Smarter listenport selection.

Now if we aren't given an explicit port, we always initiate the port search
at 12300 and count upward looking for an available port.

Normally the kernel will assign us a random port, but that's not ideal
in our case because we'd like to use the same port numbers whenever
possible; that avoids piling up crap inside iptables in the (hopefully
unlikely) event that we die without cleaning up correctly.

client.py

@@ -1,4 +1,4 @@
-import struct, socket, select, subprocess
+import struct, socket, select, subprocess, errno
 from ssnet import SockWrapper, Handler, Proxy
 from helpers import *
 
@@ -58,9 +58,28 @@ def main(listenip, remotename, subnets):
     log('Starting sshuttle proxy.\n')
     listener = socket.socket()
     listener.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
-    listener.bind(listenip)
+    if listenip[1]:
+        ports = [listenip[1]]
+    else:
+        ports = xrange(12300,65536)
+    last_e = None
+    bound = False
+    log('Binding:')
+    for port in ports:
+        log(' %d' % port)
+        try:
+            listener.bind((listenip[0], port))
+            bound = True
+            break
+        except socket.error, e:
+            last_e = e
+    log('\n')
+    if not bound:
+        assert(last_e)
+        raise last_e
     listener.listen(10)
-    log('Listening on %r.\n' % (listener.getsockname(),))
+    listenip = listener.getsockname()
+    log('Listening on %r.\n' % (listenip,))
 
     iptables_setup(listenip[1], subnets)
 

iptables.py

@@ -21,7 +21,20 @@ def ipt(*args):
         raise Exception('%r returned %d' % (argv, rv))
 
 
-# FIXME: this prints scary-looking errors
+# 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)

main.py

@@ -66,10 +66,10 @@ elif opt.iptables:
                            parse_subnets(extra[1:])))
 else:
     if len(extra) < 1:
-        o.fatal('at least one argument expected')
+        o.fatal('at least one subnet expected')
     remotename = extra[0]
     if remotename == '' or remotename == '-':
         remotename = None
-    sys.exit(client.main(parse_ipport(opt.listen or '0.0.0.0:1234'),
+    sys.exit(client.main(parse_ipport(opt.listen or '0.0.0.0:0'),
                          remotename,
                          parse_subnets(extra)))