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Welcome to the Slackware Documentation Project

Serial Console

Slackware can be installed on various embedded devices. A lot of those don't have a traditional console, like a monitor and keyboard. Instead, the console is often routed to the serial port. In order to take advantage of this, some post-install configuration is necessary.

This howto can also be used to setup a serial console on an existing headless server as a backup for administering the machine.

Preparation

You need to know the serial device and the speed. The device will mostly be ttyS0, the first serial device. If you are using a serial to USB adapter, it will likely be ttyUSB0. Consult the documentation of your board if you are not sure.

Some devices have an option in the BIOS to change the speed and other serial settings. Make sure you know the speed that is currently set.

In the below examples we will assume ttyS0 with speed 38400.

Host configuration

Activate serial port

Edit /etc/inittab and uncomment the correct serial console line. Change the speed setting when required.

/etc/inittab
# Local serial lines:
s1:12345:respawn:/sbin/agetty -L ttyS0 38400 vt100
#s2:12345:respawn:/sbin/agetty -L ttyS1 9600 vt100

Allow root login

Edit /etc/securetty and uncomment your serial device to allow root to login via the serial console. You can omit this step if you will access the machine with a normal user id.

/etc/securetty
# These are some remote ttys, and uncommenting them might be less than fully secure:
ttyS0
#ttyS1 

Setup LILO boot loader

Edit /etc/lilo.conf. Add a line to the general section:

/etc/lilo.conf
serial=0,38400n8

The 0 refers to the first serial device, ttyS0.

Add the following parameter to the kernel line:

/etc/lilo.conf
append="console=ttyS0,38400n8"

If the append line already exists, add the parameter as follows:

/etc/lilo.conf
append="mem=1024M console=ttyS0,38400n8"

Now run /sbin/lilo to write the new bootloader configuration.

To have the console on normal tty as well as serial port, adjust the kernel line as follows:

/etc/lilo.conf
append="console=tty0 console=ttyS0,38400"

Setup GRUB-legacy boot loader

Edit /boot/grub/menu.lst and change it as follows:

Add lines to the general section as follows:

/boot/grub/menu.lst
serial --speed=38400
terminal serial

Add the following parameter to the kernel line:

/boot/grub/menu.lst
console=ttyS0,38400n8

The complete kernel line might look like this (substitute your root partition and vmlinuz file):

/boot/grub/menu.lst
kernel /boot/vmlinuz root=/dev/sda2 ro vga=normal console=ttyS0,38400n8

To have the console on normal tty as well as serial port, adjust the terminal and kernel lines as follows:

/boot/grub/menu.lst
terminal --timeout=5 serial console
/boot/grub/menu.lst
kernel xxx console=tty0 console=ttyS0,38400n8

When the terminal timeout expires, the first option (in this case serial) will be chosen as the console. The normal bootloader timeout will start when the terminal timeout ends.

Setup GRUB2 boot loader

Edit file /etc/default/grub and change the following line to define the console:

grub
GRUB_CMDLINE_LINUX_DEFAULT="console=ttyS0,38400n8"

Add the following lines to define the console settings:

grub
GRUB_TERMINAL=serial
GRUB_SERIAL_COMMAND="serial --speed=38400 --unit=0 --word=8 --parity=no --stop=1"

Now rebuild the GRUB2 configuration:

# grub-mkconfig -o /boot/grub/grub.cfg

Connect from a remote computer

To use the serial console, you need to attach a second computer to the serial port of the host. Use a null modem cable. Most modern computers no longer have a serial port so you might need a serial to USB connector as well.

Once you have physically connected the machines, you need a communications package to interact with the console. Two popular options are screen and minicom.

Connect using screen

This is the easiest option because it doesn't need any configuration. Start screen and specify the serial device and the desired speed on the command line.

user@darkstar$ screen /dev/ttyUSB0 38400

Connect using minicom

Before using minicom, you need to configure it. Start with the -s switch as follows:

user@darkstar$ minicom -s

You will see the main menu.

 +-----[configuration]------+
 | Filenames and paths      |
 | File transfer protocols  |
 | Serial port setup        |
 | Modem and dialing        |
 | Screen and keyboard      |
 | Save setup as dfl        |
 | Save setup as..          |
 | Exit                     |
 | Exit from Minicom        |
 +--------------------------+

Select Serial port setup. You will see the serial port settings.

    +-----------------------------------------------------------------------+
    | A -    Serial Device      : /dev/ttyS1                                |
    | B - Lockfile Location     : /var/lock                                 |
    | C -   Callin Program      :                                           |
    | D -  Callout Program      :                                           |
    | E -    Bps/Par/Bits       : 115200 8N1                                |
    | F - Hardware Flow Control : Yes                                       |
    | G - Software Flow Control : No                                        |
    |                                                                       |
    |    Change which setting?                                              |
    +-----------------------------------------------------------------------+

Use the A to G keys to change various settings. Make sure the device and Bps/Par/Bits settings are correct. Hit Enter to return to the main screen and select the Modem and dialing option. On the detail screen, make sure the Init string and Reset string options are empty. Hit Enter again to return to the main screen. Now you can save your options. Use Save setup as dfl to save this as your new default. Use Save setup as.. to save as a certain profile.

Call minicom again and the new options will take effect.

user@darkstar$ minicom

Or you can start with a certain configuration

user@darkstar$ minicom myprofile

Use Ctrl+A, Z for help. Use Ctrl+A,X to quit.

Troubleshooting

Garbled characters in terminal

When you see progress in the terminal but the characters are garbled and unreadable, make sure you have the same speed set in the following sections: BIOS, bootloader and inittab. Also make sure your client connects with the same speed.

Unable to sign on as root

Check the securetty settings. See section above.

Unable to use screen or minicom as a regular user

When you get authorization problems using the serial device, make sure your user is in the dialout group.

Further reading

Sources


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