IPSEC MTU routing issue

[Andrei Brezan]

Weird subject, maybe.

I'm running FreeBSD-10.0-RELEASE with PF as firewall and racoon for 
IPSEC. The IPSEC tunnel is between the FreeBSD box and a Fortinet appliance.

The IPSEC tunnel comes up and on a quick test it seems to be working, 
icmp between networks is ok, you can successfully telnet on services on 
the other side. However when you need to transfer some data strange 
things happen. I'm really trying to wrap my head around it and I still 
don't understand why it happens (http://pastebin.com/NAspcM9w). The 
packets smaller than 1260 and larger than 1417 are delivered to vlan103, 
the ones in between are not.

If anyone has any idea why this might happen please shed some light.

# tcpdump -nttti gif0

00:00:00.000000 IP "a.b.c.d" > "e.f.g.h": ICMP echo request, id 21034, 
seq 1, length 1108
00:00:43.603248 IP "a.b.c.d" > "e.f.g.h": ICMP echo request, id 22826, 
seq 1, length 1308

# tcpdump -nttti enc0

00:00:00.000000 (authentic,confidential): SPI 0x0d06e35d: IP 
109.235.79.81 > 193.239.202.174: IP "a.b.c.d" > "e.f.g.h": ICMP echo 
request, id 21034, seq 1, length 1108 (ipip-proto-4)
00:00:00.000139 (authentic,confidential): SPI 0x86741d6b: IP "e.f.g.h" > 
"a.b.c.d": ICMP echo reply, id 21034, seq 1, length 1108
00:00:00.000006 (authentic,confidential): SPI 0x86741d6b: IP 
193.239.202.174 > 109.235.79.81: IP "e.f.g.h" > "a.b.c.d": ICMP echo 
reply, id 21034, seq 1, length 1108 (ipip-proto-4)
00:00:43.603102 (authentic,confidential): SPI 0x0d06e35d: IP 
109.235.79.81 > 193.239.202.174: IP "a.b.c.d" > "e.f.g.h": ICMP echo 
request, id 22826, seq 1, length 1308 (ipip-proto-4)

# tcpdump -nttti vlan103 host "a.b.c.d"

00:00:00.000000 IP "a.b.c.d" > "e.f.g.h": ICMP echo request, id 21034, 
seq 1, length 1108
00:00:00.000109 IP "e.f.g.h" > "a.b.c.d": ICMP echo reply, id 21034, seq 
1, length 1108

Thanks,

-- 
Andrei