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courier: DNS temporary errors should cause temporary delivery failures

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In the SMTP courier, when the MX lookup fails with a DNS temporary
error, we should propagate that up and cause delivery to fail with a
temporary error too.

That way the delivery can be attempted later by the queue.

However, the code today doesn't make the distinction and will treat any
temporary DNS error as a permanent delivery failure, which is a bug.

The bug was found by ludusrusso and reported in
https://github.com/albertito/chasquid/issues/4

This patch fixes the bug by propagating the error properly.
This commit is contained in:
Alberto Bertogli
2020-03-12 22:04:24 +00:00
parent 6641d858ad
commit 8fab350b59
2 changed files with 27 additions and 12 deletions
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14
internal/courier/smtp.go
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@@ -68,13 +68,13 @@ func (s *SMTP) Deliver(from string, to string, data []byte) (error, bool) {
a.from = ""
}
mxs, err := lookupMXs(a.tr, a.toDomain)
mxs, err, perm := lookupMXs(a.tr, a.toDomain)
if err != nil || len(mxs) == 0 {
// Note this is considered a permanent error.
// This is in line with what other servers (Exim) do. However, the
// downside is that temporary DNS issues can affect delivery, so we
// have to make sure we try hard enough on the lookup above.
return a.tr.Errorf("Could not find mail server: %v", err), true
return a.tr.Errorf("Could not find mail server: %v", err), perm
}
// Issue an EHLO with a valid domain; otherwise, some servers like postfix
@@ -245,10 +245,10 @@ func (s *SMTP) fetchSTSPolicy(tr *trace.Trace, domain string) *sts.Policy {
return policy
}
func lookupMXs(tr *trace.Trace, domain string) ([]string, error) {
func lookupMXs(tr *trace.Trace, domain string) ([]string, error, bool) {
domain, err := idna.ToASCII(domain)
if err != nil {
return nil, err
return nil, err, true
}
mxs := []string{}
@@ -264,10 +264,10 @@ func lookupMXs(tr *trace.Trace, domain string) ([]string, error) {
dnsErr, ok := err.(*net.DNSError)
if !ok {
tr.Debugf("MX lookup error: %v", err)
return nil, err
return nil, err, true
} else if dnsErr.Temporary() {
tr.Debugf("temporary DNS error: %v", dnsErr)
return nil, err
return nil, err, false
}
// Permanent error, we assume MX does not exist and fall back to A.
@@ -292,5 +292,5 @@ func lookupMXs(tr *trace.Trace, domain string) ([]string, error) {
}
tr.Debugf("MXs: %v", mxs)
return mxs, nil
return mxs, nil, true
}