buglwIP - A Lightweight TCP/IP stack - Bugs: bug #61946, IPv4 fragmentation fails for...

 
 

You are not allowed to post comments on this tracker with your current authentication level.

bug #61946: IPv4 fragmentation fails for certain MTU / packet length combinations

Submitter:  Carlos Sánchez de La Lama <csanchez>
Submitted:  Thu 27 Jan 2022 06:26:07 PM UTC
   
 
Category:  IPv4 Severity:  3 - Normal
Item Group:  Faulty Behaviour Status:  None
Privacy:  Public Assigned to:  None
Open/Closed:  Open Planned Release:  None
lwIP version:  git head

Thu 27 Jan 2022 06:26:07 PM UTC, original submission:  

The calculations used compute the size of the next fragment and to check whether current fragment is the last one differ. This causes that under certain size combinations, the one before last packet is (incorrectly) detected as last and therefore emitted without the "more fragments" flag.

This seems to have been introduced by commit 576f49ee2bad3839490e915c24ad1837d655637a

It happens when ((MTU-IPHDRLEN) % 8 != 0) and the packet length is such that last fragment would fit in (MTU-IPHDRLEN) but does not fit in (((MTU-IPHDRLEN)/8)*8), so two more fragments are generated.

Attached patch (ip4_wrong_fragmentation_test.patch) over master (34e435c78611fbf21c49c5ddb6c395a097e24cc7) adds code to ip4 fragmentation test which writes 4 .hex files to pwd.

mtu300len8000.hex -> bug does not trigger (29 fragments)
mtu300len1124.hex -> bug does not trigger (5 fragments)
mtu304len8000.hex -> bug does not trigger (29 fragments)
mtu304len1124.hex -> bug TRIGGERS (5 fragments but last is lost)

Those have hex dump format and can be imported into Wireshark for analysis. For convenience, I also attach .pcapng versions of mtu300len1124 and mtu304len1124, so the problem can be quickly observed. 

Carlos Sánchez de La Lama <csanchez>

 

(Note: upload size limit is set to 16384 kB, after insertion of the required escape characters.)

Attached Files
file #52757:  mtu300len1124.pcapng added by csanchez (2KiB - application/x-pcapng)
file #52758:  mtu304len1124.pcapng added by csanchez (2KiB - application/x-pcapng)

 

Depends on the following items: None found

Items that depend on this one: None found

 

Carbon-Copy List
  • -email is unavailable- added by csanchez (Submitted the item)
  • -email is unavailable- added by csanchez
  • -email is unavailable- added by csanchez
  •  

    There are 0 votes so far. Votes easily highlight which items people would like to see resolved in priority, independently of the priority of the item set by tracker managers.

     

    Follow 5 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2022-01-27 csanchez Attached File- Added ip4_wrong_fragmentation_test.patch, #52756
        Attached File- Added mtu300len1124.pcapng, #52757
        Attached File- Added mtu304len1124.pcapng, #52758
        Carbon-Copy- Added -email is unavailable-
        Carbon-Copy- Added -email is unavailable-

    Back to the top

    Powered by Savane 3.13-4b48.
    Corresponding source code