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queue: Use a local envelope-from when forwarding

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If there's an alias to forward email to a non-local domain, using the original
From is problematic, as we may not be an authorized sender for it.

Some MTAs (like Exim) will do it anyway, others (like gmail) will construct a
special address based on the original address.

This patch implements the latter approach, which is safer and allows the
receiver to properly enforce SPF.

We construct a (hopefully) reasonable From based on the local user, and
embedding the original From (but transformed for IDNA, as the receiver may not
support SMTPUTF8).
This commit is contained in:
Alberto Bertogli
2016-10-03 00:49:32 +01:00
parent afd79dfd8d
commit 55b03c8cf0
6 changed files with 83 additions and 41 deletions
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33
internal/queue/queue.go
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@@ -28,6 +28,7 @@ import (
"github.com/golang/glog"
"github.com/golang/protobuf/ptypes"
"github.com/golang/protobuf/ptypes/timestamp"
"golang.org/x/net/idna"
"golang.org/x/net/trace"
)
@@ -170,8 +171,9 @@ func (q *Queue) Put(hostname, from string, to []string, data []byte) (string, er
// not very pretty but at least it's self contained.
for _, aliasRcpt := range rcpts {
r := &Recipient{
Address: aliasRcpt.Addr,
Status: Recipient_PENDING,
Address: aliasRcpt.Addr,
Status: Recipient_PENDING,
OriginalAddress: t,
}
switch aliasRcpt.Type {
case aliases.EMAIL:
@@ -397,7 +399,24 @@ func (item *Item) deliver(q *Queue, rcpt *Recipient) (err error, permanent bool)
if envelope.DomainIn(rcpt.Address, q.localDomains) {
return q.localC.Deliver(item.From, rcpt.Address, item.Data)
} else {
return q.remoteC.Deliver(item.From, rcpt.Address, item.Data)
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)))
}
return q.remoteC.Deliver(from, rcpt.Address, item.Data)
}
}
}
@@ -420,3 +439,11 @@ func timestampNow() *timestamp.Timestamp {
ts, _ := ptypes.TimestampProto(now)
return ts
}
func mustIDNAToASCII(s string) string {
a, err := idna.ToASCII(s)
if err != nil {
return a
}
return s
}