[TUHS] conventions around zero padding in ip4

[Steffen Nurpmeso]

Ralph Corderoy wrote in
 <20220508102208.50A2822158 at orac.inputplus.co.uk>:
 |> I first learned in the 80s that 127.1 meant 127.0.0.1. I always
 |> assumed zero padding was defined in a standard *somewhere*, but am
 |> finding out maybe not.
 |
 |It has been standardised; see inet_addr(3p) where ‘p’ means the POSIX
 |version of the man page or
 |https://pubs.opengroup.org/onlinepubs/9699919799/functions/inet_addr.html
 |
 |Briefly, the string must be one of
 |
 |    a.b.c.d
 |    a.b.cd
 |    a.bcd
 |    abcd
 |
 |where the number of bytes represented is the number of characters.
 |Each number is as defined by ISO C, e.g. ‘0x...’ means hex, thus
 |‘ping 017777777776’.
 |
 |That's all there is to it.  It's simple to explain and I've used it for
 |years too.  Given POSIX defines it, without deprecation, programming
 |languages which don't use the C library and programs which must parse
 |the string themselves should follow POSIX to avoid those annoying
 |programs which deviate from the long-established norm.
 |
 |> IP4 padding came up recently: the ip command interprets 10.2 as
 |> 10.2.0.0, whereas most things (golang libraries, ping, ...) interpret
 |> it as 10.0.0.2.
 |
 |Bug the ip(1) folks, pointing to POSIX.  :-)

"However", RFC 2553 says

  RFC 2553       Basic Socket Interface Extensions for IPv6     March 1999

     The address conversion functions -- inet_ntoa() and inet_addr() --
     convert IPv4 addresses between binary and printable form.  These
     functions are quite specific to 32-bit IPv4 addresses.  We have
     designed two analogous functions that convert both IPv4 and IPv6
     addresses, and carry an address type parameter so that they can be
     extended to other protocol families as well.

And for this POSIX says

  If the af argument of inet_pton( ) is AF_INET, the src string
  shall be in the standard IPv4 dotted-decimal form:
  ddd.ddd.ddd.ddd where "ddd" is a one to three digit decimal
  number between 0 and 255 (see inet_addr( )). The inet_pton()
  function does not accept other formats [.]

I am too lazy / busy to check whether inet_pton() with AF_INET
gets the short version right.
(I personally had IPAdress:: .. pub boolean fromString(const char
*_straddr, ui4 _straddrlen=M1::ui4):

    * Tries to parse the address as stored in \a _straddr.
    * If \a _straddr was successfully parsed a true boolean is returned.
    * Otherwise \THIS has not been modified.
    *
    * This function can parse all usual address representations,
    * in particular all of those which can be produced with toString().
    * It can even parse "127.  0.  0.  1", "127.000.000.001",
    * but \e cannot parse "127.  0.0000.  1" (room for four digits).
    * And ditto for IPv6.
    *
    * \remarks If result is IPv6, flowInfo() and scopeId() are set to 0.
    * \note
    * ARPA strings are not understood!
    * I.e., strings produced by toArpaString() cannot be re-parsed to
    * a plain address with this one!
    *
    * \note
    * ::0.0.0.0 and ::FFFF:0.0.0.0 will fail.
    * ::0.0.0.1 and ::FFFF:0.0.0.1 will also fail.

and now find the need to do

    /* xxx Client had this already, simply binary pass it, too? */
    c_af = (su_cs_find_c(pgp->pg_ca, ':') != NIL) ? AF_INET6 : AF_INET;
    if(inet_pton(c_af, pgp->pg_ca, (c_af == AF_INET ? S(void*,&c_sip.v4)
             : S(void*,&c_sip.v6))) != 1){
       su_log_write(su_LOG_CRIT, _("Cannot re-parse an already "
          "prepared IP address?: "), pgp->pg_ca);
       goto jleave0;
    }
    c_ip = (c_af == AF_INET) ? R(u32*,&c_sip.v4.s_addr)
          : R(u32*,c_sip.v6.s6_addr);

quite lengthy and complicated.  Of course inet_pton() can be made
"extended to other protocol families", and maybe they are already
even, but i find this distressing as either you need to provide
some kind of _(un)?register() or even hardwire, maybe with a tree
of #ifdef, those other families.  And better watch out for
EAFNOSUPPORT at runtime.  Of course i came late and wanted /
needed something specific so i am fine out.

--steffen
|
|Der Kragenbaer,                The moon bear,
|der holt sich munter           he cheerfully and one by one
|einen nach dem anderen runter  wa.ks himself off
|(By Robert Gernhardt)