Interface match in POSTROUTING

["\"Oleg A. Arkhangelsky\"" <sysoleg@yandex.ru>]

Hello,

I have a question regarding this piece of code in iptables.c:

                if (strcmp(chain, "POSTROUTING") == 0
                    || strcmp(chain, "OUTPUT") == 0) {
                        /* -i not valid with outgoing packets */
                        if (cs.options & OPT_VIANAMEIN)
                                xtables_error(PARAMETER_PROBLEM,
                                           "Can't use -%c with %s\n",
                                           opt2char(OPT_VIANAMEIN),
                                           chain);
                }

I agree that "-i" is not valid for locally generated packets
(that is what OUTPUT chain for). But for forwarded packets
going through POSTROUTING this is not true. As far as I
understand we can match for both input and output interface
here (as in FORWARD).

So why we need this restriction?

-- 
wbr, Oleg.