linux serial console_grub.conf

Introduction
Have you ever needed to connect a dumb terminal (like a Wyse 50) to a Linux host? Do you need to login to a

Linux server from a laptop to perform administrative functions, because there is no monitor or keyboard

[@more@]

Introduction
Have you ever needed to connect a dumb terminal (like a Wyse 50) to a Linux host? Do you need to login to a

Linux server from a laptop to perform administrative functions, because there is no monitor or keyboard

attached to the server? If you are accustomed to administering routers, switches, or firewalls in this manner,

then you may be interested in doing the same with some of your GNU/Linux hosts. This HOWTO will explain,

step-by-step, how to setup a serial console for Red Hat 9, although most of it should apply to other

distributions as well.

Why did I write this document? Although there are lots of documents available on the Internet dealing with

Linux serial ports, most of them seemed to be either out of date, or focused on modem dial-in/dial-out. I

wanted consise documentation on how to setup simple terminal access via RS-232-C serial ports for Red Hat 9.

Assumptions/Setup
I was using Red Hat 9 for this test. My test machine consisted of:

Motherboard: Gigabyte Technology GA-7VA motherboard (Rev. 2.0)
Chipset: VIA KT400A
CPU: AMD-K7 (Duron 1400)
RAM: 256MB DDR333
Serial Ports: 2 built-in ports with 16550A UARTs, DB-9 male
Linux kernel: 2.4.20-24.9

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Step 1: Check your system's serial support
First, let's make sure that your operating system recognizes serial ports in your hardware. You should make a

visual inspection and make sure that you have one or more serial ports on your motherboard or add-in PCI card.

Most motherboards have two built-in ports, which are called COM1: and COM2: in the DOS/Windows world. You may

need to enable them in BIOS before the OS can recognize them. After your system boots, you can check for serial

ports with the following commands:

[root@oscar root]# dmesg | grep tty
ttyS0 at 0x03f8 (irq = 4) is a 16550A
ttyS1 at 0x02f8 (irq = 3) is a 16550A

[root@oscar root]# setserial -g /dev/ttyS[01]
/dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4
/dev/ttyS1, UART: 16550A, Port: 0x02f8, IRQ: 3
As you can see, the two built-in serial ports are /dev/ttyS0 and /dev/ttyS1.

Step 2: Configure your inittab to support serial console logins
The /etc/inittab file must be reconfigured to allow serial console logins. You will note that the mingetty

daemon is used to listen for virtual consoles (like the 6 that run by default with your keyboard and monitor).

You will need to configure agetty or mgetty to listen on the serial ports, because they are capable of

responding to input on physical serial ports. In the past, I have used both full-featured gettys. In this

document, I will only discuss agetty, since it is already included in the default Red Hat 9 installation. It

handles console/dumb terminal connections as well as dial-in modem connections.

What is a getty?

A getty is is a program that opens a tty port, prompts for a login name, and runs the /bin/login command. It is

normally invoked by init.

Before you edit /etc/inittab, which is a very important config file, you should make a backup copy:

[root@oscar etc]# cp /etc/inittab /etc/inittab.org
The required /etc/inittab additions are highlighted in red:

id:3:initdefault:

# System initialization.
si::sysinit:/etc/rc.d/rc.sysinit

l0:0:wait:/etc/rc.d/rc 0
l1:1:wait:/etc/rc.d/rc 1
l2:2:wait:/etc/rc.d/rc 2
l3:3:wait:/etc/rc.d/rc 3
l4:4:wait:/etc/rc.d/rc 4
l5:5:wait:/etc/rc.d/rc 5
l6:6:wait:/etc/rc.d/rc 6

# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -r now

pf::powerfail:/sbin/shutdown -f -h +2 "Power Failure; System Shutting Down"

# If power was restored before the shutdown kicked in, cancel it.
pr:12345:powerokwait:/sbin/shutdown -c "Power Restored; Shutdown Cancelled"

