FreeBSD hang without panic

[Andrew Liles]

On a FreeBSD 5.2.1 box that has been in production a long time, I have had
a new type of hang on two days this week, the type of which I have not
seen before.  (No recent software/hardware changes).

Symptom: The machine is a web/email server only.  It stops receiving new
SSH/HTTP/IMAP/POP connections, existing shells cease working.

On the machine itself no messages are shown and the machine responds to
repeated ENTER key presses with the login prompt.  Once you enter "root"
[ENTER] then no further responses (except line feeds as you press ENTER).

What kind of hang is this? Could it be a DoS attack? Or is it some
internal process hanging?  I have a debug kernel but as the only solution
is a power-off reset, I never get any dump.

Any suggestions please where I might start to look, or services to
experiment disabling?

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