Add the argument --namespace-pid

The argument '--namespace-pid' allows sshuttle to attach to the same net
namespace used by a running process.

sshuttle/cmdline.py

@@ -39,10 +39,11 @@ def main():
 
     try:
         namespace = getattr(opt, 'namespace', None)
-        if namespace:
+        namespace_pid = getattr(opt, 'namespace_pid', None)
+        if namespace or namespace_pid:
             prefix = helpers.logprefix
             helpers.logprefix = 'ns: '
-            enter_namespace(namespace)
+            enter_namespace(namespace, namespace_pid)
             helpers.logprefix = prefix
 
         if opt.firewall:

sshuttle/namespace.py

@@ -9,8 +9,11 @@ CLONE_NEWNET = 0x40000000
 NETNS_RUN_DIR = "/var/run/netns"
 
 
-def enter_namespace(namespace):
+def enter_namespace(namespace, namespace_pid):
+    if namespace:
         namespace_dir = f'{NETNS_RUN_DIR}/{namespace}'
+    else:
+        namespace_dir = f'/proc/{namespace_pid}/ns/net'
     
     if not os.path.exists(namespace_dir):
         raise Fatal('The namespace %r does not exists.' % namespace_dir)
@@ -29,8 +32,9 @@ def enter_namespace(namespace):
     
     libc.setns.errcheck = errcheck # type: ignore
 
-    debug1('Entering namespace %r' % namespace)
+    debug1('Entering namespace %r' % namespace_dir)
+
     with open(namespace_dir) as fd:
         libc.setns(fd.fileno(), CLONE_NEWNET)
 
-    debug1('Namespace %r successfully set' % namespace)
+    debug1('Namespace %r successfully set' % namespace_dir)

sshuttle/options.py

@@ -474,5 +474,12 @@ if sys.platform == 'linux':
     parser.add_argument(
         '--namespace',
         type=parse_namespace,
-        help="Run it inside of a namespace."
+        help="Run inside of a net namespace with the given name."
+    )
+    parser.add_argument(
+        '--namespace-pid',
+        type=int,
+        help="""
+        Run inside the net namespace used by the process with
+        the given pid."""
     )