docs: add dnsaddr protocol spec (#102)

* docs: add dnsaddr protocol spec

...to the best of my knowledge.

Added as separate doc in `protocols` dir so existing link in README becomes not 404 :)

* fix: clarify address being queried

* fix: better syntax highlighting on gh

* Update protocols/DNSADDR.md

Co-Authored-By: Marcin Rataj <[email protected]>

README.md

@@ -114,7 +114,7 @@ TODO: most of these are way underspecified
 
 - /ip4, /ip6
 - /dns4, /dns6
-- /dnsaddr
+- [/dnsaddr](protocols/DNSADDR.md)
 - /tcp
 - /udp
 - /utp

protocols/DNSADDR.md

@@ -0,0 +1,38 @@
+# `dnsaddr`
+
+`dnsaddr` is a protocol that instructs the resolver to lookup multiaddr(s) in DNS TXT records for the domain name in it's value section.
+
+To resolve a `dnsaddr` multiaddr, the domain name in the value section must first be prefixed with `_dnsaddr.`. Then a DNS query to lookup TXT records for the domain must be made. There may be multiple DNS TXT records for the domain. Valid `dnsaddr` TXT records begin with `dnsaddr=`, followed by a single multiaddr. Recursive lookups are allowed.
+
+## Example
+
+`/dnsaddr/bootstrap.libp2p.io` would result in a DNS TXT record query for `_dnsaddr.bootstrap.libp2p.io`.
+
+```sh
+# TXT records for `_dnsaddr.bootstrap.libp2p.io`:
+dnsaddr=/dnsaddr/sjc-1.bootstrap.libp2p.io/tcp/4001/ipfs/QmNnooDu7bfjPFoTZYxMNLWUQJyrVwtbZg5gBMjTezGAJN
+dnsaddr=/dnsaddr/ams-2.bootstrap.libp2p.io/tcp/4001/ipfs/QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb
+# ...
+```
+
+...which further resolve to:
+
+```sh
+# TXT records for `_dnsaddr.sjc-1.bootstrap.libp2p.io`:
+dnsaddr=/ip6/2604:1380:1000:6000::1/tcp/4001/ipfs/QmNnooDu7bfjPFoTZYxMNLWUQJyrVwtbZg5gBMjTezGAJN
+dnsaddr=/ip4/147.75.69.143/tcp/4001/ipfs/QmNnooDu7bfjPFoTZYxMNLWUQJyrVwtbZg5gBMjTezGAJN
+
+# TXT records for `_dnsaddr.ams-2.bootstrap.libp2p.io`:
+dnsaddr=/ip4/147.75.83.83/tcp/4001/ipfs/QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb
+dnsaddr=/ip6/2604:1380:2000:7a00::1/tcp/4001/ipfs/QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb
+```
+
+So, `/dnsaddr/bootstrap.libp2p.io` resolves to (at least) four multiaddrs:
+
+```
+/ip6/2604:1380:1000:6000::1/tcp/4001/ipfs/QmNnooDu7bfjPFoTZYxMNLWUQJyrVwtbZg5gBMjTezGAJN
+/ip4/147.75.69.143/tcp/4001/ipfs/QmNnooDu7bfjPFoTZYxMNLWUQJyrVwtbZg5gBMjTezGAJN
+
+/ip4/147.75.83.83/tcp/4001/ipfs/QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb
+/ip6/2604:1380:2000:7a00::1/tcp/4001/ipfs/QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb
+```