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Date: Sun, 11 Sep 2011 19:03:32 +0000 (+0000)
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+This is a howto about booting Linux without a
+traditional screen and through a serial line.
+
+It is often the only method for servers. But it can be generally
+useful for debugging, by allowing logging of the boot messages, which
+scroll too fast on a VGA screen and are not fully kept in
+/var/log/messages.
+
+The method is unlikely to be useful for out of the box modern
+desktops/laptops/notebooks, as there have no more a serial
+port. Emulation of a serial port through USB can't work here, as we
+are too early in the boot process. In that case, and if your hardware
+allows, you will have to plug in some serial port extension card.
+
+I will in the following limit the scope to what I use: the IA-32
+and derived architectures, but any information about other ones
+welcome.
+
+# Controlling boot through the serial line 
+
+## Serial line configuration at the booting computer side
+
+To have a complete boot experience through the serial line, you have to:
+
+1. manage to get the BIOS I/O go through the serial line
+1. configure a bootloader like GRUB to use the serial line
+1. configure the kernel to output boot messages to the serial line
+1. get a login prompt on the serial line
+
+### Manage to get the BIOS I/O go through the serial line
+
+On servers it may be the only method, but it is unlikely to exist on a
+desktop BIOS. In that case, you will miss the BIOS messages on the
+serial line.
+
+### Configure a bootloader like GRUB to use the serial line
+
+On GRUB 1 or GRUB 2 you simply have to add these lines in the
+GRUB configuration file:
+
+    # insmod is GRUB 2 specific, comment out for GRUB 1
+    insmod serial
+    serial --unit=0 --speed=9600 --word=8 --parity=no --stop=1
+    terminal serial
+
+### Configure the kernel to output boot messages to the serial line
+
+To get kernel messages on the serial line, you have to add a
+new option to the kernel command line, like:
+
+    console=ttyS0,9600n8
+
+which means 9600 bauds, no parity and 8 bits, to fit the previous
+section 4 lines. This will also involve editing GRUB configuration
+file. It may be unnecessary to do that if you use a server without a
+VGA card, Linux is clever enough to try a serial port in that case.
+
+If you are using Debian, you can configure items 2 and 3 the Debian
+way, see for instance:
+[[http://linux.xvx.cz/2009/08/debian-with-grub2-and-serial-connection/]]
+
+Notice that you can't use arrows to navigate the GRUB menus when on
+serial line, other standard keys are offered to that effect on the
+greeting page.
+
+### Get a login prompt on the serial line
+
+It is covered in that  page: [[recovering_when_locked_out_from_your_computer]].
+
+## The other end of the serial line: hardware terminal or emulation
+
+In any case, you will need a [emergency null modem] to link the
+booting host and the terminal (real dumb serial terminal or computer
+running an emulator).
+
+Even if your computer emulating a terminal has no serial port, you can
+use an USB port with a hardware adapter and a Linux driver, look
+here: [[recovering_when_locked_out_from_your_computer]]. 
+
+# Recording the boot messages
+
+At that point, you should be able to get boot messages through the
+serial line and then a login prompt. 
+
+When you boot through a traditional VGA screen, you can only study
+back at most aroung 50 lines, depending on screen definition. While
+through an emulated serial terminal, you are free to record the full
+booting process.
+
+Minicom (see:
+[[recovering_when_locked_out_from_your_computer]]) can record what is sent/received on the serial line, see
+option "Capture on/off" in the menu, it has hotkey L. 
+
+But other tools are more convenient. I suggest you use ttyrec. To
+register in file ttyrecord what happens during a minicom session, run:
+
+    ttyrec -e minicom
+
+When you are done with minicom, you can replay the session with:
+
+    ttyplay ttyrecord
+
+To stop replay at some point, hit Ctrl-S; to resume hit Ctrl-Q. See
+ttyrec man page for how to tune the general speed of the replay.
+