Controlling the contents of DNS-SD SRV records

[efuss]

In an office environment, it may be undesirable for the DNS-SD SRV records to refer to the .local (mDNS) name of the CUPS server and preferable to hand out a unicast-DNS-resolvable name instead.
We, for instance, resolve cups.math.uni-bonn.de to different addresses (of the same server) for different sets of clients; we also like to control whether that name resolves to an IPv6 address for a given client and suppress special-purpose alias addresses of the server (on which CUPS doesn't listen).

The attached patch tries to address that by introducing a BrowseDNSSDSrvName option.
The patch works (on NetBSD) with mDNSResponder; the Avahi part hasn't even been compile-tested.
dnssd-srv-name.txt

[michaelrsweet]

At least with mDNSResponder, the default (NULL) registration should cover both the .local and the fully-qualified domain name for the system (or at least that's how it works on the Mac). But will consider including this in a future release of CUPS to override the default...

[efuss]

At least with mDNSResponder, the default (NULL) registration should cover both the .local and the fully-qualified domain name for the system (or at least that's how it works on the Mac).

I'm not sure what exactly you mean by ``cover''. The point is the contents of the resulting SRV RRs. Without the modification, the SRV RRs emitted bs mdnsd on CUPS' behalf refer to <servername>.local, like (according to dns-sd -Z _ipp._tcp) in: Testdrucker\032@\032cupstest-ea\.math\.uni-bonn\.de._ipp._tcp SRV 0 0 631 trave.local. But trave.local. gets resolved by mDNS and I have limited control over the result. With the modification (to cupsd and cupsd.conf), I can make them refer to a unicast-DNS-resolvable name, as in: Testdrucker\032@\032cupstest-ea\.math\.uni-bonn\.de._ipp._tcp SRV 0 0 631 cupstest-ea.math.uni-bonn.de. Then I have full control over what cupstest-ea.math.uni-bonn.de resolves to depending on the client's DNS view.

But will consider including this in a future release of CUPS to override the default...

Thanks.

[michaelrsweet]

OK, looking at the code it appears we already have a hostname for DNS-SD in the code, we just aren't using it for the service registrations. And if the mDNSResponder/Avahi-level hostname hasn't been set (just defaulting to something.local) we use the ServerName value.

So I think just passing DNSSDHostName for our registration will Do The Right Thing.

[michaelrsweet]

[master c5d2b23] Use the DNS-SD hostname when registering shared printers (Issue #5071)

[efuss]

And if the mDNSResponder/Avahi-level hostname hasn't been set (just defaulting to something.local) we use the ServerName value.

The problem is that host_fqdn is set to the return value of avahi_client_get_host_name_fqdn(), which I can't control. That value will be trave.math.uni-bonn.de, while what I want to go into the SRV records is cups-ea.math.uni-bonn.de. So I need something used at a higher priority than host_fqdn.

[michaelrsweet]

I actually think you can configure the FQDN for Avahi in the avahi.conf file... Need to look a bit closer at that... mDNSResponder likewise has configuration options to override the default .local hostname...

[efuss]

I actually think you can configure the FQDN for Avahi in the avahi.conf file

But I may want other Avahi customers use different names. The point is the CUPS process always using cups-ea.math.uni-bonn.de no matter what server it runs on.

[michaelrsweet]

[master d7b3868] Add support for DNSSDHostName configuration directive (Issue #5071)

[efuss]

Add support for DNSSDHostName configuration directive (Issue #5071)

Thanks!

+ { "DNSSDHostName", &DNSSDSubTypes, CUPSD_VARTYPE_STRING },

That’s a copy-paste-typo, no?

[michaelrsweet]

Oops, yes. Fixed...

[master 513f1b6] Fix cut/paste error (Issue #5071)