[cisco-voip] session target dns

[Anthony Holloway]

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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