Merge branch 'dns'

* dns:
  dns on MacOS: use divert sockets instead of 'fwd' rules.
  client.py: do DNS listener on the same port as the TCP listener.
  Move client._islocal() to helpers.islocal() in preparation for sharing.
  dns: add support for MacOS (but it doesn't work...)
  Oops, dns_done() crashed if the request had already been timed out.
  dns: trim DNS channel handlers after a response, or after a timeout.
  dns: extract 'nameserver' lines from /etc/resolv.conf
  Extremely basic, but functional, DNS proxying support (--dns option)

Author: apenwarr

client.py

@@ -1,26 +1,11 @@
-import struct, socket, select, errno, re, signal
+import struct, socket, select, errno, re, signal, time
 import compat.ssubprocess as ssubprocess
 import helpers, ssnet, ssh, ssyslog
 from ssnet import SockWrapper, Handler, Proxy, Mux, MuxWrapper
 from helpers import *
 
 _extra_fd = os.open('/dev/null', os.O_RDONLY)
 
-def _islocal(ip):
-    sock = socket.socket()
-    try:
-        try:
-            sock.bind((ip, 0))
-        except socket.error, e:
-            if e.args[0] == errno.EADDRNOTAVAIL:
-                return False  # not a local IP
-            else:
-                raise
-    finally:
-        sock.close()
-    return True  # it's a local IP, or there would have been an error
-
-
 def got_signal(signum, frame):
     log('exiting on signal %d\n' % signum)
     sys.exit(1)
@@ -111,14 +96,15 @@ def original_dst(sock):
 
 
 class FirewallClient:
-    def __init__(self, port, subnets_include, subnets_exclude):
+    def __init__(self, port, subnets_include, subnets_exclude, dnsport):
         self.port = port
         self.auto_nets = []
         self.subnets_include = subnets_include
         self.subnets_exclude = subnets_exclude
+        self.dnsport = dnsport
         argvbase = ([sys.argv[0]] +
                     ['-v'] * (helpers.verbose or 0) +
-                    ['--firewall', str(port)])
+                    ['--firewall', str(port), str(dnsport)])
         if ssyslog._p:
             argvbase += ['--syslog']
         argv_tries = [
@@ -190,7 +176,7 @@ def done(self):
 
 
 def _main(listener, fw, ssh_cmd, remotename, python, latency_control,
-          seed_hosts, auto_nets,
+          dnslistener, seed_hosts, auto_nets,
           syslog, daemon):
     handlers = []
     if helpers.verbose >= 1:
@@ -282,7 +268,7 @@ def onaccept():
         dstip = original_dst(sock)
         debug1('Accept: %s:%r -> %s:%r.\n' % (srcip[0],srcip[1],
                                               dstip[0],dstip[1]))
-        if dstip[1] == listener.getsockname()[1] and _islocal(dstip[0]):
+        if dstip[1] == listener.getsockname()[1] and islocal(dstip[0]):
             debug1("-- ignored: that's my address!\n")
             sock.close()
             return
@@ -292,6 +278,30 @@ def onaccept():
         handlers.append(Proxy(SockWrapper(sock, sock), outwrap))
     handlers.append(Handler([listener], onaccept))
 
+    dnsreqs = {}
+    def dns_done(chan, data):
+        peer,timeout = dnsreqs.get(chan) or (None,None)
+        debug3('dns_done: channel=%r peer=%r\n' % (chan, peer))
+        if peer:
+            del dnsreqs[chan]
+            debug3('doing sendto %r\n' % (peer,))
+            dnslistener.sendto(data, peer)
+    def ondns():
+        pkt,peer = dnslistener.recvfrom(4096)
+        now = time.time()
+        if pkt:
+            debug1('DNS request from %r: %d bytes\n' % (peer, len(pkt)))
+            chan = mux.next_channel()
+            dnsreqs[chan] = peer,now+30
+            mux.send(chan, ssnet.CMD_DNS_REQ, pkt)
+            mux.channels[chan] = lambda cmd,data: dns_done(chan,data)
+        for chan,(peer,timeout) in dnsreqs.items():
+            if timeout < now:
+                del dnsreqs[chan]
+        debug3('Remaining DNS requests: %d\n' % len(dnsreqs))
+    if dnslistener:
+        handlers.append(Handler([dnslistener], ondns))
+
     if seed_hosts != None:
         debug1('seed_hosts: %r\n' % seed_hosts)
         mux.send(0, ssnet.CMD_HOST_REQ, '\n'.join(seed_hosts))
@@ -307,7 +317,7 @@ def onaccept():
         mux.callback()
 
 
-def main(listenip, ssh_cmd, remotename, python, latency_control,
+def main(listenip, ssh_cmd, remotename, python, latency_control, dns,
          seed_hosts, auto_nets,
          subnets_include, subnets_exclude, syslog, daemon, pidfile):
     if syslog:
@@ -319,8 +329,7 @@ def main(listenip, ssh_cmd, remotename, python, latency_control,
             log("%s\n" % e)
             return 5
     debug1('Starting sshuttle proxy.\n')
-    listener = socket.socket()
-    listener.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+    
     if listenip[1]:
         ports = [listenip[1]]
     else:
@@ -330,8 +339,13 @@ def main(listenip, ssh_cmd, remotename, python, latency_control,
     debug2('Binding:')
     for port in ports:
         debug2(' %d' % port)
+        listener = socket.socket()
+        listener.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+        dnslistener = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+        dnslistener.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
         try:
             listener.bind((listenip[0], port))
+            dnslistener.bind((listenip[0], port))
             bound = True
             break
         except socket.error, e:
@@ -344,11 +358,20 @@ def main(listenip, ssh_cmd, remotename, python, latency_control,
     listenip = listener.getsockname()
     debug1('Listening on %r.\n' % (listenip,))
 
