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courier: Use DNSError.IsNotFound to identify NXDOMAIN

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When resolving MX records, we need to distinguish between "no such
domain" and other kinds of errors. Before Go 1.13, this was not
possible, so we had a workaround that assumed any permanent error was a
"no such domain", which is not great, but functional.

Now that our minimum supported version is Go 1.15, we can remove the
workaround.

This patch replaces the workaround with proper logic using
DNSError.IsNotFound to identify NXDOMAIN results when resolving MX
records.

This requires to adjust a few tests, that used to work on environments
where resolving unknown domains (used for testing) returned a permanent
error, and now they no longer do so. Instead of relying on this
environmental property, we make the affected tests use our own DNS
server, which should make them more hermetic and reproducible.
This commit is contained in:
Alberto Bertogli
2021-10-08 23:03:49 +01:00
parent 6633f0785c
commit ed38945fca
10 changed files with 58 additions and 17 deletions
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19
internal/courier/smtp.go
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@@ -249,22 +249,19 @@ func lookupMXs(tr *trace.Trace, domain string) ([]string, error, bool) {
if err != 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: Use dnsErr.IsNotFound once we can use Go >= 1.13.
dnsErr, ok := err.(*net.DNSError)
if !ok {
tr.Debugf("MX lookup error: %v", err)
return nil, err, true
} else if dnsErr.Temporary() {
tr.Debugf("temporary DNS error: %v", dnsErr)
tr.Debugf("Error resolving MX on %q: %v", domain, err)
return nil, err, false
} else if dnsErr.IsNotFound {
// MX not found, fall back to A.
tr.Debugf("MX for %s not found, falling back to A", domain)
mxs = []string{domain}
} else {
tr.Debugf("MX lookup error on %q: %v", domain, dnsErr)
return nil, err, !dnsErr.Temporary()
}
// Permanent error, we assume MX does not exist and fall back to A.
tr.Debugf("failed to resolve MX for %s, falling back to A", domain)
mxs = []string{domain}
} else {
// Convert the DNS records to a plain string slice. They're already
// sorted by priority.