netgraph: Add RFC 6598/Carrier Grade NAT support to ng_nat

[nc]

Summary

netgraph: Add RFC 6598/Carrier Grade NAT support to ng_nat.

This extends upon the RFC 6598 support to libalias/ipfw in r357092.

Submitted by: Neel Chauhan <neel AT neelc DOT org>

Test Plan

Load the kernel module

kldunload ng_ipfw

Replace WAN_IP with the WAN IPv4, and WAN_IF with the WAN interface.

Set the netgraph rules.

ngctl mkpeer ipfw: nat 60 out
ngctl name ipfw:60 nat
ngctl connect ipfw: nat: 61 in
ngctl msg nat: setaliasaddr WAN_IP
ngctl msg nat: setmode "{flags=0x100}"

Set the ipfw rules.

ipfw add 300 netgraph 61 all from any to any in via WAN_IF
ipfw add 400 netgraph 60 all from any to any out via WAN_IF

Set the sysctl:

sysctl net.inet.ip.fw.one_pass=0

[donner]

Of course, the change is straight forward, but can we please have some words in the man page explaining how the names of the flags match the correspondent names in libalias.

[nc]

I added the explanation in the man page.

[bcr]

Manpages is also good with this change. Thanks for the implementation.

[melifaro]

Can you please add "TESTING" section?

[nc]

Here, added the testing section.

[kp]

Approved by: kp (mentor)

Don't forget to update the man page date before you commit.