Bug 55 - Certain large ping sizes are not forwarded properly
Summary: Certain large ping sizes are not forwarded properly
Alias: None
Product: passt
Classification: Unclassified
Component: ICMP (show other bugs)
Version: unspecified
Hardware: All Linux
: Low minor
Assignee: nobody
Depends on: 65 62
  Show dependency tree
Reported: 2023-06-20 05:14 UTC by David Gibson
Modified: 2023-07-04 04:39 UTC (History)
0 users

See Also:


Description David Gibson 2023-06-20 05:14:28 UTC
How to reproduce:

$ pasta --config-net
# ping <gw address> -s 65493

Expected results:

ping success fully

Actual results:

Replies are truncated

65496 bytes from icmp_seq=1 ttl=255 (truncated)

Further info:

Ping sizes of 65492 and less bytes appear to work correctly.

Ping sizes from 65493..65507 bytes inclusive should work, but appear to be truncated.  It looks like they might be dropped entirely with passt (rather than pasta).

Ping sizes 65508 and up are bigger than the maximum theoretical IPv4 ping size (the packet size with headers exceeds 65535 bytes), and fail as expected.
Comment 1 David Gibson 2023-06-29 06:27:43 UTC
Ok, I've investigated this, and it appears to be a consequence of the fact that we don't handle IP fragmentation.  ping -s 65492 is the largest ping size which can be sent unfragmented with our guest-side MTU of 65520 bytes.  Beyond that the ping in the guest is fragmented, and because we don't handle defragmentation in passt/pasta, we only forward the contents of the first fragment to the ping socket, so the reply comes back similarly truncated.
Comment 2 David Gibson 2023-07-04 04:39:28 UTC
The fix I've posted for bug 62 means this will now give a warning message while dropping the packet, rather the silently misbehaving.

Note You need to log in before you can comment on or make changes to this bug.