# Run gettys in standard runlevels
1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2
3:2345:respawn:/sbin/mingetty tty3
4:2345:respawn:/sbin/mingetty tty4
5:2345:respawn:/sbin/mingetty tty5
6:2345:respawn:/sbin/mingetty tty6

# Run agetty on COM1/ttyS0 and COM2/ttyS1
s0:2345:respawn:/sbin/agetty -L -f /etc/issueserial 9600 ttyS0 vt100
s1:2345:respawn:/sbin/agetty -L -f /etc/issueserial 38400 ttyS1 vt100
#s1:2345:respawn:/sbin/agetty -L -i 38400 ttyS1 vt100

# Run xdm in runlevel 5
x:5:respawn:/etc/X11/prefdm -nodaemon

agetty options explained:

-L force line to be local line with no need for carrier detect (when you have no modem).
-f alternative /etc/issue file. This is what a user sees at the login prompt.
-i do not display any messages at the login prompt.
9600 serial line rate in bps. Set this to your dumb terminal or terminal emulator line rate.
ttyS0 this is the serial port identifier.
vt100 is the terminal emulation. You can use others, but VT100 is the most common or "standard". Another

widely used termial type is VT102.
Possible serial line rates (sometimes called baud rates) for the 16550A UART:

110 bps
300 bps
1200 bps
2400 bps
4800 bps
9600 bps
19,200 bps
38,400 bps
57,600 bps
115,200 bps
I have tried all of these line rates. 9600 bps is generally O.K., and is a very common setting for networking

hardware. 38,400 bps is the speed of the standard Linux console, so it is my second choice. If your dumb

terminal or terminal emulator cannot handle 38,400 bps, then try 19,200 bps: it is reasonably speedy and you

will not be annoyed.

Here was my custom issue file, /etc/issueserial. It uses escape sequences defined in the agetty manpage to add

some useful information, such as the serial port number, line speed, and how many users are currently logged

on:

Oscar
Connected on l at b bps
U

Now, you must activate the changes that you made in /etc/inittab. This is done with the following command,

which forces the init process to re-read the configuration file:

[root@oscar root]# init q
Now, let's make sure that the agetty process is listening on the serial ports:

[root@oscar root]$ ps -ef | grep agetty
root 958 1 0 Dec13 ttyS0 00:00:00 /sbin/agetty -L -f /etc/issueserial 9600 ttyS0 vt100
root 1427 1 0 Dec13 ttyS1 00:00:00 /sbin/agetty -L -f /etc/issueserial 38400 ttyS1 vt100

Step 3: Test serial port login with an external dumb terminal or terminal emulator

I have tested this setup with a WYSE dumb terminal, a Linux laptop running Minicom, and Windows 2000/XP laptops

running HyperTerminal. They all worked just fine.

Terminal settings: should be 9600, N, 8, 1. Terminal emulation should be set to VT100 or VT102. Turn flow

control off. If you want to use the 38,400 bps serial port on ttyS1, then your settings should be adjusted to

38400, N, 8, 1.

Cable: To connect a laptop to the serial port on the Linux host, you need to have a null-modem cable. The

purpose of a null-modem cable is to permit two RS-232 DTE devices to communicate with each other without modems

between them. While you can construct this yourself, a good, sturdy manufactured null-modem cable is

inexpensive and will last longer.

If you insist on making the cable yourself, then check out Nullmodem.Com for the wiring and pinout diagram.

Connectors: Motherboard serial ports are typically male DB-9 connectors, but some serial ports use DB-25

connectors. You may need some DB-9 to DB-25 converters or gender-changers in order to connect to your terminal.

For a typical laptop to server connection, a DB-9 null-modem cable should be sufficient.

