the djb way


howdyd: a howdy daemon, part 2

Hi there! Welcome back.

Let's recap part 1 and the pieces that went into making a TCP/IP service with daemontools and ucspi-tcp:

The howdy daemon demonstrated all of these components, as well as other standard practices used in putting up a secure service with daemontools:

But while our first cut at the howdy daemon may have used these features --and while it was certainly friendly enough-- the howdyd server we have seen so far isn't especially useful.

In fact, though, a howdy server can actually be nice to have around. Think of the applications where you might want a quick way to get a status check on a remote system without having to log in. The howdyd framework also provides a great way to thoroughly explore all the details and options of setting up a server with daemontools and ucspi-tcp.

To see some possibilities, install this slightly enhanced version of in /var/svc.d/howdyd/

# a howdy daemon
# ===
echo "*** A visitor from ${TCPREMOTEIP}!" >&2
echo "Hi there!  Welcome to `hostname`!"
echo "The time here: `date`"
echo "Our uptime is: `uptime`"
echo "The howdyd environment:"
printenv | sort
echo "The howdyd user:"
who -Hm
echo "Our users:"
w -h
echo "Bye!"
echo "*** The visitor from ${TCPREMOTEIP} departs!" >&2
exit 0
### that's all, folks!

Now open three other terminals. Use one of these to control the service, restarting it every time you modify the settings:

# svc -t /service/howdyd

Use another to follow the log (use -f for OpenBSD):

$ tail -F /var/multilog/howdyd/current | tai64nlocal
2004-01-06 16:09:25.408525500 *** Starting howdyd service...
2004-01-06 16:09:25.415041500 tcpserver: status: 0/13

And use another to contact the server with your "client":

$ tcpcat 1789
Hi there!  Welcome to!
The time here: Tue Jan  6 16:13:19 EAT 2004
Our uptime is:  4:13PM  up  7:23, 5 users, load averages: 0.11, 0.10, 0.08

The howdyd environment:

The howdyd user:
USER     LINE     WHEN           FROM
howdyd   tty??    Jan  6 16:13 

Our users:
wcm      C0 -                 8:53AM  7:20 xinit /home/wcm//.xinitrc -- 
wcm      p0 :0.0              8:53AM     4 tail -f /var/multilog/howdyd/current
wcm      p1 :0.0              8:54AM  7:18 tail -f /var/multilog/fetchmail/curr
wcm      p2 :0.0              8:56AM  4:42 -bash 
wcm      p3 :0.0             11:58AM     0 bash -l 

What's another word for "thesaurus"?
		-- Steven Wright

This shows the environment that tcpserver sets up for the howdyd server. Note the $SOMEVAR variable, set as we specified in the howdy.rules configured for this service.

The message is also sprinkled with a few other system info commands, as well as a fortune, before a cheerful farewell.

Now switch back to the log to see what was recorded for this connection:

2004-01-06 16:09:25.408525500 *** Starting howdyd service...
2004-01-06 16:09:25.415041500 tcpserver: status: 0/13
2004-01-06 16:13:00.490932500 tcpserver: status: 1/13
2004-01-06 16:13:00.491776500 tcpserver: pid 10390 from
2004-01-06 16:13:00.500501500 tcpserver: ok 10390 0:
2004-01-06 16:13:00.503919500 *** A visitor from!
2004-01-06 16:13:00.532722500 *** The visitor from departs!
2004-01-06 16:13:00.533943500 tcpserver: end 10390 status 0
2004-01-06 16:13:00.533959500 tcpserver: status: 0/13

Most of the lines are tcpserver status messages. These show the current connection number (slash) maximum connections, such as 1/13. In this case, the 13 comes from the $CONLIMIT parameter in the run script.

The log also shows a few lines generated by the script itself. Your server only needs to write to stderr, and multilog will pick it up for the log.

The other lines in the log pull data from the $TCP* variables. The completeness of these variables depend on DNS records for the clients, and the data gathering options used with tcpserver:

So now what to do is just mess around, changing things and remembering to restart the server each time with svc -t, studying the results:

Of course, doesn't have to be a shell script, either. Use your favorite language, Perl, Python, Ruby. Why, you could write your server using C!

The more time you spend playing around with this framework, the more you will understand the use of daemontools services.

Copyright © 2003, 2004, Wayne Marshall.
All rights reserved.

Last edit 2004.03.08, wcm.