Serial Consoles

From PDP/Grid Wiki
Revision as of 22:05, 12 November 2010 by (talk | contribs) (→‎Serial Consoles on a regular kernel)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to navigationJump to search

Serial Consoles, setup and use

Serial-over-LAN and IPMI 2.0

SoL is a standard feature of IPMI 2.0 and can be used with any IPMI 2.0 client over the LAN+ interface, provided the BMC is correctly configured with an IPv4 address. SoL is only avaialble to privileged users (on Dell: "root", hooi-ei: "root" and on Valentine: "ADMIN"), and of course such accounts on the BMC must be protected with a passphrase, which they are. Only one SoL client can be connected at any one time.

FYI: the default password for the Dell DRAC "root" user is "calvin". You can change that in bulk by

ipmitool -I lanplus -H -U root -P calvin user set password 2 'NEWPASSWD'

Setting up the BMC LAN+ channel

The trivial way is to do it with the BMC BIOS interface (press Ctrl-E halfway in the boot process), but it can also be done using the IPMI device driver local interface as described in this Lone Sysadmin article on configuring IPMI on a Dell PE with RHEL.

For some reason the BMC LAN configuration can make the BMC hang, and in other cases I had to enable ARP responses and gratuitious ARP announcements to get the IP address to be recognised by clients. For the initial tests, I've just used an IP address out of the regular range for the host addresses (e.g. out of for the farmnet servers like kaf), but this can and should be changed later as it will otherwise eat IP addresses like mad.

If you think you've configured everything correctly using "ipmitool -I open ...", but the network still refuses to answer, a reboot of the BMC will usually help.

Enabling SoL

SoL should be enabled in the BIOS or using the IPMI interface, but it usually on already. You can activate (connect to) it any time.

The default baud rate is 19200 8N1, and it's really best to keep it that way.

SoL clients

There are many SoL clients:

  • OpenIPMI provides ipmitool:
ipmitool -I lanplus -H IPMI-BMC-IPADDR -U BMCPRIVUSER sol activate

and if somebody else is already connected, throw them off with

ipmitool -I lanplus -H IPMI-BMC-IPADDR -U BMCPRIVUSER sol deactivate

and they're gone -- and you can start playing with the console.

  • Various vendors have IPMI viewers, like SuperMicro's IPMIview 2.0

Serial Consoles


The Dell PE serial console redirection has two modes of operation. As explained in the Dell BMC Users Guide, Console Rdirection is set in the BIOS, accessible using F2 during boot.

Assuming we will use COM2 for the SoL console, and leave COM1 happily attached to the physical RS232 interface, we use the recipe there:

Set Serial Communication-> Serial Communication to On with Console Redirection via COM2
Set Serial Communication-> External Serial Connector to COM2
NOTE: If the console redirection is used for SOL then the External Serial Connector setting 
      does not need to be configured.

and so we set

 Set Serial Communication-> External Serial Connector to COM1

But then there are two options for

 Serial Communication -> Redirection After Boot

and each corresponds to a very specific way of configuring grub. If you do it the wrong way round, you will either have to press a key during boot on either console, or you'll just hang the server (but you can use "ipmitool -I lan -H XXX.XXX.XXX.XXX -U root chassis power cycle" to reboot any time).

Redirection After Boot Enabled

Grub essentially should see a single console, called "console", as the BIOS will take care of sending any output also to the serial line. If you would configure grub now to also talk to the serial (SoL) port, they'll fight for the input, and you will have to wait for a very, very long time. Actually, you can wait forever, infinitely long.

If you have Redirection After Boot enabled, your grub should really look like:

# grub.conf assuming a Xen kernel mess
serial --unit=1 --speed=19200
terminal --timeout=2 console

title CentOS (2.6.18-8.1.15.el5xen)
       root (hd0,0)
       kernel /xen.gz-2.6.18-8.1.15.el5 com2=19200,8n1 console=com2,vga
       module /vmlinuz-2.6.18-8.1.15.el5xen ro root=/dev/md1 console=xvc xencons=xvc pnpacpi=off
       module /initrd-2.6.18-8.1.15.el5xen.img

where grub only uses the console, but of course the (Xen) kernel uses COM2: explicitly, as by then you'll be in protected mode. The CentOS kernel lines are thus standard for a Xen kernel.

Redirection After Boot Disabled

If you have defailt redirection after boot disabled, grub will have to take care of talking to the serial console (which, in case of SoL, will happily be waiting for you on COM2:). Now, you must set the "terminal" line as follows:

  terminal --timeout=5 serial console

But, it does not change a thing for the kernel commandline arguments, as the BIOS redirection will only have effect for as long as you're in real mode. Once you switch to protected mode, the BIOS redirection will be out of the loop.

