VPN blocking MMS text messages

[snagglewest]

Hi all. Lots of googling has failed me so I'm asking for the groups help.

For several years I ran OpenVPN for access to my local network on an old bare metal PC. It worked perfectly and I never had a problem with anything routed thru it. A side benefit was also that the add blocking also worked on anything connected to the VPN. When you're used to not being spammed with adds you (and the spouse) get spoiled quick

Unfortunately the PC died last week so I need a new solution. I decided to try the OpenVPN option within the router. Setup went easy and it seemed to be the perfect solution. I was suddenly wondering why I had gone to the trouble of setting up the stand alone VPN server years ago!

Until... the first time the wife had the VPN on her cell phone active and a MMS message tried to come thru. For some reason the VPN is blocking the MMS messages. SMS works fine. It's obviously something either with my configuration or the Asus OpenVPN but I'm at a loss.

Can anyone shed some light on what's causing this and maybe a solution?

Thanks

Here's my settings"

RT-AC86U
Asus/RMerlin 386.12_4
amtm 4.3
Diversion 5.0
Skynet 7.5.6
scMerlin 2.4.0
spdMerlin 4.4.5
YazDHCP 1.0.6


OpenVPN server settings on the router:
Client will use VPN to access - both (lan/wan)
Interface Type - TUN
Protocol - UDP
Server Port (Default : 1194)
Authentication Mode - TLS
Username/Password Authentication - No
Enable IPv6 Server mode - Yes
Enable NAT IPv6 - Yes
Advertise DNS to clients - Yes
Compression - disabled
Manage Client-Specific Options - No

 

[Viktor Jaep]

Generally, both SMS/MMS messages are transmitted over the mobile network. Unless you can specify that they are to be transmitted over wifi, then they shouldn't be able to go that route.

 

[sfx2000]

Until... the first time the wife had the VPN on her cell phone active and a MMS message tried to come thru. For some reason the VPN is blocking the MMS messages. SMS works fine. It's obviously something either with my configuration or the Asus OpenVPN but I'm at a loss.

See if WiFi calling is enabled on the handset - MMS uses data burst (typically over the SMS channel) to notifiy the handset that there is an MMS message, and when that data burst is processed, the SMS/MMS client then will open up a data channel to download the message content.

With the VPN in place, it might be the carrier's MMSC that is blocking the download request - I do see this sometimes with WiFi calling enabled on certain Android and KaiOS (simple phones) handsets with certain carriers...

 

[snagglewest]

Wifi calling is not enabled and I'm sure it's not the carrier. The strange part is worked fine when I had the stand alone OpenVPN server running on my LAN. When I switched over to the OpenVPN server within Asus-Merlin is when the problem started. I'm at a loss unless the Asus-Merlin version of OpenVPN is not the same as the normal package.

 

[snagglewest]

OK, new symptom. When receiving s MMS I just noticed a alert on the phone saying "Your APN settings are incorrect, please set them manually. Google doens't seem to be much help on this one either.

 

[Viktor Jaep]

OK, new symptom. When receiving s MMS I just noticed a alert on the phone saying "Your APN settings are incorrect, please set them manually. Google doens't seem to be much help on this one either.

What carrier do you use? And what kind of phone is it?

 

[snagglewest]

It's not a carrier or phone problem but Verizon/Samsung android just for the sake of argument.
After hours of messing around trying different settings and setups, UDP, TCP etc I took the path of least resistance. Spun up a minimal Ubuntu 22.04 VM on the NAS and it's working perfectly just as before.
The VM is running OpenVPN V2.5.9. The only thing I can think is maybe Asus is running an older version of OpenVPN.

 

[Viktor Jaep]

388.5 was updated to OpenVPN 2.6.8

Changelog (388.x) | Asuswrt-Merlin

www.asuswrt-merlin.net

 

[Piotrek]

Asus/RMerlin 386.12_4

The only thing I can think is maybe Asus is running an older version of OpenVPN.

Asuswrt (stock) - OpenVPN 2.4.12
Asuswrt-Merlin 386.12_4 - OpenVPN 2.6.8

Changelog (386.xx) | Asuswrt-Merlin

www.asuswrt-merlin.net