Strange leakage of private source addresses w/ipfw and natd

[freebsd at dwec.ru]

Ok, maybe not THAT important but definitely a Bad Surprise.
Here's the sample (and in current configuration only ICMP packets from time
to time are being passed through unaltered):

snort: [1:0:0] POSSIBLE address leakage - ICMP {ICMP} 192.168.5.2 ->
208.115.104.193
[**] POSSIBLE address leakage - ICMP [**]
10/25-22:55:08.782139 192.168.5.2 -> 208.115.104.193
ICMP TTL:255 TOS:0x0 ID:17365 IpLen:20 DgmLen:60
Type:11  Code:0  TTL EXCEEDED IN TRANSIT

192.168.5.2 is Cisco 2509 if it matters.

box details: ipfw2+natd, acts as a gateway.
OS version 4.9-PRERELEASE FreeBSD 4.9-PRERELEASE #0: Thu Sep 25 08:58:21 MSD
2003,but it doesn't matter as I've seen this behaviour before.

PS
 I can provide more details if needed.

> > This doesn't have a (user-) noticeable impact on traffic, but
> > installing a silent network recorder outside my firewall shows that
> > some RFC 1918 addrs are getting through.
> don't worry, just block them on the external interface.
>
> > I'll post details when I've got them, but I'm wondering if anyone
> > else has seen this?
> it happens, and with my installation they are coming from the outside.
>   clemens