Linux, Unix, /etc/
$ To set up this Linux system, do I really have to type long,
cryptic, frequently inconsistent and undocumented commands with
occasional long strings of hex digits?
# Yeah. You got a problem with that?
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Multi-User Linux: Supporting Terminals on a Linux Box
intro
This article is about supporting multiple users on a Linux box by
attaching terminals. While this approach to multi-user computing has
been somewhat overshadowed in recent years by the client-sever model,
in which each user has their own computer which avails of network
services from its peers and/or a central server, the central host /
dumb terminal scheme has certain advantages, not least among them
being lower cost and easier maintanance. This article examines how
to set up such a system using Linux as the central host OS.
Terminals are not just of use to companies with large numbers of staff.
One of the disadvantages of the home computer is that it really needs
a room of its own, or at least a room with a considerable portion
of space given over to it and its paraphernalia. This is clearly
not an ideal way of using the machine: one would not think of going
to a special room to read a magazine, so why do so when you wish to
read through articles on a web site? Linux's inherent multi-user
capabilities and its support for terminals offers an answer.
Server
Let's start by listing what we need on the server.
hardware
The hardware is simple: we need a least one serial port.
serial ports
The standard PC comes with two serial ports. If all you want is
two terminals, then this is fine. For most uses, though, you'll
need more. Many multi-port serial cards are supported by Linux.
For some, such as the AST Fourport and clones, support comes as an
optional compile-time feature to the standard kernel; for others,
the manufacturer may provide a driver, perhaps as a kernel module or
as a patch to the standard source tree..
software
Configuring the serial ports
The extra serial ports must have corresponding devices created in
/dev, and these must be configured using the setserial program.
The devices may already exist; if not, creating them is simple.
The mknod command is used (look up the man page!) e.g.
mknod -m 666 /dev/ttyS4 c 4 68
Where c says it is a character special device (the alternative is b
for block, e.g. a disk), and the two numbers following the device file
name are the major and minor numbers respectively. For serial input,
the major number is 4; for serial output, 5. The minor number is
the port number plus 64 (you ask why? I don't know!) The -m option,
as you've probably guessed from the look of it, sets ther permissions
on the file.
The following devices exist on my system. Note the major and minor
numbers. /dev/ttyS4-7 and /dev/cua4-7 are the control devices for
my additional serial lines.
Input
crw—w--w- 1 root root 4, 64 Sep 24 19:44 /dev/ttyS0
crw—w--w- 1 root root 4, 65 Sep 20 19:42 /dev/ttyS1
crw-rw-rw- 1 root root 4, 66 Jan 23 1980 /dev/ttyS2
crw-rw-rw- 1 root root 4, 67 Sep 24 19:36 /dev/ttyS3
crw-rw-rw- 1 root other 4, 68 Nov 17 16:53 /dev/ttyS4
crw-rw-rw- 1 root other 4, 69 Nov 17 16:56 /dev/ttyS5
crw-rw-rw- 1 root other 4, 70 Nov 17 16:56 /dev/ttyS6
crw-rw-rw- 1 root other 4, 71 Nov 17 16:56 /dev/ttyS7
Output
crw-rw-rw- 1 root tty 5, 64 Apr 1 1993 /dev/cua0
crw-rw-rw- 2 uucp staff 5, 65 Oct 7 21:42 /dev/cua1
crw-rw-rw- 1 root tty 5, 66 Apr 11 1993 /dev/cua2
crw-rw-rw- 1 paul staff 5, 67 Nov 21 09:16 /dev/cua3
crw-rw-rw- 1 root tty 5, 68 Sep 20 13:11 /dev/cua4
crw-r—r-- 1 root root 5, 69 Sep 20 13:12 /dev/cua5
crw-r—r-- 1 root root 5, 70 Sep 20 13:12 /dev/cua6
crw-r—r-- 1 root root 5, 71 Sep 20 13:12 /dev/cua7
Setserial
The setserial program is essential to configure the additional
serial ports. The README file that comes with the source says it
all, really: "[setserial] allows you to look at and change various
attributes of a serial device, including its port, its IRQ, and other
serial port options." It comes with an rc.serial script which with
a little tuning will do everything you need.
