I've had an interesting situation with osmo-ggsn and kernel GTP module that I
wonder if someone could shed some light on.
I've set up osmo-ggsn (v1.7.1) on Debian 10. There are two ethernet interfaces
on the system, ens224 (for the Gn Interface) and ens256 (for the Gi
Interface).
Using the following ggsn configuration seems to work fine:
ggsn ggsn0
description Keith's Test APN
gtp state-dir /tmp
gtp bind-ip <ens224 IP>
gtp control-ip <ens224 IP>
apn keithtest
gtpu-mode tun
tun-device apn0
type-support v4
ip prefix dynamic 192.168.42.0/24
ip dns 0 <redacted>
ip dns 1 <redacted>
ip ifconfig 192.168.42.0/24
no shutdown
default-apn keithtest
no shutdown ggsn
I get the following tun device:
5: apn0: <POINTOPOINT,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN
group default qlen 500
link/none
inet 192.168.42.0/24 scope global apn0
valid_lft forever preferred_lft forever
inet6 fe80::fada:2e69:ad71:a61b/64 scope link stable-privacy
valid_lft forever preferred_lft forever
Traffic routes, all is good with the world.
However, if I change to using Kernel GTP, by changing the gtpu-mode to
"kernel-gtp", traffic will not route (and, yes, I have done modprobe gtp). After
some exploration, it appears the tun device is being created with an MTU of
zero which then, cannot pass traffic:
10: apn0: <POINTOPOINT,UP,LOWER_UP> mtu 0 qdisc noqueue state UNKNOWN group
default qlen 1000
link/none
inet 192.168.42.0/24 scope global apn0
valid_lft forever preferred_lft forever
Here are the logs:
Tue Apr 13 11:02:06 2021 DGGSN NOTICE <0002> ggsn.c:830 GGSN(ggsn0):
Successfully started
Tue Apr 13 11:02:06 2021 DGGSN INFO <0002> ggsn.c:186 APN(keithtest): Starting
Tue Apr 13 11:02:06 2021 DGGSN INFO <0002> ggsn.c:204 APN(keithtest): Opening
Kernel GTP device apn0
Tue Apr 13 11:02:06 2021 DGGSN NOTICE <0002> gtp-kernel.c:79 Initialized GTP
kernel mode (genl ID is 24)
Tue Apr 13 11:02:06 2021 DTUN NOTICE <0001> tun.c:217 GTP kernel configured
Tue Apr 13 11:02:06 2021 DGGSN INFO <0002> ggsn.c:236 APN(keithtest): Setting
tun IP address 192.168.42.0/24
Tue Apr 13 11:02:06 2021 DGGSN INFO <0002> ggsn.c:294 APN(keithtest): Creating
IPv4 pool 192.168.42.0/24
Tue Apr 13 11:02:06 2021 DGGSN INFO <0002> ggsn.c:168 APN(keithtest):
Blacklist tun IP 192.168.42.0/24
Tue Apr 13 11:02:06 2021 DGGSN NOTICE <0002> ggsn.c:325 APN(keithtest):
Successfully started
Note that if I manually alter the MTU, everything starts working again:
ip link set dev apn0 mtu 1500
So I can put this into a script and run it via ipup-script, and this works
fine. I just get the impression I shouldn't have to do this, and the system
should be setting the MTU, just as it does when I use tun mode.
Any thoughts on what I'm doing wrong here?
Cheers,
Keith
