Refactor automatic method selection.

Add an "is_supported()" function to the different methods so that each
method can include whatever logic they wish to indicate if they are
supported on a particular machine. Previously, methods/__init__.py
contained all of the logic for selecting individual methods. Now, it
iterates through a list of possible options and stops on the first
method that it finds that is_supported().

Currently, the decision is made based on the presence of programs in
the PATH. In the future, things such as the platform sshuttle is
running on could be considered.

sshuttle/firewall.py

@@ -114,6 +114,11 @@ def main(method_name, syslog):
         ssyslog.start_syslog()
         ssyslog.stderr_to_syslog()
 
+    if not method.is_supported():
+        raise Fatal("The %s method is not supported on this machine. "
+                    "Check that the appropriate programs are in your "
+                    "PATH." % method_name)
+
     debug1('ready method name %s.\n' % method.name)
     stdout.write('READY %s\n' % method.name)
     stdout.flush()

sshuttle/methods/__init__.py

@@ -3,7 +3,7 @@ import socket
 import struct
 import errno
 import ipaddress
-from sshuttle.helpers import Fatal, debug3, which
+from sshuttle.helpers import Fatal, debug3
 
 
 def original_dst(sock):
@@ -54,6 +54,12 @@ class BaseMethod(object):
         result.user = False
         return result
 
+    @staticmethod
+    def is_supported():
+        """Returns true if it appears that this method will work on this
+        machine."""
+        return False
+
     @staticmethod
     def get_tcp_dstip(sock):
         return original_dst(sock)
@@ -102,16 +108,15 @@ def get_method(method_name):
 
 
 def get_auto_method():
-    if which('iptables'):
-        method_name = "nat"
-    elif which('nft'):
-        method_name = "nft"
-    elif which('pfctl'):
-        method_name = "pf"
-    elif which('ipfw'):
-        method_name = "ipfw"
-    else:
-        raise Fatal(
-            "can't find either iptables, nft or pfctl; check your PATH")
+    debug3("Selecting a method automatically...\n")
+    # Try these methods, in order:
+    methods_to_try = ["nat", "nft", "pf", "ipfw"]
+    for m in methods_to_try:
+        method = get_method(m)
+        if method.is_supported():
+            debug3("Method '%s' was automatically selected.\n" % m)
+            return method
 
-    return get_method(method_name)
+    raise Fatal("Unable to automatically find a supported method. Check that "
+                "the appropriate programs are in your PATH. We tried "
+                "methods: %s" % str(methods_to_try))

sshuttle/methods/ipfw.py

@@ -1,8 +1,8 @@
 import os
 import subprocess as ssubprocess
 from sshuttle.methods import BaseMethod
-from sshuttle.helpers import log, debug1, debug3, \
-    Fatal, family_to_string, get_env
+from sshuttle.helpers import log, debug1, debug2, debug3, \
+    Fatal, family_to_string, get_env, which
 
 recvmsg = None
 try:
@@ -253,3 +253,10 @@ class Method(BaseMethod):
         ipfw_noexit('table', '124', 'flush')
         ipfw_noexit('table', '125', 'flush')
         ipfw_noexit('table', '126', 'flush')
+
+    def is_supported(self):
+        if which("ipfw"):
+            return True
+        debug2("ipfw method not supported because 'ipfw' command is "
+               "missing.\n")
+        return False

sshuttle/methods/nat.py

@@ -1,6 +1,6 @@
 import socket
 from sshuttle.firewall import subnet_weight
-from sshuttle.helpers import family_to_string
+from sshuttle.helpers import family_to_string, which, debug2
 from sshuttle.linux import ipt, ipt_ttl, ipt_chain_exists, nonfatal
 from sshuttle.methods import BaseMethod
 
@@ -124,3 +124,10 @@ class Method(BaseMethod):
         result = super(Method, self).get_supported_features()
         result.user = True
         return result
+
+    def is_supported(self):
+        if which("iptables"):
+            return True
+        debug2("nat method not supported because 'iptables' command "
+               "is missing.\n")
+        return False

sshuttle/methods/nft.py

@@ -2,6 +2,7 @@ import socket
 from sshuttle.firewall import subnet_weight
 from sshuttle.linux import nft, nonfatal
 from sshuttle.methods import BaseMethod
+from sshuttle.helpers import debug2, which
 
 
 class Method(BaseMethod):
@@ -113,3 +114,9 @@ class Method(BaseMethod):
         result = super(Method, self).get_supported_features()
         result.ipv6 = True
         return result
+
+    def is_supported(self):
+        if which("nft"):
+            return True
+        debug2("nft method not supported because 'nft' command is missing.\n")
+        return False

sshuttle/methods/pf.py

@@ -12,7 +12,7 @@ from ctypes import c_char, c_uint8, c_uint16, c_uint32, Union, Structure, \
     sizeof, addressof, memmove
 from sshuttle.firewall import subnet_weight
 from sshuttle.helpers import debug1, debug2, debug3, Fatal, family_to_string, \
-    get_env
+    get_env, which
 from sshuttle.methods import BaseMethod
 
 
@@ -491,3 +491,9 @@ class Method(BaseMethod):
             return True
         else:
             return False
+
+    def is_supported(self):
+        if which("pfctl"):
+            return True
+        debug2("pf method not supported because 'pfctl' command is missing.\n")
+        return False

sshuttle/methods/tproxy.py

@@ -3,7 +3,7 @@ from sshuttle.firewall import subnet_weight
 from sshuttle.helpers import family_to_string
 from sshuttle.linux import ipt, ipt_ttl, ipt_chain_exists
 from sshuttle.methods import BaseMethod
-from sshuttle.helpers import debug1, debug3, Fatal
+from sshuttle.helpers import debug1, debug2, debug3, Fatal, which
 
 recvmsg = None
 try:
@@ -294,3 +294,10 @@ class Method(BaseMethod):
         if ipt_chain_exists(family, table, divert_chain):
             _ipt('-F', divert_chain)
             _ipt('-X', divert_chain)
+
+    def is_supported(self):
+        if which("iptables") and which("ip6tables"):
+            return True
+        debug2("tproxy method not supported because 'iptables' "
+               "or 'ip6tables' commands are missing.\n")
+        return False

tests/client/test_firewall.py

@@ -116,6 +116,8 @@ def test_main(mock_get_method, mock_setup_daemon, mock_rewrite_etc_hosts):
     assert mock_setup_daemon.mock_calls == [call()]
     assert mock_get_method.mock_calls == [
         call('not_auto'),
+        call().is_supported(),
+        call().is_supported().__bool__(),
         call().setup_firewall(
             1024, 1026,
             [(AF_INET6, u'2404:6800:4004:80c::33')],