In /etc/rc.local, I have the following lines:
if [ -f /etc/rc.serial ]; then
/etc/rc.serial
fi
init config
/etc/inittab
# Serial lines
s4:2345:respawn:/sbin/uugetty ttyS4 9600
s2:45:respawn:/sbin/agetty 19200 ttyS1 vt100
# Dialup lines
d1:45:respawn:/sbin/agetty -mt60 38400,19200,9600,2400,1200 ttyS0 vt100
d2:45:respawn:/sbin/agetty -mt60 38400,19200,9600,2400,1200 ttyS1 vt100
How Linux talks to terminals: getty
A standalone Linux box already supports multiple terminals, due to the
virtual console support built into the kernel. Each of the screens
accessible on the console monitor through an Alt-Fkey combination is
treated as a separate terminal. This can be illustrated by a ps
listing, which should show some lines like this:
5831 p 5 S 0:00 /sbin/agetty 38400 tty5
Support for real terminals starts along similar lines. First, it
is necessary to tell the system to run a getty process on each
port to which a terminal will be attached. The ports would normally
be serial lines. A normal PC has only two of these, but there are
cheap multi-port serial cards available, which provide 4, 8, 16 or
even more additional serial lines, while taking up only one interrupt
(and an expansion slot on the motherboard, of course). I use a cheap
clone of the AST Fourport card, which as the name implies gives me
four extra serial ports.
Why agetty?
You will notice that I am referring to agetty(8), rather than the
standard getty(8). Both are available in a standard Linux install,
and agetty has several features and is simpler to configure.
Just like the standard getty, agetty opens a tty port, prompts for a
login name and invokes the /bin/login command. In addition, agetty
has several useful non-standard features. These include:
Adapting the tty settings to parity bits and to erase, kill,
end-of-line and uppercase characters when it reads a login name.
Agetty can handle 7-bit characters with even, odd, none or space par
ity, and 8-bit characters with no parity. The fol lowing special
characters are recognized: @ and Control-U (kill); #, DEL and back
space (erase); carriage return and line feed (end of line).
Optionally deduces the baud rate from the CONNECT messages produced
by Hayes(tm)-compatible modems.
Optionally does not hang up when it is given an already opened line
(useful for call-back applica tions).
Optionally turns on hard-ware flow control
Optionally forces the line to be local with no need for carrier
detect.
Note that agetty does <EM>not</EM> use the /etc/gettydefs or
/etc/gettytab files.
agetty is usually called by init(8). Here are some sample command
entries for the /etc/inittab file.
1) For a hard-wired line or a console tty:
/sbin/agetty 9600 ttyS1
2) For a directly connected terminal without proper carriage detect
wiring: (try this if your terminal just sleeps instead of giving you
a password: prompt.)
/sbin/agetty -L 9600 ttyS1 vt100
One handy feature of agetty is its support for escape codes in the
/etc/issue file. These are:
- b Insert the baudrate of the current line.
- d Insert the current date.
- s Insert the system name, the name of the operating
system.
- l Insert the name of the current tty line.
- m Insert the architecture identifier of the machine,
eg. i486
- n Insert the nodename of the machine, also known as
the hostname.
- o Insert the domainname of the machine.
- r Insert the release number of the OS, eg. 1.1.9.
- t Insert the current time.
- u Insert the number of current users logged in.
- U Insert the string "1 user" or "<n> users" where <n>
is the number of current users logged in.
- v Insert the version of the OS, eg. the build-date
etc.
For example, my /etc/issue reads
Running \s \r It is now \t \U logged on
which displays
Running Linux 2.0.36 It is now 15:23:13 1 user logged on
Depending on how the program was configured, all diagnos tics are
written to the console device or reported via the syslog(3) facility.
