Merge pull request #1831 from matrix-org/travis/msc/wk-before-srv

MSC1831: Change the order of .well-known and SRV discovery techniques

proposals/1831-srv-after-wellknown.md

@@ -0,0 +1,25 @@
+# Proposal to do SRV lookups after .well-known to discover homeservers
+
+Currently there is a logistical error proposed by [MSC1708](https://github.com/matrix-org/matrix-doc/pull/1708)
+which results in some homeservers unable to migrate to the new functionality
+proposed by [MSC1711](https://github.com/matrix-org/matrix-doc/pull/1711). This
+can happen if the delegated homeserver cannot obtain a valid TLS certificate for
+the domain, and an SRV record is used for backwards compatibility reasons.
+
+Specifically, in order to be compatible with requests from both Synapse 0.34 and 1.0,
+servers can have both a SRV and a .well-known file, with Synapse presenting a certificate
+corresponding to the target of the .well-known. Synapse 0.34 is then happy because it
+will follow the SRV (and won't care about the incorrect certificate); Synapse 1.0 is
+happy because it will follow the .well-known (and will see the correct cert).
+
+## Proposal
+
+We change the order of operations to perform a .well-known lookup before falling
+back to resolving the SRV record. This allows for domains to delegate to other
+hostnames and maintains backwards compatibility with older homeservers.
+
+## Tradeoffs
+
+More HTTP hits will be made due to the .well-known lookup being first. This is
+somewhat mitigated by servers caching the responses appropriately, and using
+connection pools where possible.