main.c

/*
 * Dovecot checkpassword implementaion
 * For the protocol, see https://wiki2.dovecot.org/AuthDatabase/CheckPassword
 *
 * Expects two inputs on fd 3, a username and password.
 *
 * Connects to a socket on localhost and echos the inputs, plus an IP address
 * we find in the environment.
 *
 * Expect back (on the socket) either "fail\n" or "success\n".
 *
 * On fail, exit with code 1.
 *
 * On success, read "KEY=VALUE\n" lines from the socket, write them
 * into the environment, and then run the file found on our command line.
 *
 * Any sign of trouble, exit with code 111.
 *
 * File:   main.c
 * Author: Osric Wilkinson (osric@fluffypeople.com)
 *
 * Created on 07 November 2017, 06:50
 */
#define _GNU_SOURCE
#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <syslog.h>

#define AUTH_FAIL 1
#define TEMP_ERROR 111

#define PORT 15408
#define LOCALHOST "127.0.0.1"

int create_socket() {
    int sock;
    struct sockaddr_in dest;

    if ((sock = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
        perror("Socket");
        exit(TEMP_ERROR);
    }

    dest.sin_family = AF_INET;
    dest.sin_port = htons(PORT);
    dest.sin_addr.s_addr = inet_addr("127.0.0.1");
    memset(dest.sin_zero, '\0', sizeof dest.sin_zero);

    if (connect(sock, (struct sockaddr*) &dest, sizeof (dest)) != 0) {
        perror("Connect");
        exit(TEMP_ERROR);
    }

    return sock;
}

void trimNewline(char *str) {
    str[strcspn(str, "\n")] = 0;
}

void readEnvironment(FILE *socket) {
    char *env;
    size_t env_len = 0;
    while (getline(&env, &env_len, socket) != -1) {
        char *name, *value;

        trimNewline(env);

        // Split env on equals.
        name = env;
        size_t eq = strcspn(env, "=");
        name[eq] = 0;
        value = env + (eq + 1);

        if (setenv(name, value, 1) == -1) {
            perror("Can't set environment");
            exit(TEMP_ERROR);
        }

        free(env);
        env_len = 0;
    }
}

void timeout(int signum) {
    syslog(LOG_WARNING, "%s", "Timeout");
    exit(TEMP_ERROR);
}

int main(int argc, char** argv) {
    char *user, *passwd, *remoteIP;

    openlog(NULL, LOG_NDELAY | LOG_PERROR, 0);
    
    // Give ourselves 8 seconds
    signal(SIGALRM, &timeout);
    alarm(8);

    FILE *input = fdopen(3, "r");

    size_t len = 0;
    if (getdelim(&user, &len, '\0', input) < 0) {
        syslog(LOG_WARNING, "%s: %s", "Can't read username", strerror(errno));
        exit(TEMP_ERROR);
    }

    len = 0;
    if (getdelim(&passwd, &len, '\0', input) < 0) {
        syslog(LOG_WARNING, "%s: %s", "Can't read password", strerror(errno));
        exit(TEMP_ERROR);
    }

    remoteIP = getenv("TCPREMOTEIP");

    int socket_fp = create_socket();

    FILE *socket = fdopen(socket_fp, "r+");

    // Write to socket
    if (fprintf(socket, "%s\n%s\n%s\n", user, passwd, remoteIP == NULL ? "unknown" : remoteIP) < 0) {
        syslog(LOG_WARNING, "%s: %s", "Can't write to socket", strerror(errno));
        exit(TEMP_ERROR);
    }
    if (fflush(socket) == EOF) {
        syslog(LOG_WARNING, "%s: %s", "Can't flush socket", strerror(errno));
        exit(TEMP_ERROR);
    }

    // Check for success
    char *result;
    size_t result_len = 0;
    if (getline(&result, &result_len, socket) == -1) {
        syslog(LOG_WARNING, "%s: %s", "Can't read result", strerror(errno));
        exit(TEMP_ERROR);
    }

    trimNewline(result);

    if (strcmp("success", result) == 0) {
        clearenv();
        readEnvironment(socket);

        closelog();
        
        if (execl(argv[1], argv[1], NULL) == -1) {
            syslog(LOG_WARNING, "%s: %s", "Can't execute next program", strerror(errno));
            exit(TEMP_ERROR);
        }
        // end of process
    } else {
        return (AUTH_FAIL);
    }
}