Mali mail server

/*
**
** Datum: 20023-06-13
**
** Program salje HTML mail sa aplikacionog servera.
** mailx ne radi na produkcionom aplikacionom servru, a direktna konekcija
** na port 25 mail servera radi.
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <netdb.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>

#define MAIL_SERVER "mail.foo.com"
#define MAIL_PORT 25
#define BUFFER_SIZE 65536

/* Error handling */
void error(const char *msg) {
    perror(msg);
    exit(1);
}

/* Send and receive bytes from socket */
void SMTPsend(int sock, const char *line, int waitresponse) {
    char buffer[BUFFER_SIZE];
    int n;

    if (line) {
        printf("%s", line);
        n = write(sock, line, strlen(line));
        if (n < 0) error("ERROR writing to socket");
    }
    bzero(buffer, BUFFER_SIZE);
    if (waitresponse) {
        n = read(sock, buffer, BUFFER_SIZE - 1);
        if (n < 0) error("ERROR reading from socket");
        printf("%s", buffer);
    }
}

/* Opening socket */
int smtp_connect() {
    int sockfd;
    struct sockaddr_in serv_addr;
    struct hostent *server;

    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) error("ERROR opening socket");

    server = gethostbyname(MAIL_SERVER);
    if (server == NULL) error("ERROR, no such host");

    bzero((char *) &serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    bcopy((char *) server->h_addr, (char *) &serv_addr.sin_addr.s_addr, server->h_length);
    serv_addr.sin_port = htons(MAIL_PORT);

    if (connect(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0)
        error("ERROR connecting");
    return sockfd;
}

void usage(const char *name) {
    printf("\n\n\tUsage: \n\n");
    printf("\t%s [-s SUBJECT] [-f FILENAME] MAIL_ADDRESS...\n\n", name);
    printf("\tProgram sends HTML mail, where the mail body is in the FILENAME.\n");
    printf("\tIf FILENAME is omitted, then program reads input from STDIN\n");
    printf("\tMandatory parameter is list of email addresses,\n\n\n");
    exit(1);
}

int main(int argc, char *argv[]) {
    char buffer[BUFFER_SIZE];
    int sockfd;
    FILE *input_stream = stdin;
    int opt;
    char *subject = "[No Subject]", *file_name = NULL;
    char *recipients[100];
    char *sender = "Some-Sender";
    int recipient_count = 0;

    while ((opt = getopt(argc, argv, "s:f:")) != -1) {
        switch (opt) {
            case 's': /* Subject */
                subject = strdup(optarg);
                break;
            case 'f': /* File name */
                file_name = strdup(optarg);
                if ((input_stream = fopen(file_name, "r")) == NULL) {
                    snprintf(buffer, sizeof(buffer), "ERROR opening %s\n", file_name);
                    error(buffer);
                }
                break;
            default:
                usage(argv[0]);
        }
    }

    if (optind == argc) usage(argv[0]);

    for (int i = optind; i < argc; i++) {
        recipients[recipient_count] = argv[i];
        recipient_count++;
    }

    sockfd = smtp_connect();

    /* Create mail header */
    snprintf(buffer, sizeof(buffer), "From: %s\r\nTo: ", sender);
    for (int i = 0; i < recipient_count; i++) {
        strcat(buffer, recipients[i]);
        if (i < recipient_count - 1) strcat(buffer, ", ");
    }
    strcat(buffer, "\r\nSubject: ");
    strcat(buffer, subject);
    strcat(buffer, "\r\nMIME-Version: 1.0\r\nContent-Type: text/html\r\n\r\n");

    /* Read mail server greeting */
    SMTPsend(sockfd, NULL, 1);

    /* Greet mail server with EHLO */
    SMTPsend(sockfd, "EHLO\r\n", 1);

    /* Send FROM part */
    SMTPsend(sockfd, "MAIL FROM:user@localhost\r\n", 1);

    /* Send TO part */
    for (int i = 0; i < recipient_count; i++) {
        SMTPsend(sockfd, "RCPT TO:", 0);
        SMTPsend(sockfd, recipients[i], 0);
        SMTPsend(sockfd, "\r\n", 1);
    }

    /* Intro into DATA transmission */
    SMTPsend(sockfd, "DATA\r\n", 0);

    /* Send actual mail */
    SMTPsend(sockfd, buffer, 0);

    /* Send email body */
    while (fgets(buffer, sizeof(buffer), input_stream) != NULL) {
        SMTPsend(sockfd, buffer, 0);
    }

    /* Finish mail */
    SMTPsend(sockfd, "\r\n.\r\nQUIT\r\n", 1);

    /* Free allocated memory */
    free(subject);
    if (file_name) free(file_name);

    return 0;
}