srv lookup in record

[Marc Roos]

Is it possible to use srv lookups, like eg cname. I do not want to
create SRV record, I just want to 'get' the ip addresses, that I would
get vai srv lookup.

Say I have this task

[@temp3]$ dig +short server.test.marathon.mesos
192.168.123.101
192.168.124.50
192.168.124.52
192.168.124.51
192.168.123.100
192.168.123.102

[@temp3]$ dig +short srv _http-apps._server.test._tcp.marathon.mesos
0 1 31024 server.test-usbzr-s3.marathon.mesos.
0 1 31852 server.test-z9x84-s3.marathon.mesos.
0 1 31790 server.test-k7g8r-s4.marathon.mesos.

[marc@os0 temp3]$ dig +short srv
_http-demo._server.test._tcp.marathon.mesos
0 1 31791 server.test-c8g8b-s4.marathon.mesos.
0 1 31025 server.test-wtbza-s3.marathon.mesos.
0 1 31853 server.test-d0x87-s3.marathon.mesos.

I would like to only make available the ip addresses that are in the
same range. If I would use a cname like this:

server.local. CNAME server.test.marathon.mesos.

I would get 6 of which 3 ip addresses are not in the same range. So I
need to have something like

server.local. ??? _http-apps._server.test._tcp.marathon.mesos.

Is this possible in bind-9.8.2-0.68.rc1.el6_10.3.x86_64?

[Dave Warren]

On 2020-08-21 16:26, Marc Roos wrote:
> Is it possible to use srv lookups, like eg cname. I do not want to
> create SRV record, I just want to 'get' the ip addresses, that I would
> get vai srv lookup.

I don't think so, nor does it seem to make sense to me that you would
want such a thing (in the general case, you may have a use-case).

SRV records are more than just pointers to a specific server, there is
also the priority and weight that need to be considered at the
application level.

[Marc Roos]

> I don't think so, nor does it seem to make sense to me that you would
> want such a thing (in the general case, you may have a use-case).

What would be better way to solve this then? To filter out only the ip
addresses that are in the same netmask?

[John Levine]

In article <mailman.821.159806...@lists.isc.org> you write:
>On 2020-08-21 16:26, Marc Roos wrote:
>> Is it possible to use srv lookups, like eg cname. I do not want to
>> create SRV record, I just want to 'get' the ip addresses, that I would
>> get vai srv lookup.
>

>SRV records are more than just pointers to a specific server, there is
>also the priority and weight that need to be considered at the
>application level.

More importantly, SRV records have port numbers. In SIP, which seems to be
the largest current use of SRV, as often as not the port number is different
from the default 5060.

SRV really is not like CNAME.

[Grant Taylor]

On 8/21/20 4:26 PM, Marc Roos wrote:
> Is it possible to use srv lookups, like eg cname. I do not want to
> create SRV record, I just want to 'get' the ip addresses, that I
> would get vai srv lookup.

I don't know of any over the counter - if you will - way to do what - I
think - you want to do.

> Say I have this task
>
> [@temp3]$ dig +short server.test.marathon.mesos
> 192.168.123.101
> 192.168.124.50
> 192.168.124.52
> 192.168.124.51
> 192.168.123.100
> 192.168.123.102

Are these the IP addresses that the names in the following SRV records
resolve to?

> [@temp3]$ dig +short srv _http-apps._server.test._tcp.marathon.mesos
> 0 1 31024 server.test-usbzr-s3.marathon.mesos.
> 0 1 31852 server.test-z9x84-s3.marathon.mesos.
> 0 1 31790 server.test-k7g8r-s4.marathon.mesos.
>
> [marc@os0 temp3]$ dig +short srv
> _http-demo._server.test._tcp.marathon.mesos
> 0 1 31791 server.test-c8g8b-s4.marathon.mesos.
> 0 1 31025 server.test-wtbza-s3.marathon.mesos.
> 0 1 31853 server.test-d0x87-s3.marathon.mesos.
>
> I would like to only make available the ip addresses that are in the
> same range. If I would use a cname like this:

What does "same range" mean in this context?

Is it the client's IP range? Or is it the server's IP range? Or some
reference to _http-apps vs _http-demo?

To me, the following three owner names are completely independent of
each other.

- server.test.marathon.mesos
- _http-apps._server.test._tcp.marathon.mesos
- _http-demo._server.test._tcp.marathon.mesos

I see how we as humans can probably correlate the three. But I don't
see how BIND will do it.

> server.local. CNAME server.test.marathon.mesos.

That seems like a simple enough alias. Simple enough that I think that
it's existence can largely be ignored and focus on the IP extraction
from SRV record(s) that I think you're asking about.

> I would get 6 of which 3 ip addresses are not in the same range. So I
> need to have something like
>
> server.local. ??? _http-apps._server.test._tcp.marathon.mesos.

How are you going to convey the "???" portion in the owner name of the
DNS query?

> Is this possible in bind-9.8.2-0.68.rc1.el6_10.3.x86_64?

I don't think what - I'm speculating - you want is possible as is with
stock BIND.

I do wonder if you might be able to write a custom Dynamic Loadable Zone
database (?) driver that might be able to do what you want.

Specifically, if you could create a DLZ driver that could take the query
name, apply some logic to it (to determine the "???" above), perform the
proper resolution of the SRV record(s), post process, and return the
desired result to the original DNS query.

$Marc's_Request IN CNAME bind-dlz.sourceforge.net

Link - BIND DLZ Home
- http://bind-dlz.sourceforge.net/

Aside: I think this is atypical of BIND. But I also think that it
/may/ be in BIND's wheel house to ... hack a possible solution to.

Further aside: I have wondered about a custom DLZ that would return the
client's IP address (as seen by the server). I'd like to roll my own
"what's my IP" type service. }:-)



--
Grant. . . .
unix || die

[John Levine]

In article <mailman.838.159839...@lists.isc.org> you write:
>> [@temp3]$ dig +short srv _http-apps._server.test._tcp.marathon.mesos
>> 0 1 31024 server.test-usbzr-s3.marathon.mesos.
>> 0 1 31852 server.test-z9x84-s3.marathon.mesos.
>> 0 1 31790 server.test-k7g8r-s4.marathon.mesos.

These SRV records say that the service is on ports 31024, 31852, and 31790 on
the respective servers. CNAME does not give you a port number. There is no
way to fake SRV using CNAME.

R's,
John

[Grant Taylor]

On 8/25/20 8:43 PM, John Levine wrote:
> These SRV records say that the service is on ports 31024, 31852,
> and 31790 on the respective servers. CNAME does not give you a
> port number. There is no way to fake SRV using CNAME.

Agreed.

I've had some off-line conversations with Marc about some related
things, so I thought he was only looking for the IP aspect and not
worried about the port aspect of the SRV records.