Add support for user defined network namespace

Introduce the --netns option to allow interface creation
into a user-defined network namespace.
This allows the VPN data plane to be isolated from the main
OpenVPN process namespace.

The current netlink library integration supports interface creation
and deletion in a target namespace. However, subsequent configuration
operations (e.g. address or mtu set) are executed in the caller's
namespace, as they rely on the default netlink socket context.
As a result, interface-related configuration performed after creation
may be applied in the wrong namespace.
Introduce helper functions to temporarily switch the process to the
requested network namespace using setns(2), execute the required
netlink operations, and then restore the original namespace.
The namespace switch is temporary and scoped to each netlink
operation. Once the operation completes, the original namespace
is restored to preserve the process execution context.

Note: This feature is Linux-only and depends on setns(2).
It is not compatible (yet) with Data Channel Offload (DCO).

Change-Id: I8b0d1cad7062856abcc40c4e16ec93b45295bbd3
Signed-off-by: Gianmarco De Gregori <gianmarco@mandelbit.com>

Author: itsGiaan

doc/man-sections/vpn-network-options.rst

@@ -304,6 +304,12 @@ routing.
   Specify the link layer address, more commonly known as the MAC address.
   Only applied to TAP devices.
 
+--netns name
+  Specify the network namespace in which the tunnel interface will be created. 
+  The namespace must already exist.
+
+  (Supported on Linux only, on other platforms this is a no-op).
+
 --persist-tun
   Don't close and reopen TUN/TAP device or run up/down scripts across
   :code:`SIGUSR1` or ``--ping-restart`` restarts.

src/openvpn/dco.c

@@ -388,6 +388,12 @@ dco_check_startup_option(msglvl_t msglevel, const struct options *o)
                 ret);
         }
     }
+
+    if (o->netns)
+    {
+        msg(msglevel, "Note: --netns not supported by DCO, disabling data channel offload.");
+        return false;
+    }
 #endif /* if defined(_WIN32) */
 
 #if defined(HAVE_LIBCAPNG)

src/openvpn/networking.h

@@ -39,10 +39,10 @@ typedef void *openvpn_net_ctx_t;
 typedef void *openvpn_net_iface_t;
 #endif /* ifdef ENABLE_SITNL */
 
-/* Only the iproute2 backend implements these functions,
+/* Only the iproute2 and sitnl backend implements these functions,
  * the rest can rely on these stubs
  */
-#if !defined(ENABLE_IPROUTE)
+#if !defined(ENABLE_IPROUTE) && !defined(ENABLE_SITNL) || defined(TARGET_ANDROID)
 static inline int
 net_ctx_init(struct context *c, openvpn_net_ctx_t *ctx)
 {
@@ -63,7 +63,7 @@ net_ctx_free(openvpn_net_ctx_t *ctx)
 {
     (void)ctx;
 }
-#endif /* !defined(ENABLE_IPROUTE) */
+#endif /* !defined(ENABLE_IPROUTE) && !defined(ENABLE_SITNL) || defined(TARGET_ANDROID) */
 
 #if defined(ENABLE_SITNL) || defined(ENABLE_IPROUTE)