fix(wsl-pro-service): Find host address when NAT + DNS tunneling ON (#…

…777)

WSL version 2.2.1 (pre-release) turns DNS tunneling ON by default
(again). With DNS tunneling activated, the Windows host IP no longer
matches the nameserver IP found in /etc/resolv.conf but still matches
the default gateway. When that was the case the nameserver IP would
contain a loopback address, but since that version it contains an IP
address of type 10.0.0.0/8 . Attempting to connect to such address
always fails. If that behavior remains, we should change the
wsl-pro-service IP selection to rely on the default gateway IP address
for all cases when networking mode is not “mirrored”, for which case it
must remain at 127.0.0.1 .
That’s of now the preferred method to find the host IP address as
documented in
https://learn.microsoft.com/en-us/windows/wsl/networking#accessing-windows-networking-apps-from-linux-host-ip

The gist is: we no longer need to read `/etc/resolv.conf`.

---
UDENG-2761

docs/howto/set-up-up4w.md

@@ -49,10 +49,8 @@ Sure:
 	  wslinfo --networking-mode
 	  ```
         - If it says `mirrored`, the relevant IP is `127.0.0.1`. Take note of this address.
-        - Otherwise, open file `/etc/resolv.conf` in the WSL instance named _Ubuntu-22.04_. Find the line starting with `nameserver` followed by an IP address.
-           - If the IP address does not start with `127`, take note of this address.
-           - Otherwise, run the command `ip route | grep ^default` and take note of the IP address that is printed.
-   3. Set up a Landscape Beta server: 
+	- Otherwise, run the command `ip route | grep ^default` and take note of the IP address that is printed.
+   3. Set up a Landscape Beta server:
       1. Start a shell in your _Ubuntu-22.04_ distro.
       2. Install the Landscape (beta) following the steps in the Landscape Quickstart deployment with the following considerations:
          - Make sure you install the beta version.
@@ -185,9 +183,7 @@ On your Windows host, in the Microsoft Store, search for the `Ubuntu-Preview` or
 <!-- 	  wslinfo --networking-mode -->
 <!-- 	  ``` -->
 <!--         - If it says `mirrored`, the relevant IP is `127.0.0.1`. Take note of this address. -->
-<!--         - Otherwise, open file `/etc/resolv.conf` in the WSL instance named _Ubuntu-22.04_. Find the line starting with `nameserver` followed by an IP address. -->
-<!--            - If the IP address does not start with `127`, take note of this address. -->
-<!--            - Otherwise, run the command `ip route | grep ^default` and take note of the IP address that is printed. -->
+<!--         - Otherwise, run the command `ip route | grep ^default` and take note of the IP address that is printed. -->
 <!--    3. Set up a Landscape Beta server.  -->
 <!--       1. Start a shell in your _Ubuntu-22.04_ distro. -->
 <!--       2. Install the Landscape (beta) following the steps in the Landscape Quickstart deployment with the following considerations: -->

wsl-pro-service/internal/system/networking.go

@@ -4,7 +4,6 @@ import (
 	"bufio"
 	"context"
 	"encoding/binary"
-	"errors"
 	"fmt"
 	"net"
 	"os"
@@ -27,15 +26,6 @@ func (s *System) WindowsHostAddress(ctx context.Context) (ip net.IP, err error)
 		return net.IPv4(127, 0, 0, 1), nil
 	}
 
-	nameserver, err := s.nameServer()
-	if err != nil {
-		return nil, fmt.Errorf("could not find nameserver: %v", err)
-	}
-
-	if !nameserver.IsLoopback() {
-		return nameserver, nil
-	}
-
 	return s.defaultGateway()
 }
 
@@ -50,59 +40,6 @@ func (s *System) networkingMode(ctx context.Context) (string, error) {
 	return strings.TrimSpace(string(out)), nil
 }
 
-// nameServer parses /etc/resolv.conf to get the IP address of the nameserver.
-func (s *System) nameServer() (ip net.IP, err error) {
-	/*
-		We parse the IP address of the nameserver from /etc/resolv.conf. Here is an example
-		of what this file may look like:
-
-		# This file was automatically generated by WSL. To stop automatic generation of this file, add the following entry to /etc/wsl.conf:
-		# [network]
-		# generateResolvConf = false
-		nameserver 172.22.16.1
-	*/
-	const fileName = "/etc/resolv.conf"
-
-	r, err := os.Open(s.Path(fileName))
-	if err != nil {
-		// Error mesage already says could not open <filename>
-		return nil, err
-	}
-	defer r.Close()
-
-	var address string
-	var line int
-
-	sc := bufio.NewScanner(r)
-	for sc.Scan() {
-		line++
-
-		suffix, found := strings.CutPrefix(sc.Text(), "nameserver")
-		if !found {
-			continue
-		}
-		address = strings.TrimSpace(suffix)
-		break
-	}
-
-	defer decorate.OnError(&err, "could not parse %s", fileName)
-
-	if err := sc.Err(); err != nil {
-		return nil, fmt.Errorf("line %d: %v", line, err)
-	}
-
-	if address == "" {
-		return nil, errors.New("no line matches 'nameserver <IP>'")
-	}
-
-	ip = net.ParseIP(address)
-	if ip == nil {
-		return nil, fmt.Errorf("line %d: could not parse address %q", line, address)
-	}
-
-	return ip, nil
-}
-
 // defaultGateway returns the default gateway of the machine.
 func (s *System) defaultGateway() (ip net.IP, err error) {
 	/*

wsl-pro-service/internal/system/system_test.go

@@ -468,7 +468,6 @@ func TestWindowsHostAddress(t *testing.T) {
 		fileNotExist
 		fileBroken
 		fileIPbroken
-		fileIPisLoopback
 	)
 
