[asterisk-users] Asterisk and several clients behind NAT

[Ishfaq Malik]

I've just had this Static/Dynamic IP issue in the last couple of days

Any time the IP address changes the phone needs to be re-registered. 
This normally isn't that much of a problem as most people only reboot 
their routers about twice a year. However, here's a warning to anyone UK 
based. BT are now recycling their dynamic IPs on a nightly basis so if a 
customer has a SIP phone going through a BT dynamic IP service, they end 
up having to re-register on a daily basis.

Best idea is to always use a static IP.

Ish

jonas kellens wrote:
> Asterisk can 'ping' the clients behind NAT with the qualify-option so 
> the NAT-tables and routes are kept open.
>
> What happens when one resets the router (where the NAT-tables are 
> kept)  ?? Do NAT-tables get flushed when a router is reset ??
>
> Does the public IP-address needs to be a static IP-address ??? How can 
> Asterisk use qualify to clients that are behind a dynamic public 
> IP-address once registered ?? The clients are not aware that the 
> public IP-address has changed and will not re-register automatically 
> ?! Would dyndns be a solution ?
>
> Thanks for the feedback !
>
> Jonas.
>
>
> On Tue, 2009-07-14 at 06:33 -0400, Alex Balashov wrote:
>> jonas kellens wrote:
>>
>> > Is it possible to have several clients behind NAT to register to an 
>> > Asterisk-server with a public IP-address ?
>> > 
>> > When Asterisk receives an incoming call, how will it know @ which 
>> > private IP-address the client is reachable ?
>> > 
>> > I guess it is impossible for Asterisk to directly contact the private 
>> > client behind the NAT ?! Or to distinguish between the private clients ?!
>> > 
>> > Is there an easy solution to this ? How does hosted IP-PBX services work 
>> > then ?!
>>
>> Yes, this problem has a solution.  The NAT gateway creates a UDP state 
>> mapping between internal source ports and external source (and 
>> destination, since most user agents are symmetrical nowadays) ports.
>>
>> The NAT gateway then allocates different external UDP ports for 
>> different "connections" being tracked in this manner.
>>
>> Consider, for example, two phones - 192.168.1.10 and 192.168.1.11 - 
>> registering to an outside SIP UAS through a NAT gateway whose public 
>> address is 67.194.23.55.  The NAT gateway maps the source ports in a 
>> random or pseudorandom manner akin to:
>>
>>     192.168.1.10:5060 --> 67.194.23.55:32947
>>     192.168.1.11:5060 --> 67.194.23.55:47948
>>
>> If far-end NAT traversal is enabled on the UAS (in the case of Asterisk, 
>> that's nat=yes in sip.conf), the Contact URI supplied in the REGISTER 
>> message is ignored and the actual "received" IP and port on the network 
>> and transport layer is used in its place.  The latter is what is stored 
>> as the contact binding.
>>
>> Later, a call comes in and the UAS maps it back to 67.194.23.55:47948 or 
>> 32947 depending on which registrant it is destined to go to.
>>
>> This scenario is not without its problems.  Some user agents do not 
>> behave symmetrically.  Some firewall/NAT router ALGs (application layer 
>> gateways) break this process, though they mean well and try to be 
>> helpful.  But by far the most pressing problem is that many NAT gateways 
>> rather quickly age the temporary state information (internal:external 
>> UDP port mapping) out after a relatively short period of inactivity. 
>> That is why many far-end NAT traversal approaches implement a policy of 
>> periodically "pinging" the stored ("received") contact with some sort of 
>> message that causes a bidirectional exchange of communication, and 
>> therefore causes the NAT gateway to reset its expiration timer for that 
>> "connection" state.  In Asterisk, the OPTIONS messages generated when 
>> the qualify=yes option is enabled in sip.conf fulfill this function.
>>
>> Hope that helps,
>>
>>     
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Ishfaq Malik
Software Developer
PackNet Ltd

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