mpd-netgraph related issue

[Alexander Pavlov]

Hello,

I'm using a couple of freebsd boxes for providing pptp vpn services. 
Some of them have nearly identical hardware and software configuration. 
Others don't (I use amd64 and i386 architectures). But the same problem 
is observed on either of them. The problem is that at arbitrary moment 
of time (almost daily) box losses connectivity. I've tried to upgrade 
from 6.1-RELEASE to 6.2-RELEASE, but problem remains.
Now I'm running:
#uname -v
FreeBSD 6.2-RELEASE-p9 #1: Sat Jan 26 17:31:44 UTC 2008     
lup at torment.skynet:/usr/obj/usr/src/sys/CLONED-CORE
# mpd4 -v
Version 4.3

For NAT I use ipnat:
#ipf -V
ipf: IP Filter: v4.1.13 (416)

Two em-NIC's are on board
# pciconf -l|grep em
em0 at pci4:0:0:   class=0x020000 card=0x348d8086 chip=0x108c8086 rev=0x03 
hdr=0x00
em1 at pci5:5:0:   class=0x020000 card=0x348d8086 chip=0x10768086 rev=0x05 
hdr=0x00

I observe three different scenarios:
1. Complete loss of connectivity (box is working).
Box itself is not frozen, but I can't even ping localhost. I'm not 
running outof mbufs -- output of netstat -m seems to be ok. Other kernel 
limitations are not exceeded also (but not sure that I've checked all of 
them).

2. Complete loss of connectivity (box hangs up).
Symtoms are same, except a very strange behaviour of box. When I'm 
trying to log in locally, I do not observe any reaction on my 
keystrokes, but when I'm trying to shutdown box, it says
acpi: suspend request ignored (not ready yet)
and shows me everything I typed. Using power button as enter key %)) I 
can login locally and run some commands. top shows load averages beyond 
1, but no network connectivity though.

3. Particial loss of connectivity.
I can connect to ports 22, 1723 and other opened ports via LAN interface 
(on which mpd operates) using telnet. ICMP also works. But I can't login 
using ssh and I can't connect using pptp client. ssh via wan interface 
works perfectly.

There are some symptoms, which are same for all of the scenarios above:
1. I can't shutdown mpd. Sending KILL signal does nothing.
2. When I try to run ngctl (e.g. ngctl types), it hangs, but I can kill 
ngctl process.
3. When I try to reboot box, it hangs (but still responds to icmp echo 
requests) until I hard-reset it.

Can anybody explain this abnormal behaviour?