SIP ALG Routers

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What's a SIP ALG?

Many of today's commercial routers implement SIP ALG (Application Layer Gateway), coming with this feature enabled by default. While ALG could help in solving NAT related problems, the fact is that many routers' ALG implementations are wrong and break SIP.

There are various solutions for SIP clients behind NAT, some of them in client side (STUN, TURN, ICE), others in server side (Proxy RTP as RtpProxy, MediaProxy). ALG works typically in the client LAN router or gateway. In some scenarios some client side solutions are not valid, for example STUN with symmetrical NAT router. If the SIP proxy doesn't provide a server side NAT solution, then an ALG solution could have a place.

An ALG understands the protocol used by the specific applications that it supports (in this case SIP) and does a protocol packet-inspection of traffic through it. A NAT router with a built-in SIP ALG can re-write information within the SIP messages (SIP headers and SDP body) making signaling and audio traffic between the client behind NAT and the SIP endpoint possible.

SIP ALG problems

The main problem is the poor implementation at SIP protocol level of most commercial routers and the fact that this technology is just useful for outgoing calls, but not for incoming calls:

- Lack of incoming calls: When a UA is switched on it sends a REGISTER to the proxy in order to be localizable and receive incoming calls. This REGISTER is modified by the ALG feature (if not the user wouldn't be reachable by the proxy since it indicated a private IP in REGISTER "Contact" header). Common routers just mantain the UDP "conntection" open for a while (30-60 seconds) so after that time the port forwarding is ended and incoming packets are discarded by the router. Many SIP proxies maintain the UDP keepalive by sending OPTIONS or NOTIFY messages to the UA, but they just do it when the UA has been detected as being behind NAT during the registration. A SIP ALG router rewrites the REGISTER request so the proxy doesn't detect the NAT and doesn't maintain the keepalive - incoming calls will fail.
- Breaking SIP signaling: Many of the actual common routers with inbuilt SIP ALG modify SIP headers and the SDP body incorrectly, breaking SIP and making communication just impossible. Some of them do a whole replacing by searching a private address in all SIP headers and body and replacing them with the router public mapped address (for example, replacing the private address if it appears in "Call-ID" header, which makes no sense at all). Many SIP ALG routers corrupt the SIP message when writing into it (i.e. missed semi-colon ";" in header parameters). writing incorrect port values greater than 65536 is also common in many of these routers.
- Disallows server side solutions: Even if you don't need a client side NAT solution (your SIP proxy gives you a server NAT solution), if your router has SIP ALG enabled that breaks SIP signaling, it will make communication with your proxy impossible. 

List of routers with SIP ALG enabled

The following is a list containing SIP ALG router models, their issues and how to disable SIP ALG (enabled by default in most of the cases).

Models: ST530 v6 (firmware >= comes with SIP ALG enabled by default.
NAT type: symmetrical

- No incoming calls.
- It replaces the private IP appearing in SIP headers with the public IP using a dumb text replacement. If for example the private IP appears in the "Call-ID" it replaces it too (that it's completely unnecessary). 

To disable SIP ALG:
~# telnet router
-> connection unbind application=SIP port=5060
-> saveall

Models: 660 family comes with SIP ALG enabed by default.
NAT type: symmetrical

- No incoming calls.
- SIP protocol broken making 50% of outgoing calls impossible because the wrong values are inserted into SIP headers. 

To disable SIP ALG:
~# telnet router
Menu option "24. System Maintenance".
Menu option "8. Command Interpreter Mode".
ip nat service sip active 0

Models: WGR614v9 Wireless-G Router, DGN2000 Wireless-N ADSL2+ Modem Router
Firmware V1.0.18_8.0.9NA
To disable SIP ALG: From Wan Setup Menu, NAT Filtering, uncheck the box next to "Disable SIP ALG"

NAT type: No symmetrical

- The ALG doesn't replace the private address in "Call-ID" header (that is correct) but it does replace the "call-id" value in "Refer-To" header so SIP transfer is broken. 

Models: WRV200
NAT type: Symmetrical

- The ALG replaces the private address in "Call-ID" header (not needed at all). Some phones (as Linksys with latest firmware) encode the "Call-ID" value in the "Refer-To" header (by escaping the dots) so the private IP appearing there is not replaced with the public IP. This causes that the call transfer fails since the proxy/PBX/endpoint will not recognize the dialog info.

Peplink Multi-WAN routers
Models: All multi-WAN models
To disable SIP ALG, go to http:///cgi-bin/MANGA/support.cgi
Click the "Disable" button under "SIP ALG Support"

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