auto-hosts: don't add hosts that aren't being routed by sshuttle.

I've been meaning to add this patch for a long time, but it's especially
important once we add FQDN support to --auto-hosts.  Basically, auto-hosts
will still discover all the hostnames it can, but we'll only add them to
/etc/hosts if their IP address is in one of the routed subnet ranges.  That
prevents polluting the /etc/hosts file with cruft.

firewall.py

@@ -430,6 +430,19 @@ def restore_etc_hosts(port):
     rewrite_etc_hosts(port)
 
 
+def _mask(ip, width):
+    nip = struct.unpack('!I', socket.inet_aton(ip))[0]
+    masked = nip & shl(shl(1, width) - 1, 32-width)
+    return socket.inet_ntoa(struct.pack('!I', masked))
+
+
+def ip_in_subnets(ip, subnets):
+    for swidth,sexclude,snet in sorted(subnets, reverse=True):
+        if _mask(snet, swidth) == _mask(ip, swidth):
+            return not sexclude
+    return False
+
+
 # This is some voodoo for setting up the kernel's transparent
 # proxying stuff.  If subnets is empty, we just delete our sshuttle rules;
 # otherwise we delete it, then make them from scratch.
@@ -521,8 +534,9 @@ def main(port, dnsport, syslog):
             line = sys.stdin.readline(128)
             if line.startswith('HOST '):
                 (name,ip) = line[5:].strip().split(',', 1)
-                hostmap[name] = ip
-                rewrite_etc_hosts(port)
+                if ip_in_subnets(ip, subnets):
+                    hostmap[name] = ip
+                    rewrite_etc_hosts(port)
             elif line:
                 raise Fatal('expected EOF, got %r' % line)
             else:

helpers.py

@@ -78,3 +78,10 @@ def islocal(ip):
     return True  # it's a local IP, or there would have been an error
 
 
+def shl(n, bits):
+    # we use our own implementation of left-shift because
+    # results may be different between older and newer versions
+    # of python for numbers like 1<<32.  We use long() because
+    # int(2**32) doesn't work in older python, which has limited
+    # int sizes.
+    return n * long(2**bits)

server.py

@@ -37,20 +37,11 @@ def _maskbits(netmask):
     if not netmask:
         return 32
     for i in range(32):
-        if netmask[0] & _shl(1, i):
+        if netmask[0] & shl(1, i):
             return 32-i
     return 0
 
 
-def _shl(n, bits):
-    # we use our own implementation of left-shift because
-    # results may be different between older and newer versions
-    # of python for numbers like 1<<32.  We use long() because
-    # int(2**32) doesn't work in older python, which has limited
-    # int sizes.
-    return n * long(2**bits)
-
-
 def _list_routes():
     argv = ['netstat', '-rn']
     p = ssubprocess.Popen(argv, stdout=ssubprocess.PIPE)
@@ -63,7 +54,7 @@ def _list_routes():
         maskw = _ipmatch(cols[2])  # linux only
         mask = _maskbits(maskw)   # returns 32 if maskw is null
         width = min(ipw[1], mask)
-        ip = ipw[0] & _shl(_shl(1, width) - 1, 32-width)
+        ip = ipw[0] & shl(shl(1, width) - 1, 32-width)
         routes.append((socket.inet_ntoa(struct.pack('!I', ip)), width))
     rv = p.wait()
     if rv != 0: