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/**
* femtomail - Minimal sendmail replacement for forwarding mail to a single
* Maildir box. Note: this program does not try to implement sendmail.
*
* Installation commands:
* $ cc -DUSERNAME=\"$USER\" femtomail.c -o femtomail
* (Optional override: -DMAILBOX_PATH=\".Maildir/inbox\")
* # install -m 755 femtomail /usr/bin/sendmail
* # setcap cap_setuid,cap_setgid=ep /usr/bin/sendmail
*
* Copyright (C) 2013 Peter Wu <lekensteyn@gmail.com>
*
* 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/>.
*/
#define _GNU_SOURCE /* for setresgid/setresuid */
#include <time.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdbool.h>
#include <ctype.h>
#include <sys/types.h>
#include <pwd.h>
#include <sys/prctl.h>
#ifndef USERNAME
# error Please define the user to deliver mail to with USERNAME
#endif
/* Maildir directory relative to home dir of USERNAME (see above) */
#ifndef MAILBOX_PATH
# define MAILBOX_PATH ".local/share/local-mail/inbox"
#endif
/* change user/group context to username and fill in maildir path */
int
init_user(const char *username, char *maildir, size_t maildir_len) {
struct passwd *pwd;
uid_t uid;
gid_t gid;
if ((pwd = getpwnam(username)) == NULL) {
fprintf(stderr, "Unknown user %s, cannot locate Maildir.\n", username);
return 1;
}
uid = pwd->pw_uid;
gid = pwd->pw_gid;
if (setresgid(gid, gid, gid) || setresuid(uid, uid, uid)) {
perror("Failed to change uid/gid");
return 1;
}
snprintf(maildir, maildir_len, "%s/" MAILBOX_PATH "/new", pwd->pw_dir);
return 0;
}
/* get a random number between 0 and 999 (inclusive) */
unsigned
get_random(void) {
FILE *fp;
int rnd;
fp = fopen("/dev/urandom", "r");
if (fp != NULL) {
unsigned char buf[2];
fread(buf, 2, 1, fp);
rnd = (buf[0] << 8) | buf[1];
fclose(fp);
} else {
rnd = rand();
}
return rnd % 1000;
}
/* get the current hostname, sanitizing '/' and ':' chars */
void
get_hostname(char *hostname, size_t hostname_len) {
if (gethostname(hostname, hostname_len) >= 0) {
char *p;
/* sanitized per http://cr.yp.to/proto/maildir.html */
for (p = hostname; *p; p++) {
if (*p == '/') *p = '\057';
if (*p == ':') *p = '\072';
}
} else {
strncpy(hostname, "unknown", hostname_len);
}
}
/* generate a filename suitable for Maildir consisting of a timestamp, a random
* middle part and a hostname (per http://cr.yp.to/proto/maildir.html) */
void
make_name(char *name, size_t name_len, time_t tm) {
char hostname[128];
get_hostname(hostname, sizeof(hostname));
snprintf(name, name_len, "%llu.R%u.%s",
(unsigned long long) tm, get_random(), hostname);
}
/* write Received header to file */
void
write_headers(FILE *mail_fp, const char *address, time_t tm) {
char timestr[200];
struct tm *tmp;
tmp = localtime(&tm);
if (tmp == NULL) {
perror("localtime");
return;
}
if (!strftime(timestr, sizeof(timestr), "%a, %d %b %Y %T %z", tmp)) {
fprintf(stderr, "strftime() failed!\n");
return;
}
fprintf(mail_fp, "Received: for %s with local (femtomail); %s\n", address, timestr);
}
/* read text from stdin and write to file */
int
read_and_write(FILE *mail_fp) {
size_t r;
char buf[4096];
while ((r = fread(buf, 1, sizeof(buf), stdin)) > 0) {
if (fwrite(buf, 1, r, mail_fp) != r) {
/* disk full? */
return 1;
}
}
return 0;
}
/* validate the recipient address as in `sendmail [address]` */
bool
valid_address(const char *address) {
char c;
while ((c = *address++) != 0) {
/* reject unprintable chars, including newlines */
if (!isgraph(c)) {
return false;
}
}
return true;
}
int
main(int argc, char **argv) {
char maildir[256];
char *address = NULL, file_path[256];
time_t tm;
int i, mail_fd, ret;
FILE *mail_fp;
for (i = 1; i < argc; i++) {
if (argv[i][0] == '-') {
/* ignore arg, hopefully next arg does not contain a value */
continue;
}
if (!valid_address(argv[i])) {
fprintf(stderr, "Illegal characters in address!\n");
return (EXIT_FAILURE);
}
address = argv[i];
break;
}
if (!address) {
fprintf(stderr, "Missing recipient address.\n");
return (EXIT_FAILURE);
}
if (init_user((USERNAME), maildir, sizeof(maildir))) {
return (EXIT_FAILURE);
}
if (chdir(maildir)) {
fprintf(stderr, "chdir(%s): %s\n", maildir, strerror(errno));
return (EXIT_FAILURE);
}
tm = time(NULL);
make_name(file_path, sizeof(file_path), tm);
/* ensure that no file gets overwritten */
mail_fd = open(file_path, O_WRONLY | O_CREAT | O_EXCL, 0644);
if (mail_fd < 0) {
perror("open");
return (EXIT_FAILURE);
}
mail_fp = fdopen(mail_fd, "w");
write_headers(mail_fp, address, tm);
ret = read_and_write(mail_fp);
if (ret) {
perror("fwrite");
}
fclose(mail_fp);
return ret;
}
/* vim: set sw=4 ts=4 et: */
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