Add support for dynamic "NAT" interfaces (-L/local interface).

When a local interface is configured, vmd configures a /31 address on
the tap(4) interface of the host and provides another IP in the same
subnet via DHCP (BOOTP) to the VM.  vmd runs an internal BOOTP server
that replies with IP, gateway, and DNS addresses to the VM.  The
built-in server only ever responds to the VM on the inside and cannot
leak its DHCP responses to the outside.

Thanks to Uwe Werler, Josh Grosse, and some others for testing!

OK deraadt@

1 files changed, 9 insertions, 3 deletions

diff --git a/usr.sbin/vmctl/vmctl.8 b/usr.sbin/vmctl/vmctl.8
index aed2816f15f..71a0aa4e7f6 100644
--- a/usr.sbin/vmctl/vmctl.8
+++ b/usr.sbin/vmctl/vmctl.8
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: vmctl.8,v 1.28 2017/04/14 00:53:28 mlarkin Exp $
+.\"	$OpenBSD: vmctl.8,v 1.29 2017/04/19 15:38:32 reyk Exp $
 .\"
 .\" Copyright (c) 2015 Mike Larkin <mlarkin@openbsd.org>
 .\"
@@ -14,7 +14,7 @@
 .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: April 14 2017 $
+.Dd $Mdocdate: April 19 2017 $
 .Dt VMCTL 8
 .Os
 .Sh NAME
@@ -72,8 +72,8 @@ Reset the configured switches.
 .It Cm reset vms
 Reset and terminate all VMs.
 .It Xo Cm start Ar name
+.Op Fl Lc
 .Op Fl b Ar path
-.Op Fl c
 .Op Fl d Ar path
 .Op Fl i Ar count
 .Op Fl m Ar size
@@ -91,6 +91,12 @@ Automatically connect to the VM console.
 Disk image file (may be specified multiple times to add multiple disk images).
 .It Fl i Ar count
 Number of network interfaces to add to the VM.
+.It Fl L
+Add a local network interface.
+.Xr vmd 8
+will auto-generate an IPv4 subnet for the interface,
+configure a gateway address on the VM host side,
+and run a simple DHCP (BOOTP) server for the VM.
 .It Fl m Ar size
 Memory
 .Ar size