1 Daemon Processes Computer Network Programming

2 Motivation You wrote a server and you want to have it running all the time so that clients can connect and use your service at any time. –Examples: Web serves, telnet servers, ftp servers, domain name servers, etc. You don’t want your server to be killed when you logout or when you press ^C. You want your server to be running in the background silently without disturbing others and without getting disturbed by others. You want to run it for a long-time: for days, months, years maybe. You want it to be started automatically when the system boots up.

3 Daemon Processes Solution: You need to run your server as a daemon! A daemon is a process that runs in the background and is independent of control from all terminals. A daemon process usually performs server tasks and administrative tasks There are daemon processes running in a Unix system: silently.

4 root Mar 22 ? 0:07 /usr/sbin/inetd -s root Mar 22 ? 0:01 /usr/lib/nfs/statd root Mar 22 ? 1:49 /usr/lib/autofs/automountd root Mar 22 ? 0:00 /usr/lib/nfs/lockd root Mar 22 ? 0:07 /usr/lib/utmpd root Mar 22 ? 0:22 /usr/sbin/syslogd -n -z 18 root Mar 22 ? 0:02 /usr/sbin/cron root Mar 22 ? 0:01 /usr/lib/snmp/snmpdx -y -c /etc/snmp/conf root Mar 22 ? 0:03 /usr/lib/nfs/mountd root Mar 22 ? 0:00 /usr/lib/nfs/nfsd -a 16 root Mar 22 ? 0:00 /usr/lib/dmi/snmpXdmid -s aspendos root Mar 26 ? 1:00 /usr/lib/lpsched root Mar 22 ? 0:00 /usr/lib/dmi/dmispd root Mar 22 ? 0:08 /usr/dt/bin/dtlogin -daemon root Mar 27 ? 0:00 in.rlogind root Mar 26 ? 0:00 /aspendosh/apache/bin/httpd root Mar 23 ? 0:18 /usr/dt/bin/rpc.ttdbserverd root :10:32 ? 0:00 in.telnetd ………. truncated output Some daemons running on a Unix system

5 Why independence from all terminals Assume you start your daemon from a terminal (xterm, dump terminal, etc). –You want to use the same terminal for other tasks. Hence you don’t want the daemon to print out messages to the screen while you are doing some other tasks –When the user presses interrupt keys (^C), you don’t want your daemon to be killed.

6 How to start a daemon –Start it from a system initialization script during system startup. –inetd, syslogd, sendmail, httpd are examples –inetd starts the daemon (service) –ftpd, telnetd, etc. are started from the inetd process. –cron daemon starts them at regular intervals –at command can be used to execute a server at a well-specified time in the future. –Can be started from user terminal: »to test the daemon »to restart a died daemon

7 syslogd daemon Used to log messages (error messages, information messages, all kinds of messages) Daemons usually don’t print messages on the screen. They send the messages to the syslogd and it logs everything or prints to console if necessary

8 syslogd Daemon Process Daemon Process Daemon kernel console Log file Udp Socket Unix Domain Socket Messages /dev/klog Uses select() to multiplex input from multiple sources - input from UDP socket (port 514) - input from Unix domain socket (/var/run/log) - input from Kernel (/dev/klog device) - output to log file(s) - output to console - output to user - output to an other syslogd Syslogs reads a configuration file /etc/syslog.conf to learn what to do with each kind of message

9 syslog() function A daemon does not have a controlling terminal (a keyboard/screen for example), hence it can not call printf.. It calls syslog() function to output a error messages or log messages to the syslogd and syslogd stores the message or writes it to the console. Syslog() function sends the message to the syslogd. void syslog(int priority, const char* message, ….); priority is combination of level and facility flags. message is a format string like printf(). then some arguments may follow

10 Level denotes the severity of the message: crital, error, info, debug message etc. Facility identifies the type of the process sending the message ftp daemon, kernel, mail system, a user process, local use, etc. Example: syslog (LOG_INFO | LOG_LOCAL2, “rename error: %s %s”, file1, file2) syslogd uses the configuration file /etc/syslog.conf and level and facility parameters to decide what to do with each incoming message. /etc/syslogd.conf file content *.err;kern.notice;auth.notice /dev/console *.err;kern.debug;daemon.notice;mail.crit /var/adm/message

11 static void err_doit(int errnoflag, int level, const char *fmt, va_list ap) { int errno_save, n; char buf[MAXLINE]; errno_save = errno; vsprintf(buf, fmt, ap); n = strlen(buf); if (errnoflag) snprintf(buf+n, sizeof(buf)-n, ": %s", strerror(errno_save)); strcat(buf, "\n"); if (daemon_proc) { syslog (level, buf); /* send to syslogd daemon for logging */ } else { fflush(stdout); /* print out to the screen */ fputs(buf, stdout); fflush(stdout); } return; } Use of syslog() function