 	// copyFile is a helper that copies the appropriate version of a fixture to the desired destination.
@@ -487,8 +486,6 @@ func TestWindowsHostAddress(t *testing.T) {
 			suffix = ".bad"
 		case fileIPbroken:
 			suffix = ".bad-ip"
-		case fileIPisLoopback:
-			suffix = ".loopback"
 		}
 
 		from = from + suffix
@@ -501,36 +498,28 @@ func TestWindowsHostAddress(t *testing.T) {
 	// These are the addresses hard-coded on the fixtures labelled as "good"
 	const (
 		localhost   = "127.0.0.1"
-		nameserver  = "172.22.16.1"
-		degaultGway = "172.25.32.1"
+		defaultGway = "172.25.32.1"
 	)
 
 	testCases := map[string]struct {
 		networkNotNAT bool
 		breakWslInfo  bool
 
-		etcResolv    fileState
 		procNetRoute fileState
 
 		want    string
 		wantErr bool
 	}{
-		"Success without NAT":                          {networkNotNAT: true, want: localhost},
-		"Success with NAT, nameserver is not loopback": {want: nameserver},
-		"Success with NAT, nameserver is loopback":     {etcResolv: fileIPisLoopback, want: degaultGway},
+		"Without NAT": {networkNotNAT: true, want: localhost},
+		"With NAT":    {want: defaultGway},
 
 		// WSL info errors
 		"Error when wslinfo returns an error": {breakWslInfo: true, wantErr: true},
 
-		// NAT errors with broken /etc/resolv.conf
-		"Error with NAT when /etc/resolv.conf does not exist":       {etcResolv: fileNotExist, wantErr: true},
-		"Error with NAT when /etc/resolv.conf is ill-formed":        {etcResolv: fileBroken, wantErr: true},
-		"Error with NAT when /etc/resolv.conf has an ill-formed IP": {etcResolv: fileIPbroken, wantErr: true},
-
 		// NAT errors with loopback nameserver and broken /proc/net/route
-		"Error with NAT when /proc/net/route does not exist":       {etcResolv: fileIPisLoopback, procNetRoute: fileNotExist, wantErr: true},
-		"Error with NAT when /proc/net/route is ill-formed":        {etcResolv: fileIPisLoopback, procNetRoute: fileBroken, wantErr: true},
-		"Error with NAT when /proc/net/route has an ill-formed IP": {etcResolv: fileIPisLoopback, procNetRoute: fileIPbroken, wantErr: true},
+		"Error with NAT when /proc/net/route does not exist":       {procNetRoute: fileNotExist, wantErr: true},
+		"Error with NAT when /proc/net/route is ill-formed":        {procNetRoute: fileBroken, wantErr: true},
+		"Error with NAT when /proc/net/route has an ill-formed IP": {procNetRoute: fileIPbroken, wantErr: true},
 	}
 
 	for name, tc := range testCases {
@@ -547,7 +536,6 @@ func TestWindowsHostAddress(t *testing.T) {
 				mock.SetControlArg(testutils.WslInfoIsNAT)
 			}
 
-			copyFile(t, tc.etcResolv, filepath.Join(commontestutils.TestFamilyPath(t), "etc-resolv.conf"), mock.Path("/etc/resolv.conf"))
 			copyFile(t, tc.procNetRoute, filepath.Join(commontestutils.TestFamilyPath(t), "proc-net-route"), mock.Path("/proc/net/route"))
 
 			got, err := sys.WindowsHostAddress(ctx)

wsl-pro-service/internal/testutils/mock_executables.go

@@ -58,9 +58,6 @@ var (
 	//go:embed filesystem_defaults/os-release
 	defaultOsReleaseContents []byte
 
-	//go:embed filesystem_defaults/resolv.conf
-	defaultResolvConfContents []byte
-
 	//go:embed filesystem_defaults/proc.mounts
 	defaultProcMountsContents []byte
 
@@ -655,9 +652,6 @@ func mockFilesystemRoot(t *testing.T) (rootDir string) {
 	err := os.MkdirAll(filepath.Join(rootDir, "etc"), 0750)
 	require.NoError(t, err, "Setup: could not create mock /etc/")
 
-	err = os.WriteFile(filepath.Join(rootDir, "etc/resolv.conf"), defaultResolvConfContents, 0600)
-	require.NoError(t, err, "Setup: could not write mock /etc/resolv.conf")
-
 	err = os.WriteFile(filepath.Join(rootDir, "etc/os-release"), defaultOsReleaseContents, 0600)
 	require.NoError(t, err, "Setup: could not write mock /etc/os-release")