Extremely basic, but functional, DNS proxying support (--dns option)

Limitations:
- uses a hardcoded DNS server IP on both client and server
- never expires request/response objects, so leaks memory and sockets
- works only with iptables, not with ipfw

5 files changed, 89 insertions, 15 deletions

diff --git a/client.py b/client.py
index e584933..7872bae 100644
--- a/client.py
+++ b/client.py
@@ -111,14 +111,15 @@ 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 +191,7 @@ class FirewallClient:
 
 
 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:
@@ -292,6 +293,25 @@ def _main(listener, fw, ssh_cmd, remotename, python, latency_control,
         handlers.append(Proxy(SockWrapper(sock, sock), outwrap))
     handlers.append(Handler([listener], onaccept))
 
+    dnspeers = {}
+    def dns_done(chan, data):
+        peer = dnspeers.get(chan)
+        debug1('dns_done: channel=%r peer=%r\n' % (chan, peer))
+        if peer:
+            del dnspeers[chan]
+            debug1('doing sendto %r\n' % (peer,))
+            dnslistener.sendto(data, peer)
+    def ondns():
+        pkt,peer = dnslistener.recvfrom(4096)
+        if pkt:
+            debug1('Got DNS request from %r: %d bytes\n' % (peer, len(pkt)))
+            chan = mux.next_channel()
+            dnspeers[chan] = peer
+            mux.send(chan, ssnet.CMD_DNS_REQ, pkt)
+            mux.channels[chan] = lambda cmd,data: dns_done(chan,data)
+    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 +327,7 @@ def _main(listener, fw, ssh_cmd, remotename, python, latency_control,
         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:
@@ -319,6 +339,7 @@ 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]:
@@ -344,11 +365,20 @@ 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)
+    dnsport = 0
+    dnslistener = None
+    if dns:
+        dnslistener = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+        dnslistener.bind((listenip[0], 0))
+        dnsip = dnslistener.getsockname()
+        debug1('DNS listening on %r.\n' % (dnsip,))
+        dnsport = dnsip[1]
+
+    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:
diff --git a/firewall.py b/firewall.py
index b63bffa..8ec67bc 100644
--- a/firewall.py
+++ b/firewall.py
@@ -49,7 +49,7 @@ 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
@@ -80,6 +80,13 @@ def do_iptables(port, subnets):
                         '--dest', '%s/%s' % (snet,swidth),
                         '-p', 'tcp',
                         '--to-ports', str(port))
+                
+    if dnsport:
+        ipt_ttl('-A', chain, '-j', 'REDIRECT',
+                '--dest', '192.168.42.1/32',
+                '-p', 'udp',
+                '--dport', '53',
+                '--to-ports', str(dnsport))
 
 
 def ipfw_rule_exists(n):
@@ -145,7 +152,7 @@ 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)
 
@@ -235,9 +242,11 @@ 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 +300,7 @@ def main(port, syslog):
     try:
         if line:
             debug1('firewall manager: starting transproxy.\n')
-            do_it(port, subnets)
+            do_it(port, dnsport, subnets)
             sys.stdout.write('STARTED\n')
         
