tailcfg: add DNSConfig.ExtraRecords

Updates #1748
Updates #1235
Updates #2055

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -1750,6 +1750,25 @@ func (b *LocalBackend) authReconfig() {
 	for _, peer := range nm.Peers {
 		set(peer.Name, peer.Addresses)
 	}
+	for _, rec := range nm.DNS.ExtraRecords {
+		switch rec.Type {
+		case "", "A", "AAAA":
+			// Treat these all the same for now: infer from the value
+		default:
+			// TODO: more
+			continue
+		}
+		ip, err := netaddr.ParseIP(rec.Value)
+		if err != nil {
+			// Ignore.
+			continue
+		}
+		fqdn, err := dnsname.ToFQDN(rec.Name)
+		if err != nil {
+			continue
+		}
+		dcfg.Hosts[fqdn] = append(dcfg.Hosts[fqdn], ip)
+	}
 
 	if uc.CorpDNS {
 		addDefault := func(resolvers []tailcfg.DNSResolver) {

tailcfg/tailcfg.go

@@ -44,7 +44,8 @@ import (
 //    19: 2021-04-21: MapResponse.Debug.SleepSeconds
 //    20: 2021-06-11: MapResponse.LastSeen used even less (https://github.com/tailscale/tailscale/issues/2107)
 //    21: 2021-06-15: added MapResponse.DNSConfig.CertDomains
-const CurrentMapRequestVersion = 21
+//    22: 2021-06-16: added MapResponse.DNSConfig.ExtraRecords
+const CurrentMapRequestVersion = 22
 
 type StableID string
 
@@ -882,6 +883,28 @@ type DNSConfig struct {
 	// These names are FQDNs without trailing periods, and without
 	// any "_acme-challenge." prefix.
 	CertDomains []string `json:",omitempty"`
+
+	// ExtraRecords contains extra DNS records to add to the
+	// MagicDNS config.
+	ExtraRecords []DNSRecord `json:",omitempty"`
+}
+
+// DNSRecord is an extra DNS record to add to MagicDNS.
+type DNSRecord struct {
+	// Name is the fully qualified domain name of
+	// the record to add. The trailing dot is optional.
+	Name string
+
+	// Type is the DNS record type.
+	// Empty means A or AAAA, depending on value.
+	// Other values are currently ignored.
+	Type string `json:",omitempty"`
+
+	// Value is the IP address in string form.
+	// TODO(bradfitz): if we ever add support for record types
+	// with non-UTF8 binary data, add ValueBytes []byte that
+	// would take precedence.
+	Value string
 }
 
 // PingRequest is a request to send an HTTP request to prove the

tailcfg/tailcfg_clone.go

@@ -205,6 +205,7 @@ func (src *DNSConfig) Clone() *DNSConfig {
 	dst.Domains = append(src.Domains[:0:0], src.Domains...)
 	dst.Nameservers = append(src.Nameservers[:0:0], src.Nameservers...)
 	dst.CertDomains = append(src.CertDomains[:0:0], src.CertDomains...)
+	dst.ExtraRecords = append(src.ExtraRecords[:0:0], src.ExtraRecords...)
 	return dst
 }
 
@@ -219,6 +220,7 @@ var _DNSConfigNeedsRegeneration = DNSConfig(struct {
 	Nameservers       []netaddr.IP
 	PerDomain         bool
 	CertDomains       []string
+	ExtraRecords      []DNSRecord
 }{})
 
 // Clone makes a deep copy of DNSResolver.