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Implement a sockaddr_ntop() function that works like inet_ntop() but prints
sockaddrs. Works for all sockaddrs so can be used to print sockaddrs nicely.
OK phessler@

/*	$OpenBSD: inet_ntop.c,v 1.4 2016/09/04 17:05:24 claudio Exp $	*/

/* Copyright (c) 1996 by Internet Software Consortium.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND INTERNET SOFTWARE CONSORTIUM DISCLAIMS
 * ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL INTERNET SOFTWARE
 * CONSORTIUM BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
 * DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
 * PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS
 * ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
 * SOFTWARE.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/socket.h>

#include <netinet/in.h>

#define IN6ADDRSZ	16
#define INT16SZ		2

/*
 * WARNING: Don't even consider trying to compile this on a system where
 * sizeof(int) < 4.  sizeof(int) > 4 is fine; all the world's not a VAX.
 */

static const char *inet_ntop4(const u_char *src, char *dst, size_t size);
#ifdef INET6
static const char *inet_ntop6(const u_char *src, char *dst, size_t size);
#endif /* INET6 */

/* char *
 * inet_ntop(af, src, dst, size)
 *	convert a network format address to presentation format.
 * return:
 *	pointer to presentation format address (`dst'), or NULL (see errno).
 * author:
 *	Paul Vixie, 1996.
 */
const char *
inet_ntop(int af, const void *src, char *dst, socklen_t size)
{
	switch (af) {
	case AF_INET:
		return (inet_ntop4(src, dst, (size_t)size));
#ifdef INET6
	case AF_INET6:
		return (inet_ntop6(src, dst, (size_t)size));
#endif /* INET6 */
	default:
		return (NULL);
	}
	/* NOTREACHED */
}

const char *
sockaddr_ntop(struct sockaddr *sa, char *dst, size_t size)
{
	u_int8_t l;
	size_t n;

	if (sa->sa_len < 2)
		return "bad sa";
	switch (sa->sa_family) {
	case AF_INET:
		return inet_ntop4((u_char *)&satosin(sa)->sin_addr, dst, size);
#ifdef INET6
	case AF_INET6:
		return inet_ntop6((u_char *)&satosin6(sa)->sin6_addr, dst, size);
#endif
	default:
		n = snprintf(dst, size, "%d ", sa->sa_family);
		for (l = 0; l < sa->sa_len - offsetof(struct sockaddr, sa_data); l++) {
			int r = snprintf(dst + n, size - n, "%02x", sa->sa_data[l]);
			if (r == -1)
				return "bad sa";
			n += r;
			if (n > size)
				n = size;
		}
		return (dst);
	}
}

/* const char *
 * inet_ntop4(src, dst, size)
 *	format an IPv4 address, more or less like inet_ntoa()
 * return:
 *	`dst' (as a const)
 * notes:
 *	(1) uses no statics
 *	(2) takes a u_char* not an in_addr as input
 * author:
 *	Paul Vixie, 1996.
 */
static const char *
inet_ntop4(const u_char *src, char *dst, size_t size)
{
	char tmp[sizeof "255.255.255.255"];
	int l;

	l = snprintf(tmp, sizeof(tmp), "%u.%u.%u.%u",
	    src[0], src[1], src[2], src[3]);
	if (l <= 0 || l >= size) {
		return (NULL);
	}
	strlcpy(dst, tmp, size);
	return (dst);
}

#ifdef INET6
/* const char *
 * inet_ntop6(src, dst, size)
 *	convert IPv6 binary address into presentation (printable) format
 * author:
 *	Paul Vixie, 1996.
 */
static const char *
inet_ntop6(const u_char *src, char *dst, size_t size)
{
	/*
	 * Note that int32_t and int16_t need only be "at least" large enough
	 * to contain a value of the specified size.  On some systems, like
	 * Crays, there is no such thing as an integer variable with 16 bits.
	 * Keep this in mind if you think this function should have been coded
	 * to use pointer overlays.  All the world's not a VAX.
	 */
	char tmp[sizeof "ffff:ffff:ffff:ffff:ffff:ffff:255.255.255.255"];
	char *tp, *ep;
	struct { int base, len; } best, cur;
	u_int words[IN6ADDRSZ / INT16SZ];
	int i;
	int advance;

	/*
	 * Preprocess:
	 *	Copy the input (bytewise) array into a wordwise array.
	 *	Find the longest run of 0x00's in src[] for :: shorthanding.
	 */
	memset(words, '\0', sizeof words);
	for (i = 0; i < IN6ADDRSZ; i++)
		words[i / 2] |= (src[i] << ((1 - (i % 2)) << 3));
	best.base = -1;
	cur.base = -1;
	for (i = 0; i < (IN6ADDRSZ / INT16SZ); i++) {
		if (words[i] == 0) {
			if (cur.base == -1)
				cur.base = i, cur.len = 1;
			else
				cur.len++;
		} else {
			if (cur.base != -1) {
				if (best.base == -1 || cur.len > best.len)
					best = cur;
				cur.base = -1;
			}
		}
	}
	if (cur.base != -1) {
		if (best.base == -1 || cur.len > best.len)
			best = cur;
	}
	if (best.base != -1 && best.len < 2)
		best.base = -1;

	/*
	 * Format the result.
	 */
	tp = tmp;
	ep = tmp + sizeof(tmp);
	for (i = 0; i < (IN6ADDRSZ / INT16SZ) && tp < ep; i++) {
		/* Are we inside the best run of 0x00's? */
		if (best.base != -1 && i >= best.base &&
		    i < (best.base + best.len)) {
			if (i == best.base) {
				if (tp + 1 >= ep)
					return (NULL);
				*tp++ = ':';
			}
			continue;
		}
		/* Are we following an initial run of 0x00s or any real hex? */
		if (i != 0) {
			if (tp + 1 >= ep)
				return (NULL);
			*tp++ = ':';
		}
		/* Is this address an encapsulated IPv4? */
		if (i == 6 && best.base == 0 &&
		    (best.len == 6 || (best.len == 5 && words[5] == 0xffff))) {
			if (!inet_ntop4(src+12, tp, (size_t)(ep - tp)))
				return (NULL);
			tp += strlen(tp);
			break;
		}
		advance = snprintf(tp, ep - tp, "%x", words[i]);
		if (advance <= 0 || advance >= ep - tp)
			return (NULL);
		tp += advance;
	}
	/* Was it a trailing run of 0x00's? */
	if (best.base != -1 && (best.base + best.len) == (IN6ADDRSZ / INT16SZ)) {
		if (tp + 1 >= ep)
			return (NULL);
		*tp++ = ':';
	}
	if (tp + 1 >= ep)
		return (NULL);
	*tp++ = '\0';

	/*
	 * Check for overflow, copy, and we're done.
	 */
	if ((size_t)(tp - tmp) > size) {
		return (NULL);
	}
	strlcpy(dst, tmp, size);
	return (dst);
}
#endif /* INET6 */