As part of my day to day job, I’ve been working on a headless Linux client that can be transported from site to site to automate some network testing. I can’t really go into detail on what’s being tested, or why, but I did want to write up a (hopefully) useful entry about headless client and some of the changes I made to the basic CentOS install to get everything to work.
First up was the issue of headless operation. We’re using Cappuccino SlimPRO SP-625 units with the triple Gigabit Ethernet option. They’re not bad little machines, though I do have a gripe with the back cover on them. It doesn’t properly cover all of the ports on the back, leaving rather large holes where dust and dirt can get in. What’s worse is that the power plug is not surrounded and held in place by the case, so I can foresee the board cracking at some point from the stress of the power cord… But, for a sub-$800 machine, it’s not all that bad.
Anyway, on to the fun. These machines will be transported to various locations where testing is to be performed. On-site, there will be no keyboard, no mouse, and no monitor for them. However, sometimes things go wrong and subtle adjustments may need to be made. This means we need a way to get into the machine, locally, just in case there’s a problem with the network connection. Luckily, there’s a pretty simple means of accessing a headless Linux machine without the need to lug around an extra monitor, keyboard, and mouse. If you’ve ever worked on a switch or router, you’ll know where I’m going with this.
Most technician have access to a laptop, especially if they have to configure routers or switches. Why not access a Linux box the same way? Using the agetty command, you can. A getty is a program that manages terminals within Unix. Those terminals can be physical, like the local keyboard, or virtual, like a telnet or ssh session. The agetty program is an alternative getty program that has some non-standard features such as baud rate detection, adaptive tty, and more. In short, it’s perfect for direct serial, or even dial-in connections.
Setting this all up is a snap, too. By default, CentOS (and most Linux distros) set up six gettys for virtual terminals. These virtual terminals use yet another getty, mingetty, which is a minimalized getty program with only enough features for virtual terminals. In order to provide serial access, we need to add a few lines to enable agettys on the serial ports.
But wait, what serial ports do we have? Well, assuming they are enabled in the BIOS, we can see them using the dmesg and setserial commands. The dmesg command prints out the current kernel message buffer to the screen. This is usually the output from the boot sequence, but if your system has been up a while, it may contain more recent messages. We can use dmesg to determine the serial interfaces like this :
[friz@test ~]$ dmesg | grep serial
serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
serial8250: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A
As you can see from the output above, we have both a ttyS0 and a ttyS1 port available on this particular machine. Now, we use setserial to make sure the system recognizes the ports:
[friz@test ~]$ sudo setserial -g /dev/ttyS[0-1]
/dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4
/dev/ttyS1, UART: 16550A, Port: 0x02f8, IRQ: 3
The output is similar to dmesg, but setserial actually polls the port to get the necessary information, thereby ensuring that it’s active. Also note, you will likely need to run this command as root to make it work.
Now that we know what serial ports we have, we just need to add them to the inittab and reload the init daemon. Adding these to the inittab is pretty simple. Your inittab will look something like this:
# inittab This file describes how the INIT process should set up
# the system in a certain run-level.
# Author: Miquel van Smoorenburg, <firstname.lastname@example.org>
# Modified for RHS Linux by Marc Ewing and Donnie Barnes
# Default runlevel. The runlevels used by RHS are:
# 0 – halt (Do NOT set initdefault to this)
# 1 – Single user mode
# 2 – Multiuser, without NFS (The same as 3, if you do not have networking)
# 3 – Full multiuser mode
# 4 – unused
# 5 – X11
# 6 – reboot (Do NOT set initdefault to this)
# System initialization.
# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -r now
# When our UPS tells us power has failed, assume we have a few minutes
# of power left. Schedule a shutdown for 2 minutes from now.
# This does, of course, assume you have powerd installed and your
# UPS connected and working correctly.
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
# Run xdm in runlevel 5
Just add the following after the original gettys lines:
# Run agettys in standard runlevels
s0:2345:respawn:/sbin/agetty -L -f /etc/issue.serial 9600 ttyS0 vt100
s1:2345:respawn:/sbin/agetty -L -f /etc/issue.serial 9600 ttyS1 vt100
Let me explain quickly what the above means. Each line is broken into multiple fields, separated by a colon. At the very beginning of the line is an identifier, s0 and s1 in our case. Next comes a list of the runlevels for which this program should be spawned. Finally, the command to run is last.
The agetty command takes a number of arguments:
- The -L switch disables carrier detect for the getty.
- The next switch, -f, tells agetty to display the contents of a file before the login prompt, /etc/issue.serial in our case.
- Next is the baud rate to use. 9600 bps is a good default value to use. You can specify speeds up to 152,200 bps, but they may not work with all terminal programs.
- Next up is the serial port, ttyS0 and ttyS1 in our example.
- Finally, the terminal emulation to use. VT100 is probably the most common, but you can use others.
Now that you’ve added the necessary lines, reload the init daemon via this command:
[friz@test ~]$ sudo /sbin/init q
You can also redirect the kernel to output all console messages to the serial port as well. This is accomplished by adding a switch to the kernel line in your /etc/grub.conf file like this:
# grub.conf generated by anaconda
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, eg.
# root (hd0,0)
# kernel /vmlinuz-version ro root=/dev/hdc3
# initrd /initrd-version.img
title CentOS (2.6.18-53.1.21.el5)
kernel /vmlinuz-2.6.18-53.1.21.el5 ro root=LABEL=/ console=ttyS0,9600
The necessary change is highlighted above. The console switch tells the kernel that you want to re-direct the console output. The first option is the serial port to re-direct to, and the second is the baud rate to use.
And now you have a headless Linux system! These come in handy when you need a Linux machine for remote access, but you don’t want to deal with having a mouse, keyboard, and monitor handy to access the machine locally.