cmd/ctrld: add --iface for setting DNS on specific interface

cmd/ctrld/os_linux.go

@@ -1,7 +1,18 @@
 package main
 
 import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"net"
+	"net/netip"
 	"os/exec"
+	"strings"
+
+	"tailscale.com/net/dns"
+	"tailscale.com/util/dnsname"
+
+	"github.com/Control-D-Inc/ctrld/internal/resolvconffile"
 )
 
 // allocate loopback ip
@@ -23,3 +34,84 @@ func deAllocateIP(ip string) error {
 	}
 	return nil
 }
+
+// set the dns server for the provided network interface
+func setDNS(iface *net.Interface, nameservers []string) error {
+	logf := func(format string, args ...any) {
+		mainLog.Debug().Msgf(format, args...)
+	}
+
+	r, err := dns.NewOSConfigurator(logf, iface.Name)
+	if err != nil {
+		mainLog.Error().Err(err).Msg("failed to create DNS OS configurator")
+		return err
+	}
+
+	ns := make([]netip.Addr, 0, len(nameservers))
+	for _, nameserver := range nameservers {
+		ns = append(ns, netip.MustParseAddr(nameserver))
+	}
+	return r.SetDNS(dns.OSConfig{
+		Nameservers:   ns,
+		SearchDomains: []dnsname.FQDN{},
+	})
+}
+
+func resetDNS(iface *net.Interface, nameservers []string) error {
+	if err := setDNS(iface, nameservers); err != nil {
+		mainLog.Error().Err(err).Msg("resetDNS failed.")
+		return err
+	}
+	return nil
+}
+
+func currentDNS(iface *net.Interface) []string {
+	for _, fn := range []getDNS{getDNSByResolvectl, getDNSByNmcli, resolvconffile.NameServers} {
+		if ns := fn(iface.Name); len(ns) > 0 {
+			return ns
+		}
+	}
+	return nil
+}
+
+func getDNSByResolvectl(iface string) []string {
+	b, err := exec.Command("resolvectl", "dns", "-i", iface).Output()
+	if err != nil {
+		return nil
+	}
+	parts := strings.SplitN(string(b), "%", 2)
+	if len(parts) != 2 {
+		return nil
+	}
+	parts = strings.Fields(parts[0])
+	if len(parts) > 2 {
+		fmt.Println(parts)
+		return parts[3:]
+	}
+	return nil
+}
+
+func getDNSByNmcli(iface string) []string {
+	b, err := exec.Command("nmcli", "dev", "show", iface).Output()
+	if err != nil {
+		return nil
+	}
+	s := bufio.NewScanner(bytes.NewReader(b))
+	var dns []string
+	do := func(line string) {
+		parts := strings.SplitN(line, ":", 2)
+		if len(parts) > 1 {
+			dns = append(dns, strings.TrimSpace(parts[1]))
+		}
+	}
+	for s.Scan() {
+		line := s.Text()
+		switch {
+		case strings.HasPrefix(line, "IP4.DNS"):
+			fallthrough
+		case strings.HasPrefix(line, "IP6.DNS"):
+			do(line)
+		}
+	}
+	return dns
+}