blacklist servers which are down for several days

[richvdh]

Why don't we add servers which fail to respond to federation requests for several days to a blacklist, and stop trying?

This could significantly reduce network traffic, CPU usage, and the amount of cruft that gets logged.

We would need to unblock such servers when we receive a valid request from them.

[aaronraimist]

See https://gist.github.com/Sharparam/b144e294189d78ee6c73df0e109ee2af for one week of data showing the number of times per day that @Sharparam's server contacted a bunch of dead servers.

See convo starting from https://matrix.to/#/!HsxjoYRFsDtWBgDQPh:matrix.org/$155655145216EFGaa:matrix.sharparam.com?via=matrix.org&via=chat.weho.st&via=hackerspaces.be

[richvdh]

Note that we already have an exponential-backoff algorithm, but it tops out at a 24hr retry period.

After the first failed request, we back off for 10 minutes. We then increase the backoff by a factor of between 4 and 7 after each failed request, until we get to 24 hours. The retry intervals are therefore:

• 10min
• ~ 55min
• ~ 5hr
• 24hr
• 24hr
• ...

An easy way to implement this would be to go to 999 years after 24 hours.

[richvdh]

I should also mention that each request is retried several times (normally 10), with its own exponential-backoff loop. The per-host exponential backoff is only increased after a request fails completely.

[mguentner]

The worst backoff time of a server should be calculated to know how long
operators of homeservers have time to fix issues. This default needs to be
communicated to homeserver admins.

Also, if a server is down for one week or so this shouldn't result in a blacklist if it is no easily reversible.
Such a reversal could be that the blacklisted server contacts the server again that put its blacklist.

An observation:
I moved an homeserver I operate it to a dedicated machine some days ago.
Before it was running on domain.tld, now it runs on synapse.domain.tld

To do that, I created a .well-known/matrix/server file and changed the
SRV record from domain.tld to synapse.domain.tld.
The main domain (without the host part, synapse) however appears now in the gist referenced in #5113 (comment)
My homeserver happily federates with the network as far as I can see and the fed tester
is also green.
Adding a .well-known/matrix/server file or changing / adding the SRV record mustn't
result in a ban, even if cached DNS results or GET requests of .well-known/matrix/server
let requests fail.

[Sharparam]

@mguentner What is your server hostname so I can check the logs more closely for errors related to it?

[kroeckx]

@Sharparam: My server (roeckx.be) seems to have high numbers in that file, but the server has always been up.

[Sharparam]

@kroeckx Your server is responding with 400: Bad Request to federation attempts.

Edit: The latest entry from the dumped logs:

2019-04-29 15:32:44,450 - synapse.http.matrixfederationclient - 472 - WARNING - federation_transaction_transmission_loop-242514- {PUT-O-173776} [roeckx.be] Request failed: PUT matrix://roeckx.be/_matrix/federation/v1/send/1556427433382: HttpResponseException("400: b'Bad Request'",)

[kroeckx]

Looking in my logs, I see lots of: 2019-05-01 21:48:21,978 - synapse.access.https.8448 - 233 - INFO - PUT-4120380- 80.245.199.234 - 8448 - Received request: PUT /_matrix/federation/v1/send/1554208235995 2019-05-01 21:48:21,979 - synapse.http.server - 85 - INFO - PUT-4120380- <SynapseRequest at 0x7f4ad26864a8 method='PUT' uri='/_matrix/federation/v1/send/1554208235995' clientproto='HTTP/1.1' site=8448> SynapseError: 400 - Unrecognized request 2019-05-01 21:48:21,980 - synapse.access.https.8448 - 302 - INFO - PUT-4120380- 80.245.199.234 - 8448 - {None} Processed request: 0.002sec/0.001sec (0.004sec, 0.000sec) (0.000sec/0.000sec/0) 59B 400 "PUT /_matrix/federation/v1/send/1554208235995 HTTP/1.1" "Synapse/0.99.3" [0 dbevts] Why is generating that error, and why doesn't the federation tester give any error?

[Sharparam]

For the record, some discussion related to this in #synapse:matrix.org starting from this message.

Edit: Looks like you might need to update your Synapse instance @kroeckx. You are on 0.99.2 and the latest Synapse is 0.99.3, apparently something regarding federation was changed.

[mguentner]

@Sharparam Aha! My instance was still on 0.99.2 as well but now runs on 0.99.3
It would have been a bad reason to ban my homeserver though ;)

[Sharparam]

@mguentner Yeah, my analysis only looks at the errors logged by Synapse and that doesn't take into account that special case where there is a failed call immediately followed by a successful one. The blacklisting code would have to take this into account somehow. (This actually might resolve itself if the servers are removed from blacklist as soon as a successful call is made or received.)

[richvdh]

The blacklisting code would have to take this into account somehow.

the current backoff code ignores 400s, ftr.