all: do not depend on vyatta-dhcpd service on EdgeOS

The only reason that forces ctrld to depend on vyatta-dhcpd service on
EdgeOS is allowing ctrld to watch lease files properly, because those
files may not be created at the time client info table initialized.

However, on some EdgeOS version, vyatta-dhcpd could not start with an
empty config file, causing restart loop itself, flooding systemd log,
making the router run out of memory.

To fix this, instead of depending on vyatta-dhcpd, we should just watch
for lease files creation, then adding them to watch list.

While at it, also making ctrld starts after nss-lookup, ensuring we have
a working DNS before starting ctrld.

internal/router/edgeos/edgeos.go

@@ -169,6 +169,13 @@ func ContentFilteringEnabled() bool {
 	return err == nil && !st.IsDir()
 }
 
+func LeaseFileDir() string {
+	if checkUSG() {
+		return ""
+	}
+	return "/run"
+}
+
 func checkUSG() bool {
 	out, _ := exec.Command("mca-cli-op", "info").Output()
 	return bytes.Contains(out, []byte("UniFi-Gateway-"))

internal/router/router.go

@@ -173,20 +173,6 @@ func CanListenLocalhost() bool {
 	}
 }
 
-// ServiceDependencies returns list of dependencies that ctrld services needs on this router.
-// See https://pkg.go.dev/github.com/kardianos/service#Config for list format.
-func ServiceDependencies() []string {
-	if Name() == edgeos.Name {
-		// On EdeOS, ctrld needs to start after vyatta-dhcpd, so it can read leases file.
-		return []string{
-			"Wants=vyatta-dhcpd.service",
-			"After=vyatta-dhcpd.service",
-			"Wants=dnsmasq.service",
-		}
-	}
-	return nil
-}
-
 // SelfInterfaces return list of *net.Interface that will be source of requests from router itself.
 func SelfInterfaces() []*net.Interface {
 	switch Name() {
@@ -197,6 +183,14 @@ func SelfInterfaces() []*net.Interface {
 	}
 }
 
+// LeaseFilesDir is the directory which contains lease files.
+func LeaseFilesDir() string {
+	if Name() == edgeos.Name {
+		edgeos.LeaseFileDir()
+	}
+	return ""
+}
+
 func distroName() string {
 	switch {
 	case bytes.HasPrefix(unameO(), []byte("DD-WRT")):