[cisco-voip] session target dns

Anthony Holloway avholloway+cisco-voip at gmail.com
Thu Mar 15 11:12:03 EDT 2018 

Wow.  So you pointed out a flaw in the provider network.  Presumably, they
were hosting other customers with the same setup; so how in the world was
it working for the others?  Or maybe you are the beta tester?

On Thu, Mar 15, 2018 at 9:54 AM Ed Leatherman <ealeatherman at gmail.com>
wrote:

> Follow-up for posterity..
>
> I had a feeling this was the case but got some confirmation from TAC:
> "This is working as designed when this is an incoming call, the reason
> why it works that way is because on the incoming leg the call comes from 1
> specific IP address, and if CUBE does a DNS query for SRV it might resolve
> a different IP address than the one where INVITE is coming from and might
> cause inbound calls to fail. Instead, if CUBE does DNS query for A record
> it will resolve one specific IP address."
>
>
> The service provider changed their SBC to send an IP address in the
> Contact field URI which resolved the issue.
>
>
> Ed
>
> On Thu, Mar 8, 2018 at 2:16 PM, Ed Leatherman <ealeatherman at gmail.com>
> wrote:
>
>>
>> Follow-up to this SRV/CUBE topic..
>>
>> Outbound calls work fine with this setup (after I enabled ip
>> domain-lookup ;-) )
>>
>> For inbound calls, the service provider is using the hostname for the SRV
>> record (peer.isc.lumos.net) in the contact field of the invite.
>> Apparently, CUBE only does an A record lookup on that field?
>>
>> 022206: Mar  8 13:44:04 est:
>> //25051/829EEEDD9B28/SIP/Info/verbose/4608/sipSPIProcessContactInfo:
>> Previous Hop peer.isc.lumos.net:5060
>> ...
>> 022210: Mar  8 13:44:04 est:
>> //-1/xxxxxxxxxxxx/SIP/Info/info/8192/sip_dns_type_a_aaaa_query: DNS query
>> for peer.isc.lumos.net and type:1
>> 022211: Mar  8 13:44:04 est:
>> //-1/xxxxxxxxxxxx/SIP/Error/sip_dns_type_a_query:
>>  TYPE A query failed for peer.isc.lumos.net
>> 022212: Mar  8 13:44:04 est: //-1/xxxxxxxxxxxx/SIP/Error/_send_dns_fail:
>>  DNS Query for peer.isc.lumos.net failed
>>
>> CUBE is basically shutting down the call saying it can't resolve the
>> contact field. If I put a local host entry for that name using their
>> currently active SBC, inbound calls work. Shouldn't CUBE be doing a SRV
>> lookup here, or should the service provider send me an hostname instead of
>> an SRV in this field?
>>
>>
>>
>>
>> On Tue, Mar 6, 2018 at 2:25 PM, Anthony Holloway <
>> avholloway+cisco-voip at gmail.com> wrote:
>>
>>> 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
>>>>>>
>>>>>> _______________________________________________
>>>>>> cisco-voip mailing list
>>>>>> cisco-voip at puck.nether.net
>>>>>> https://puck.nether.net/mailman/listinfo/cisco-voip
>>>>>>
>>>>>> _______________________________________________
>>>>>> cisco-voip mailing list
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>>>>>> https://puck.nether.net/mailman/listinfo/cisco-voip
>>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>> Ed Leatherman
>>>>
>>>
>>
>>
>> --
>> Ed Leatherman
>>
>
>
>
> --
> Ed Leatherman
> _______________________________________________
> cisco-voip mailing list
> cisco-voip at puck.nether.net
> https://puck.nether.net/mailman/listinfo/cisco-voip
>
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