federation: Server without IPv6 support retries failed IPv6 connections without back off, spamming .well-known lookups many times a second

[Ablu]

Background information

• Dendrite version or git SHA: 0.13.5+b7054f4
• SQLite3 or Postgres?: unknown
• Running in Docker?: yes
• go version: unknown
• Client used (if applicable): na

Description

• What is the problem:

If a server does not support IPv6, but another server is only reachable via IPv6 connections are attempted in a tight loop without any back-off. This results in many .well-known lookups on the target server per second.

• Who is affected: The IPv6-only server is getting hammered by .well-known request non-stop. The IPv4-only server gets a huge amount of log errors
• How is this bug manifesting: Spam in logs, rate-limiting kicking in and DDoSing the target server.
• When did this first appear: unknown, noticed it when I got rate limited by my webhosting provider that hosted the .well-known file.

Steps to reproduce

• Run dendrite on a IPv4 only host
• Attempt federation with an IPv6 only host (For example, message @ablu:ablu.org)
• Observe non-stop log errors:

time="2023-12-18T15:14:28.942253091Z" level=debug msg="Error sending request to https://matrix.ablu.org:443/_matrix/key/v2/server: dial tcp [2001:9e8:d58b:8100:51be:e7e4:9b58:7507]:443: connect: network is unreachable" out.req.ID=lpqzvlnCpkM4 out.req.method=GET out.req.uri="matrix://ablu.org/_matrix/key/v2/server" req.id=1tjUd9EkbTnE req.method=PUT req.path=/_matrix/federation/v1/send/1699241410034
time="2023-12-18T15:14:28.949413244Z" level=debug msg="Error sending request to https://matrix.ablu.org:443/_matrix/key/v2/server: dial tcp [2001:9e8:d58b:8100:51be:e7e4:9b58:7507]:443: connect: network is unreachable" out.req.ID=lpqzvlnCpkM4 out.req.method=GET out.req.uri="matrix://ablu.org/_matrix/key/v2/server" req.id=1tjUd9EkbTnE req.method=PUT req.path=/_matrix/federation/v1/send/1699241410034
time="2023-12-18T15:14:28.949483724Z" level=debug msg="Outgoing request failed" error="Get \"matrix://ablu.org/_matrix/key/v2/server\": dial tcp [2001:9e8:d58b:8100:51be:e7e4:9b58:7507]:443: connect: network is unreachable" out.req.ID=lpqzvlnCpkM4 out.req.method=GET out.req.uri="matrix://ablu.org/_matrix/key/v2/server" req.id=1tjUd9EkbTnE req.method=PUT req.path=/_matrix/federation/v1/send/1699241410034
time="2023-12-18T15:14:28.979877904Z" level=debug msg="Error sending request to https://matrix.ablu.org:443/_matrix/key/v2/query: dial tcp [2001:9e8:d58b:8100:51be:e7e4:9b58:7507]:443: connect: network is unreachable" out.req.ID=av7ZmKa6ZhFR out.req.method=POST out.req.uri="matrix://ablu.org/_matrix/key/v2/query" req.id=1tjUd9EkbTnE req.method=PUT req.path=/_matrix/federation/v1/send/1699241410034
time="2023-12-18T15:14:28.981335511Z" level=debug msg="Error sending request to https://matrix.ablu.org:443/_matrix/key/v2/query: dial tcp [2001:9e8:d58b:8100:51be:e7e4:9b58:7507]:443: connect: network is unreachable" out.req.ID=av7ZmKa6ZhFR out.req.method=POST out.req.uri="matrix://ablu.org/_matrix/key/v2/query" req.id=1tjUd9EkbTnE req.method=PUT req.path=/_matrix/federation/v1/send/1699241410034

• Observe matching .well-known lookups on the target server

Expected behaviour:

The connection should fail and exponentially back off. The .well-known entry should probably be cached as well.

/cc @davralin

[davralin]

So, the dendrite-server in question is mine, and the server it targeted is @Ablu 's.

To fill in the gaps:

• Postgresql-database.
• Same go-version as is present in ghcr.io/matrix-org/dendrite-monolith:v0.13.5.
• Container is running on a talos-node in Oracle Cloud (free tier), which is overwritten from a "normal" linux install.
  I never bothered with IPv6-connectivity there, so nothing is configured on the host, IPv6-wize.