[Freeswitch-users] Best practices question about SIP registration

Lawrence Conroy lconroy at insensate.co.uk
Wed Jan 9 00:56:02 MSK 2013

Hi there,
 at the risk of butting in on someone else's party ...
Nope; your interpretations is NOT best practice.
I have some sympathy, as the term domain is overloaded within fS.

A sip address consists of a userpart and a domain part -- e.g., <sip:user at sipdomain>
The sip domain is similar to an email domain -- e.g., <mailto:user at maildomain>
With email, you need to do a lookup of the MX record in DNS to find the FQDN of the machine that handles mail for the domain.
With SIP (see RFC 3263), you do a lookup on the SRV record (at _sip._udp.<sipdomain>) to find the machine that handles SIP
registrations/incalls for the domain. That also gives you the port on which that machine is listening.
(Yup, you can also have a NAPTR record in the domain to tell you where the SRV record is, but many folks don't bother -- for Best Practice, you should, but ...)

There IS a "get out" clause in the SIP specs for RFC 2543 (AKA legacy) support that means most SIP clients will look for the SRV and, if it can't be found (or there's an IP address rather than a DNS -style domain, in which case the SIP client won't bother hunting the SRV), the client will guess that the domain has a machine (i.e. it will look for an A or AAAA record), and also guess it's listening on 5060 (the default port).
Email is the same (mail to fred at example.com, and strictly the sender will do a check for a MX and then look for an A record for example.com, and try there).

However, relying on that default "get out" clause is definitely NOT what you should do for BCP.
Using the hostname as the sip domain is a kludge -- the FQDN with A record usually works, but it's not what you want to do.

SO ... get yourself a domain, put a D2U NAPTR at that domain, put a SRV at _sip._udp.<domain>, and you're done. No need for an A record at that domain at all.

(RFC 3263 is not too hard to read, for a change -- it's certainly shorter than RFC 3261, and it even has an ASCII art diagram :).

all the best,

On 8 Jan 2013, at 21:05, Steven Schoch wrote:

> On Fri, Dec 28, 2012 at 8:47 PM, Tim St. Pierre <
> fs-list at communicatefreely.net> wrote:
>> Hi Steven,
>> I would recommend using a proper domain name as much as possible.  For
>> one, it looks
>> nicer!  A SIP URI is supposed to be user at domain like an e-mail address
>> is, and I hope that
>> one day URI dialing will be common place, so we might as well do it right
>> the first time.
> What you're saying is that "domain" should really be a fully-qualified host
> name that points via DNS to the actual host on which FreeSwitch is running.
> That is, the domain should be "pbx.example.com" instead of just "
> example.com", as the last example would most likely point to a web server,
> not the SIP server.  Do I have that right?
> Next, in the configuration for Polycom phones (for example), there are 2
> fields that both have the userid.  In the example in
> http://wiki.freeswitch.org/wiki/Polycom_configuration it has:
> reg.1.auth.userId="1000"
> and
> reg.1.address="1000 at fs.domain.local"
> How is the "address" value used?  Is that sent in the SIP registration
> message?  If that's the case, what does Freeswitch do with it?
> -- 
> Steve
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