Bug 36724

The ipnat(5) manpage features, on -stable, a section wrt the grammar
recognized by the ipnat parser.

This grammar is incomplete and does not represent properly ipnat
behavior.

1- Undefined non-terminals:

a- Major:

these actually prevent one from clearly knowing what the syntax is

- portnum (which I guess is portnumbers)
- numports (idem)
- nummask

b- Minor (we can easily guess what these are):

bits, mask, ip, host-name, ipaddr, hexnumber, numbers

Note that ip and ipaddr seem to be redundant here.

2- inconsistence in the syntax

In the same grammar, curly brackets and brackest are used to represent
repetition, at least I guess.

For example, I guessed that:

ip [ "," ip ]

meant that ip could be repeated, seperated by commas.

But, on the other hand, we havge:

fname ::= 'A' - 'Z' { 'A' - 'Z' } numbers .

which I guessed means that "'A' - 'Z'" can be repeated any number of
times.

I think this section of the manpage needs a complete rewrite.

Fix: 

Unknown. I will inspect the parser if I find the time, but since it's a
flat .c file (natparse.c) and not a .y, it's harder. ;)

Any idea why ipnat doesn't use something like yacc or bison?

I'm not sure what format the grammar is following right now, but it
would gain at following a standard notation such as the BNF.
How-To-Repeat: 
Follow the grammar given in the grammar.

Also see:

docs/33371 guido    ipnat(5) requires mapped-to port in rdr rules
docs/36127 darrenr  ipnat(5) man page glitches

for other glitches.