Hi,
I am wondering what the SIP service actually does on RouterOS. There is a brief mention on the NAT page but that does not really help me to understand it.
[admin@router] > /ip firewall service-port print where name=sip
Columns: NAME, PORTS
# NAME PORTS
4 sip 5060
5061
The background of my question: I am trying to run a FritzBox in the internal network as a VoIP telephone client. Should the SIP service be off or on for that purpose?
The SIP service (SIP ALG) in RouterOS acts as a helper that inspects SIP packets passing through NAT and rewrites the internal IP addresses in SIP headers to your public IP. It also dynamically opens ports for RTP (voice traffic).
In practice though, it causes more problems than it solves — things like one-way audio, registration failures, and broken calls. Most modern VoIP devices (including FritzBox) handle NAT traversal on their own using STUN or outbound proxies.
My advice: turn it off.
/ip/firewall/service-port/set sip disabled=yes
I run a Grandstream UCM6301 behind a MikroTik with SIP ALG disabled and everything works perfectly — SIP trunks, internal extensions, the lot. FritzBox should be fine too.
If you run into one-way audio after disabling it, make sure your FritzBox has a STUN server configured or that your VoIP provider supports NAT keepalive.
How does this relate to IPv6? The setting is located under /ip firewall, thus I assume that it relates only to IPv4. Is this assumption correct?
The FritzBox has an IPv6 address and I would assume that calls are routed over IPv6 (Deutsche Telekom). But I suppose this would only work if all related connections are established from the LAN side.
Ok, the VoIP infrastructure seems to be IPv4 only at the moment, so all of that is still relevant.