all: move nameserver resolution to public API

Make nameserver resolution functions more consistent and accessible:
- Rename currentNameserversFromResolvconf to CurrentNameserversFromResolvconf
- Move function to public API for better reusability
- Update all internal references to use the new public API
- Add comprehensive godoc comments for nameserver functions
- Improve code organization by centralizing DNS resolution logic

This change makes the nameserver resolution functionality more maintainable
and easier to use across different parts of the codebase.

cmd/cli/os_darwin.go

@@ -9,7 +9,6 @@ import (
 	"strings"
 
 	"github.com/Control-D-Inc/ctrld"
-	"github.com/Control-D-Inc/ctrld/internal/resolvconffile"
 )
 
 // allocate loopback ip
@@ -92,7 +91,7 @@ func restoreDNS(iface *net.Interface) (err error) {
 }
 
 func currentDNS(_ *net.Interface) []string {
-	return resolvconffile.NameServers()
+	return ctrld.CurrentNameserversFromResolvconf()
 }
 
 // currentStaticDNS returns the current static DNS settings of given interface.

cmd/cli/os_freebsd.go

@@ -9,8 +9,8 @@ import (
 	"tailscale.com/health"
 	"tailscale.com/util/dnsname"
 
+	"github.com/Control-D-Inc/ctrld"
 	"github.com/Control-D-Inc/ctrld/internal/dns"
-	"github.com/Control-D-Inc/ctrld/internal/resolvconffile"
 )
 
 // allocate loopback ip
@@ -94,7 +94,7 @@ func restoreDNS(iface *net.Interface) (err error) {
 }
 
 func currentDNS(_ *net.Interface) []string {
-	return resolvconffile.NameServers()
+	return ctrld.CurrentNameserversFromResolvconf()
 }
 
 // currentStaticDNS returns the current static DNS settings of given interface.

cmd/cli/os_linux.go

@@ -21,9 +21,9 @@ import (
 	"tailscale.com/health"
 	"tailscale.com/util/dnsname"
 
+	"github.com/Control-D-Inc/ctrld"
 	"github.com/Control-D-Inc/ctrld/internal/dns"
 	ctrldnet "github.com/Control-D-Inc/ctrld/internal/net"
-	"github.com/Control-D-Inc/ctrld/internal/resolvconffile"
 )
 
 const resolvConfBackupFailedMsg = "open /etc/resolv.pre-ctrld-backup.conf: read-only file system"
@@ -201,7 +201,7 @@ func restoreDNS(iface *net.Interface) (err error) {
 }
 
 func currentDNS(iface *net.Interface) []string {
-	resolvconfFunc := func(_ string) []string { return resolvconffile.NameServers() }
+	resolvconfFunc := func(_ string) []string { return ctrld.CurrentNameserversFromResolvconf() }
 	for _, fn := range []getDNS{getDNSByResolvectl, getDNSBySystemdResolved, getDNSByNmcli, resolvconfFunc} {
 		if ns := fn(iface.Name); len(ns) > 0 {
 			return ns

internal/resolvconffile/dns.go

@@ -9,6 +9,7 @@ import (
 
 const resolvconfPath = "/etc/resolv.conf"
 
+// NameServersWithPort retrieves a list of nameservers with the default DNS port 53 appended to each address.
 func NameServersWithPort() []string {
 	c, err := resolvconffile.ParseFile(resolvconfPath)
 	if err != nil {
@@ -21,11 +22,17 @@ func NameServersWithPort() []string {
 	return ns
 }
 
+// NameServers retrieves a list of nameservers from the /etc/resolv.conf file
+// Returns an empty slice if reading fails.
 func NameServers() []string {
 	nss, _ := NameserversFromFile(resolvconfPath)
 	return nss
 }
 
+// NameserversFromFile reads nameserver addresses from the specified resolv.conf file
+// and returns them as a slice of strings.
+//
+// Returns an error if the file cannot be parsed.
 func NameserversFromFile(path string) ([]string, error) {
 	c, err := resolvconffile.ParseFile(path)
 	if err != nil {

nameservers.go

@@ -1,6 +1,10 @@
 package ctrld
 
-import "context"
+import (
+	"context"
+
+	"github.com/Control-D-Inc/ctrld/internal/resolvconffile"
+)
 
 type dnsFn func(ctx context.Context) []string
 
@@ -28,3 +32,8 @@ func nameservers(ctx context.Context) []string {
 
 	return dns
 }
+
+// CurrentNameserversFromResolvconf returns the current nameservers set from /etc/resolv.conf file.
+func CurrentNameserversFromResolvconf() []string {
+	return resolvconffile.NameServers()
+}

nameservers_unix.go

@@ -9,15 +9,8 @@ import (
 	"time"
 
 	"tailscale.com/net/netmon"
-
-	"github.com/Control-D-Inc/ctrld/internal/resolvconffile"
 )
 
-// currentNameserversFromResolvconf returns the current nameservers set from /etc/resolv.conf file.
-func currentNameserversFromResolvconf() []string {
-	return resolvconffile.NameServers()
-}
-
 // dnsFromResolvConf reads usable nameservers from /etc/resolv.conf file.
 // A nameserver is usable if it's not one of current machine's IP addresses
 // and loopback IP addresses.
@@ -35,7 +28,7 @@ func dnsFromResolvConf(_ context.Context) []string {
 			time.Sleep(retryInterval)
 		}
 
-		nss := resolvconffile.NameServers()
+		nss := CurrentNameserversFromResolvconf()
 		var localDNS []string
 		seen := make(map[string]bool)
 

nameservers_windows.go

@@ -297,7 +297,7 @@ func getDNSServers(ctx context.Context) ([]string, error) {
 	return ns, nil
 }
 
-// currentNameserversFromResolvconf returns a nil slice of strings.
+// CurrentNameserversFromResolvconf returns a nil slice of strings.
 func currentNameserversFromResolvconf() []string {
 	return nil
 }

resolver.go

@@ -647,7 +647,7 @@ func lookupIP(ctx context.Context, domain string, timeout int, bootstrapDNS []st
 // This is useful for doing PTR lookup in LAN network.
 func NewPrivateResolver(ctx context.Context) Resolver {
 	nss := initDefaultOsResolver(ctx)
-	resolveConfNss := currentNameserversFromResolvconf()
+	resolveConfNss := CurrentNameserversFromResolvconf()
 	localRfc1918Addrs := Rfc1918Addresses()
 	n := 0
 	for _, ns := range nss {