language C : simple function to log message to remote syslog server via UDP port 514

rsyslog_example.c

// rsyslog_example.c
// 202011171619 
//
// gcc -o rsyslog_example rsyslog_example.c
//
// simple function to log message to remote syslog server via UDP port 514
//
// why is this NOT part of the standard syslog library?????
//

#define VERSION_MAJOR 1
#define VERSION_MINOR 0

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <syslog.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h> 

// logging setup
// LOG_EMERG
// A panic condition was reported to all processes.
// LOG_ALERT
// A condition that should be corrected immediately.
// LOG_CRIT
// A critical condition.
// LOG_ERR
// An error message.
// LOG_WARNING
// A warning message.
// LOG_NOTICE
// A condition requiring special handling.
// LOG_INFO
// A general information message.
// LOG_DEBUG
// A message useful for debugging programs.

#define RSYSLOG_ADDRESS "192.168.yyy.xxx"
#define LOGMESSAGESIZE 512
char log_message[LOGMESSAGESIZE];

// this function sends a log message to a remote syslog server
// call with:
//  log level
//  hostname of logging server
//  program name that is sending log message
//  message

void send_remote_syslog_message(int log_level, char *hostname,  char *program_name, char *message)
{
    int sockfd, portno, n;
    int serverlen;
    int message_length;
    struct sockaddr_in serveraddr;
    struct hostent *server;
    #define LOGBUFSIZE 1024
    char buf[LOGBUFSIZE];
    #define RSYSLOGPORT 514

    // socket: create the socket
    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0)
    { 
        fprintf(stderr, "ERROR opening socket for remote syslog write");
        exit(1);
    }

    // gethostbyname: get the server's DNS entry
    server = gethostbyname(hostname);
    if (server == NULL)
    {
        fprintf(stderr,"ERROR, no such host as %s for remote syslog write\n", hostname);
        exit(1);
    }

    // build the server's Internet address
    bzero((char *) &serveraddr, sizeof(serveraddr));
    serveraddr.sin_family = AF_INET;
    bcopy((char *)server->h_addr, 
      (char *)&serveraddr.sin_addr.s_addr, server->h_length);
    serveraddr.sin_port = htons(RSYSLOGPORT);

    // build the syslog message
    message_length = snprintf(buf, LOGBUFSIZE, "<%d>%s %s", LOG_USER + log_level, program_name, message);

    // fprintf(stdout, "%s\n", buf);

    // send the message to the server
    serverlen = sizeof(serveraddr);
    n = sendto(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&serveraddr, serverlen);
    if (n < 0)
    {
      fprintf(stderr,"ERROR in sendto for remote syslog write\n");
      exit(1);
    }

    return;
}


int main(int argc, char **argv) {

    setlogmask (LOG_UPTO (LOG_INFO));
    openlog (__FILE__, LOG_CONS | LOG_PID | LOG_NDELAY, LOG_LOCAL1);

    snprintf(log_message, LOGMESSAGESIZE, "%s v: %d.%d Starting.", __FILE__, VERSION_MAJOR, VERSION_MINOR);
    send_remote_syslog_message(LOG_INFO, RSYSLOG_ADDRESS, __FILE__, log_message);
    syslog (LOG_INFO, log_message);

    //
    //
    //
    //


    snprintf(log_message, LOGMESSAGESIZE, "%s v: %d.%d <ctrl>-c signal received, exiting.", __FILE__, VERSION_MAJOR, VERSION_MINOR);
    send_remote_syslog_message(LOG_INFO, RSYSLOG_ADDRESS, __FILE__, log_message);
    syslog (LOG_INFO, log_message);

    return 0;
}