cmd/cli: watch changes to /etc/resolv.conf

On some routers, change to network may trigger re-rendering
/etc/resolv.conf file, causing requests from router itself stop using
ctrld.

Fixing this by watching changes to /etc/resolv.conf, then revert them.

cmd/cli/os_linux.go

@@ -9,10 +9,12 @@ import (
 	"net"
 	"net/netip"
 	"os/exec"
+	"path/filepath"
 	"strings"
 	"syscall"
 	"time"
 
+	"github.com/fsnotify/fsnotify"
 	"github.com/insomniacslk/dhcp/dhcpv4/nclient4"
 	"github.com/insomniacslk/dhcp/dhcpv6"
 	"github.com/insomniacslk/dhcp/dhcpv6/client6"
@@ -23,7 +25,10 @@ import (
 	"github.com/Control-D-Inc/ctrld/internal/resolvconffile"
 )
 
-const resolvConfBackupFailedMsg = "open /etc/resolv.pre-ctrld-backup.conf: read-only file system"
+const (
+	resolvConfPath            = "/etc/resolv.conf"
+	resolvConfBackupFailedMsg = "open /etc/resolv.pre-ctrld-backup.conf: read-only file system"
+)
 
 // allocate loopback ip
 // sudo ip a add 127.0.0.2/24 dev lo
@@ -64,6 +69,11 @@ func setDNS(iface *net.Interface, nameservers []string) error {
 		Nameservers:   ns,
 		SearchDomains: []dnsname.FQDN{},
 	}
+	defer func() {
+		if r.Mode() == "direct" {
+			go watchResolveConf(osConfig)
+		}
+	}()
 
 	trySystemdResolve := false
 	for i := 0; i < maxSetDNSAttempts; i++ {
@@ -299,3 +309,59 @@ func sliceIndex[S ~[]E, E comparable](s S, v E) int {
 	}
 	return -1
 }
+
+// watchResolveConf watches any changes to /etc/resolv.conf file,
+// and reverting to the original config set by ctrld.
+func watchResolveConf(oc dns.OSConfig) {
+	mainLog.Load().Debug().Msg("start watching /etc/resolv.conf file")
+	watcher, err := fsnotify.NewWatcher()
+	if err != nil {
+		mainLog.Load().Warn().Err(err).Msg("could not create watcher for /etc/resolv.conf")
+		return
+	}
+
+	// We watch /etc instead of /etc/resolv.conf directly,
+	// see: https://github.com/fsnotify/fsnotify#watching-a-file-doesnt-work-well
+	watchDir := filepath.Dir(resolvConfPath)
+	if err := watcher.Add(watchDir); err != nil {
+		mainLog.Load().Warn().Err(err).Msg("could not add /etc/resolv.conf to watcher list")
+		return
+	}
+
+	r, err := dns.NewOSConfigurator(func(format string, args ...any) {}, "lo") // interface name does not matter.
+	if err != nil {
+		mainLog.Load().Error().Err(err).Msg("failed to create DNS OS configurator")
+		return
+	}
+
+	for {
+		select {
+		case event, ok := <-watcher.Events:
+			if !ok {
+				return
+			}
+			if event.Name != resolvConfPath { // skip if not /etc/resolv.conf changes.
+				continue
+			}
+			if event.Has(fsnotify.Write) || event.Has(fsnotify.Create) {
+				mainLog.Load().Debug().Msg("/etc/resolv.conf changes detected, reverting to ctrld setting")
+				if err := watcher.Remove(watchDir); err != nil {
+					mainLog.Load().Error().Err(err).Msg("failed to pause watcher")
+					continue
+				}
+				if err := r.SetDNS(oc); err != nil {
+					mainLog.Load().Error().Err(err).Msg("failed to revert /etc/resolv.conf changes")
+				}
+				if err := watcher.Add(watchDir); err != nil {
+					mainLog.Load().Error().Err(err).Msg("failed to continue running watcher")
+					return
+				}
+			}
+		case err, ok := <-watcher.Errors:
+			if !ok {
+				return
+			}
+			mainLog.Load().Err(err).Msg("could not get event for /etc/resolv.conf")
+		}
+	}
+}