iptables: more resilient startup/cleanup.

Now the sudo iptables subprocess persists for the entire life of sshuttle.
The benefits of this are:

- no need to authenticate again at shutdown (failure of which could cause us
  to not clean up iptables)

- if the main process dies unexpectedly, iptables still gets cleaned up

- the password prompt can happen *before* starting the ssh/server process,
  which means it'll stand out and the password prompt won't be overwritten.

client.py

@@ -13,17 +13,40 @@ def original_dst(sock):
     return (ip,port)
 
 
-def iptables_setup(port, subnets):
-    subnets_str = ['%s/%d' % (ip,width) for ip,width in subnets]
-    argv = (['sudo', sys.argv[0]] +
-            ['-v'] * (helpers.verbose or 0) +
-            ['--iptables', str(port)] + subnets_str)
-    rv = subprocess.call(argv)
-    if rv != 0:
-        raise Fatal('%r returned %d' % (argv, rv))
+class IPTables:
+    def __init__(self, port, subnets):
+        self.port = port
+        self.subnets = subnets
+        subnets_str = ['%s/%d' % (ip,width) for ip,width in subnets]
+        self.argv = (['sudo', sys.argv[0]] +
+                     ['-v'] * (helpers.verbose or 0) +
+                     ['--iptables', str(port)] + subnets_str)
+        self.p = subprocess.Popen(self.argv,
+                                  stdin=subprocess.PIPE,
+                                  stdout=subprocess.PIPE)
+        line = self.p.stdout.readline()
+        self.check()
+        if line != 'READY\n':
+            raise Fatal('%r expected READY, got %r' % (self.argv, line))
+
+    def check(self):
+        rv = self.p.poll()
+        if rv:
+            raise Fatal('%r returned %d' % (self.argv, rv))
+
+    def start(self):
+        self.p.stdin.write('GO\n')
+        self.p.stdin.flush()
+
+    def done(self):
+        self.p.stdin.close()
+        self.p.stdout.close()
+        rv = self.p.wait()
+        if rv:
+            raise Fatal('cleanup: %r returned %d' % (self.argv, rv))
 
 
-def _main(listener, listenport, use_server, remotename, subnets):
+def _main(listener, ipt, use_server, remotename):
     handlers = []
     if use_server:
         if helpers.verbose >= 1:
@@ -47,7 +70,7 @@ def _main(listener, listenport, use_server, remotename, subnets):
 
     # we definitely want to do this *after* starting ssh, or we might end
     # up intercepting the ssh connection!
-    iptables_setup(listenport, subnets)
+    ipt.start()
 
     def onaccept():
         sock,srcip = listener.accept()
@@ -118,7 +141,9 @@ def main(listenip, use_server, remotename, subnets):
     listenip = listener.getsockname()
     debug1('Listening on %r.\n' % (listenip,))
 
+    ipt = IPTables(listenip[1], subnets)
+    
     try:
-        return _main(listener, listenip[1], use_server, remotename, subnets)
+        return _main(listener, ipt, use_server, remotename)
     finally:
-        iptables_setup(listenip[1], [])
+        ipt.done()

iptables.py

@@ -22,24 +22,7 @@ def ipt(*args):
         raise Exception('%r returned %d' % (argv, rv))
 
 
-# This is some iptables voodoo for setting up the Linux kernel's transparent
-# proxying stuff.  If subnets is empty, we just delete our sshuttle chain;
-# otherwise we delete it, then make it from scratch.
-#
-# 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-
-# recently-started one will win (because we use "-I OUTPUT 1" instead of
-# "-A OUTPUT").
-#
-# sshuttle is supposed to clean up after itself by deleting extra chains on
-# exit.  In case that fails, it's not the end of the world; future runs will
-# supercede it in the transproxy list, at least, so the leftover iptables
-# chains are mostly harmless.
-def main(port, subnets):
-    assert(port > 0)
-    assert(port <= 65535)
-
+def do_it(port, subnets):
     chain = 'sshuttle-%s' % port
 
     # basic cleanup/setup of chains
@@ -63,4 +46,45 @@ def main(port, subnets):
                 '--to-ports', str(port),
                 '-m', 'ttl', '!', '--ttl', '42'  # to prevent infinite loops
                 )
-    subnets_str = ['%s/%d' % (ip,width) for ip,width in subnets]
+
+
+# This is some iptables voodoo for setting up the Linux kernel's transparent
+# proxying stuff.  If subnets is empty, we just delete our sshuttle chain;
+# otherwise we delete it, then make it from scratch.
+#
+# 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-
+# recently-started one will win (because we use "-I OUTPUT 1" instead of
+# "-A OUTPUT").
+#
+# This code is supposed to clean up after itself by deleting extra chains on
+# exit.  In case that fails, it's not the end of the world; future runs will
+# supercede it in the transproxy list, at least, so the leftover iptables
+# chains are mostly harmless.
+def main(port, subnets):
+    assert(port > 0)
+    assert(port <= 65535)
+
+    debug1('iptables manager ready.\n')
+    sys.stdout.write('READY\n')
+    sys.stdout.flush()
+
+    # ctrl-c shouldn't be passed along to me.  When the main sshuttle dies,
+    # I'll die automatically.
+    os.setsid()
+
+    # we wait until we get some input before creating the rules.  That way,
+    # sshuttle can launch us as early as possible (and get sudo password
+    # authentication as early in the startup process as possible).
+    sys.stdin.readline()
+    try:
+        do_it(port, subnets)
+
+        # Now we wait until EOF or any other kind of exception.  We need
+        # to stay running so that we don't need a *second* password
+        # authentication at shutdown time - that cleanup is important!
+        while sys.stdin.readline():
+            pass
+    finally:
+        do_it(port, [])

main.py

@@ -83,5 +83,6 @@ except Fatal, e:
     log('fatal: %s\n' % e)
     sys.exit(99)
 except KeyboardInterrupt:
-    log('\nKeyboard interrupt: exiting.\n')
+    log('\n')
+    log('Keyboard interrupt: exiting.\n')
     sys.exit(1)

ssh.py

@@ -32,7 +32,8 @@ def connect(rhost):
         os.setsid()
     s1a,s1b = os.dup(s1.fileno()), os.dup(s1.fileno())
     s1.close()
-    p = subprocess.Popen(argv, stdin=s1a, stdout=s1b, preexec_fn=setup)
+    p = subprocess.Popen(argv, stdin=s1a, stdout=s1b, preexec_fn=setup,
+                         close_fds=True)
     os.close(s1a)
     os.close(s1b)
     return p, s2