Add simple support for CNAMEs as DNS aliases (#7)

- supports create/read/destroy and DNS lookup of CNAME record for hosts
- supports one level of "aliasing" by resolving CNAME in mock DNS server

README.md

@@ -8,7 +8,9 @@
 The `challtestsrv` package offers a library/command that can be used by test
 code to respond to HTTP-01, DNS-01, and TLS-ALPN-01 ACME challenges. The
 `challtestsrv` package can also be used as a mock DNS server letting
-developers mock `A`, `AAAA`, and `CAA` DNS data for specific hostnames.
+developers mock `A`, `AAAA`, `CNAME`, and `CAA` DNS data for specific hostnames.
+The mock server will resolve up to one level of `CNAME` aliasing for accepted
+DNS request types.
 
 **Important note: The `challtestsrv` command and library are for TEST USAGE
 ONLY. It is trivially insecure, offering no authentication. Only use

challenge-servers.go

@@ -74,6 +74,8 @@ type mockDNSData struct {
 	aaaaRecords map[string][]string
 	// A map of host to CAA policies for CAA responses.
 	caaRecords map[string][]MockCAAPolicy
+	// A map of host to CNAME records.
+	cnameRecords map[string]string
 }
 
 // MockCAAPolicy holds a tag and a value for a CAA record. See
@@ -131,11 +133,12 @@ func New(config Config) (*ChallSrv, error) {
 		tlsALPNOne:     make(map[string]string),
 		redirects:      make(map[string]string),
 		dnsMocks: mockDNSData{
-			defaultIPv4: defaultIPv4,
-			defaultIPv6: defaultIPv6,
-			aRecords:    make(map[string][]string),
-			aaaaRecords: make(map[string][]string),
-			caaRecords:  make(map[string][]MockCAAPolicy),
+			defaultIPv4:  defaultIPv4,
+			defaultIPv6:  defaultIPv6,
+			aRecords:     make(map[string][]string),
+			aaaaRecords:  make(map[string][]string),
+			caaRecords:   make(map[string][]MockCAAPolicy),
+			cnameRecords: make(map[string]string),
 		},
 	}
 

dns.go

@@ -28,6 +28,28 @@ func mockSOA() *dns.SOA {
 // more RRs for the response.
 type dnsAnswerFunc func(question dns.Question) []dns.RR
 
+// cnameAnswers is a dnsAnswerFunc that creates CNAME RR's for the given question
+// using the ChallSrv's dns mock data. If there is no mock CNAME data for the
+// given hostname in the question no RR's will be returned.
+func (s *ChallSrv) cnameAnswers(q dns.Question) []dns.RR {
+	var records []dns.RR
+
+	if value := s.GetDNSCNAMERecord(q.Name); value != "" {
+		record := &dns.CNAME{
+			Hdr: dns.RR_Header{
+				Name:   q.Name,
+				Rrtype: dns.TypeCNAME,
+				Class:  dns.ClassINET,
+			},
+			Target: value,
+		}
+
+		records = append(records, record)
+	}
+
+	return records
+}
+
 // txtAnswers is a dnsAnswerFunc that creates TXT RR's for the given question
 // using the ChallSrv's dns mock data. If there is no mock TXT data for the
 // given hostname in the question no RR's will be returned.
@@ -133,8 +155,10 @@ func (s *ChallSrv) caaAnswers(q dns.Question) []dns.RR {
 }
 
 // dnsHandler is a miekg/dns handler that can process a dns.Msg request and
-// write a response to the provided dns.ResponseWriter. TXT, A, AAAA, and CAA
-// queries types are supported and answered using the ChallSrv's mock DNS data.
+// write a response to the provided dns.ResponseWriter. TXT, A, AAAA, CNAME,
+// and CAA queries types are supported and answered using the ChallSrv's mock
+// DNS data. A host that is aliased by a CNAME record will follow that alias
+// one level and return the requested record types for that alias' target
 func (s *ChallSrv) dnsHandler(w dns.ResponseWriter, r *dns.Msg) {
 	m := new(dns.Msg)
 	m.SetReply(r)
@@ -146,8 +170,19 @@ func (s *ChallSrv) dnsHandler(w dns.ResponseWriter, r *dns.Msg) {
 			Question: q,
 		})
 
+		// If a CNAME exists for the question include the CNAME record and modify
+		// the question to instead lookup based on that CNAME's target
+		if cname := s.GetDNSCNAMERecord(q.Name); cname != "" {
+			cnameRecords := s.cnameAnswers(q)
+			m.Answer = append(m.Answer, cnameRecords...)
+
+			q = dns.Question{Name: cname, Qtype: q.Qtype}
+		}
+
 		var answerFunc dnsAnswerFunc
 		switch q.Qtype {
+		case dns.TypeCNAME:
+			answerFunc = s.cnameAnswers
 		case dns.TypeTXT:
 			answerFunc = s.txtAnswers
 		case dns.TypeA:

mockdns.go

@@ -38,6 +38,33 @@ func (s *ChallSrv) GetDefaultDNSIPv6() string {
 	return s.dnsMocks.defaultIPv6
 }
 
+// AddDNSCNAMERecord sets a CNAME record that will be used like an alias when
+// querying for other DNS records for the given host.
+func (s *ChallSrv) AddDNSCNAMERecord(host string, value string) {
+	s.challMu.Lock()
+	defer s.challMu.Unlock()
+	host = dns.Fqdn(host)
+	value = dns.Fqdn(value)
+	s.dnsMocks.cnameRecords[host] = value
+}
+
+// GetDNSCNAMERecord returns a target host if a CNAME is set for the querying
+// host and an empty string otherwise.
+func (s *ChallSrv) GetDNSCNAMERecord(host string) string {
+	s.challMu.RLock()
+	host = dns.Fqdn(host)
+	defer s.challMu.RUnlock()
+	return s.dnsMocks.cnameRecords[host]
+}
+
+// DeleteDNSCAMERecord deletes any CNAME alias set for the given host.
+func (s *ChallSrv) DeleteDNSCNAMERecord(host string) {
+	s.challMu.Lock()
+	defer s.challMu.Unlock()
+	host = dns.Fqdn(host)
+	delete(s.dnsMocks.cnameRecords, host)
+}
+
 // AddDNSARecord adds IPv4 addresses that will be returned when querying for
 // A records for the given host.
 func (s *ChallSrv) AddDNSARecord(host string, addresses []string) {