Before, when resetting the NAT table to handle an out-of-memory
condition we tried to allocate the new table while still holding
the reference to the old one. It should be more reliable to drop
the old reference first.
Log showed:
2016-01-31 19:33.47: INF [firewall] added NAT redirect 10.137.3.12:32860 -> 53:firewall:52517 -> 53:net-vm
2016-01-31 19:33.52: WRN [firewall] Out_of_memory adding NAT rule. Dropping NAT table...
--- End dump ---
Fatal error: exception Out of memory
Raised by primitive operation at file "hashtbl.ml", line 63, characters 52-70
Called from file "router.ml", line 47, characters 11-30
Called from file "src/core/lwt.ml", line 907, characters 20-24
Mirage exiting with status 2
Do_exit called!
Added explicit NAT target, allowing NAT even within client net and
making it clear that NAT is used externally.
Changed Redirect_to_netvm to NAT_to, and allow specifying any target
host.
