queue: retry with another IP when first attempt fails for dualstack remote servers #152
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@mjl- in the end, I reused the retry mechanism already present in the code for errors received at the start of the connexion. No changes in the configuration are needed. I don't know if it's better, or not, compared to https://github.com/mjl-/mox/issues/149#issuecomment-2041007381 I'm also wondering how to write a test for this commit. Any idea will be appreciated. |
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this change also seems useful, or did you run into problems with this? i can imagine that it's better to just not attempt delivering over ipv6 if it can cause trouble. but a mail server can connect dualstack, we can retry for these failures both during EHLO, and when it happens later.
I'm also wondering how to write a test for this commit. Any idea will be appreciated.
this isn't too easy indeed. i think the approach would be to write a test like those in TestQueue where you create a fake smtp server that expects two connections, that responds with a 5xx 7.x.x on first invocation, and expects another connection attempt which completes the smtp transaction successfully. the smtpclient.DialHook could store which address family was dialed, to check if both ipv4 and ipv6 was tried. a mock resolver that resolves to both ipv4 and ipv6 would be needed.
we currently also don't have a check if the retry. would be good to have. the TestQueue code is a bit read-only... if you want to give a try, it may be easier to copy some of that function into a new one: the various cases are connected, and they may require quite a refactor to fit another new case in...
…emote servers mox was already giving another try for received errors after the `HELO`/`EHLO` command. Now mox do the same for received errors when trying to deliver the message to the remote SMTP server. This should help to deliver messages to SMTP server that rejects incoming messages because of bad ipv4 or ipv6 configuration (for example for servers checking reverse DNS records). mox will now try to deliver messages on both ip family instead before considering the error as permanent. fix #149
I didn't encounter any problems during my tests with this PR. But, I would prefer to use the other PR for these reasons:
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this change didn't look too dangerous to me wrt changing existing behaviour. and it will be useful to also detect this error code during smtp rcpt to in case of dns block listings of ipv6 vs ipv4, so i merged it, thanks! |
mox was already giving another try for received errors after the
HELO/EHLOcommand. Now mox do the same for received errors when trying to deliver the message to the remote SMTP server.This should help to deliver messages to SMTP server that rejects incoming messages because of bad ipv4 or ipv6 configuration (for example for servers checking reverse DNS records). mox will now try to deliver messages on both ip family instead before considering the error as permanent.
fix #149