
    oo|    Yari Adán Petralanda, a.k.a. mustang
   /`'\          
  (\_;/)           http://www.yadan.org
              Registered Linux User #257290                     
--
Nothing so needs reforming as other people's habits.
  -- Mark Twain, "Pudd'nhead Wilson's Calendar"

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