Question about OsmoGGSN project

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morteza ali Ahmadi morteza.ali.ahmadi at
Sun Feb 7 18:44:59 UTC 2021

Hi, osmocom team members

We have run OsmoGGSN project using the attached config file in this email
and the following command:


As you can see in the config file, we have set IP for GGSN.
After executing the above command, a tunnel named tun4 is created.

To communicate with SGSN, we use the SGSN emulator in OsmoGGSN as follow:

sgsnemu -l -r

After executing the above command, we see that 2 packets are sent from SGSN
to GGSN. These packets are "Echo request" and "Create PDP context request"
and GGSN responses to SGSN with the packets "Echo response" and "Create PDP
context response".

To test tun4, we send a GTP <DNS> packet containing an arbitrary query to
GGSN and we see that this GTP <DNS> packet is received in Loopback (lo)
interface and also, the DNS packet is received in tun4 interface. But there
is no DNS response to our query neither in tun4 interface nor lo interface.
A screenshot of the lo interface and tun4 interface in the wireshark is
attached in this email (right is lo and left is tun4).

How can we receive DNS responses to our DNS query? (Does tun4 require
routing or something else?)

*When there is much light, The shadow is deep...*
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