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ICMP error message, do not access th_flags. The field is beyond the first eight bytes of the header that are required to be present and were pulled up in the mbuf. A random value of th_flags can have TH_SYN set, which made the sequence number comparison not apply the window scaling factor, which led to legitimate ICMP(v6) packets getting blocked with "BAD ICMP" debug log messages (if enabled with pfctl -xm), thus breaking PMTU discovery. Triggering the bug requires TCP window scaling to be enabled (sysctl net.inet.tcp.rfc1323, enabled by default) on both end- points of the TCP connection. Large scaling factors increase the probability of triggering the bug. PR: kern/115413: [ipv6] ipv6 pmtu not working Tested by: Jacek Zapala Reviewed by: mlaier Approved by: re (kensmith)
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