Use id command for user lookup on MacOS (#384)

When building client without CGO, user.Lookup
attempts to get user from /etc/passwd
Which doesn't have the user as MacOS uses
opendirectoryd as user directory

client/ssh/lookup.go

@@ -0,0 +1,10 @@
+//go:build !darwin
+// +build !darwin
+
+package ssh
+
+import "os/user"
+
+func userNameLookup(username string) (*user.User, error) {
+	return user.Lookup(username)
+}

client/ssh/lookup_darwin.go

@@ -0,0 +1,47 @@
+//go:build darwin
+// +build darwin
+
+package ssh
+
+import (
+	"bytes"
+	"fmt"
+	"os/exec"
+	"os/user"
+	"strings"
+)
+
+func userNameLookup(username string) (*user.User, error) {
+	var userObject *user.User
+	userObject, err := user.Lookup(username)
+	if err != nil && err.Error() == user.UnknownUserError(username).Error() {
+		return idUserNameLookup(username)
+	} else if err != nil {
+		return nil, err
+	}
+
+	return userObject, nil
+}
+
+func idUserNameLookup(username string) (*user.User, error) {
+	cmd := exec.Command("id", "-P", username)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return nil, fmt.Errorf("error while retrieving user with id -P command, error: %v", err)
+	}
+	colon := ":"
+
+	if !bytes.Contains(out, []byte(username+colon)) {
+		return nil, fmt.Errorf("unable to find user in returned string")
+	}
+	// netbird:********:501:20::0:0:netbird:/Users/netbird:/bin/zsh
+	parts := strings.SplitN(string(out), colon, 10)
+	userObject := &user.User{
+		Username: parts[0],
+		Uid:      parts[2],
+		Gid:      parts[3],
+		Name:     parts[7],
+		HomeDir:  parts[8],
+	}
+	return userObject, nil
+}

client/ssh/server.go

@@ -137,7 +137,7 @@ func (srv *DefaultServer) sessionHandler(session ssh.Session) {
 		}
 	}()
 
-	localUser, err := user.Lookup(session.User())
+	localUser, err := userNameLookup(session.User())
 	if err != nil {
 		_, err = fmt.Fprintf(session, "remote SSH server couldn't find local user %s\n", session.User()) //nolint
 		err = session.Exit(1)