Add missing MD5 support (#1866)

docs/_providers/axfrddns.md

@@ -211,3 +211,7 @@ When AutoDNSSEC is enabled, the AXFR+DDNS provider will emit a warning when no R
 When AutoDNSSEC is disabled, the AXFR+DDNS provider will emit a warning when RRSIG, DNSKEY or NSEC records are found in the zone.
 
 When AutoDNSSEC is not enabled or disabled, no checking is done.
+
+## FYI: MD5 Support
+
+By default the used DNS Go package by miekg has deprecated supporting the (insecure) MD5 algorithm [https://github.com/miekg/dns/commit/93945c284489394b77653323d11d5de83a2a6fb5](Link). Some providers like the Leibniz Supercomputing Centre (LRZ) located in Munich still use this algorithm to authenticate internal dynamic DNS updates. To compensate the lack of MD5 a custom MD5 TSIG Provider was added into DNSControl.  

providers/axfrddns/axfrddnsProvider.go

@@ -232,6 +232,9 @@ func (c *axfrddnsProvider) FetchZoneRecords(domain string) ([]dns.RR, error) {
 		transfer.TsigSecret =
 			map[string]string{c.transferKey.id: c.transferKey.secret}
 		request.SetTsig(c.transferKey.id, c.transferKey.algo, 300, time.Now().Unix())
+		if c.transferKey.algo == dns.HmacMD5 {
+			transfer.TsigProvider = md5Provider(c.transferKey.secret)
+		}
 	}
 
 	envelope, err := transfer.In(request, c.master)
@@ -428,6 +431,9 @@ func (c *axfrddnsProvider) GetDomainCorrections(dc *models.DomainConfig) ([]*mod
 							client.TsigSecret =
 								map[string]string{c.updateKey.id: c.updateKey.secret}
 							update.SetTsig(c.updateKey.id, c.updateKey.algo, 300, time.Now().Unix())
+							if c.updateKey.algo == dns.HmacMD5 {
+								client.TsigProvider = md5Provider(c.updateKey.secret)
+							}
 						}
 
 						msg, _, err := client.Exchange(update, c.master)

providers/axfrddns/md5Provider.go

@@ -0,0 +1,46 @@
+package axfrddns
+
+import (
+	"crypto/hmac"
+	"crypto/md5" //#nosec
+	"encoding/base64"
+	"encoding/hex"
+
+	"github.com/miekg/dns"
+)
+
+type md5Provider string
+
+func fromBase64(s []byte) (buf []byte, err error) {
+	buflen := base64.StdEncoding.DecodedLen(len(s))
+	buf = make([]byte, buflen)
+	n, err := base64.StdEncoding.Decode(buf, s)
+	buf = buf[:n]
+	return
+}
+
+func (key md5Provider) Generate(msg []byte, _ *dns.TSIG) ([]byte, error) {
+	rawsecret, err := fromBase64([]byte(key))
+	if err != nil {
+		return nil, err
+	}
+	h := hmac.New(md5.New, rawsecret)
+
+	h.Write(msg)
+	return h.Sum(nil), nil
+}
+
+func (key md5Provider) Verify(msg []byte, t *dns.TSIG) error {
+	b, err := key.Generate(msg, t)
+	if err != nil {
+		return err
+	}
+	mac, err := hex.DecodeString(t.MAC)
+	if err != nil {
+		return err
+	}
+	if !hmac.Equal(b, mac) {
+		return dns.ErrSig
+	}
+	return nil
+}