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sts: Don't pre-filter MX list, but skip them if needed

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Instead of pre-filtering the MX list based on STS policy, just check
if it's allowed before each attempt, and skip it if not.

This simplifies the code.
This commit is contained in:
Alberto Bertogli
2018-05-27 10:43:46 +01:00
parent 252ab5d3e3
commit 8bf584bd86
2 changed files with 17 additions and 35 deletions
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46
internal/courier/smtp.go
View File

@@ -73,9 +73,7 @@ func (s *SMTP) Deliver(from string, to string, data []byte) (error, bool) {
a.from = ""
}
a.stsPolicy = s.fetchSTSPolicy(a.tr, a.toDomain)
mxs, err := lookupMXs(a.tr, a.toDomain, a.stsPolicy)
mxs, err := 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
@@ -95,7 +93,14 @@ func (s *SMTP) Deliver(from string, to string, data []byte) (error, bool) {
a.helloDomain, _ = os.Hostname()
}
a.stsPolicy = s.fetchSTSPolicy(a.tr, a.toDomain)
for _, mx := range mxs {
if a.stsPolicy != nil && !a.stsPolicy.MXIsAllowed(mx) {
a.tr.Printf("%q skipped as per MTA-STA policy", mx)
continue
}
var permanent bool
err, permanent = a.deliver(mx)
if err == nil {
@@ -248,7 +253,7 @@ func (s *SMTP) fetchSTSPolicy(tr *trace.Trace, domain string) *sts.Policy {
return policy
}
func lookupMXs(tr *trace.Trace, domain string, policy *sts.Policy) ([]string, error) {
func lookupMXs(tr *trace.Trace, domain string) ([]string, error) {
domain, err := idna.ToASCII(domain)
if err != nil {
return nil, err
@@ -288,39 +293,12 @@ func lookupMXs(tr *trace.Trace, domain string, policy *sts.Policy) ([]string, er
// This case is explicitly covered by the SMTP RFC.
// https://tools.ietf.org/html/rfc5321#section-5.1
mxs = filterMXs(tr, policy, mxs)
if len(mxs) == 0 {
tr.Errorf("domain %q has no valid MX/A record", domain)
} else if len(mxs) > 5 {
// Cap the list of MXs to 5 hosts, to keep delivery attempt times
// sane and prevent abuse.
// Cap the list of MXs to 5 hosts, to keep delivery attempt times
// sane and prevent abuse.
if len(mxs) > 5 {
mxs = mxs[:5]
}
tr.Debugf("MXs: %v", mxs)
return mxs, nil
}
func filterMXs(tr *trace.Trace, p *sts.Policy, mxs []string) []string {
if p == nil {
return mxs
}
filtered := []string{}
for _, mx := range mxs {
if p.MXIsAllowed(mx) {
filtered = append(filtered, mx)
} else {
tr.Printf("MX %q not allowed by policy, skipping", mx)
}
}
// We don't want to return an empty set if the mode is not enforce.
// This prevents failures for policies in reporting mode.
// https://tools.ietf.org/html/draft-ietf-uta-mta-sts-18#section-5.1
if len(filtered) == 0 && p.Mode != sts.Enforce {
filtered = mxs
}
return filtered
}

6
internal/sts/sts.go
View File

@@ -140,9 +140,13 @@ func (p *Policy) Check() error {
return nil
}
// MXMatches checks if the given MX is allowed, according to the policy.
// MXIsAllowed checks if the given MX is allowed, according to the policy.
// https://tools.ietf.org/html/draft-ietf-uta-mta-sts-18#section-4.1
func (p *Policy) MXIsAllowed(mx string) bool {
if p.Mode != Enforce {
return true
}
for _, pattern := range p.MXs {
if matchDomain(mx, pattern) {
return true