Very simple and secure identd

Save the code below as identd.c. Then compile it:

$ cc identd.c -o fake_identd
$ sudo cp fake_identd /usr/local/libexec

Then use it in /etc/inetd.conf:

ident		stream	tcp	nowait	_identd	/usr/local/libexec/fake_identd	fake_identd

identd.c

/*
** A proper identification protocol (RFC 1413) implementation,
** providing all the utility, security, and authentication
** of all other implementations of this protocol, without
** all the expensive and pointless grovelling in /dev/kmem.
**
** Careful design and software engineering makes this software
** efficient, fast, compact, scalable, and completely maintenance free.
**
** Anyone who feels the need to use a daemon more complex than this one
** should go read section 6 of RFC 1413 very carefully, and think again.
**
** Erik E. Fair <fair@clock.org>, February 1, 2000
*/
 
/*
** This runs under inetd, with the following configuration:
** ident  stream tcp  nowait  nobody  /usr/local/libexec/identd  identd
**
** There are two mutually exclusive options:
** -h	- always reply that the user is hidden
** -u	- always reply that there was an unknown error
*/
 
#include <sys/types.h>
#include <sys/time.h>
#include <sys/uio.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
 
#define	STDIN	0
#define	STDOUT	1
 
const	char	hiding[] = ":ERROR:HIDDEN-USER\r\n";
const	char	unknown[] = ":ERROR:UNKNOWN-ERROR\r\n";
 
const	char	resperr[] = "0,0:ERROR:INVALID-PORT\r\n";
const	char	response[] = "%s:USERID:OTHER:%lu-ident-is-a-completely-pointless-protocol-that-offers-no-security-or-traceability-at-all-so-take-this-and-log-it!\r\n";
 
 
#define	MAXREAD	1024
static	char	buf[MAXREAD];
 
char *
MakeItAllUp(query)
char	*query;
{
	struct timeval tod;
	pid_t	pid = getpid();	/* fail? perish the thought! */
	unsigned long	nonsense;
	static	char	silliness[MAXREAD];
 
	/* never let it be said that I won't give anyone the time of day */
	if (gettimeofday(&tod, NULL) < 0) {
		exit(4);
	}
 
	nonsense = tod.tv_usec % pid;
	snprintf(silliness, MAXREAD, response, query, nonsense);
	return(silliness);
}
 
int
main(ac, av)
int	ac;
char	*av[];
{
	char	*p;
	ssize_t	resplen = read(STDIN, buf, MAXREAD);
 
	if (resplen <= 0) { exit(1); }
 
	p = memchr(buf, '\r', MAXREAD);
 
	if (p == NULL) { exit(2); }
 
	*p = '\0';
 
	resplen = strlen(buf) + sizeof(response);
 
	if (resplen > MAXREAD) {
		(void)write(STDOUT, resperr, sizeof(resperr));
		exit(3);
	}
 
	if ((ac > 1) && (strlen(av[1]) > 1)) {
		switch (av[1][1]) {
		case 'h':
			resplen = strlen(buf) + sizeof(hiding);
			p = strcat(buf, hiding);
			break;
		case 'u':
			resplen = strlen(buf) + sizeof(unknown);
			p = strcat(buf, unknown);
			break;
		default:
			p = MakeItAllUp(buf);
			break;
		}
	} else {
		p = MakeItAllUp(buf);
	}
 
	(void)write(STDOUT, p, strlen(p));
 
	exit(0);
}