Simple socket program lisning on port 25

smtp.c

#include <strings.h>   
#include <unistd.h>    
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>     
#include <syslog.h>    
#include <libgen.h>    
#include <arpa/inet.h> 
#include <sys/time.h>  
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <signal.h>
#include <errno.h>
#include <stdarg.h>
#include <sys/stat.h>
#include <fcntl.h>

int main () {

    int listener_fd, new_fd;
    int on;
    int remotelen = sizeof(struct sockaddr_in);

    struct sockaddr_in local_sin, remote_sin;

    if(( listener_fd = socket( AF_INET, SOCK_STREAM,0 )) < 0 ) {
        printf("error socket");
        exit(1);
    }

    on = 1;

    if((setsockopt(listener_fd, SOL_SOCKET, SO_REUSEADDR, (char *)&on, sizeof(on))) <0){
        printf("error setsockopt");
        exit(1);
    }

    memset((char *)&local_sin, 0x0, sizeof(struct sockaddr_in));
    memset((char *)&remote_sin, 0x0, sizeof(struct sockaddr_in));

    local_sin.sin_port   = htons((short)25);
    local_sin.sin_family = AF_INET;
    local_sin.sin_addr.s_addr = htonl(INADDR_ANY);

    if(bind(listener_fd,(struct sockaddr *)&local_sin,sizeof(struct sockaddr_in)) < 0 ){
        printf("error bind");
        exit(1);
    }

    if(listen(listener_fd, 5) < 0) {
        printf("error listen");
        exit(1);

    }

    while (1) {

        if((new_fd = accept(listener_fd,(struct sockaddr *)&remote_sin, &remotelen)) < 0){

            perror("accept");
            exit(-1);
        }
        printf("new fd=%d\n", new_fd);

        int reading = 1;

        while(reading){
            char databuf[10];
            int rsize;
            char prev;

            rsize = recv(new_fd, databuf, 10, 0);

            if(rsize == 0){
                perror("connection closed");
            } else if(rsize < 0){
                perror("recv\n");
                exit(-1);
            }
            int i;
            for( i = 0 ; i < rsize ; i++){
                printf("%c", databuf[i]);

                if(databuf[i] == '.' && prev == '\n'){
                    close(new_fd);
                    reading = 0;
                    break;
                }
                prev = databuf[i];
            }
        }
        printf("\n");
    }
}

gcc -lsocket -lnsl -o smtp smpt.c