Ever wondered why `ping 127.1` works?

README.md

inet-pnp

Ever wondered why this works?

$ ping 127.1
PING 127.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.047 ms

$ ping 1.1
PING 1.1 (1.0.0.1) 56(84) bytes of data.
64 bytes from 1.0.0.1: icmp_seq=1 ttl=48 time=5.04 ms

What's doing on here?

getaddrinfo(3):

node specifies either a numerical network address (for IPv4, numbers-and-dots notation as supported by inet_aton(3); for IPv6, hexadecimal string format as supported by inet_pton(3))

inet(3):

The address supplied in cp can have one of the following forms:

a.b.c.d Each of the four numeric parts specifies a byte of the address; the bytes are assigned in left-to-right order to produce the binary address.

a.b.c Parts a and b specify the first two bytes of the binary address. Part c is interpreted as a 16-bit value that defines the rightmost two bytes of the binary address. This notation is suitable for specifying (outmoded) Class B network addresses.

a.b Part a specifies the first byte of the binary address. Part b is interpreted as a 24-bit value that defines the rightmost three bytes of the binary address. This notation is suitable for specifying (outmoded) Class A network addresses.

a The valuqe a is interpreted as a 32-bit value that is stored directly into the binary address without any byte rearrangement.

inet_ntop(3):

AF_INET src points to a struct in_addr (in network byte order) which is converted to an IPv4 network address in the dotted-decimal format, "ddd.ddd.ddd.ddd". The buffer dst must be at least INET_ADDRSTRLEN bytes long.

AF_INET6 src points to a struct in6_addr (in network byte order) which is converted to a representation of this address in the most appropriate IPv6 network address format for this address. The buffer dst must be at least INET6_ADDRSTRLEN bytes long.

It's because inet_ntop() only accepts "ddd.ddd.ddd.ddd" format whereas inet_aton() accepts "a", "a.b", ..., "a.b.c.d". Any program that calls getaddrinfo() could potentially accept the inet_aton() style.

The code

Build:

cc -std=c17 inet-pnp -o inet-pnp.c

Run:

# normal data
echo '1.2.3.4' | ./inet-pnp
# inet_aton() style
echo '127.1' | ./inet-pnp

Result

Team "loose"(Windows, Linux, FreeBSD):

pton: 1.2.3.4, gai: 1.2.3.4
pton: Bad message, gai: 127.0.0.1

Team "strict"(OpenBSD):

pton: 1.2.3.4, gai: 1.2.3.4
pton: Bad message, gai: name or service is not known

Takeaway

The inet_aton() style IPv4 address representation is not portable. Always use 'a.b.c.d' format. Use inet_ntop() over inet_aton() where possible.

Some history

https://datatracker.ietf.org/doc/html/draft-main-ipaddr-text-rep-00

inet-pnp.c

#define _POSIX_C_SOURCE 200112L
#include <stdio.h>
#include <stdint.h>
#include <stdbool.h>
#include <string.h>
#include <assert.h>
#include <errno.h>
#ifdef WIN32
#include <winsock2.h>
#include <ws2tcpip.h>
#include <windows.h>
#else
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#endif

#define ARGV0 "inet-pnp"

static bool do_pn (const char *in, const int af) {
	uint8_t buf[16];
	char out[INET6_ADDRSTRLEN];

	out[0] = 0;
	errno = EBADMSG;
	return
		inet_pton(af, in, buf) > 0 &&
		inet_ntop(af, buf, out, sizeof(out)) != NULL &&
		printf("%s", out);
}

static bool do_gn (const char *in) {
	static const struct addrinfo hints = {
		.ai_flags = AI_NUMERICHOST
	};
	union {
		struct sockaddr_storage storage;
		struct sockaddr a;
		struct sockaddr_in in;
		struct sockaddr_in6 in6;
	} s;
	struct addrinfo *ai = NULL;
	char out[INET6_ADDRSTRLEN];
	bool ret = false;
	const void *addrbuf;
	const int fr = getaddrinfo(in, NULL, &hints, &ai);

	if (fr != 0) {
		const char *errmsg = gai_strerror(fr);
		printf("%s", errmsg);
		ret = true;
		goto END;
	}

	memcpy(&s, ai->ai_addr, ai->ai_addrlen);
	switch (ai->ai_family) {
	case AF_INET: addrbuf = &s.in.sin_addr; break;
	case AF_INET6: addrbuf = &s.in6.sin6_addr; break;
	default:
		errno = EAFNOSUPPORT;
		goto END;
	}
	out[0] = 0;

	errno = EBADMSG;
	if (inet_ntop(ai->ai_family, addrbuf, out, sizeof(out)) == NULL) {
		goto END;
	}

	printf("%s", out);
	ret = true;

END:
	if (ai != NULL) {
		freeaddrinfo(ai);
	}
	return ret;
}

#ifdef WIN32
static void start_wsa (void) {
	int fr;
	WSADATA wsaData = {0};

	fr = WSAStartup(MAKEWORD(2, 2), &wsaData);
	assert(fr == 0);
	(void)fr;
}
#endif

int main (const int argc, const char **argv) {
	char line[2][256];
	bool err = false;
	bool proc = false;

#ifdef WIN32
	start_wsa();
#endif

	while (fgets(line[0], sizeof(line[0]), stdin) != NULL) {
		unsigned int cnt = 0;
		// trim
		line[1][0] = 0;
		if (sscanf(line[0], "%255s", line[1]) != 1) {
			// empty line. all whitespace string cannot be captured using %s
			putc('\n', stdout);
			continue;
		}

		printf("pton: ");
		if (do_pn(line[1], AF_INET) || do_pn(line[1], AF_INET6)) {
			cnt += 1;
		}
		else {
			printf("%s", strerror(errno));
		}
		printf(", gai: ");
		if (do_gn(line[1])) {
			cnt += 1;
		}
		else {
			printf("%s", strerror(errno));
		}

		if (cnt > 0) {
			proc = true;
		}
		else {
			err = true;
		}
		putc('\n', stdout);
	}

	if (err) {
		if (proc) {
			return 3;
		}
		return 1;
	}
	return 0;
}