DHCPDISCOVER with bad udp checksum issue

[Philip Prindeville]

It is a red-herring.

Tcpdump (via BPF) sees the packet as IP passes it down to the Ethernet driver (and the hardware does the checksum calculation as part of the offload, as previously identified).

So you see it as it gets passed to the driver, not as it appears on the wire.

If you use a separate network tap you'll see it's correct (or on the receiver works as well).

You can verify this by running another UDP-based protocol (like DNS or Traceroute) and verify that they work despite tcpdump complaining about checksums.



> On Oct 6, 2021, at 1:15 PM, Richard Laager <rlaager at wiktel.com> wrote:
> 
> The bad checksum may be a red herring. It sounds like you've already looked at checksum offload. I would check that again. If the checksums really are bad, then you need to figure out why the transmitting side is sending packets with bad checksums.
> -- 
> Richard