GPRS problem: SGSN tunnel packets received from mobile device, going nowhere?

[rainnw]

Hello,

I am running YateBTS build 503 with BladeRF on Ubuntu. Devices are able to register and attach. They have IP addresses and they can send packets, as seen on sgsntun:

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root@sheldon-B85M-D3H:/home/sheldon# tcpdump -ni sgsntun
tcpdump: WARNING: sgsntun: no IPv4 address assigned
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on sgsntun, link-type RAW (Raw IP), capture size 65535 bytes
18:14:51.308269 IP 10.1.255.11.39078 > 8.8.8.8.53: 12748+ A? google.com. (28)
18:14:51.308296 IP 10.1.255.11.3833 > 8.8.8.8.53: 29513+ A? google.com. (28)
18:14:52.287408 IP 10.1.255.11.24805 > 8.8.8.8.53: 1136+ A? google.com. (28)
18:14:52.329324 IP 10.1.255.11.2474 > 8.8.4.4.53: 30185+ A? google.com. (28)
18:14:52.385379 IP 10.1.255.11.10889 > 8.8.8.8.53: 7742+ A? www.googleapis.com. (36)

I have the following configured:

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# cat /proc/sys/net/ipv4/ip_forward
1

And I am using the OpenBTS IP Tables and loading them in with iptables-restore:

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# Generated by iptables-save v1.4.4
*nat
:PREROUTING ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A POSTROUTING -o eth0 -j MASQUERADE
COMMIT
# Generated by iptables-save v1.4.4
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
COMMIT

However, as you can see above, traffic goes nowhere. There is no traffic routed from the mobile devices onto eth0 with nat turned on or off. The packets just seem to die on the sgsntun interface.

Any idea what the problem could be?