Added new --auto-hosts and --seed-hosts options to the client.

Now if you use --auto-hosts (-H), the client will ask the server to spawn a
hostwatcher to add names.  That, in turn, will send names back to the
server, which sends them back to the client, which sends them to the
firewall subprocess, which will write them to /etc/hosts.  Whew!

Only the firewall process can write to /etc/hosts, of course, because only
he's running as root.

Since the name discovery process is kind of slow, we cache the names in
~/.sshuttle.hosts on the remote server.

Right now, most of the names are discovered using nmblookup and smbclient,
as well as by reading the existing entries in /etc/hosts.  What would really
be nice would be to query active directory or mdns somehow... but I don't
really know how those work, so this is what you get for now :)  It's pretty
neat, at least.

firewall.py

@@ -1,4 +1,4 @@
-import subprocess, re
+import subprocess, re, errno
 import helpers
 from helpers import *
 
@@ -134,6 +134,38 @@ def program_exists(name):
         if os.path.exists(fn):
             return not os.path.isdir(fn) and os.access(fn, os.X_OK)
 
+hostmap = {}
+def rewrite_etc_hosts(port):
+    HOSTSFILE='/etc/hosts'
+    BAKFILE='%s.sbak' % HOSTSFILE
+    APPEND='# sshuttle-firewall-%d AUTOCREATED' % port
+    old_content = ''
+    try:
+        old_content = open(HOSTSFILE).read()
+    except IOError, e:
+        if e.errno == errno.ENOENT:
+            pass
+        else:
+            raise
+    if old_content.strip() and not os.path.exists(BAKFILE):
+        open(BAKFILE, 'w').write(old_content)
+    tmpname = "%s.%d.tmp" % (HOSTSFILE, port)
+    f = open(tmpname, 'w')
+    for line in old_content.rstrip().split('\n'):
+        if line.find(APPEND) >= 0:
+            continue
+        f.write('%s\n' % line)
+    for (name,ip) in sorted(hostmap.items()):
+        f.write('%-30s %s\n' % ('%s %s' % (ip,name), APPEND))
+    f.close()
+    os.rename(tmpname, HOSTSFILE)
+
+
+def restore_etc_hosts(port):
+    global hostmap
+    hostmap = {}
+    rewrite_etc_hosts(port)
+
 
 # This is some voodoo for setting up the kernel's transparent
 # proxying stuff.  If subnets is empty, we just delete our sshuttle rules;
@@ -199,20 +231,29 @@ def main(port):
         
         try:
             sys.stdout.flush()
-            
-            # 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(128):
-                pass
         except IOError:
             # the parent process died for some reason; he's surely been loud
             # enough, so no reason to report another error
             return
 
+        # 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 1:
+            line = sys.stdin.readline(128)
+            if line.startswith('HOST '):
+                (name,ip) = line[5:].strip().split(',', 1)
+                hostmap[name] = ip
+                rewrite_etc_hosts(port)
+            elif line:
+                raise Fatal('expected EOF, got %r' % line)
+            else:
+                break
+
     finally:
         try:
             debug1('firewall manager: undoing changes.\n')
         except:
             pass
         do_it(port, [])
+        restore_etc_hosts(port)