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aliases: Implement "via" aliases

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This patch implements "via" aliases, which let us explicitly select a
server to use for delivery.

This feature is useful in different scenarios, such as a secondary MX
server that forwards all incoming email to a primary.

For now, it is experimental and the syntax and semantics are subject to
change.
This commit is contained in:
Alberto Bertogli
2025-04-06 12:35:51 +01:00
parent 1cf24ba94a
commit 9999a69086
28 changed files with 671 additions and 74 deletions
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54
internal/queue/queue.go
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@@ -191,6 +191,9 @@ func (q *Queue) Put(tr *trace.Trace, from string, to []string, data []byte) (str
r.Type = Recipient_EMAIL
case aliases.PIPE:
r.Type = Recipient_PIPE
case aliases.FORWARD:
r.Type = Recipient_FORWARD
r.Via = aliasRcpt.Via
default:
log.Errorf("unknown alias type %v when resolving %q",
aliasRcpt.Type, t)
@@ -387,6 +390,21 @@ func (item *Item) deliver(q *Queue, rcpt *Recipient) (err error, permanent bool)
return cmd.Run(), true
}
// Recipient type is FORWARD: we always use the remote courier, and pass
// the list of servers that was given to us.
if rcpt.Type == Recipient_FORWARD {
deliverAttempts.Add("forward", 1)
// When forwarding with an explicit list of servers, we use SRS if
// we're sending from a non-local domain (regardless of the
// destination).
from := item.From
if !envelope.DomainIn(item.From, q.localDomains) {
from = rewriteSender(item.From, rcpt.OriginalAddress)
}
return q.remoteC.Forward(from, rcpt.Address, item.Data, rcpt.Via)
}
// Recipient type is EMAIL.
if envelope.DomainIn(rcpt.Address, q.localDomains) {
deliverAttempts.Add("email:local", 1)
@@ -396,24 +414,32 @@ func (item *Item) deliver(q *Queue, rcpt *Recipient) (err error, permanent bool)
deliverAttempts.Add("email:remote", 1)
from := item.From
if !envelope.DomainIn(item.From, q.localDomains) {
// We're sending from a non-local to a non-local. This should
// happen only when there's an alias to forward email to a
// non-local domain. In this case, using the original From is
// problematic, as we may not be an authorized sender for this.
// Some MTAs (like Exim) will do it anyway, others (like
// gmail) will construct a special address based on the
// original address. We go with the latter.
// Note this assumes "+" is an alias suffix separator.
// We use the IDNA version of the domain if possible, because
// we can't know if the other side will support SMTPUTF8.
from = fmt.Sprintf("%s+fwd_from=%s@%s",
envelope.UserOf(rcpt.OriginalAddress),
strings.Replace(from, "@", "=", -1),
mustIDNAToASCII(envelope.DomainOf(rcpt.OriginalAddress)))
// We're sending from a non-local to a non-local, need to do SRS.
from = rewriteSender(item.From, rcpt.OriginalAddress)
}
return q.remoteC.Deliver(from, rcpt.Address, item.Data)
}
func rewriteSender(from, originalAddr string) string {
// Apply a send-only Sender Rewriting Scheme (SRS).
// This is used when we are sending from a (potentially) non-local domain,
// to a non-local domain.
// This should happen only when there's an alias to forward email to a
// non-local domain (either a normal "email" alias with a remote
// destination, or a "forward" alias with a list of servers).
// In this case, using the original From is problematic, as we may not be
// an authorized sender for this.
// To do this, we use a sender rewriting scheme, similar to what other
// MTAs do (e.g. gmail or postfix).
// Note this assumes "+" is an alias suffix separator.
// We use the IDNA version of the domain if possible, because
// we can't know if the other side will support SMTPUTF8.
return fmt.Sprintf("%s+fwd_from=%s@%s",
envelope.UserOf(originalAddr),
strings.Replace(from, "@", "=", -1),
mustIDNAToASCII(envelope.DomainOf(originalAddr)))
}
// countRcpt counts how many recipients are in the given status.
func (item *Item) countRcpt(statuses ...Recipient_Status) int {
c := 0