12 How can we daemonize a process #define MAXFD 64 extern int daemon_proc; /* defined in error.c */ void daemon_init(const char *pname, int facility) { int i; pid_t pid; if ( (pid = Fork()) != 0) exit(0); /* parent terminates */ /* 1st child continues */ setsid(); /* become session leader */ Signal(SIGHUP, SIG_IGN); if ( (pid = Fork()) != 0) exit(0); /* 1st child terminates */ /* 2nd child continues */ daemon_proc = 1; /* for our err_XXX() functions */ chdir("/"); /* change working directory */ umask(0); /* clear our file mode creation mask */ for (i = 0; i < MAXFD; i++) close(i); openlog(pname, LOG_PID, facility); }

13 A daemon process will never receive the following signals from the kernel: SIG_HUP, SIG_INT, SIH_WINCH. Usually SIG_HUP signal is given to the daemon by system administrators to make the daemon reread its confıguration file(s). Exampe: kill -HUP inetd

14 int main(int argc, char **argv) { int listenfd, connfd; socklen_t addrlen, len; struct sockaddr *cliaddr; char buff[MAXLINE]; time_t ticks; daemon_init(argv[0], 0); if (argc == 2) listenfd = Tcp_listen(NULL, argv[1], &addrlen); else if (argc == 3) listenfd = Tcp_listen(argv[1], argv[2], &addrlen); else err_quit("usage: daytimetcpsrv2 [ ] "); cliaddr = Malloc(addrlen); for ( ; ; ) { len = addrlen; connfd = Accept(listenfd, cliaddr, &len); err_msg("connection from %s", Sock_ntop(cliaddr, len)); ticks = time(NULL); snprintf(buff, sizeof(buff), "%.24s\r\n", ctime(&ticks)); Write(connfd, buff, strlen(buff)); Close(connfd); } Daytime tcp server as a daemon

15 inetd daemon internet superserver There are a lot of internet servers that will be running on a typical Unix system - FTP, telnet, rlogin, TFTP, daytime, time, echo Servers In earlier systems, each service had a process (daemon) associated with. - each process does the same startup tasks creating sockets, binding listening…. - each process took an entry in the process table - these processes are asleep most of the time waiting for requests. After 4.3 BSD Unix system, a single server inetd daemon is used to wait for requests for all of these services and then create the corresponding server process on demand.

16 inetd ftpstreamtcpnowaitroot/usr/sbin/in.ftpdin.ftpd telnetstreamtcpnowaitroot/usr/sbin/in.telnetdin.telnetd namedgramudpwaitroot/usr/sbin/in.tnamedin.tnamed shellstreamtcpnowaitroot/usr/sbin/in.rshdin.rshd loginstreamtcpnowaitroot/usr/sbin/in.rlogindin.rlogind tftpdgramudpwaitroot/usr/sbin/in.tftpdin.tftpd -s /tftpboot /etc/inetd.conf file ftpdtelnetdrlogind ……. fork() and exec() reads the configuration file upon startup Has a listening socket for each TCP service or a UDP socket for each UDP service and uses select()

17 inetd and its child services aspendos{korpe}:> ps -ef | grep 139 user ppid pid tty process root Mar 22 ? 0:07 /usr/sbin/inetd -s korpe :15:11 ? 0:00 in.ftpd root :14:16 ? 0:00 in.ftpd root :26:13 ? 0:00 in.rlogind root :13:55 ? 0:00 in.telnetd root Mar 26 ? 0:00 in.rlogind root :00:56 ? 0:00 in.telnetd root :13:18 ? 0:00 in.telnetd root :16:04 ? 0:00 in.telnetd We have a separate child server process for each client. The parent of these servers is inetd.

18 socket() steps performed by inetd bind() listen() (if TCP) select() accept() (if TCP) fork() close all descriptors other than socket Dup socket to descriptors 0, 1, 2 close socket setgid, setusid exec() server close connected socket (if TCP) For each service listed in the /etc/inetd.conf file parentchild

19 –For services that uses TCP, when inetd creates a child process, it does not wait until the child terminates before accepting a new service request. –The child uses a different connected socket than the listening socket of the inetd. –For services that uses UDP, when inetd creates a child process, inetd waits until the child terminates before accepting a new service request. –The child uses the same UDP socket that the inetd was waiting on for requests. Wait and Nowait

20 int main(int argc, char **argv) { socklen_t len; struct sockaddr * cliaddr; char buff[MAXLINE]; time_t ticks; daemon_inetd(argv[0], 0); cliaddr = Malloc(MAXSOCKADDR); len = MAXSOCKADDR; Getpeername(0, cliaddr, &len); err_msg("connection from %s", Sock_ntop(cliaddr, len)); ticks = time(NULL); snprintf(buff, sizeof(buff), "%.24s\r\n", ctime(&ticks)); Write(0, buff, strlen(buff)); Close(0); /* close TCP connection */ exit(0); } Daytime Server as Daemon invoked by inetd extern int daemon_proc; /* defined in error.c */ void daemon_inetd(const char *pname, int facility) { daemon_proc = 1; openlog(pname, LOG_PID, facility); } We are writing to descriptor 0 since it corresponds to the socket. (normally it corresponds to stdin = keyboard)