[cisco-voip] session target dns

Tue Mar 6 14:25:33 EST 2018

Just so you know, they're not going to know if you use SRV records or not,
or host records for that matter.  They probably only care about two things:

1) They control which peers you send traffic to via DNS updates

2) They receive the proper/expected host portion in your traffic to them

For all intents and purposes, the inclusion of a name in the host portion
of a SIP URI is separate from the DNS query.

The fact that you point your system at a name (or IP for that matter) and
that it then becomes the RHS of the URI is nice, but not required.

Therefore, if you ask them to commit to telling you about IP address
changes completely negates their desire to use SRV records.  Just say'n.

On Tue, Mar 6, 2018 at 6:30 AM Ed Leatherman <ealeatherman at gmail.com> wrote:

> Thanks Anthony, That was spot on what I was trying to figure out. I've
> been using server-groups up until now (and will continue on the CUCM facing
> side), the service provider is forcing the change on the side facing them.
>
> Loren: That's an interesting idea to lock in the host resolution on the
> CUBE itself, but in this case I think it might set me up for an outage if
> the service provider changes their IP Addressing. Maybe I can get them to
> commit to telling me before they change those..
>
> On Mon, Mar 5, 2018 at 2:31 PM, Anthony Holloway <
> avholloway+cisco-voip at gmail.com> wrote:
>
>> Loren,
>>
>> Just out of curiosity, why didn't you just use session server groups?
>> Based on the config you shared, it looks like it would achieve the same
>> thing, but with less config, and not adding in the DNS stack within IOS.
>>
>> Ed,
>>
>> *Note, you cannot use DNS in server groups, so it's one or the other.
>>
>> I also think it's important to know that the IOS code is written such
>> that it will look for SRV records first, and then fallback to looking for
>> an A (host) record once the DNS timeouts.
>>
>> E.g.,
>>
>> You enter "session target dns:collab.domain.com"
>>
>> IOS looks for _sip._udp.collab.domain.com SRV record first, timesout,
>> then looks for collab.domain.com host record second.
>>
>> *Note that the outgoing session transport on IOS is UDP by default,
>> unless you change it to TCP with the command "session transport tcp" at the
>> "voice service voip" level, or at the dial-peer level.  So, having a
>> _sip._tcp SRV record on your CUBE would create a confusing scenario.
>> Contrast this with the incoming connection, which can be either.  However,
>> SRV records, like Loren is showing, are for outbound connection
>> establishments.
>>
>> I have not done an extensive amount of testing here, but I would be
>> curious to know if IOS handles the TTL for the DNS record correctly, or if
>> it queries DNS for every setup like how that one defect was hitting CUCM
>> SIP trunks for a while there.  I looked for "TTL" in the CVP Config guide,
>> but it didn't say.
>>
>> On Mon, Mar 5, 2018 at 11:19 AM Loren Hillukka <lchillukka at hotmail.com>
>> wrote:
>>
>>> You can have your gw query your DNS server, and you have to add SRV
>>> records to your central DNS server (like with the jabber entries required
>>> to get jabber sign-in to work).
>>>
>>> Here’s the example of doing local DNS to static entries on the gateway
>>> itself, from the CVP 10 config guide.  CVP is where I first started doing
>>> dns srv on the local gateway, as I preferred breaking the call center
>>> myself instead of having the AD/DNS teams do it for me without me knowing
>>> ;-)
>>>
>>> ===========
>>>
>>> You can also configure the Gateway statically instead of using DNS. The
>>> following example shows how both the A and SRV type records could be
>>> configured:
>>>
>>> ip host cvp4cc2.cisco.com 10.4.33.132
>>>
>>> ip host cvp4cc3.cisco.com 10.4.33.133
>>>
>>> ip host cvp4cc1.cisco.com 10.4.33.131
>>>
>>> For SIP/TCP:
>>>
>>> ip host _sip._tcp.cvp.cisco.com srv 50 50 5060 <50%2050%205060>
>>> cvp4cc3.cisco.com
>>>
>>> ip host _sip._tcp.cvp.cisco.com srv 50 50 5060 <50%2050%205060>
>>> cvp4cc2.cisco.com
>>>
>>> ip host _sip._tcp.cvp.cisco.com srv 50 50 5060 <50%2050%205060>
>>> cvp4cc1.cisco.com
>>>
>>> For SIP/UDP:
>>>
>>> ip host _sip._udp.cvp.cisco.com srv 50 50 5060 <50%2050%205060>
>>> cvp4cc3.cisco.com
>>>
>>> ip host _sip._udp.cvp.cisco.com srv 50 50 5060 <50%2050%205060>
>>> cvp4cc2.cisco.com
>>>
>>> ip host _sip._udp.cvp.cisco.com srv 50 50 5060 <50%2050%205060>
>>> cvp4cc1.cisco.com
>>>
>>>  ============
>>>
>>> Then your dial-peer would have session target dns:cvp.cisco.com which
>>> would point to the SRV record, which would use the weight/priority values
>>> to choose the final host, and resolve the selected host to an IP using the
>>> normal "ip host name x.x.x.x" entry
>>>
>>>
>>> Loren
>>>
>>> On Mar 5, 2018, at 10:15 AM, Ed Leatherman <ealeatherman at gmail.com>
>>> wrote:
>>>
>>> Hi everyone,
>>>
>>> Hopefully a quick question - in a dial-peer on CUBE (16.3.5) how does
>>> session target dns: resolve to an IP? I've never used DNS as target before
>>> for this.
>>>
>>> Does CUBE just do a query for the A record by default, or does it do a
>>> SRV query by default? I have a SIP provider that wants to start using SRV
>>> for their SBC(s) and I'm researching how to setup my end in IOS. If it
>>> doesn't query SRV default, where do I toggle that behavior?
>>>
>>> The command reference just says  "Configures the host device housing the
>>> domain name system (DNS) server that resolves the name of the dial peer to
>>> receive calls."
>>>
>>> I've found the knob to tell it what SRV format to use in the sip-ua
>>> section.
>>>
>>> Thanks!
>>>
>>>
>>>
>>> --
>>> Ed Leatherman
>>>
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>>>
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>>>
>>
>
>
> --
> Ed Leatherman
>
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