Error messages are produced if the port argument does not specify a
terminal device; if there is no utmp entry for the current process
(System V only); and so on.
Terminal
Most standard serial terminals will work under Linux. If in doubt,
consult the manufacturer.
The Connection
The cabling needed for a basic serial connection to a terminal is
very simple. Just three wires must be connected, as follows:
RxD Receive Data 2 - 3 TxD Transmit Data
TxD Transmit Data 3 - 2 RxD Receive Data
GND Signal Ground 7 - 7 GND Signal Ground
This will work, but allows for no hardware handshaking. I once had
a small set of old Wyse 30s hocked up to my machine, and this basic
cabling worked fine with them. Your terminals may also supported
advanced features through additional lines on the serial cable.
A full null-modem configuration is as follows:
DCD Carrier Detect 1 - 20 DTR Data Terminal Ready
RxD Receive Data 2 - 2 TxD Transmit Data
TxD Transmit Data 3 - 3 RxD Receive Data
DTR Data Terminal Ready 4 - 6 DSR Data Set Ready
DTR Data Terminal Ready 4 - 8 DCD Carrier Detect
GND Signal Ground 5 - 7 GND Signal Ground
DSR Data Set Ready 6 - 20 DTR Data Terminal Ready
RTS Request To Send 7 - 5 CTS Clear To Send
CTS Clear To Send 8 - 4 RTS Request To Send
If you want to have hardware handshaking signals, you must have a
full null modem cable:
PC male DB25 Terminal DB25
TxD Transmit Data 2 —> 3 RxD Receive Data
RxD Receive Data 3 <— 2 TxD Transmit Data
RTS Request To Send 4 —> 5 CTS Clear To Send
CTS Clear To Send 5 <— 4 RTS Request To Send
DSR Data Set Ready 6
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DCD Carrier Detect 8 <— 20 DTR Data Terminal Ready
SG Signal Ground 7 -— 7 SG Signal Ground
6 DSR Data Set Ready
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DTR Data Terminal Ready 20 —> 8 DCD Carrier Detect
If you have a DB9 connector on your serial port, try the following:
PC DB9 Terminal DB25
RxD Receive Data 2 <— 2 TxD Transmit Data
TxD Transmit Data 3 —> 3 RxD Receive Data
SG Signal Ground 5 -— 7 SG Signal Ground
Alternatively, a full DB9-DB25 null modem cable:
PC DB9 Terminal DB25
RxD Receive Data 2 <— 2 TxD Transmit Data
TxD Transmit Data 3 —> 3 RxD Receive Data
6 DSR Data Set Ready
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DTR Data Terminal Ready 4 —> 8 DCD Carrier Detect
GND Signal Ground 5 -— 7 GND Signal Ground
DCD Carrier Detect 1
|
DSR Data Set Ready 6 <— 20 DTR Data Terminal Ready
RTS Request To Send 7 —> 5 CTS Clear To Send
CTS Clear To Send 8 <— 4 RTS Request To Send
(RI Ring Indicator 9 not needed)
(Yes, the pins 2 and 3 really do have the opposite meanings in DB9 connectors
than in DB25 connectors!)
As my terminals could be configured to use no handshaking, as I was
fixing up my own cables with their appropriate connectors, and as I
hate soldering, I took the former route! It works just fine.
Configuring agetty
Edit your /etc/inittab file to run agetty on the serial port
(substituting in the correct information for your environment - port,
speed, and default terminal type):
s2:45:respawn:/sbin/agetty 19200 ttyS1 vt100
Restart your system. That's it!
Conclusion
So, at the end of it all, we have a cheap and simple way to set up
a multi-user system. I think that any REAL operating system has to
offer this capability (multi-user support just "slots in" on top of
multi-tasking, after all). Certainly, it has made my use of my Linux
box much more productive, by allowing me access to it from several
different locations in my house.
Paul Dunne 2000
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