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.

hostwatch.py

@@ -4,6 +4,7 @@ if not globals().get('skip_imports'):
     from helpers import *
 
 POLL_TIME = 60*15
+CACHEFILE=os.path.expanduser('~/.sshuttle.hosts')
 
 
 _nmb_ok = True
@@ -17,6 +18,39 @@ def _is_ip(s):
     return re.match(r'\d+\.\d+\.\d+\.\d+$', s)
 
 
+def write_host_cache():
+    tmpname = '%s.%d.tmp' % (CACHEFILE, os.getpid())
+    try:
+        f = open(tmpname, 'wb')
+        for name,ip in sorted(hostnames.items()):
+            f.write('%s,%s\n' % (name, ip))
+        f.close()
+        os.rename(tmpname, CACHEFILE)
+    finally:
+        try:
+            os.unlink(tmpname)
+        except:
+            pass
+
+
+def read_host_cache():
+    try:
+        f = open(CACHEFILE)
+    except IOError, e:
+        if e.errno == errno.ENOENT:
+            return
+        else:
+            raise
+    for line in f:
+        words = line.strip().split(',')
+        if len(words) == 2:
+            (name,ip) = words
+            name = re.sub(r'[^-\w]', '-', name).strip()
+            ip = re.sub(r'[^0-9.]', '', ip).strip()
+            if name and ip:
+                found_host(name, ip)
+                
+
 def found_host(hostname, ip):
     hostname = re.sub(r'\..*', '', hostname)
     hostname = re.sub(r'[^-\w]', '_', hostname)
@@ -27,6 +61,7 @@ def found_host(hostname, ip):
         hostnames[hostname] = ip
         debug1('Found: %s: %s\n' % (hostname, ip))
         sys.stdout.write('%s,%s\n' % (hostname, ip))
+        write_host_cache()
 
 
 def _check_etc_hosts():
@@ -187,6 +222,8 @@ def hw_main(seed_hosts):
         helpers.logprefix = 'HH: '
     else:
         helpers.logprefix = 'hostwatch: '
+
+    read_host_cache()
         
     _enqueue(_check_etc_hosts)
     check_host('localhost')