Grandstream VoIP over OpenVPN

Grandstream VoIP telephones are very popular because of their high build quality compared to an excellent price. In some cases you want to use an encrypted communication channel between your device and the PBX (e.g. asterisk). The current grandstream firmware includes basic OpenVPN support (client mode, tun) which allows you to tunnel the whole SIP/RTP traffic over an encrypted channel. This is also the best solution to avoid any kind of NAT/routing issues because all devices are directly accessible within the virtual ip subnet.

OpenVPN Server Config#

Use the following (minimal) configuration as template. The important options are set to work with the current grandstream firmware (1.0.4.106). Certificate based authentication is preferred for security (login/password not needed)!

tls-server
dev tunX
topology subnet
server 172.16.1.0 255.255.255.0
port 10111
proto udp

# cert based auth
pkcs12 server.p12

# 1024 and 2048 bit dh params are supported
dh dh2048.pem
keepalive 10 120
script-security 2

# bh-cbc as well as aes-128-cbc are supported by the current firmware
cipher aes-256-cbc

# well sha1 is a bit weak but its set within grandstream firmware
auth sha1

# compression has to be enabled
comp-lzo

tun-mtu 1500
mtu-disc yes

# custom logging
verb 3

# retain TOS flags (VoIP)
passtos

# internal network (VOIP Server)
push "route 10.16.0.1 255.255.255.0"

Notes#

• Don’t forget to alter your firewall rules. The new OpenVPN subnet needs to be accessible by your VoIP Server (e.g. asterisk) and vice versa
• Add Quality-of-Service rules to your router which matches the OpenVPN port set above. The traffic should be marked with class EF (realtime, expected forwarding) to avoid package lost. Default VoIP rules will not match because of the encrypted channel!