net/dns/resolver: use forwarded dns txid directly

Previously, we hashed the question and combined it with the original
txid which was useful when concurrent queries were multiplexed on a
single local source port. We encountered some situations where the DNS
server canonicalizes the question in the response (uppercase converted
to lowercase in this case), which resulted in responses that we couldn't
match to the original request due to hash mismatches. This includes a
new test to cover that situation.

Fixes #2597

Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>

net/dns/resolver/tsdns_test.go

@@ -440,6 +440,8 @@ func TestDelegate(t *testing.T) {
 	records := []interface{}{
 		"test.site.",
 		resolveToIP(testipv4, testipv6, "dns.test.site."),
+		"LCtesT.SiTe.",
+		resolveToIPLowercase(testipv4, testipv6, "dns.test.site."),
 		"nxdomain.site.", resolveToNXDOMAIN,
 		"small.txt.", resolveToTXT(smallTXT, noEdns),
 		"smalledns.txt.", resolveToTXT(smallTXT, 512),
@@ -485,6 +487,21 @@ func TestDelegate(t *testing.T) {
 			dnspacket("test.site.", dns.TypeNS, noEdns),
 			dnsResponse{name: "dns.test.site.", rcode: dns.RCodeSuccess},
 		},
+		{
+			"ipv4",
+			dnspacket("LCtesT.SiTe.", dns.TypeA, noEdns),
+			dnsResponse{ip: testipv4, rcode: dns.RCodeSuccess},
+		},
+		{
+			"ipv6",
+			dnspacket("LCtesT.SiTe.", dns.TypeAAAA, noEdns),
+			dnsResponse{ip: testipv6, rcode: dns.RCodeSuccess},
+		},
+		{
+			"ns",
+			dnspacket("LCtesT.SiTe.", dns.TypeNS, noEdns),
+			dnsResponse{name: "dns.test.site.", rcode: dns.RCodeSuccess},
+		},
 		{
 			"nxdomain",
 			dnspacket("nxdomain.site.", dns.TypeA, noEdns),