[SNMP4J] Invalid trap checksum??

[Jochen Katz]

Hi,

> I ran tcpdump on the receiving system and got this:

are you sure that this was on the receiving system?

> [root at nbc-verisign ~]# tcpdump -vvv port 162
> tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 
> 65535 bytes
> 10:41:34.724763 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto 
> UDP (17), length 357)
>      nbc.vrsn.com.49834 > 1.2.3.4.snmp: [bad udp cksum 52d!]{ SNMPv1 
> C=<communityString> { Trap(310)  .2.11etc. etc.
> 
> The incoming string looks to be correct. But I'm concerned about the 
> 'bad udp cksum' section. Is there a way to create the checksum before 
> the message is sent? If not, what would be the error here, and how do I 
> correct it??

As the application is not responsible for the UDP checksum, you cannot
correct it in the application.

With not too old hardware, the network card generates the checksum,
therefore the network stack of the operating system passes packets with
invalid checksums to the network card. So on the sending system, tcpdump
will allways report an invalid checksum.

Regards,
  Jochen