mDNS service discovery middleware

[rtreffer]

For my home network I used to run a zone that is filled by mDNS (excluding private IPv6 addresses). That way exposing a host was a matter of allowing access.
Not all applications are mDNS aware thus having a DNS zone is helpful in those cases, too.

I also used to run a caddy proxy for some services. Having different internal and external resolutions for the same names makes them work seemingly.

I'd thus like to see a mDNS service discovery middleware for CoreDNS.

[miekg]

[ Quoting notifications@github.com in "[miekg/coredns] mDNS service discov..." ]

For my home network I used to run a zone that is filled by mDNS (excluding private IPv6 addresses). That way exposing a host was a matter of allowing access.
Not all applications are mDNS aware thus having a DNS zone is helpful in those cases, too.

I also used to run a caddy proxy for some services. Having different internal and external resolutions for the same names makes them work seemingly.

I'd thus like to see a mDNS service discovery middleware for CoreDNS.

I'm not intimate familiar with mDNS - this also involves registration with
CoreDNS?

I.e. it would help to populate this bug with some design requirements; what is
needed in the middleware to make it happen. Is an in-memory store enough, or do
we need disk persistence or remote storage?

/Miek

Miek Gieben

[rtreffer]

mDNS is basically DNS as a multicasting service on port 5353.

To receive e.g. printers one would broadcast a PTR query for "_printer._tcp.local". For hosts one could simple do a broadcast of a ANY query for the name, e.g.

dig @224.0.0.251 -p 5353 home.local ANY

; <<>> DiG 9.10.3-P4-Ubuntu <<>> @224.0.0.251 -p 5353 home.local ANY
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 43155
;; flags: qr aa; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;home.local.                    IN      ANY

;; ANSWER SECTION:
home.local.             10      IN      HINFO   "MIPS" "LINUX"
home.local.             10      IN      A       192.168.2.1
home.local.             10      IN      AAAA    fd00:b0ef:54ac::1

;; Query time: 2 msec
;; SERVER: 192.168.2.1#5353(224.0.0.251)
;; WHEN: Mon Oct 03 14:37:26 CEST 2016
;; MSG SIZE  rcvd: 95

An implementation would need at least

• An internal cache that respects TTL (implementations may rate-limit responses)
• An mDNS client that can re-fetch entries via mDNS, bound to port 5353 (with reuse port)
• Translation of zone-relative entries (strip zone name + append .local for queries and the other way around for responses)

Others have build mDNS clients on top of your DNS library, e.g.: https://github.com/hashicorp/mdns

[miekg]

feel free to create PR that implements this, I think that would be a useful addition.

[miekg]

This is probably non-trivial, esp seeing discussions about mDNS popping up on Go DNS (github.com/miekg/dns)