cmd/cli: fix issue with editing /etc/resolv.conf directly on Darwin

On Darwin, modifying /etc/resolv.conf directly does not change interface
network settings. Thus the networksetup command uses to set DNS does not
do anything.

To fix this, after setting DNS using networksetup, re-check the content
of /etc/resolv.conf file to see if the nameservers are what we expected.
Otherwise, re-generate the file with proper nameservers.

cmd/cli/prog.go

@@ -651,6 +651,8 @@ func (p *prog) dnsWatchdog(iface *net.Interface, nameservers []string, allIfaces
 					if dnsChanged(i, ns) {
 						if err := setDnsIgnoreUnusableInterface(i, nameservers); err != nil {
 							mainLog.Load().Error().Err(err).Str("iface", i.Name).Msgf("could not re-apply DNS settings")
+						} else {
+							mainLog.Load().Debug().Msgf("re-applying DNS for interface %q successfully", i.Name)
 						}
 					}
 					return nil

cmd/cli/resolvconf_darwin.go

@@ -3,15 +3,44 @@ package cli
 import (
 	"net"
 	"net/netip"
+	"os"
+	"slices"
+
+	"github.com/Control-D-Inc/ctrld/internal/dns/resolvconffile"
 )
 
+const resolvConfPath = "/etc/resolv.conf"
+
 // setResolvConf sets the content of resolv.conf file using the given nameservers list.
 func setResolvConf(iface *net.Interface, ns []netip.Addr) error {
 	servers := make([]string, len(ns))
 	for i := range ns {
 		servers[i] = ns[i].String()
 	}
-	return setDNS(iface, servers)
+	if err := setDNS(iface, servers); err != nil {
+		return err
+	}
+	slices.Sort(servers)
+	curNs := currentDNS(iface)
+	slices.Sort(curNs)
+	if !slices.Equal(curNs, servers) {
+		c, err := resolvconffile.ParseFile(resolvConfPath)
+		if err != nil {
+			return err
+		}
+		c.Nameservers = ns
+		f, err := os.Create(resolvConfPath)
+		if err != nil {
+			return err
+		}
+		defer f.Close()
+
+		if err := c.Write(f); err != nil {
+			return err
+		}
+		return f.Close()
+	}
+	return nil
 }
 
 // shouldWatchResolvconf reports whether ctrld should watch changes to resolv.conf file with given OS configurator.