Fix dnsFromResolvConf not filtering loopback IPs

The continue statement only broke out of the inner loop, so
loopback/local IPs (e.g. 127.0.0.1) were never filtered.
This caused ctrld to use itself as bootstrap DNS when already
installed as the system resolver — a self-referential loop.

Use the same isLocal flag pattern as getDNSFromScutil() and
getAllDHCPNameservers().

nameservers_unix.go

@@ -5,12 +5,38 @@ package ctrld
 import (
 	"context"
 	"net"
+	"net/netip"
 	"slices"
 	"time"
 
 	"tailscale.com/net/netmon"
 )
 
+// localNameservers filters a list of nameserver strings, returning only those
+// that are not loopback or local machine IP addresses.
+func localNameservers(nss []string, regularIPs, loopbackIPs []netip.Addr) []string {
+	var result []string
+	seen := make(map[string]bool)
+
+	for _, ns := range nss {
+		if ip := net.ParseIP(ns); ip != nil {
+			// skip loopback and local IPs
+			isLocal := false
+			for _, v := range slices.Concat(regularIPs, loopbackIPs) {
+				if ip.String() == v.String() {
+					isLocal = true
+					break
+				}
+			}
+			if !isLocal && !seen[ip.String()] {
+				seen[ip.String()] = true
+				result = append(result, ip.String())
+			}
+		}
+	}
+	return result
+}
+
 // dnsFromResolvConf reads usable nameservers from /etc/resolv.conf file.
 // A nameserver is usable if it's not one of current machine's IP addresses
 // and loopback IP addresses.
@@ -29,24 +55,7 @@ func dnsFromResolvConf(_ context.Context) []string {
 		}
 
 		nss := CurrentNameserversFromResolvconf()
-		var localDNS []string
-		seen := make(map[string]bool)
-
-		for _, ns := range nss {
-			if ip := net.ParseIP(ns); ip != nil {
-				// skip loopback IPs
-				for _, v := range slices.Concat(regularIPs, loopbackIPs) {
-					ipStr := v.String()
-					if ip.String() == ipStr {
-						continue
-					}
-				}
-				if !seen[ip.String()] {
-					seen[ip.String()] = true
-					localDNS = append(localDNS, ip.String())
-				}
-			}
-		}
+		localDNS := localNameservers(nss, regularIPs, loopbackIPs)
 
 		// If we successfully read the file and found nameservers, return them
 		if len(localDNS) > 0 {

nameservers_unix_test.go

@@ -0,0 +1,105 @@
+//go:build unix
+
+package ctrld
+
+import (
+	"net/netip"
+	"testing"
+)
+
+func Test_localNameservers(t *testing.T) {
+	loopbackIPs := []netip.Addr{
+		netip.MustParseAddr("127.0.0.1"),
+		netip.MustParseAddr("::1"),
+	}
+	regularIPs := []netip.Addr{
+		netip.MustParseAddr("192.168.1.100"),
+		netip.MustParseAddr("10.0.0.5"),
+	}
+
+	tests := []struct {
+		name       string
+		nss        []string
+		regularIPs []netip.Addr
+		loopbackIPs []netip.Addr
+		want       []string
+	}{
+		{
+			name:        "filters loopback IPv4",
+			nss:         []string{"127.0.0.1", "8.8.8.8"},
+			regularIPs:  nil,
+			loopbackIPs: loopbackIPs,
+			want:        []string{"8.8.8.8"},
+		},
+		{
+			name:        "filters loopback IPv6",
+			nss:         []string{"::1", "1.1.1.1"},
+			regularIPs:  nil,
+			loopbackIPs: loopbackIPs,
+			want:        []string{"1.1.1.1"},
+		},
+		{
+			name:        "filters local machine IPs",
+			nss:         []string{"192.168.1.100", "8.8.4.4"},
+			regularIPs:  regularIPs,
+			loopbackIPs: nil,
+			want:        []string{"8.8.4.4"},
+		},
+		{
+			name:        "filters both loopback and local IPs",
+			nss:         []string{"127.0.0.1", "192.168.1.100", "8.8.8.8"},
+			regularIPs:  regularIPs,
+			loopbackIPs: loopbackIPs,
+			want:        []string{"8.8.8.8"},
+		},
+		{
+			name:        "deduplicates results",
+			nss:         []string{"8.8.8.8", "8.8.8.8", "1.1.1.1"},
+			regularIPs:  regularIPs,
+			loopbackIPs: loopbackIPs,
+			want:        []string{"8.8.8.8", "1.1.1.1"},
+		},
+		{
+			name:        "all filtered returns nil",
+			nss:         []string{"127.0.0.1", "::1", "192.168.1.100"},
+			regularIPs:  regularIPs,
+			loopbackIPs: loopbackIPs,
+			want:        nil,
+		},
+		{
+			name:        "empty input returns nil",
+			nss:         nil,
+			regularIPs:  regularIPs,
+			loopbackIPs: loopbackIPs,
+			want:        nil,
+		},
+		{
+			name:        "skips unparseable entries",
+			nss:         []string{"not-an-ip", "8.8.8.8"},
+			regularIPs:  regularIPs,
+			loopbackIPs: loopbackIPs,
+			want:        []string{"8.8.8.8"},
+		},
+		{
+			name:        "no local IPs filters nothing",
+			nss:         []string{"8.8.8.8", "1.1.1.1"},
+			regularIPs:  nil,
+			loopbackIPs: nil,
+			want:        []string{"8.8.8.8", "1.1.1.1"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := localNameservers(tt.nss, tt.regularIPs, tt.loopbackIPs)
+			if len(got) != len(tt.want) {
+				t.Fatalf("localNameservers() = %v, want %v", got, tt.want)
+			}
+			for i := range got {
+				if got[i] != tt.want[i] {
+					t.Errorf("localNameservers()[%d] = %q, want %q", i, got[i], tt.want[i])
+				}
+			}
+		})
+	}
+}