What Issues Are Caused By Hardware Checksum Offloading?

  • The checkbox under advanced to disable Hardware Checksum Offloading gives this explanation: Checking this option will disable hardware checksum offloading. Checksum offloading is broken in some hardware, particularly some Realtek cards. Rarely, drivers may have problems with checksum offloading and some specific NICs.

    Does anyone have any examples of the problems that drivers can have?  Is it a better idea to leave this disabled than not?

  • A search in the forums for "hardware checksum" will probably show a variety of problems.

  • I've seen the Realtek cards generate bad checksums for certain payloads. As a result, some packets will never get through and connections will drop. This showed up for me because Samba sessions could normally transfer smaller files fine, but large several-GB files would always fail. Did some investigation and the hardware offload turned out to be the culprit. Probably some kind of hardware errata or driver bug, but this is the sort of thing that happens.

    If you're using known-good hardware like the Intel or Via (vr) NICs I would leave it enabled, but if your gear is questionable I'd probably turn it off to be safe.

  • Rebel Alliance Developer Netgate

    There is also a known issue when bridging ath nics to vr nics (and perhaps others), you have to disable hardware checksums or else traffic will not be able to get out of your local network.

    There is was a trouble ticket about that, but it has been addressed (it disables the checksums internally in that situation rather than requiring you to change that setting by hand).

  • If you aren't having problems, leave it enabled.