-    fw = FirewallClient(listenip[1], subnets_include, subnets_exclude)
+    if dns:
+        dnsip = dnslistener.getsockname()
+        debug1('DNS listening on %r.\n' % (dnsip,))
+        dnsport = dnsip[1]
+    else:
+        dnsport = 0
+        dnslistener = None
+        dnslistener.bind((listenip[0], 0))
+
+    fw = FirewallClient(listenip[1], subnets_include, subnets_exclude, dnsport)
 
     try:
         return _main(listener, fw, ssh_cmd, remotename,
-                     python, latency_control,
+                     python, latency_control, dnslistener,
                      seed_hosts, auto_nets, syslog, daemon)
     finally:
         try:

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']
@@ -49,7 +52,7 @@ def ipt_ttl(*args):
 # 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").
-def do_iptables(port, subnets):
+def do_iptables(port, dnsport, subnets):
     chain = 'sshuttle-%s' % port
 
     # basic cleanup/setup of chains
@@ -59,12 +62,13 @@ def do_iptables(port, subnets):
         ipt('-F', chain)
         ipt('-X', chain)
 
-    if subnets:
+    if subnets or dnsport:
         ipt('-N', chain)
         ipt('-F', chain)
         ipt('-I', 'OUTPUT', '1', '-j', chain)
         ipt('-I', 'PREROUTING', '1', '-j', chain)
 
+    if subnets:
         # create new subnet entries.  Note that we're sorting in a very
         # particular order: we need to go from most-specific (largest swidth)
         # to least-specific, and at any given level of specificity, we want
@@ -80,6 +84,15 @@ def do_iptables(port, subnets):
                         '--dest', '%s/%s' % (snet,swidth),
                         '-p', 'tcp',
                         '--to-ports', str(port))
+                
+    if dnsport:
+        nslist = resolvconf_nameservers()
+        for ip in nslist:
+            ipt_ttl('-A', chain, '-j', 'REDIRECT',
+                    '--dest', '%s/32' % ip,
+                    '-p', 'udp',
+                    '--dport', '53',
+                    '--to-ports', str(dnsport))
 
 
 def ipfw_rule_exists(n):
@@ -88,7 +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
+            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())
@@ -135,6 +148,39 @@ def sysctl_set(name, val):
     if val != oldval:
         _changedctls.append(name)
         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):
@@ -145,7 +191,7 @@ def ipfw(*args):
         raise Fatal('%r returned %d' % (argv, rv))
 
 
-def do_ipfw(port, subnets):
+def do_ipfw(port, dnsport, subnets):
     sport = str(port)
     xsport = str(port+1)
 
@@ -158,13 +204,14 @@ def do_ipfw(port, subnets):
         oldval = _oldctls[name]
         _sysctl_set(name, oldval)
 
-    if subnets:
+    if subnets or dnsport:
         sysctl_set('net.inet.ip.fw.enable', 1)
         sysctl_set('net.inet.ip.scopedroute', 0)
 
         ipfw('add', sport, 'check-state', 'ip',
              'from', 'any', 'to', 'any')
-        
+
+    if subnets:
         # create new subnet entries
         for swidth,sexclude,snet in sorted(subnets, reverse=True):
             if sexclude:
@@ -177,6 +224,65 @@ def do_ipfw(port, 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:
+            # relabel and then catch outgoing DNS requests
+            ipfw('add', sport, 'divert', sport,
+                 'log', 'udp',
+                 'from', 'any', 'to', '%s/32' % ip, '53',
+                 '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):
     paths = (os.getenv('PATH') or os.defpath).split(os.pathsep)
@@ -185,6 +291,7 @@ 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'
@@ -235,9 +342,11 @@ def restore_etc_hosts(port):
 # 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 rules
 # are hopefully harmless.
-def main(port, syslog):
+def main(port, dnsport, syslog):
     assert(port > 0)
     assert(port <= 65535)
+    assert(dnsport >= 0)
+    assert(dnsport <= 65535)
 
     if os.getuid() != 0:
         raise Fatal('you must be root (or enable su/sudo) to set the firewall')
@@ -291,7 +400,7 @@ def main(port, syslog):
     try:
         if line:
             debug1('firewall manager: starting transproxy.\n')
-            do_it(port, subnets)
+            do_wait = do_it(port, dnsport, subnets)
             sys.stdout.write('STARTED\n')
 
         try:
@@ -305,6 +414,7 @@ def main(port, 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)
@@ -319,5 +429,5 @@ def main(port, syslog):
             debug1('firewall manager: undoing changes.\n')
         except:
             pass
-        do_it(port, [])
+        do_it(port, 0, [])
         restore_etc_hosts(port)