[Qemu-devel] hostfwd_add and -netdev user

[Markus Armbruster]

Maybe I'm missing something, but it looks like hostfwd_add doesn't fully
work with -netdev.

Command definition from hmp-commands.hx:

    {
        .name       = "hostfwd_add",
        .args_type  = "arg1:s,arg2:s?,arg3:s?",
        .params     = "[vlan_id name] 
[tcp|udp]:[hostaddr]:hostport-[guestaddr]:guestport",
        .help       = "redirect TCP or UDP connections from host to guest 
(requires -net user)",
        .mhandler.cmd = net_slirp_hostfwd_add,
    },

Note: params says command takes either 1 or 3 arguments.  args_type
can't express that, so it asks for 1-3.

Command handler:

void net_slirp_hostfwd_add(Monitor *mon, const QDict *qdict)
{
    const char *redir_str;
    SlirpState *s;
    const char *arg1 = qdict_get_str(qdict, "arg1");
    const char *arg2 = qdict_get_try_str(qdict, "arg2");
    const char *arg3 = qdict_get_try_str(qdict, "arg3");

    if (arg2) {
        s = slirp_lookup(mon, arg1, arg2);
        redir_str = arg3;
    } else {
        s = slirp_lookup(mon, NULL, NULL);
        redir_str = arg1;
    }
    if (s) {
        slirp_hostfwd(s, redir_str, 0);
    }

}

If I understand the code correctly, the command has the following forms:

* hostfwd_add REDIR

  This form uses the default stack (first member of slirp_stacks).
  Seems to work okay with -netdev:

    $ qemu-system-x86_64 --nodefaults --enable-kvm -vnc :0 -S -m 384 -monitor 
stdio -netdev user,id=net.0 -device virtio-net-pci,netdev=net.0,id=nic.0
    QEMU 0.15.92 monitor - type 'help' for more information
    (qemu) hostfwd_add tcp::12345-:22
    (qemu) info usernet
    VLAN -1 (net.0):
      Protocol[State]    FD  Source Address  Port   Dest. Address  Port RecvQ 
SendQ
      TCP[HOST_FORWARD]  12               * 12345       10.0.2.15    22     0   
  0

* hostfwd_add VLAN STACK REDIR

  This form was added in commit f13b572c "slirp: Make hostfwd_add/remove
  multi-instance-aware".

  I don't like how it overloads the command; adding the optional
  arguments at the end would have been cleaner.  Water under the bridge.

  Note that slirp_lookup() converts VLAN to an int (without error
  checking, i.e. non-integers are silently interpreted as zero, integers
  out of bounds are silently truncated).

  Anyway, I can't make it work with -netdev:

    (qemu) hostfwd_add -1 net.0 tcp::12345-:22
    unknown VLAN -1

* Just for giggles: what if I give two arguments?

    $ qemu-system-x86_64--nodefaults --enable-kvm -vnc :0 -S -m 384 -monitor 
stdio -net user -device virtio-net-pci,id=nic.0,vlan=0QEMU 0.15.92 monitor - 
type 'help' for more information
    (qemu) hostfwd_add 0 user.0
    invalid host forwarding rule '(null)'

  The (null) is from a printf-like function printing a null pointer with
  conversion %s.  Crash bug on some systems.