Serial Consoles on a regular kernel

There are plenty of guides for this. Use

kernel /vmlinuz-2.6.18-8.1.15.el5 ro root=/dev/md1 console=ttyS1,19200n8 console=tty1

and you should be ready for action. If you want to run anaconda (the Red Hat installer), the console=tty1 bit should be left out for otherwise the installer will use that instead of the serial console.

If you see nothing but garbage on the serial console after the kernel boots, you may need to look at the line speeds. For IPMI, you need to use this:

ipmitool -I lanplus -H -U root sol set volatile-bit-rate 9.6
ipmitool -I lanplus -H -U root sol set non-volatile-bit-rate 9.6

Or other bit rates as set in the BIOS.

If you want to log in over the serial line, also add to /etc/inittab

co:2345:respawn:/sbin/agetty -L ttyS1 19200 vt100

and send SIGHUP to init, and don't forget to add /dev/ttyS1 to /etc/securetty ...

Serial Consoles with a Xen kernel

This is a lot more challenging, as Xen will do two things:

  • virtualize the regular tty, so that the OS in Dom0 can talk to the native console
  • open a fake serial connection for itself on a ttySx to that a host-OS in Dom0 can look at the status diagnostics from the root Xen kernel (xen.gz)

We need to change both in order to get the serial console to work again.

title CentOS (2.6.18-8.1.15.el5xen)
  root (hd0,0)
  kernel /xen.gz-2.6.18-8.1.15.el5 com2=19200,8n1 console=com2,vga
  module /vmlinuz-2.6.18-8.1.15.el5xen ro root=/dev/md1 console=xvc xencons=xvc pnpacpi=off

will do these two things: instruct Xen to send it's status output to COM2: (the SoL console), and simultaneously also send it to VGA. The COM2: serial settings are the default 19200 8N1. Secondly, the actual kernel in Dom-0 will use the Xen virtual console (xvc) to direct all it's output to. Via the Xen kernel (in xen.gz) all this output will end up on the SoL console as well. the "pnpacpi=off" hack seems to be necessary on some systems.

The login prompt on Dom-0 will be connected to /dev/xvc0 since RHEL automatically adds to inittab

 co:2345:respawn:/sbin/agetty xvc0 9600 vt100-nav

and also the RHEL5 distribution will automatically add /dev/xvc0 to /etc/securetty so that root can log in.

If you cannot get anything to work, and echo-ing characters to /dev/ttyS1 gives "no such device or address" on a Xen enabled system in Dom-0, it's a sign that Xen is still eating the serials for it's own status messages and you need to fiddle with the arguments to both the xen.gz and the "module vmlinuz ..." line.

login prompt

If you use the Xen kernel, and use xencons=xvc, RHEL5 will do all the magic for you. If you build your own Xen stuff and run Xen with RHEL4 in Dom-0, try "xencons=off" and a direct "console=ttyS1,19200n8" as a (real) kernel argument. It appears to work that way, but you miss most of the startup messages and the Xen status info.

M600 blade configuration with CentOS5

Configuration takes place in grub.conf (with a Xen kernel) like:

serial --unit=1 --speed=115200
terminal --timeout=2 console
title CentOS (2.6.18-128.1.10.el5xen)
       root (hd0,0)
       kernel /xen.gz-2.6.18-128.1.10.el5 com1=115200,8n1 console=com1,vga
       module /vmlinuz-2.6.18-128.1.10.el5xen ro root=/dev/vg0/lvroot console=xvc xencons=xvc pnpacpi=off
       module /initrd-2.6.18-128.1.10.el5xen.img

Useful Links

And general IPMI links:

Example configurations from


# 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/md1
#          initrd /initrd-version.img
serial --unit=1 --speed=19200
terminal --timeout=2 console

title CentOS (2.6.18-8.1.15.el5xen)
       root (hd0,0)
       kernel /xen.gz-2.6.18-8.1.15.el5 com2=19200,8n1 console=com2,vga
       module /vmlinuz-2.6.18-8.1.15.el5xen ro root=/dev/md1 console=xvc xencons=xvc pnpacpi=off
       module /initrd-2.6.18-8.1.15.el5xen.img

title CentOS monitor (2.6.18-8.1.15.el5xen)
       root (hd0,0)
       kernel /xen.gz-2.6.18-8.1.15.el5
       module /vmlinuz-2.6.18-8.1.15.el5xen ro root=/dev/md1
       module /initrd-2.6.18-8.1.15.el5xen.img


Set Serial Communication-> Serial Communication to On with Console Redirection via COM2
Set Serial Communication-> External Serial Connector to COM1
Serial Communication -> Redirection After Boot is Enabled

and in the BMC settings (Ctrl-E), LAN is enabled and set to and netmask for the time being. SoL is enabled (you cannot turn it off IIRC), and the root password is set there as well (but can be changed using the ipmitool locally).