IP header checksum: 0x0000

Question

I have a JAX-RS web service which is secured via TLS. Hence encryption is very important I decided to check the network traffic with RawCap and analyze it with WireShark. Doing this, I stumbled over the following message:

Header checksum: 0x0000 [incorrect, should be 0xac15 (may be caused by "IP checksum offload"?)]

1. What is the reason for this message?
2. Are there any further consequences?

I'm pretty sure that it isn't a problem with my RESTEasy client, because retrieving a ressource via FireFox causes the same message.

Answer 1

This doesn't come from your application - it is caused by the TCP/IP stack. Many implementations do not (or not always) fill in the header checksum, leaving it a 0x0000.

As Wireshark indicated, one reason for this is, that some combinations of OS and NIC driver make the OS think, that the checksum will be filled in by the NIC (hardware-accelerated), but in fact it will be not.

This is not a real problem, as long as your transmission path is reliable. AFAIK it is not a security risk.

Answer 2

Was this an outgoing packet?

As the error message suggests, IP checksum offload is enabled. This means that the computer’s TCP/IP stack does not calculate the checksum. Instead the NIC hardware does the calculation before sending the packet out.

This is not a real error. You can safely ignore it.