doc: Add new DNSseed policy

doc/dnsseed-policy.md

@@ -0,0 +1,52 @@
+Expectations for DNS Seed operators
+====================================
+
+Bitcoin Core attempts to minimize the level of trust in DNS seeds,
+but DNS seeds still pose a small amount of risk for the network.
+Other implementations of Bitcoin software may also use the same
+seeds and may be more exposed. In light of this exposure this
+document establishes some basic expectations for the expectations
+for the operation of dnsseeds.
+
+0. A DNS seed operating organization or person is expected
+to follow good host security practices and maintain control of
+their serving infrastructure and not sell or transfer control of their
+DNS seed. Any hosting services contracted by the operator are
+equally expected to uphold these expectations.
+
+1. The DNS seed results must consist exclusively of fairly selected and
+functioning Bitcoin nodes from the public network to the best of the
+operators understanding and capability.
+
+2. For the avoidance of doubt, the results may be randomized but must not
+single-out any group of hosts to receive different results unless due to an
+urgent technical necessity and disclosed.
+
+3. The results may not be served with a DNS TTL of less than one minute.
+
+4. Any logging of DNS queries should be only that which is necessary
+for the operation of the service or urgent health of the Bitcoin
+network and must not be retained longer than necessary or disclosed
+to any third party.
+
+5. Information gathered as a result of the operators node-spidering
+(not from DNS queries) may be freely published or retained, but only
+if this data was not made more complete by biasing node connectivity
+(a violation of expectation (1)).
+
+6. Operators are encouraged, but not required, to publicly document the
+details of their operating practices.
+
+7. A reachable email contact address must be published for inquiries
+related to the DNS seed operation.
+
+If these expectations cannot be satisfied the operator should
+discontinue providing services and contact the active Bitcoin
+Core development team as well as posting on bitcoin-development.
+
+Behavior outside of these expectations may be reasonable in some
+situations but should be discussed in public in advance.
+
+See also
+----------
+- [bitcoin-seeder](https://github.com/sipa/bitcoin-seeder) is a reference implementation of a DNS seed.