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smtpsrv: Disable TLS session tickets to work around Microsoft problems

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Microsoft SMTP servers have a bug that prevents them from successfully
establishing a TLS connection against modern Go TLS servers, and some
OpenSSL versions. It also doesn't fall back to plain-text, so this has
been causing deliverablity issues.

The problem started by the end of 2024 and it's still not fixed.

Unfortunately, because they're quite a big provider and are not fixing
their problem, it is worth to do a server-side workaround.

This patch implements that workaround: it disables TLS session tickets.

There is no security impact for doing so, and there is a small
performance penalty which is likely to be insignificant for chasquid's
main use cases.

This workaround should be removed once Microsoft fixes their problem.

We are going to make a 1.15.1 release for this, which this patch also
documents.

Thanks to Michael (l6d-dev@github) for reporting this issue and
suggesting this workaround!

See https://github.com/albertito/chasquid/issues/64 and
https://github.com/golang/go/issues/70232 for more details.
This commit is contained in:
Alberto Bertogli
2025-03-29 23:21:06 +00:00
parent deb2a2f22b
commit 48fe124c47
2 changed files with 32 additions and 3 deletions
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10
docs/relnotes.md
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@@ -11,6 +11,16 @@ noting backward-incompatible changes or known security issues.
- Log how many things were loaded for each domain.
- Add fail2ban filter configuration example.
### 1.15.1 (2025-03-30)
Implement a workaround for a Microsoft bug in TLS session ticket handling,
that is causing deliverability issues, and they are being too slow at fixing.
See this [chasquid issue](https://github.com/albertito/chasquid/issues/64),
this [Go issue](https://github.com/golang/go/issues/70232) and this
[Postfix thread](https://www.mail-archive.com/postfix-users@postfix.org/msg104308.html)
for more details.
## 1.14.0 (2024-04-21)

21
internal/smtpsrv/server.go
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@@ -96,7 +96,26 @@ func NewServer() *Server {
return &Server{
addrs: map[SocketMode][]string{},
listeners: map[SocketMode][]net.Listener{},
tlsConfig: &tls.Config{},
// Disable session tickets for now, to workaround a Microsoft bug
// causing deliverability issues.
//
// See https://github.com/golang/go/issues/70232 for more details.
//
// This doesn't impact security, it just makes the re-establishment of
// TLS sessions a bit slower, but for a server like chasquid it's not
// going to be significant.
//
// Note this is not a Go-specific problem, and affects other servers
// too (like Postfix/OpenSSL). This is a Microsoft problem that they
// need to fix. Unfortunately, because they're quite a big provider
// and are not very responsive in fixing their problems, we have to do
// a workaround here.
// TODO: Remove this once Microsoft fixes their servers.
tlsConfig: &tls.Config{
SessionTicketsDisabled: true,
},
connTimeout: 20 * time.Minute,
commandTimeout: 1 * time.Minute,
localDomains: &set.String{},