Daemon Processes, Syslog Facility & inetd Superservers

Explanation

Server software running in enterprise environments must dissociate from user sessions so that terminal logout does not trigger SIGHUP and terminate the service. The daemon_init sequence detaches the process, guarantees it is not a session leader, and establishes independent process groups. Because daemons lack a console, POSIX provides the syslog interface. The inetd (and modern systemd) superserver architecture prevents hundreds of rarely-used services from idling in memory, instead maintaining listening sockets on their behalf and executing the actual server binary via exec() upon connection arrival.

Key points

• Daemonizing steps: fork() and exit parent, setsid() to create a new session, change working directory to root '/', umask(0), and redirect stdin/stdout/stderr to /dev/null.
• Syslog facility: Daemons report errors using openlog(), syslog(LOG_ERR, ...), and closelog(), routing messages to /var/log/syslog via syslogd.
• inetd superserver: A single master daemon that listens on multiple ports listed in /etc/inetd.conf and forks the appropriate service daemon on demand.
• Advantages of inetd: Conserves system memory and process table slots by launching servers only when connection requests arrive.

Example

Code examples

void daemon_init(void) {
    pid_t pid;
    if ((pid = fork()) < 0) exit(1);
    else if (pid != 0) exit(0); // Terminate parent

    setsid(); // Become session leader
    chdir("/"); // Prevent unmounting directory
    umask(0);

    close(STDIN_FILENO);
    close(STDOUT_FILENO);
    close(STDERR_FILENO);
    open("/dev/null", O_RDWR); // fd 0
    dup(0); // fd 1
    dup(0); // fd 2
}

Exam questions

• [8 Marks] Explain the step-by-step procedure to daemonize a network process under Unix/Linux with an annotated code implementation.
• [5 Marks] Discuss the working mechanism and advantages of the inetd superserver with reference to /etc/inetd.conf.