[Bug 235607] Incorrect checksums with NAT on vtnet with offloading

bugzilla-noreply at freebsd.org bugzilla-noreply at freebsd.org
Tue Nov 12 21:46:55 UTC 2019


https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235607

mike at sentex.net changed:

           What    |Removed                     |Added
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                 CC|                            |mike at sentex.net

--- Comment #10 from mike at sentex.net ---
(In reply to Vincenzo Maffione from comment #9)

wow, this is the problem I got caught up in this morning I think.  A bunch of
FreeBSD VMs in KVM (Ubuntu 18.x).  FreeBSD firewall with vtnet0 and vtnet1
VTNET0 is the public IP and VTNET1 connected a bunch of internal VMs.

Going out from the gateway VM was not an issue. However, VMs behind the gateway
could not establish TCP connections.  On a remote host, the SYN packet would
get to the host, but the host would never respond with a SYN-ACK... It would
just ignore the SYN. When I switched all the devices to em NICs instead of
VTNET, everything worked as expected. 

A good connection initiated from the gateway across the public IP looked like

 xxx.yyy.135.74.36086 > AAA.BBB.148.55.22: Flags [S], cksum 0xbf90 (correct),
seq 489690757, win 65535, options [mss 1460,nop,wscale 6,sackOK,TS val
3421165670 ecr 0], length 0


and the bad connection with the vtnet trying to nat the internal VM as seen at
the external host

16:12:39.071314 IP (tos 0x10, ttl 60, id 0, offset 0, flags [DF], proto TCP
(6), length 60)
    xxx.yyy.135.74.50993 > AAA.BBB.148.55.22: Flags [S], cksum 0x521b
(incorrect -> 0x88ab), seq 515367701, win 65535, options [mss 1460,nop,wscale
6,sackOK,TS val 898080092 ecr 0], length 0


bad checksum....

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