/* smtp-dummy - Dummy SMTP server that delivers mail to the given file
 *
 * Copyright © 2010 Carl Worth
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see http://www.gnu.org/licenses/ .
 *
 * Authors: Carl Worth <cworth@cworth.org>
 */

/* This (non-compliant) SMTP server listens on localhost, port 25025
 * and delivers a mail received to the given filename, (specified as a
 * command-line argument). It exists after the first client connection
 * completes.
 *
 * It implements very little of the SMTP protocol, even less than
 * specified as the minimum implementation in the SMTP RFC, (not
 * implementing RSET, NOOP, nor VRFY). And it doesn't do any
 * error-checking on the input.
 *
 * That is to say, if you use this program, you will very likely find
 * cases where it doesn't do everything your SMTP client expects. You
 * have been warned.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <netinet/ip.h>
#include <netdb.h>
#include <unistd.h>

#define STRNCMP_LITERAL(var, literal) \
    strncmp ((var), (literal), sizeof (literal) - 1)

static void
receive_data_to_file (FILE *peer, FILE *output)
{
	char *line = NULL;
	size_t line_size;
	ssize_t line_len;

	while ((line_len = getline (&line, &line_size, peer)) != -1) {
		if (STRNCMP_LITERAL (line, ".\r\n") == 0)
			break;
		if (line_len < 2)
			continue;
		if (line[line_len-1] == '\n' && line[line_len-2] == '\r') {
			line[line_len-2] = '\n';
			line[line_len-1] = '\0';
		}
		fprintf (output, "%s",
			 line[0] == '.' ? line + 1 : line);
	}

	free (line);
}

static int
process_command (FILE *peer, FILE *output, const char *command)
{
	if (STRNCMP_LITERAL (command, "EHLO ") == 0) {
		fprintf (peer, "502 not implemented\r\n");
		fflush (peer);
	} else if (STRNCMP_LITERAL (command, "HELO ") == 0) {
		fprintf (peer, "250 localhost\r\n");
		fflush (peer);
	} else if (STRNCMP_LITERAL (command, "MAIL FROM:") == 0 ||
		   STRNCMP_LITERAL (command, "RCPT TO:") == 0) {
		fprintf (peer, "250 OK\r\n");
		fflush (peer);
	} else if (STRNCMP_LITERAL (command, "DATA") == 0) {
		fprintf (peer, "354 End data with <CR><LF>.<CR><LF>\r\n");
		fflush (peer);
		receive_data_to_file (peer, output);
		fprintf (peer, "250 OK\r\n");
		fflush (peer);
	} else if (STRNCMP_LITERAL (command, "QUIT") == 0) {
		fprintf (peer, "221 BYE\r\n");
		fflush (peer);
		return 1;
	} else {
		fprintf (stderr, "Unknown command: %s\n", command);
	}
	return 0;
}

static void
do_smtp_to_file (FILE *peer, FILE *output)
{
	char *line = NULL;
	size_t line_size;
	ssize_t line_len;

	fprintf (peer, "220 localhost smtp-dummy\r\n");
	fflush (peer);

	while ((line_len = getline (&line, &line_size, peer)) != -1) {
		if (process_command (peer, output, line))
			break;
	}

	free (line);
}

int
main (int argc, char *argv[])
{
	char *output_filename;
	FILE *peer_file, *output;
	int sock, peer, err;
	struct sockaddr_in addr, peer_addr;
	struct hostent *hostinfo;
	socklen_t peer_addr_len;
	int reuse;

	if (argc != 2) {
		fprintf (stderr, "Usage: %s <output-file>\n", argv[0]);
		return 1;
	}

	output_filename = argv[1];
	output = fopen (output_filename, "w");
	if (output == NULL) {
		fprintf (stderr, "Failed to open %s for writing: %s\n",
			 output_filename, strerror (errno));
		return 1;
	}

	sock = socket (AF_INET, SOCK_STREAM, 0);
	if (sock == -1) {
		fprintf (stderr, "Error: socket() failed: %s\n",
			 strerror (errno));
		return 1;
	}

	reuse = 1;
	err = setsockopt (sock, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof (reuse));
	if (err) {
		fprintf (stderr, "Error: setsockopt() failed: %s\n",
			 strerror (errno));
		return 1;
	}

	hostinfo = gethostbyname ("localhost");
	if (hostinfo == NULL) {
		fprintf (stderr, "Unknown host: localhost\n");
		return 1;
	}

	memset (&addr, 0, sizeof (addr));
	addr.sin_family = AF_INET;
	addr.sin_port = htons (25025);
	addr.sin_addr = *(struct in_addr *) hostinfo->h_addr;
	err = bind (sock, (struct sockaddr *) &addr, sizeof(addr));
	if (err) {
		fprintf (stderr, "Error: bind() failed: %s\n",
			 strerror (errno));
		close (sock);
		return 1;
	}

	err = listen (sock, 1);
	if (err) {
		fprintf (stderr, "Error: listen() failed: %s\n",
			 strerror (errno));
		close (sock);
		return 1;
	}

	peer_addr_len = sizeof (peer_addr);
	peer = accept (sock, (struct sockaddr *) &peer_addr, &peer_addr_len);
	if (peer == -1) {
		fprintf (stderr, "Error: accept() failed: %s\n",
			 strerror (errno));
		return 1;
	}

	peer_file = fdopen (peer, "w+");
	if (peer_file == NULL) {
		fprintf (stderr, "Error: fdopen() failed: %s\n",
			 strerror (errno));
		return 1;
	}

	do_smtp_to_file (peer_file, output);

	fclose (output);
	fclose (peer_file);
	close (sock);

	return 0;
}