MFC r345264:

MFC r345264:
  Add NAT64 CLAT implementation as defined in RFC6877.

  CLAT is customer-side translator that algorithmically translates 1:1
  private IPv4 addresses to global IPv6 addresses, and vice versa.
  It is implemented as part of ipfw_nat64 kernel module. When module
  is loaded or compiled into the kernel, it registers "nat64clat" external
  action. External action named instance can be created using `create`
  command and then used in ipfw rules. The create command accepts two
  IPv6 prefixes `plat_prefix` and `clat_prefix`. If plat_prefix is ommitted,
  IPv6 NAT64 Well-Known prefix 64:ff9b::/96 will be used.

    # ipfw nat64clat CLAT create clat_prefix SRC_PFX plat_prefix DST_PFX
    # ipfw add nat64clat CLAT ip4 from IPv4_PFX to any out
    # ipfw add nat64clat CLAT ip6 from DST_PFX to SRC_PFX in

  Obtained from:	Yandex LLC
  Submitted by:	Boris N. Lytochkin
  Relnotes:	yes
  Sponsored by:	Yandex LLC