notes on DNS, DHT, etc

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+
+### dat-specific configuration
+
+Default servers are specified in the `dat-swarm-defaults` module. As of
+November 2017, they are:
+
+    Domain (for mDNS, centralized DNS):
+        dat.local
+
+    DNS Servers (centralized lookup):
+        discovery1.publicbits.org
+        discovery2.publicbits.org
+
+    DHT Bootstrap Servers:
+        bootstrap1.publicbits.org:6881
+        bootstrap2.publicbits.org:6881
+        bootstrap3.publicbits.org:6881
+        bootstrap4.publicbits.org:6881
+
+### mDNS and Centralized DNS Discovery
+
+`dns-discovery` node module.
+
+Refresh period: 1 minute
+
+Take the first 40 hex characters (lowercase; 20 bytes if decoded) of the
+discovery key (a BLAKE2b hash) instead of a SHA1 hash (which is what bittorrent
+uses). Prepend this to the domain (`dat.local` in the case of dat).
+
+There are both SRV and TXT records. SRV is straightforward, a response like:
+
+    0 0 44113 172.19.0.4.
+
+TXT uses some un-documented token scheme... maybe mostly for mDNS, to update or
+"unannounce" records? Eg:
+
+    "token=kZabfUWLUw5A4E/EXM3+ka7UybMN95QJjqPk1iGmb0M=" "peers=rBMABKxR"
+
+Example lookup:
+
+    dig @discovery1.publicbits.org 905fd1b6504698425e8bec3dbb77d757e281d505.dat.local SRV
+
+Note that you might need to do lookup on *both* servers? Results can be
+different from either individually.
+
+### DHT Discovery
+
+`discovery-channel`
+
+Refresh period: 10 minutes
+