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UDP packets being dropped; 'packet dropped, err udp length' message in debug flow basic output

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Article ID: KB9027 KB Last Updated: 06 Aug 2010Version: 3.0
Summary:
packet dropped, err udp length
Symptoms:
UDP packets are dropped with the message "packet dropped, err udp length" in the debug flow basic.

****** 21099.0: <Trust/ethernet1> packet received [120]******
  ipid = 75(004b), @d781e110
  packet dropped, err udp length
Solution:

When debug flow basic and snoop are analyzed, it is determined that it is not a firewall problem.


 1099.0: 0(i):111111111111->222222222222/0800
10.10.10.1->20.20.20.1/17, tlen=120
vhl=45, tos=00, id=75, frag=0000, ttl=255
udp:ports 2746->2746, len=92

****** 21099.0: <Trust/ethernet1> packet received [120]******
ipid = 75(004b), @d781e110
packet dropped, err udp length


In the output above, the IP header shows that the IP length is 120 (shown in the tlen field); however,  the UDP header shows the UDP datagram size is 92 bytes.
UDP (92) + IP header(20) = 112, which is not equal 120!

Our firewall is doing the right job to filter out this erroneous packet.  Please check why the host is sending out erroneous packets.

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