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courier: SMTP falls back to A when MX does not exist

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Make the SMTP courier fall back to the A record when MX does not exist, as per
standard behaviour.

This is not implemented nicely, because Go's API does not give a clear signal
if the answer was that there are no MX records or something else happens.
For now, we implement it with a heuristic that should work pretty reliably,
but it's definitely not very nice.
This commit is contained in:
Alberto Bertogli
2016-10-01 21:55:47 +01:00
parent fdaca0bedf
commit 7fa40397c5
2 changed files with 23 additions and 17 deletions
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37
internal/courier/smtp.go
View File

@@ -23,10 +23,6 @@ var (
smtpPort = flag.String("testing__outgoing_smtp_port", "25",
"port to use for outgoing SMTP connections, ONLY FOR TESTING")
// Bypass the MX lookup, for testing purposes.
bypassMX = flag.Bool("testing__bypass_mx_lookup", false,
"bypass MX lookup, ONLY FOR TESTING")
// Fake MX records, used for testing only.
fakeMX = map[string]string{}
)
@@ -40,7 +36,6 @@ func (s *SMTP) Deliver(from string, to string, data []byte) (error, bool) {
defer tr.Finish()
tr.LazyPrintf("%s -> %s", from, to)
// TODO: Fall back to A if MX is not available.
mx, err := lookupMX(envelope.DomainOf(to))
if err != nil {
// Note this is considered a permanent error.
@@ -142,17 +137,29 @@ func lookupMX(domain string) (string, error) {
return v, nil
}
if *bypassMX {
return domain, nil
}
mxs, err := net.LookupMX(domain)
if err != nil {
return "", err
} else if len(mxs) == 0 {
glog.Infof("domain %q has no MX, falling back to A", domain)
return domain, nil
if err == nil {
if len(mxs) == 0 {
glog.Infof("domain %q has no MX, falling back to A", domain)
return domain, nil
}
return mxs[0].Host, nil
}
return mxs[0].Host, nil
// There was an error. It could be that the domain has no MX, in which
// case we have to fall back to A, or a bigger problem.
// Unfortunately, go's API doesn't let us easily distinguish between them.
// For now, if the error is permanent, we assume it's because there was no
// MX and fall back, otherwise we return.
// TODO: Find a better way to do this.
dnsErr, ok := err.(*net.DNSError)
if !ok {
return "", err
} else if dnsErr.Temporary() {
return "", err
}
// Permanent error, we assume MX does not exist and fall back to A.
return domain, nil
}

3
test/t-02-exim/run.sh
View File

@@ -46,8 +46,7 @@ add_user srv-chasquid someone secretpassword
# Bypass MX lookup, so it can find srv-exim (via our host alias).
mkdir -p .logs
chasquid -v=2 --log_dir=.logs --config_dir=config \
--testing__outgoing_smtp_port=2025 \
--testing__bypass_mx_lookup &
--testing__outgoing_smtp_port=2025 &
wait_until_ready 1025