         try:
@@ -319,5 +328,5 @@ def main(port, syslog):
             debug1('firewall manager: undoing changes.\n')
         except:
             pass
-        do_it(port, [])
+        do_it(port, 0, [])
         restore_etc_hosts(port)
diff --git a/main.py b/main.py
index 5597177..e76e596 100755
--- a/main.py
+++ b/main.py
@@ -54,6 +54,7 @@ sshuttle --hostwatch
 l,listen=  transproxy to this ip address and port number [127.0.0.1:0]
 H,auto-hosts scan for remote hostnames and update local /etc/hosts
 N,auto-nets  automatically determine subnets to route
+dns        capture local DNS requests and forward to the remote DNS server
 python=    path to python interpreter on the remote server [python]
 r,remote=  ssh hostname (and optional username) of remote sshuttle server
 x,exclude= exclude this subnet (can be used more than once)
@@ -82,9 +83,9 @@ try:
         server.latency_control = opt.latency_control
         sys.exit(server.main())
     elif opt.firewall:
-        if len(extra) != 1:
-            o.fatal('exactly one argument expected')
-        sys.exit(firewall.main(int(extra[0]), opt.syslog))
+        if len(extra) != 2:
+            o.fatal('exactly two arguments expected')
+        sys.exit(firewall.main(int(extra[0]), int(extra[1]), opt.syslog))
     elif opt.hostwatch:
         sys.exit(hostwatch.hw_main(extra))
     else:
@@ -111,6 +112,7 @@ try:
                              remotename,
                              opt.python,
                              opt.latency_control,
+                             opt.dns,
                              sh,
                              opt.auto_nets,
                              parse_subnets(includes),
diff --git a/server.py b/server.py
index ae7a921..3395c9e 100644
--- a/server.py
+++ b/server.py
@@ -106,6 +106,23 @@ class Hostwatch:
         self.sock = None
 
 
+class DnsProxy(Handler):
+    def __init__(self, mux, chan, request):
+        sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+        Handler.__init__(self, [sock])
+        self.sock = sock
+        self.mux = mux
+        self.chan = chan
+        self.sock.setsockopt(socket.SOL_IP, socket.IP_TTL, 42)
+        self.sock.connect(('192.168.42.1', 53))
+        self.sock.send(request)
+
+    def callback(self):
+        data = self.sock.recv(4096)
+        debug2('dns response: %d bytes\n' % len(data))
+        self.mux.send(self.chan, ssnet.CMD_DNS_RESPONSE, data)
+
+
 def main():
     if helpers.verbose >= 1:
         helpers.logprefix = ' s: '
@@ -165,6 +182,14 @@ def main():
         handlers.append(Proxy(MuxWrapper(mux, channel), outwrap))
     mux.new_channel = new_channel
 
+    dnshandlers = {}
+    def dns_req(channel, data):
+        debug1('got dns request!\n')
+        h = DnsProxy(mux, channel, data)
+        handlers.append(h)
+        dnshandlers[channel] = h
+    mux.got_dns_req = dns_req
+
     while mux.ok:
         if hw.pid:
             assert(hw.pid > 0)
diff --git a/ssnet.py b/ssnet.py
index 62fa378..554d870 100644
--- a/ssnet.py
+++ b/ssnet.py
@@ -21,6 +21,8 @@ CMD_DATA = 0x4206
 CMD_ROUTES = 0x4207
 CMD_HOST_REQ = 0x4208
 CMD_HOST_LIST = 0x4209
+CMD_DNS_REQ = 0x420a
+CMD_DNS_RESPONSE = 0x420b
 
 cmd_to_name = {
     CMD_EXIT: 'EXIT',
@@ -33,6 +35,8 @@ cmd_to_name = {
     CMD_ROUTES: 'ROUTES',
     CMD_HOST_REQ: 'HOST_REQ',
     CMD_HOST_LIST: 'HOST_LIST',
+    CMD_DNS_REQ: 'DNS_REQ',
+    CMD_DNS_RESPONSE: 'DNS_RESPONSE',
 }
     
 
@@ -281,7 +285,7 @@ class Mux(Handler):
         Handler.__init__(self, [rsock, wsock])
         self.rsock = rsock
         self.wsock = wsock
-        self.new_channel = self.got_routes = None
+        self.new_channel = self.got_dns_req = self.got_routes = None
         self.got_host_req = self.got_host_list = None
         self.channels = {}
         self.chani = 0
@@ -343,6 +347,10 @@ class Mux(Handler):
             assert(not self.channels.get(channel))
             if self.new_channel:
                 self.new_channel(channel, data)
+        elif cmd == CMD_DNS_REQ:
+            assert(not self.channels.get(channel))
+            if self.got_dns_req:
+                self.got_dns_req(channel, data)
         elif cmd == CMD_ROUTES:
             if self.got_routes:
                 self.got_routes(data)