udp packet not reach application

udp packet not reach application

[Min Wang]

HI

since Linux-net mailing list no longer active, so post here. Hope it is  
the right place.


I have following issue ( kernel 3.2.28), google around, could not find 
any good similar case.

one the host:
     all the  /proc/sys/net/ipv4/conf/*/rp_filter are 0

    a device sent udp traffics to  my udp app ( listen on port x)


At first, it was working as my app did get udp traffic.

After a while, strace showed that my app stuck at recvfrom, no udp traffic.

But tcpdump did show that the udp traffic was still coming/alive.


Even if I restart my app, strace still showed it was stuck at recv_from.


it seems the kernel stop or did not deliver the udp packet to my app at 
some point.


So I have to restart NIC interface, then I could see udp traffic 
received at my app. (strange also?)


What could be wrong? Driver? or Kernel or my app?

At which point the kernel could possibly stop delivering the packets to 
upper layer even though traffic seen from tcpdump?


Thanks a lot!



min

Re: udp packet not reach application

[Rick Jones]

What does netstat -s say in the UDP and IP sections?

What does tcpdump say about the checksums of the incoming UDP datagrams 
which are not being given to your application?

If you happen to send a UDP datagram from the local host to your 
application, is it received?  That is, sending a UDP datagram to your 
application via some path that does not go through this NIC?

Does your NIC claim to support checksum offload?

Just what is your NIC?

rick jones

Re: udp packet not reach application

[Cong Wang]

On Mon, 29 Apr 2013 at 13:36 GMT, Min Wang <ser.basis@gmail.com> wrote:
>
> At first, it was working as my app did get udp traffic.
>
> After a while, strace showed that my app stuck at recvfrom, no udp traffic.
>
> But tcpdump did show that the udp traffic was still coming/alive.
>

Try dropwatch to see where the packets get dropped.