net/dns/resolver: return symbolic PTR records for TS service IPs (#4361)

Fixes #1233

Signed-off-by: Tom DNetto <tom@tailscale.com>

Co-authored-by: Tom DNetto <tom@tailscale.com>

net/dns/resolver/tsdns.go

@@ -34,6 +34,8 @@ import (
 	"tailscale.com/wgengine/monitor"
 )
 
+const dnsSymbolicFQDN = "magicdns.localhost-tailscale-daemon."
+
 // maxResponseBytes is the maximum size of a response from a Resolver. The
 // actual buffer size will be one larger than this so that we can detect
 // truncation in a platform-agnostic way.
@@ -553,6 +555,18 @@ func (r *Resolver) resolveLocal(domain dnsname.FQDN, typ dns.Type) (netaddr.IP,
 		return netaddr.IP{}, dns.RCodeNameError
 	}
 
+	// We return a symbolic domain if someone does a reverse lookup on the
+	// DNS endpoint. To round out this special case, we also do the inverse
+	// (returning the endpoint IP if someone looks up the symbolic domain).
+	if domain == dnsSymbolicFQDN {
+		switch typ {
+		case dns.TypeA:
+			return tsaddr.TailscaleServiceIP(), dns.RCodeSuccess
+		case dns.TypeAAAA:
+			return tsaddr.TailscaleServiceIPv6(), dns.RCodeSuccess
+		}
+	}
+
 	r.mu.Lock()
 	hosts := r.hostToIP
 	localDomains := r.localDomains
@@ -644,6 +658,14 @@ func (r *Resolver) resolveLocalReverse(name dnsname.FQDN) (dnsname.FQDN, dns.RCo
 		return "", dns.RCodeRefused
 	}
 
+	// If someone curiously does a reverse lookup on the DNS IP, we
+	// return a domain that helps indicate that Tailscale is using
+	// this IP for a special purpose and it is not a node on their
+	// tailnet.
+	if ip == tsaddr.TailscaleServiceIP() || ip == tsaddr.TailscaleServiceIPv6() {
+		return dnsSymbolicFQDN, dns.RCodeSuccess
+	}
+
 	r.mu.Lock()
 	defer r.mu.Unlock()
 	ret, ok := r.ipToHost[ip]

net/dns/resolver/tsdns_test.go

@@ -344,6 +344,7 @@ func TestResolveLocal(t *testing.T) {
 		{"mx-nxdomain", "test3.ipn.dev.", dns.TypeMX, netaddr.IP{}, dns.RCodeNameError},
 		{"ns-nxdomain", "test3.ipn.dev.", dns.TypeNS, netaddr.IP{}, dns.RCodeNameError},
 		{"onion-domain", "footest.onion.", dns.TypeA, netaddr.IP{}, dns.RCodeNameError},
+		{"magicdns", dnsSymbolicFQDN, dns.TypeA, netaddr.MustParseIP("100.100.100.100"), dns.RCodeSuccess},
 	}
 
 	for _, tt := range tests {
@@ -377,6 +378,7 @@ func TestResolveLocalReverse(t *testing.T) {
 		{"ipv4_nxdomain", dnsname.FQDN("5.3.2.1.in-addr.arpa."), "", dns.RCodeNameError},
 		{"ipv6_nxdomain", dnsname.FQDN("0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.0.0.ip6.arpa."), "", dns.RCodeNameError},
 		{"nxdomain", dnsname.FQDN("2.3.4.5.in-addr.arpa."), "", dns.RCodeRefused},
+		{"magicdns", dnsname.FQDN("100.100.100.100.in-addr.arpa."), dnsSymbolicFQDN, dns.RCodeSuccess},
 	}
 
 	for _, tt := range tests {