Resolved

I've never found a bug in Monowall, so I'm prepared to accept this is something else ;o)

I have a machine running a legacy application on XP.  It restarts every 6 hours, to reclaim the memory VB has lost!

If I connect this directly to the DMZ(OPT1) port, it will get an IP address via DHCP the *first* time it starts.  When it restarts, however, the DHCP server appears to die completely.  It will not give addresses to any hosts I plug in to DMZ.  DHCP on the LAN interface is still working though.
THe only way I have found to repair DHCP when this happens is to reboot Monowall.

If I connect this machine via a hub (it says it's a hub, but it could well be switching) it works fine, and I can renew IPs.

I do not really want to run another hub here, as there is only one machine, and I don't have another one.

Running 1.3b14 on an ALIX 2C3 board.

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If I connect this machine via a hub (it says it's a hub, but it could well be switching) it works fine, and I can renew IPs.

The vr(4) driver in FreeBSD 6.3 (used for the NICs on ALIX boards) seems to have problems detecting link-up events sometimes (as it has to when you reboot that machine and it's directly connected to the ALIX, as the link will inevitably flap while it's rebooting). Unfortunately there's no solution yet; the problem only seems to be fixed in FreeBSD 7.0, and the changes are too extensive for a simple backport to 6.3.

See also http://m0n0.ch/wall/list/showmsg.php?id=344/90

Thanks for getting back to me so quickly.

After some thought, I was thinking along the same lines.

When I was investigating, I found that just pulling the cable and reconnecting didn't have this effect, so I am planning to experiment with changing the shutdown/restart scripts on the XP machine. 
Perhaps if I release the address, or disable the adaptor, before the restart it will work.
Who knows!