Allow use of sudo or doas.

This is an alternative solution to pull request #611.

Previously, sshuttle would use doas on OpenBSD and sudo on Linux.
However, some Linux distributions are opting to use doas.

This patch changes the logic so that there can be multiple attempts to
elevate privilages. If the first command fails to run, it moves on to
the next command. Part of the existing code looked like it might be
attempting to do this, but it didn't work.

It also looks for the presence of doas and sudo in the path. If we can
find doas (but cannot find sudo) or if the platform is OpenBSD, we try
doas first. Otherwise, we try sudo, then doas. We try all the options
until one succeeds (including running the command without sudo or
doas) regardless of what is in the path. I'm open to adjusting
the logic here based on feedback.

If systems have both sudo and doas, they might be configured to give
different users different permissions. For example, if a user wishes
to use doas on this system, sshuttle would try sudo first and the user
would need to enter invalid passwords to eventually cause sudo to fail
and cause sshuttle to then try doas. This might not be ideal, but it
avoids implement another sshuttle argument that the user would need to
specify. Perhaps machines actually using doas will not have sudo
installed?

sshuttle/client.py

@@ -197,55 +197,101 @@ class FirewallClient:
         if ssyslog._p:
             argvbase += ['--syslog']
 
-        # Determine how to prefix the command in order to elevate privileges.
+        # A list of commands that we can try to run to start the firewall.
-        if platform.platform().startswith('OpenBSD'):
+        argv_tries = []
-            elev_prefix = ['doas']  # OpenBSD uses built in `doas`
+
+        if os.getuid() == 0:  # No need to elevate privileges
+            argv_tries.append(argvbase)
         else:
-            elev_prefix = ['sudo', '-p', '[local sudo] Password: ']
+            # Linux typically uses sudo; OpenBSD uses doas. However, some
+            # Linux distributions are starting to use doas.
+            sudo_cmd = ['sudo', '-p', '[local sudo] Password: ']+argvbase
+            doas_cmd = ['doas']+argvbase
 
-        # Look for binary and switch to absolute path if we can find
+            # For clarity, try to replace executable name with the
-        # it.
+            # full path.
-        path = which(elev_prefix[0])
+            doas_path = which("doas")
-        if path:
+            if doas_path:
-            elev_prefix[0] = path
+                doas_cmd[0] = doas_path
+            sudo_path = which("sudo")
+            if sudo_path:
+                sudo_cmd[0] = sudo_path
 
+            # sudo_pythonpath indicates if we should set the
+            # PYTHONPATH environment variable when elevating
+            # privileges. This can be adjusted with the
+            # --no-sudo-pythonpath option.
             if sudo_pythonpath:
-            elev_prefix += ['/usr/bin/env',
+                pp_prefix = ['/usr/bin/env',
                              'PYTHONPATH=%s' %
                              os.path.dirname(os.path.dirname(__file__))]
-        argv_tries = [elev_prefix + argvbase, argvbase]
+                sudo_cmd = pp_prefix + sudo_cmd
+                doas_cmd = pp_prefix + doas_cmd
 
-        # we can't use stdin/stdout=subprocess.PIPE here, as we normally would,
+            # If we can find doas and not sudo or if we are on
-        # because stupid Linux 'su' requires that stdin be attached to a tty.
+            # OpenBSD, try using doas first.
-        # Instead, attach a *bidirectional* socket to its stdout, and use
+            if (doas_path and not sudo_path) or \
-        # that for talking in both directions.
+               platform.platform().startswith('OpenBSD'):
+                argv_tries = [doas_cmd, sudo_cmd, argvbase]
+            else:
+                argv_tries = [sudo_cmd, doas_cmd, argvbase]
+
+        # Try all commands in argv_tries in order. If a command
+        # produces an error, try the next one. If command is
+        # successful, set 'success' variable and break.
+        success = False
+        for argv in argv_tries:
+            # we can't use stdin/stdout=subprocess.PIPE here, as we
+            # normally would, because stupid Linux 'su' requires that
+            # stdin be attached to a tty. Instead, attach a
+            # *bidirectional* socket to its stdout, and use that for
+            # talking in both directions.
             (s1, s2) = socket.socketpair()
 
             def setup():
                 # run in the child process
                 s2.close()
-        if os.getuid() == 0:
+
-            argv_tries = argv_tries[-1:]  # last entry only
-        for argv in argv_tries:
             try:
-                if argv[0] == 'su':
+                debug1("Starting firewall manager with command: %r" % argv)
-                    sys.stderr.write('[local su] ')
                 self.p = ssubprocess.Popen(argv, stdout=s1, preexec_fn=setup)
                 # No env: Talking to `FirewallClient.start`, which has no i18n.
-                break
             except OSError as e:
-                log('Spawning firewall manager: %r' % argv)
+                # This exception will occur if the program isn't
-                raise Fatal(e)
+                # present or isn't executable.
+                debug1('Unable to start firewall manager. Popen failed. '
+                       'Command=%r Exception=%s' % (argv, e))
+                continue
+
             self.argv = argv
             s1.close()
             self.pfile = s2.makefile('rwb')
             line = self.pfile.readline()
-        self.check()
+
+            rv = self.p.poll()   # Check if process is still running
+            if rv:
+                # We might get here if program runs and exits before
+                # outputting anything. For example, someone might have
+                # entered the wrong password to elevate privileges.
+                debug1('Unable to start firewall manager. '
+                       'Process exited too early. '
+                       '%r returned %d' % (self.argv, rv))
+                continue
+
             if line[0:5] != b'READY':
-            raise Fatal('%r expected READY, got %r' % (self.argv, line))
+                debug1('Unable to start firewall manager. '
+                       'Expected READY, got %r. '
+                       'Command=%r' % (line, self.argv))
+                continue
+
             method_name = line[6:-1]
             self.method = get_method(method_name.decode("ASCII"))
             self.method.set_firewall(self)
+            success = True
+            break
+
+        if not success:
+            raise Fatal("All attempts to elevate privileges failed.")
 
     def setup(self, subnets_include, subnets_exclude, nslist,
               redirectport_v6, redirectport_v4, dnsport_v6, dnsport_v4, udp,