corruption of dmesg

[Sergei Vyshenski]

This started not long before 4.9-PRERELEASE

dmesg reports usual text only a few seconds after reboot.
Later it displays a single line with a fragment of ipfw log, e.g.

167 213.131.11.152 in via fxp0

which seems to change with each new activity of ipfw.

Files /var/log/dmesg.today and /var/log/dmesg.yesterday
rotate daily as usual and contain similar single line.
The file /var/run/dmesg.boot still has usual text of dmesg.

Thank you very much for any idea how to combat it.
Sergei