Here is what you should see on the dumb terminal or terminal emulator:

Oscar
Connected on ttyS1 at 38400 bps
3 users

oscar.vanemery.com login:
Note: If you want to be able to login via serial console as the root user, you will need to edit the

/etc/securetty config file. The entries to add are highlighted in red:

console
ttyS0
ttyS1
vc/1
vc/2
vc/3
vc/4
vc/5
vc/6
vc/7
vc/8
vc/9
vc/10
vc/11
tty1
tty2
tty3
tty4
tty5
tty6
tty7
tty8
tty9
tty10
tty11

Step 4: Modifying the agetty settings
If you want to change the baud rate or some other agetty setting, you will need to perform these 3 steps:

Modify the /etc/inittab configuration file
Activate the config change by forcing init to re-read the config file
Restart the agetty daemons
Here is an example of steps 2 and 3:

[root@oscar root]# init q
[root@oscar root]# pkill agetty

Optional: Configure serial port as THE system console
You can use options in /etc/grub.conf to redirect console output to one of your serial ports. This can be handy

if you do not have a keyboard or monitor available for the Linux host in question. You can also see all of the

bootup and shutdown messages from your terminal. In this example, we will make the /dev/ttyS1 port be the

console. The text to add to the config file is highlighted in red:

# grub.conf generated by anaconda
#boot=/dev/hda
default=0
timeout=10
splashimage=(hd0,0)/grub/splash.xpm.gz
title Red Hat Linux (2.4.20-24.9)
root (hd0,0)
kernel /vmlinuz-2.4.20-24.9 ro root=LABEL=/ console=tty console=ttyS0,9600
initrd /initrd-2.4.20-24.9.img
Now, if you drop your system into single user mode with the "init 1" command, you will still be able to

administer the system from your serial-connected terminal. No monitor or keyboard is required!

Warning!: The kudzu hardware detection program may "choke" on boot when the serial port becomes the console,

instead of the video adapter. To remedy this situation, you should disable kudzu (assuming that your hardware

is configured properly and won't be changing). This is how you would do that:

[root@oscar root]# chkconfig kudzu off
[root@oscar root]# chkconfig --list kudzu
kudzu 0:off 1:off 2:off 3:off 4:off 5:off 6:off
You should also know how to break into the Grub bootloader during system startup and edit the kernel line. By

deleting the console argument from the kernel line, you can boot the system with the standard console, which

uses the video card and attached keyboard. You have been warned!

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Conclusion
Now, you should be able to login from the serial ports on your GNU/Linux host. This could be useful for

maintenance or for serving a whole room full of dumb terminals. In the future, I will investigate a PCI

multiport serial card in the latter role.

Have fun!

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Resources
man agetty
man securetty
/usr/src/linux-2.4.20-24.9/Documentation/serial-console.txt
Linux Serial HOWTO
Text Terminal HOWTO
VT100.Net
Nullmodem.Com

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環境：
使用命令:

dmesg | grep serial

看看機器對串列埠的支援是否OK，如果輸出類似下面，就沒問題了

[ 54.831740] serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[ 54.853271] serial8250: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A

其中ttyS0是COM1，ttyS1是COM2
配置過程：
1. 在grub裡開啟對serial console 的支援. 在menu.lst中加入:

serial --unit=0 --speed=57600 --word=8 --parity=no --stop=1
terminal --timeout=10 serial console

速度，校驗，停止位設定好，連線時候用.第一行是開啟串列埠支援，第二行是讓grub也能使用串列埠

2. 在kernel引數里加入 "console=ttyS0, 57600n8 console=tty"，比如：

kernel /boot/vmlinuz-2.6.20-jianingy-server root=/dev/hda1 ro console=tty console=ttyS0,57600n8

console=tty是為了在本機建立一個tty，console=ttyS0,57600n8是在串列埠建立一個speed=57600, 無校驗，8位的tty。如果只有ttyS0,那麼本機在啟動時候就不輸出了。
3. ubuntu使用者把/etc/event.d/tty6,最後一行改為

respawn /sbin/getty -L ttyS0 57600 vt100

其他linux使用者可以在/etc/inittab裡改

只此一切OK了，在另外一臺機器上設定好minicom或者gtkterm，reboot機器等來來自console的資訊吧