Don't die if iptables doesn't have 'ttl match' support.

ttl matching is only needed if your server is the same machine as the
client, which is kind of useless anyway (other than for testing), so there's
no reason for it to be fatal if that doesn't work.

Reported by "Alphazo" on the mailing list, who managed to get sshuttle
working on his Nokia N900 by removing the ttl stuff.

firewall.py

@@ -23,6 +23,27 @@ def ipt(*args):
         raise Fatal('%r returned %d' % (argv, rv))
 
 
+_no_ttl_module = False
+def ipt_ttl(*args):
+    global _no_ttl_module
+    if not _no_ttl_module:
+        # we avoid infinite loops by generating server-side connections
+        # with ttl 42.  This makes the client side not recapture those
+        # connections, in case client == server.
+        try:
+            argsplus = list(args) + ['-m', 'ttl', '!', '--ttl', '42']
+            ipt(*argsplus)
+        except Fatal:
+            ipt(*args)
+            # we only get here if the non-ttl attempt succeeds
+            log('sshuttle: warning: your iptables is missing '
+                'the ttl module.\n')
+            _no_ttl_module = True
+    else:
+        ipt(*args)
+
+
+
 # We name the chain based on the transproxy port number so that it's possible
 # to run multiple copies of sshuttle at the same time.  Of course, the
 # multiple copies shouldn't have overlapping subnets, or only the most-
@@ -55,12 +76,10 @@ def do_iptables(port, subnets):
                     '--dest', '%s/%s' % (snet,swidth),
                     '-p', 'tcp')
             else:
-                ipt('-A', chain, '-j', 'REDIRECT',
+                ipt_ttl('-A', chain, '-j', 'REDIRECT',
                         '--dest', '%s/%s' % (snet,swidth),
                         '-p', 'tcp',
-                    '--to-ports', str(port),
-                    '-m', 'ttl', '!', '--ttl', '42'  # to prevent infinite loops
-                    )
+                        '--to-ports', str(port))
 
 
 def ipfw_rule_exists(n):