net/dns: fix handling non-IP single split DNS

Fixes #19834

Change-Id: I4d48efed00cd080b14c6fd713ff21e53a5a6ee3c
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>

net/dns/manager.go

@@ -367,12 +367,14 @@ func (m *Manager) compileConfig(cfg Config) (rcfg resolver.Config, ocfg OSConfig
 	// workaround.
 	isWindows := m.goos == "windows"
 	isApple := (m.goos == "darwin" || m.goos == "ios")
-	if len(cfg.singleResolverSet()) > 0 && m.os.SupportsSplitDNS() && !isWindows && !isApple {
-		// Split DNS configuration requested, where all split domains
-		// go to the same resolvers. We can let the OS do it.
-		ocfg.Nameservers = toIPsOnly(cfg.singleResolverSet())
-		ocfg.MatchDomains = cfg.matchDomains()
-		return rcfg, ocfg, nil
+	if m.os.SupportsSplitDNS() && !isWindows && !isApple {
+		if srs := toIPsOnly(cfg.singleResolverSet()); len(srs) > 0 {
+			// Split DNS configuration requested, where all split domains
+			// go to the same resolvers. We can let the OS do it.
+			ocfg.Nameservers = srs
+			ocfg.MatchDomains = cfg.matchDomains()
+			return rcfg, ocfg, nil
+		}
 	}
 
 	// Split DNS configuration with either multiple upstream routes,

net/dns/manager_test.go

@@ -957,6 +957,23 @@ func TestManager(t *testing.T) {
 			},
 			goos: "windows",
 		},
+		{
+			// Regression test for #19834
+			name: "single-doh-splitdns-no-magicdns",
+			in: Config{
+				Routes: upstreams(
+					"example.com", "http://100.101.102.103:1234/dns-query"),
+			},
+			split: true,
+			os: OSConfig{
+				Nameservers:  serviceAddr46,
+				MatchDomains: fqdns("example.com"),
+			},
+			rs: resolver.Config{
+				Routes: upstreams("example.com.", "http://100.101.102.103:1234/dns-query"),
+			},
+			goos: "linux",
+		},
 	}
 
 	trIP := cmp.Transformer("ipStr", func(ip netip.Addr) string { return ip.String() })