Bug 273618 - [pf] Port redirects for packets received on loopback have bad checksums
Summary: [pf] Port redirects for packets received on loopback have bad checksums
Status: Open
Alias: None
Product: Base System
Classification: Unclassified
Component: kern (show other bugs)
Version: 13.2-STABLE
Hardware: Any Any
: --- Affects Only Me
Assignee: freebsd-pf (Nobody)
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2023-09-07 16:10 UTC by dfr
Modified: 2023-11-03 11:38 UTC (History)
2 users (show)

See Also:

Attachments
Add an attachment (proposed patch, testcase, etc.)

Note You need to log in before you can comment on or make changes to this bug.
Description dfr 2023-09-07 16:10:43 UTC 
Normally packets received on loopback bypass the tcp checksum calculation since the packet is expected to not leave the host. The packet header is marked as having a good checksum in looutput() even though the th_csum field (for tcp packets) just contains the pseudo header partial sum.

If the packet's destination address is re-written to an address which routes it to some other host, the packet will be delivered with a bad checksum and discarded by the target's tcp stack. I can work around this with 'ifconfig lo0 -txcsum' but it would be better if there was a way for pf to detect this situation and repair the checksum.