Also check network manager on linux

dns/sysresolver_linux.go

@@ -8,6 +8,8 @@ import (
 	"net/netip"
 	"os"
 	"strings"
+
+	platform "github.com/fosrl/olm/dns/platform"
 )
 
 // readSystemDNS returns the current system DNS servers in "host:53" format.
@@ -16,9 +18,18 @@ import (
 //  1. /run/systemd/resolve/resolv.conf — maintained by systemd-resolved with
 //     the real per-link DNS servers; updated on every DHCP change and never
 //     touched by olm's D-Bus DNS override.
-//  2. /etc/resolv.conf.olm.backup — written by olm before it overrides
-//     /etc/resolv.conf on non-systemd systems.
-//  3. /etc/resolv.conf — plain fallback.
+//  2. NetworkManager, queried live over D-Bus — NetworkManager's own view of
+//     each active connection's DNS servers, independent of what is currently
+//     written to /etc/resolv.conf. This covers NetworkManager's "dnsmasq" and
+//     "unbound" DNS modes, where /etc/resolv.conf only contains a loopback
+//     stub address, and stays accurate even if olm's own override has
+//     directly overwritten /etc/resolv.conf, without going stale the way a
+//     one-time backup snapshot would if the real DNS changes mid-override
+//     (e.g. the user switches WiFi networks).
+//  3. /etc/resolv.conf.olm.backup — written by olm before it overrides
+//     /etc/resolv.conf on non-systemd systems, for when NetworkManager isn't
+//     in use at all.
+//  4. /etc/resolv.conf — plain fallback.
 //
 // Loopback and link-local addresses (e.g. 127.0.0.53, ::1) are excluded
 // because they are stub resolver addresses, not real upstream servers.
@@ -28,6 +39,10 @@ func readSystemDNS() []string {
 		return servers
 	}
 
+	if servers := readNetworkManagerDNS(); len(servers) > 0 {
+		return servers
+	}
+
 	// If olm has already overridden /etc/resolv.conf the backup holds the
 	// original pre-override DNS servers.
 	if _, err := os.Stat("/etc/resolv.conf.olm.backup"); err == nil {
@@ -39,6 +54,22 @@ func readSystemDNS() []string {
 	return parseResolvConf("/etc/resolv.conf")
 }
 
+// readNetworkManagerDNS returns the DNS servers NetworkManager reports over
+// D-Bus for its active connections, in "host:53" format. Returns nil if
+// NetworkManager isn't running or reports nothing usable.
+func readNetworkManagerDNS() []string {
+	addrs, err := platform.GetNetworkManagerNameservers()
+	if err != nil || len(addrs) == 0 {
+		return nil
+	}
+
+	result := make([]string, 0, len(addrs))
+	for _, addr := range addrs {
+		result = append(result, addrToHostPort(addr))
+	}
+	return result
+}
+
 // parseResolvConf reads nameserver lines from a resolv.conf-style file,
 // skipping loopback and link-local addresses.
 func parseResolvConf(path string) []string {