cmd/cli: only emit error for running interfaces

While at it, also ensure setDNS/resetDNS return a wrapped error on
Darwin/Windows, so the caller can decide whether to print the error to
users.

cmd/cli/cli.go

@@ -2223,3 +2223,8 @@ func absHomeDir(filename string) string {
 	}
 	return filepath.Join(dir, filename)
 }
+
+// ifaceUp reports whether the net interface is up.
+func ifaceUp(iface *net.Interface) bool {
+	return iface != nil && iface.Flags&net.FlagUp != 0
+}

cmd/cli/os_darwin.go

@@ -3,6 +3,7 @@ package cli
 import (
 	"bufio"
 	"bytes"
+	"fmt"
 	"net"
 	"os/exec"
 
@@ -36,10 +37,8 @@ func setDNS(iface *net.Interface, nameservers []string) error {
 	cmd := "networksetup"
 	args := []string{"-setdnsservers", iface.Name}
 	args = append(args, nameservers...)
-
-	if err := exec.Command(cmd, args...).Run(); err != nil {
-		mainLog.Load().Error().Err(err).Msgf("setDNS failed, ips = %q", nameservers)
-		return err
+	if out, err := exec.Command(cmd, args...).CombinedOutput(); err != nil {
+		return fmt.Errorf("%v: %w", string(out), err)
 	}
 	return nil
 }
@@ -53,10 +52,8 @@ func resetDNS(iface *net.Interface) error {
 	}
 	cmd := "networksetup"
 	args := []string{"-setdnsservers", iface.Name, "empty"}
-
-	if err := exec.Command(cmd, args...).Run(); err != nil {
-		mainLog.Load().Error().Err(err).Msgf("resetDNS failed")
-		return err
+	if out, err := exec.Command(cmd, args...).CombinedOutput(); err != nil {
+		return fmt.Errorf("%v: %w", string(out), err)
 	}
 	return nil
 }
@@ -71,7 +68,9 @@ func currentStaticDNS(iface *net.Interface) []string {
 	args := []string{"-getdnsservers", iface.Name}
 	out, err := exec.Command(cmd, args...).Output()
 	if err != nil {
-		mainLog.Load().Error().Err(err).Msg("could not get current static DNS")
+		if ifaceUp(iface) {
+			mainLog.Load().Error().Err(err).Msg("could not get current static DNS")
+		}
 		return nil
 	}
 	scanner := bufio.NewScanner(bytes.NewReader(out))

cmd/cli/os_windows.go

@@ -89,8 +89,7 @@ func resetDNS(iface *net.Interface) error {
 	// Restoring ipv4 DHCP.
 	output, err := netsh("interface", "ipv4", "set", "dnsserver", strconv.Itoa(iface.Index), "dhcp")
 	if err != nil {
-		mainLog.Load().Error().Err(err).Msgf("failed to reset ipv4 DNS: %s", string(output))
-		return err
+		return fmt.Errorf("%s: %w", string(output), err)
 	}
 	// If there's static DNS saved, restoring it.
 	if nss := savedStaticNameservers(iface); len(nss) > 0 {
@@ -178,12 +177,16 @@ func currentDNS(iface *net.Interface) []string {
 func currentStaticDNS(iface *net.Interface) []string {
 	luid, err := winipcfg.LUIDFromIndex(uint32(iface.Index))
 	if err != nil {
-		mainLog.Load().Error().Err(err).Msg("could not get interface LUID")
+		if ifaceUp(iface) {
+			mainLog.Load().Error().Err(err).Msg("could not get interface LUID")
+		}
 		return nil
 	}
 	guid, err := luid.GUID()
 	if err != nil {
-		mainLog.Load().Error().Err(err).Msg("could not get interface GUID")
+		if ifaceUp(iface) {
+			mainLog.Load().Error().Err(err).Msg("could not get interface GUID")
+		}
 		return nil
 	}
 	var ns []string

cmd/cli/prog.go

@@ -704,7 +704,9 @@ func withEachPhysicalInterfaces(excludeIfaceName, context string, f func(i *net.
 			return
 		}
 		if err := f(netIface); err != nil {
-			mainLog.Load().Warn().Err(err).Msgf("failed to %s for interface: %q", context, i.Name)
+			if ifaceUp(netIface) {
+				mainLog.Load().Warn().Err(err).Msgf("failed to %s for interface: %q", context, i.Name)
+			}
 		} else {
 			mainLog.Load().Debug().Msgf("%s for interface %q successfully", context, i.Name)
 		}