4 ms·

Dns A/AAAA record = IP address Dns SRV record = IP:port Not sure why SRV never really caught on for most internet stuff. (Edit: SRV records are hostnames, no

Dns A/AAAA record = IP address Dns SRV record = IP:port Not sure why SRV never really caught on for most internet stuff. (Edit: SRV records are hostnames, not IPs, so I guess it takes two lookups?)

inopinatus 2y ago
To be precise, SRV records don't immediately return an IP address; they return canonical name(s) (i.e. FQDN of hosts) whose IP addresses can either be included in the additional part of the DNS reply or resolved separately. In addition to canonical name & port number they also include priority & weighting values, although the usefulness of these depends on the service. SRV might not be super visible in the application developer's lane, but swim outside and there's a lot of it about. It's foundational for SIP, for example (IP telephony). Email remains a tremendously significant internet utility and SRV is used, albeit not universally, for discovery of client endpoints (SMTP submission, IMAP, CalDAV etc). However, SRV hasn't replaced MX for SMTP between MTAs, perhaps because MX already does basically the same and was established years prior. And, well, Jabber is dead, but XMPP was another fully worked demonstration of SRV's capability. In more local environments SRV is used for resolution/discovery in Active Directory (Microsoft) and Bonjour (Apple) - if you listen to the wire on any local network you'll often see a ton of SRV over mDNS or DNS-SD. Perhaps ironically, one of the objections that sometimes arose to using SRV for HTTP was from Active Directory sites with zone cuts at _tcp.example.com and facing additional complexity in any transition.
- riobard 2y ago
  I never understand the reasoning behind the design decision that SRV cannot just return IP address and forcing clients to query again for A/AAAA again. That decision forces an extra RTT before connection can be established, which makes people want to avoid SRV records. Totally self-inflicted wounds.

Dns A/AAAA record = IP address Dns SRV record = IP:port Not sure why SRV never really caught on for most internet stuff. (Edit: SRV records are hostnames, not IPs, so I guess it takes two lookups?)

inopinatus 2y ago

To be precise, SRV records don't immediately return an IP address; they return canonical name(s) (i.e. FQDN of hosts) whose IP addresses can either be included in the additional part of the DNS reply or resolved separately. In addition to canonical name & port number they also include priority & weighting values, although the usefulness of these depends on the service. SRV might not be super visible in the application developer's lane, but swim outside and there's a lot of it about. It's foundational for SIP, for example (IP telephony). Email remains a tremendously significant internet utility and SRV is used, albeit not universally, for discovery of client endpoints (SMTP submission, IMAP, CalDAV etc). However, SRV hasn't replaced MX for SMTP between MTAs, perhaps because MX already does basically the same and was established years prior. And, well, Jabber is dead, but XMPP was another fully worked demonstration of SRV's capability. In more local environments SRV is used for resolution/discovery in Active Directory (Microsoft) and Bonjour (Apple) - if you listen to the wire on any local network you'll often see a ton of SRV over mDNS or DNS-SD. Perhaps ironically, one of the objections that sometimes arose to using SRV for HTTP was from Active Directory sites with zone cuts at _tcp.example.com and facing additional complexity in any transition.

riobard 2y ago
I never understand the reasoning behind the design decision that SRV cannot just return IP address and forcing clients to query again for A/AAAA again. That decision forces an extra RTT before connection can be established, which makes people want to avoid SRV records. Totally self-inflicted wounds.