[Bug 210860] [pf] [ip6] Incorrect TCP checksums after rdr

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=210860

            Bug ID: 210860
           Summary: [pf] [ip6] Incorrect TCP checksums after rdr
           Product: Base System
           Version: 10.3-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: vieira+fbsd at yubo.be

PF seems to incorrectly computes the checksum of IPv6 TCP packets that it
rewrites as part of a simple rdr rule.

E.g., with the following rules:

table <forward_subnets> {2001:16d8:ee03::cafe:d00d/128,!::1}
rdr pass log on lo0 inet6 proto tcp to <forward_subnets> -> ::1 port 12300
pass out log route-to lo0 inet6 proto tcp to <forward_subnets> keep state

and while running nc -l ::1 12300 trying nc -vz 2001:16d8:ee03::cafe:d00d 80 to
see if any packets get to nc listening on 12300 shows that nothing is received
although tcpdump -n -e -ttt -i pflog0 -vvv shows the following incorrect
checksum in rdr:

00:00:00.000000 rule 0..16777216/0(match): pass out on vtnet0: (flowlabel
0x481b8, hlim 64, next-header TCP (6) payload length: 40)
2a03:b0c0:1:d0::425:1001.29559 > 2001:16d8:ee03::cafe:d00d.80: Flags [S], cksum
0xafd2 (incorrect -> 0x7083), seq 417394668, win 65535, options [mss
1440,nop,wscale 6,sackOK,TS val 240470 ecr 0], length 0

00:00:00.000049 rule 0..16777216/0(match): rdr in on lo0: (flowlabel 0x481b8,
hlim 64, next-header TCP (6) payload length: 40) 2a03:b0c0:1:d0::425:1001.29559
> ::1.12300: Flags [S], cksum 0x3fff (incorrect -> 0x00b0), seq 417394668, win
65535, options [mss 1440,nop,wscale 6,sackOK,TS val 240470 ecr 0], length 0


tcpdump'ing lo0 repeatedly shows "cksum 0xafd2 (incorrect -> 0x...)":

22:51:58.407819 IP6 (flowlabel 0x4766f, hlim 64, next-header TCP (6) payload
length: 40) fbsd.yubo.be.10904 > 2001:16d8:ee03::cafe:d00d.http: Flags [S],
cksum 0xafd2 (incorrect -> 0x12d1), seq 1237643503, win 65535, options [mss
1440,nop,wscale 6,sackOK,TS val 401406 ecr 0], length 0

22:52:01.438778 IP6 (flowlabel 0x4766f, hlim 64, next-header TCP (6) payload
length: 40) fbsd.yubo.be.10904 > 2001:16d8:ee03::cafe:d00d.http: Flags [S],
cksum 0xafd2 (incorrect -> 0x06fb), seq 1237643503, win 65535, options [mss
1440,nop,wscale 6,sackOK,TS val 404436 ecr 0], length 0


Note that the inet version of the same rules do work as expected, e.g.
table <forward_subnets> {188.113.88.193/32,!127.0.0.1/8}
rdr pass on lo0 inet proto tcp to <forward_subnets> -> 127.0.0.1 port 12300
pass out route-to lo0 inet proto tcp to <forward_subnets> keep state

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