Can I use a socket as stdin/stdout in lxc (linux containers)?

Question

I am interested in starting a daemon inside an lxc container with its stdin/stdout as a socket passed from the host, inetd style. Is this possible?

Feel free to ask for clarification since I am asking a bounty for the question. — alexyorke, Jul 19 '13 at 01:47
I'm thinking unix sockets. I don't think netcat would do the trick. Also that was two years ago. — joeforker, Jul 24 '13 at 18:32
How could I use netcat? I haven't had experience with that command but it sounds intriguing — alexyorke, Jul 30 '13 at 13:49

score 2 · Answer 1 · answered Aug 01 '14 at 21:22

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Be advised, if using an LXC "snapshot" clone, with a directory backing store (which thus uses overlayfs), then Unix FIFO pipes are currently broken. See:

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1214500

answered Aug 01 '14 at 21:22

Derek Simkowiak

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score 1 · Answer 2 · answered Jul 25 '13 at 03:52

1

I don't think LXC has native support, but you could always just run your lxc command under xinetd to get what you want. Or write your own server that talks sockets on one side and talks LXC (via popen() or something) on the other side.

answered Jul 25 '13 at 03:52

BraveNewCurrency

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Hans Lub · Answer 3 · 2013-07-25T12:52:11.107

inetd is a daemon that starts (non-daemonic) programs that use stdin/stdout to listen/talk to you

The LXC utilities lxc-start and lxc-execute insist on closing all open file descriptors (including stdin/stdout) making them useless with inetd. They eventually call clone(2), however, and so can you, writing your own C wrapper like this:

#define STACKSIZE 409600
/* choose your favourite brand of isolationism below */
#define SPLENDID_ISOLATION (CLONE_NEWPID|CLONE_NEWNS|CLONE_NEWNET)


int exec_command(void* arg) {
  /* don't close stdin/stdout here! */
  execl("command", "command", arg, NULL);
  return 1; 
}

void main(int  argc, char **argv) {
  void *stack = malloc(STACKSIZE) + STACKSIZE - 1; /* grows downwards */
  clone(&exec_command, stack, SIGCHLD|CLONE_VFORK|SPLENDID_ISOLATION, argv[1]);
  wait(NULL);
}

This wrapper can then be run under inetd.

Can I use a socket as stdin/stdout in lxc (linux containers)?

3 Answers3