Adjust darwin to use sysctl

dns/sysresolver.go

@@ -27,8 +27,10 @@ const dnsHealthCheckTimeout = 2 * time.Second
 //     systemd-resolved on every DHCP change), then falls back to
 //     /etc/resolv.conf.olm.backup (written before olm overrides DNS), and
 //     finally /etc/resolv.conf.
-//   - macOS: reads /etc/resolv.conf, which is never modified by olm's
-//     supplemental scutil DNS override.
+//   - macOS: reads the unscoped resolvers from `scutil --dns`, falling back
+//     to /etc/resolv.conf if scutil is unavailable. This includes olm's own
+//     supplemental scutil DNS override entry, which is expected to be
+//     filtered out via SetExcludeIP.
 //   - Windows: enumerates every network adapter's effective DNS servers
 //     (static if set, else DHCP-assigned) from the registry.
 //   - Other platforms: returns an empty list (no-op monitor).

dns/sysresolver_darwin.go

@@ -7,20 +7,78 @@ import (
 	"net"
 	"net/netip"
 	"os"
+	"os/exec"
 	"strings"
 )
 
+// scutilPath is the well-known location of scutil on macOS.
+const scutilPath = "/usr/sbin/scutil"
+
 // readSystemDNS returns the current system DNS servers in "host:53" format.
 //
-// On macOS, olm adds supplemental DNS entries via scutil without modifying
-// /etc/resolv.conf or the primary network service DNS.  Reading /etc/resolv.conf
-// therefore always yields the physical-network DNS supplied by DHCP or the user.
-// /etc/resolv.conf on macOS is a regular file managed by mDNSResponder and is
-// updated whenever the network configuration changes.
+// olm's own DNS override is itself a scutil supplemental resolver (see
+// dns/platform/darwin.go), and macOS gives supplemental resolvers priority
+// over the primary network service's resolver when generating the merged
+// configuration - which is also what gets mirrored into /etc/resolv.conf.  So
+// once olm's override is active, /etc/resolv.conf (and a naive read of just
+// the top of "scutil --dns") reflects olm's own proxy address, not the
+// physical network's real DNS.
+//
+// Instead this reads every resolver in the unscoped "DNS configuration"
+// section of `scutil --dns` (the "(for scoped queries)" section that follows
+// only duplicates per-interface resolvers and is skipped), which includes
+// both the real physical-network resolver and olm's own supplemental one.
+// olm's own address is expected to be filtered out by the caller via
+// SystemDNSMonitor.SetExcludeIP, the same mechanism used on Windows to drop
+// olm's own adapter DNS entry.
+//
+// /etc/resolv.conf is kept as a fallback for when scutil is unavailable.
 func readSystemDNS() []string {
+	if out, err := exec.Command(scutilPath, "--dns").Output(); err == nil {
+		if servers := parseScutilDNS(string(out)); len(servers) > 0 {
+			return servers
+		}
+	}
 	return parseMacResolvConf("/etc/resolv.conf")
 }
 
+// parseScutilDNS extracts nameserver addresses from the unscoped "DNS
+// configuration" section at the top of `scutil --dns` output, stopping at
+// the "DNS configuration (for scoped queries)" section that follows it.
+func parseScutilDNS(output string) []string {
+	var result []string
+	seen := make(map[string]bool)
+
+	scanner := bufio.NewScanner(strings.NewReader(output))
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if strings.HasPrefix(line, "DNS configuration (for scoped queries)") {
+			break
+		}
+		if !strings.HasPrefix(line, "nameserver[") {
+			continue
+		}
+
+		parts := strings.SplitN(line, ":", 2)
+		if len(parts) != 2 {
+			continue
+		}
+		addr, err := netip.ParseAddr(strings.TrimSpace(parts[1]))
+		if err != nil {
+			continue
+		}
+		if addr.IsLoopback() || addr.IsLinkLocalUnicast() {
+			continue
+		}
+		hp := net.JoinHostPort(addr.String(), "53")
+		if !seen[hp] {
+			seen[hp] = true
+			result = append(result, hp)
+		}
+	}
+	return result
+}
+
 func parseMacResolvConf(path string) []string {
 	f, err := os.Open(path)
 	if err != nil {