dns on MacOS: use divert sockets instead of 'fwd' rules.

It turns out diverting UDP sockets is pretty easy compared to TCP (which
makes it all the more embarrassing that they screwed up 'fwd' support for
UDP and not TCP, but oh well).  So let's use divert sockets instead of
transproxy for our DNS packets.

This is a little tricky because we have to do it all in firewall.py, since
divert sockets require root access, and only firewall.py has root access.

firewall.py

@@ -1,8 +1,11 @@
-import re, errno
+import re, errno, socket, select, struct
 import compat.ssubprocess as ssubprocess
 import helpers, ssyslog
 from helpers import *
 
+# python doesn't have a definition for this
+IPPROTO_DIVERT = 254
+
 
 def ipt_chain_exists(name):
     argv = ['iptables', '-t', 'nat', '-nL']
@@ -98,8 +101,7 @@ def ipfw_rule_exists(n):
     found = False
     for line in p.stdout:
         if line.startswith('%05d ' % n):
-            if not ('ipttl 42 setup keep-state' in line
-                    or 'ipttl 42 keep-state' in line
+            if not ('ipttl 42' in line
                     or ('skipto %d' % (n+1)) in line
                     or 'check-state' in line):
                 log('non-sshuttle ipfw rule: %r\n' % line.strip())
@@ -148,6 +150,39 @@ def sysctl_set(name, val):
         return _sysctl_set(name, val)
 
 
+def _udp_unpack(p):
+    src = (socket.inet_ntoa(p[12:16]), struct.unpack('!H', p[20:22])[0])
+    dst = (socket.inet_ntoa(p[16:20]), struct.unpack('!H', p[22:24])[0])
+    return src, dst
+
+
+def _udp_repack(p, src, dst):
+    addrs = socket.inet_aton(src[0]) + socket.inet_aton(dst[0])
+    ports = struct.pack('!HH', src[1], dst[1])
+    return p[:12] + addrs + ports + p[24:]
+
+
+_real_dns_server = [None]
+def _handle_diversion(divertsock, dnsport):
+    p,tag = divertsock.recvfrom(4096)
+    src,dst = _udp_unpack(p)
+    debug3('got diverted packet from %r to %r\n' % (src, dst))
+    if dst[1] == 53:
+        # outgoing DNS
+        debug3('...packet is a DNS request.\n')
+        _real_dns_server[0] = dst
+        dst = ('127.0.0.1', dnsport)
+    elif src[1] == dnsport:
+        if islocal(src[0]):
+            debug3('...packet is a DNS response.\n')
+            src = _real_dns_server[0]
+    else:
+        log('weird?! unexpected divert from %r to %r\n' % (src, dst))
+        assert(0)
+    newp = _udp_repack(p, src, dst)
+    divertsock.sendto(newp, tag)
+    
+
 def ipfw(*args):
     argv = ['ipfw', '-q'] + list(args)
     debug1('>> %s\n' % ' '.join(argv))
@@ -189,13 +224,64 @@ def do_ipfw(port, dnsport, subnets):
                      'from', 'any', 'to', '%s/%s' % (snet,swidth),
                      'not', 'ipttl', '42', 'keep-state', 'setup')
 
+    # This part is much crazier than it is on Linux, because MacOS (at least
+    # 10.6, and probably other versions, and maybe FreeBSD too) doesn't
+    # correctly fixup the dstip/dstport for UDP packets when it puts them
+    # through a 'fwd' rule.  It also doesn't fixup the srcip/srcport in the
+    # response packet.  In Linux iptables, all that happens magically for us,
+    # so we just redirect the packets and relax.
+    #
+    # On MacOS, we have to fix the ports ourselves.  For that, we use a
+    # 'divert' socket, which receives raw packets and lets us mangle them.
+    #
+    # Here's how it works.  Let's say the local DNS server is 1.1.1.1:53,
+    # and the remote DNS server is 2.2.2.2:53, and the local transproxy port
+    # is 10.0.0.1:12300, and a client machine is making a request from
+    # 10.0.0.5:9999. We see a packet like this:
+    #    10.0.0.5:9999 -> 1.1.1.1:53
+    # Since the destip:port matches one of our local nameservers, it will
+    # match a 'fwd' rule, thus grabbing it on the local machine.  However,
+    # the local kernel will then see a packet addressed to *:53 and
+    # not know what to do with it; there's nobody listening on port 53.  Thus,
+    # we divert it, rewriting it into this:
+    #    10.0.0.5:9999 -> 10.0.0.1:12300
+    # This gets proxied out to the server, which sends it to 2.2.2.2:53,
+    # and the answer comes back, and the proxy sends it back out like this:
+    #    10.0.0.1:12300 -> 10.0.0.5:9999
+    # But that's wrong!  The original machine expected an answer from
+    # 1.1.1.1:53, so we have to divert the *answer* and rewrite it:
+    #    1.1.1.1:53 -> 10.0.0.5:9999
+    #
+    # See?  Easy stuff.
     if dnsport:
+        divertsock = socket.socket(socket.AF_INET, socket.SOCK_RAW,
+                                   IPPROTO_DIVERT)
+        divertsock.bind(('0.0.0.0', port)) # IP field is ignored
+
         nslist = resolvconf_nameservers()
         for ip in nslist:
-            ipfw('add', sport, 'fwd', '127.0.0.1,%d' % dnsport,
+            # relabel and then catch outgoing DNS requests
+            ipfw('add', sport, 'divert', sport,
                  'log', 'udp',
                  'from', 'any', 'to', '%s/32' % ip, '53',
-                 'not', 'ipttl', '42', 'keep-state')
+                 'not', 'ipttl', '42')
+        # relabel DNS responses
+        ipfw('add', sport, 'divert', sport,
+             'log', 'udp',
+             'from', 'any', str(dnsport), 'to', 'any',
+             'not', 'ipttl', '42')
+
+        def do_wait():
+            while 1:
+                r,w,x = select.select([sys.stdin, divertsock], [], [])
+                if divertsock in r:
+                    _handle_diversion(divertsock, dnsport)
+                if sys.stdin in r:
+                    return
+    else:
+        do_wait = None
+        
+    return do_wait
 
 
 def program_exists(name):
@@ -314,7 +400,7 @@ def main(port, dnsport, syslog):
     try:
         if line:
             debug1('firewall manager: starting transproxy.\n')
-            do_it(port, dnsport, subnets)
+            do_wait = do_it(port, dnsport, subnets)
             sys.stdout.write('STARTED\n')
         
         try:
@@ -328,6 +414,7 @@ def main(port, dnsport, syslog):
         # to stay running so that we don't need a *second* password
         # authentication at shutdown time - that cleanup is important!
         while 1:
+            if do_wait: do_wait()
             line = sys.stdin.readline(128)
             if line.startswith('HOST '):
                 (name,ip) = line[5:].strip().split(',', 1)