From 30550fc539a6ff2cb7de56a0180f5ab396107933 Mon Sep 17 00:00:00 2001
From: David Crawshaw <crawshaw@tailscale.com>
Date: Sat, 12 Jun 2021 08:44:02 -0700
Subject: [PATCH] net/dns/resolver: have quad100 respond to suffix-less
 magicdns requests

This is an attempt to fix WSL2.

Unfortunately, it doesn't work in our new fancy split-dns mode.
If we went back to overriding all DNS on windows we could fix this.
---
 net/dns/resolver/tsdns.go      | 22 ++++++++++++++++++++--
 net/dns/resolver/tsdns_test.go | 10 +++++++---
 2 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/net/dns/resolver/tsdns.go b/net/dns/resolver/tsdns.go
index 105efa720..b7f9076a8 100644
--- a/net/dns/resolver/tsdns.go
+++ b/net/dns/resolver/tsdns.go
@@ -236,8 +236,26 @@ func (r *Resolver) resolveLocal(domain dnsname.FQDN, typ dns.Type) (netaddr.IP,
 				return netaddr.IP{}, dns.RCodeNameError
 			}
 		}
-		// Not authoritative, signal that forwarding is advisable.
-		return netaddr.IP{}, dns.RCodeRefused
+		if strings.IndexByte(string(domain), '.') != len(domain)-1 {
+			// This is a real domain lookup.
+			// Not authoritative, signal that forwarding is advisable.
+			return netaddr.IP{}, dns.RCodeRefused
+		}
+		// Consider this as a MagicDNS query put directly to us.
+		var magicDNSDomain dnsname.FQDN
+		for _, suffix := range localDomains {
+			if dnsname.FQDN("tailscale.net.").Contains(suffix) || dnsname.FQDN("ts.net.").Contains(suffix) {
+				magicDNSDomain = suffix
+				break
+			}
+		}
+		if magicDNSDomain != "" {
+			addrs, found = hosts[domain+magicDNSDomain]
+		}
+		if !found {
+			// Not authoritative, signal that forwarding is advisable.
+			return netaddr.IP{}, dns.RCodeRefused
+		}
 	}
 
 	// Refactoring note: this must happen after we check suffixes,
diff --git a/net/dns/resolver/tsdns_test.go b/net/dns/resolver/tsdns_test.go
index 8efe02d1e..7da6287d2 100644
--- a/net/dns/resolver/tsdns_test.go
+++ b/net/dns/resolver/tsdns_test.go
@@ -20,13 +20,15 @@
 
 var testipv4 = netaddr.MustParseIP("1.2.3.4")
 var testipv6 = netaddr.MustParseIP("0001:0203:0405:0607:0809:0a0b:0c0d:0e0f")
+var test3ipv4 = netaddr.MustParseIP("1.2.4.5")
 
 var dnsCfg = Config{
 	Hosts: map[dnsname.FQDN][]netaddr.IP{
-		"test1.ipn.dev.": []netaddr.IP{testipv4},
-		"test2.ipn.dev.": []netaddr.IP{testipv6},
+		"test1.ipn.dev.":          []netaddr.IP{testipv4},
+		"test2.ipn.dev.":          []netaddr.IP{testipv6},
+		"test3.mytailnet.ts.net.": []netaddr.IP{test3ipv4},
 	},
-	LocalDomains: []dnsname.FQDN{"ipn.dev."},
+	LocalDomains: []dnsname.FQDN{"mytailnet.ts.net.", "ipn.dev."},
 }
 
 func dnspacket(domain dnsname.FQDN, tp dns.Type) []byte {
@@ -234,6 +236,8 @@ 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},
+		{"magic", "test3.", dns.TypeA, test3ipv4, dns.RCodeSuccess},
+		{"nomagic", "test1.", dns.TypeA, netaddr.IP{}, dns.RCodeRefused},
 	}
 
 	for _, tt := range tests {