AirVPN & SIP (OSTEL)

[JamesDean 10]

​Has anyone sucessfully made and received SIP phone calls, on Android, while connected to the VPN? I'm creating a WiFi only Android phone (No SIM card, Airplane Mode) and inputting all the open access points in my area, for a data connection. I have an OSTEL.co account, which allows for encrypted (ZRTP) SIP phone calls, using the CSIP Simple application. It also interfaces with Jitsi on the desktop. I want to activate the VPN before leaving the house, and make and receive phone calls. This all works great when using my bare home ISP connection, and on my carriers data network - but as soon as I connect to the VPN, I get nothing. Is this because I would need to forward ports in my Air account?

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[OpenSourcerer 1435]

Had the exact same issue on a Fritz!Box router. Here's what I found out.

 

SIP didn't work when all traffic was routed through the tunnel. My internet telephony number could not be registered. I changed the routing of Fritz!Box and it worked partially: I added a few routes to route traffic to possible VoIP servers (SIP, STUN, etc.) - the part that worked - but encountered a very big problem which you cannot circumvent.

To transfer voice, VoIP uses RTP (Real-time Transport Protocol) which is coupled with RTCP (Real-time Transport Control Protocol). According to the specification, RTP uses an even port number while RTCP will use the next higher odd port. The problem here is that they're not bound to specific ports. They can use 1024 and 1025, 4444 and 4445 or even 65534 and 65535.

 

You cannot remote forward ports on the fly. That's why VoIP with any proxy service will not work.

[pfSense_fan 181]

Well explained mate. Better words then I mustered up when writing my pfSense guide, that is for sure.

 

It is the primary reason why I heavily, HEAVILY push pfsense users to use more than two NIC's. VOIP and other services are just not suited for VPN use.

 

My VOIP requires ports 4000 - 65535 be available and open for RTP. Air gives us a number of ports to open, but ot nearly enough for that.

[OpenSourcerer 1435]

However, I start to think about it again.

If you make your Android device tell the STUN server the OpenVPN IP the routing issue might be fixed without adding routing rules (which will be deleted upon reboot, anyways). But this won't solve the voice transfer issue... maybe we should recode VoIP protocols and make them compatible with OpenVPN?!

[JamesDean 10]

​Very informative, thanks!