types: lowercase DNS extra record names

DNS names are case-insensitive, but clients match extra records against
the lowercased query name, so records with mixed-case names (for example
"Printer.fritz.box" in an extra_records_path file) never resolved and
queries fell through to the global nameserver.

Normalize record names to lowercase where the records enter the tailcfg
DNS config, covering both dns.extra_records and extra_records_path.

Fixes #2782

(cherry picked from commit 95ba1f0566)

hscontrol/types/config.go

@@ -967,7 +967,7 @@ func dnsToTailcfgDNS(dns DNSConfig) *tailcfg.DNSConfig {
 
 	cfg.Proxied = dns.MagicDNS
 
-	cfg.ExtraRecords = dns.ExtraRecords
+	cfg.ExtraRecords = lowercaseRecordNames(dns.ExtraRecords)
 	if dns.OverrideLocalDNS {
 		cfg.Resolvers = dns.globalResolvers()
 	} else {
@@ -1512,6 +1512,22 @@ func (c *Config) SetExtraRecords(records []tailcfg.DNSRecord) {
 	defer tailcfgDNSMu.Unlock()
 
 	if c.TailcfgDNSConfig != nil {
-		c.TailcfgDNSConfig.ExtraRecords = records
+		c.TailcfgDNSConfig.ExtraRecords = lowercaseRecordNames(records)
 	}
 }
+
+// lowercaseRecordNames normalizes DNS record names to lowercase, as DNS names
+// are case-insensitive and clients match extra records by exact name.
+func lowercaseRecordNames(records []tailcfg.DNSRecord) []tailcfg.DNSRecord {
+	if len(records) == 0 {
+		return records
+	}
+
+	normalized := make([]tailcfg.DNSRecord, len(records))
+	for i, record := range records {
+		record.Name = strings.ToLower(record.Name)
+		normalized[i] = record
+	}
+
+	return normalized
+}

hscontrol/types/config_test.go

@@ -601,3 +601,32 @@ func TestTrustedProxies(t *testing.T) {
 		})
 	}
 }
+
+// DNS names are case-insensitive, but MagicDNS resolution in clients matches
+// extra records by exact name, so mixed-case record names never resolve.
+func TestExtraRecordsAreLowercased(t *testing.T) {
+	mixed := []tailcfg.DNSRecord{
+		{Name: "Printer.fritz.box", Type: "A", Value: "192.168.1.2"},
+		{Name: "NAS.FRITZ.BOX", Type: "A", Value: "192.168.1.3"},
+	}
+	want := []tailcfg.DNSRecord{
+		{Name: "printer.fritz.box", Type: "A", Value: "192.168.1.2"},
+		{Name: "nas.fritz.box", Type: "A", Value: "192.168.1.3"},
+	}
+
+	tcfg := dnsToTailcfgDNS(DNSConfig{
+		MagicDNS:     true,
+		BaseDomain:   "example.com",
+		ExtraRecords: mixed,
+	})
+	if diff := cmp.Diff(want, tcfg.ExtraRecords); diff != "" {
+		t.Errorf("dnsToTailcfgDNS extra records mismatch (-want +got):\n%s", diff)
+	}
+
+	cfg := &Config{TailcfgDNSConfig: &tailcfg.DNSConfig{}}
+	cfg.SetExtraRecords(mixed)
+
+	if diff := cmp.Diff(want, cfg.TailcfgDNSConfig.ExtraRecords); diff != "" {
+		t.Errorf("SetExtraRecords mismatch (-want +got):\n%s", diff)
+	}
+}