Add support for TPROXY via new TransTPRoxy option

Based on patch from "thomo" at #10582.

changes/10582_tproxy

@@ -0,0 +1,7 @@
+  o Minor features:
+
+    - Add support for the TPROXY transparent proxying facility on Linux.
+      See documentation for the new TransTRPOXY option for more details.
+      Implementation by "thomo". Closes ticket 10582.
+
+

doc/tor.1.txt

@@ -1183,6 +1183,19 @@ The following options are useful only for clients (that is, if
     compatibility, TransListenAddress is only allowed when TransPort is just
     a port number.)
 
+[[TransTPROXY]] **TransTPROXY** **0**|**1**::
+    TransTPROXY may only be enabled when there is transparent proxy listener
+    enabled and only for Linux.
+ +
+    Set this 1 if you wish to be able to use the TPROXY linux module to
+    transparently proxy connections that are configured using the TransPort
+    option. This setting lets the listener on the TransPort accept connections
+    for all addresses, even when the TransListenAddress is configured for an
+    internal address. Detailed information on how to configure the TPROXY
+    feature can be found in the Linux kernel source tree in the file  
+    Documentation/networking/tproxy.txt.
+    (Default: 0)
+
 [[NATDPort]] **NATDPort** \['address':]__port__|**auto** [_isolation flags_]::
     Open this port to listen for connections from old versions of ipfw (as
     included in old versions of FreeBSD, etc) using the NATD protocol.

src/or/config.c

@@ -408,6 +408,7 @@ static config_var_t option_vars_[] = {
   OBSOLETE("TrafficShaping"),
   V(TransListenAddress,          LINELIST, NULL),
   VPORT(TransPort,                   LINELIST, NULL),
+  V(TransTPROXY,                 BOOL,     "0"),
   V(TunnelDirConns,              BOOL,     "1"),
   V(UpdateBridgesFromAuthority,  BOOL,     "0"),
   V(UseBridges,                  BOOL,     "0"),
@@ -2530,10 +2531,27 @@ options_validate(or_options_t *old_options, or_options_t *options,
         "undefined, and there aren't any hidden services configured.  "
         "Tor will still run, but probably won't do anything.");
 
-#ifndef USE_TRANSPARENT
-  /* XXXX024 I think we can remove this TransListenAddress */
-  if (options->TransPort_set || options->TransListenAddress)
-    REJECT("TransPort and TransListenAddress are disabled in this build.");
+#ifdef USE_TRANSPARENT
+  if (options->TransTPROXY) {
+#ifndef __linux__
+    REJECT("TransTPROXY is a Linux-specific feature.")
+#endif
+    if (!options->TransPort_set) {
+      REJECT("Cannot use TransTPROXY without any valid TransPort or "
+             "TransListenAddress.");
+    }
+    /* Friendly suggestion about running as root initially. */
+    if (!options->User) {
+      log_warn(LD_CONFIG,
+               "You have enabled TransTPROXY but have not specified the "
+               "\"User\" option. TransTPROXY will not function without "
+               "root privileges.");
+    }
+  }
+#else
+  if (options->TransPort_set || options->TransTPROXY)
+    REJECT("TransPort, TransListenAddress, and TransTPROXY are disabled "
+           "in this build.");
 #endif
 
   if (options->TokenBucketRefillInterval <= 0

src/or/connection.c

@@ -1035,6 +1035,17 @@ connection_listener_new(const struct sockaddr *listensockaddr,
 
     make_socket_reuseable(s);
 
+#if defined USE_TRANSPARENT && defined(IP_TRANSPARENT)
+    if (options->TransTPROXY && type == CONN_TYPE_AP_TRANS_LISTENER) {
+      int one = 1;
+      if (setsockopt(s, SOL_IP, IP_TRANSPARENT, &one, sizeof(one)) < 0) {
+        int e = tor_socket_errno(s);
+        log_warn(LD_NET, "Error setting IP_TRANSPARENT flag: %s",
+                 tor_socket_strerror(e));
+      }
+    }
+#endif
+
 #ifdef IPV6_V6ONLY
     if (listensockaddr->sa_family == AF_INET6) {
 #ifdef _WIN32

src/or/or.h

@@ -3553,6 +3553,8 @@ typedef struct {
   config_line_t *SocksPort_lines;
   /** Ports to listen on for transparent pf/netfilter connections. */
   config_line_t *TransPort_lines;
+  int TransTPROXY; /** < Boolean: are we going to listen for all destinations
+                    * on the TransPort_lines are required for TPROXY? */
   config_line_t *NATDPort_lines; /**< Ports to listen on for transparent natd
                             * connections. */
   config_line_t *ControlPort_lines; /**< Ports to listen on for control