Serial Consoles

From PDP/Grid Wiki
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"), 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.

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 readon 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 194.171.97.0/24 for the farmnet servers like kaf), but this can and should be changed later as it will otherwise eat IP addresses like mad.

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 194.171.97.250 -U root sol activate

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

ipmitool -I lanplus -H 194.171.97.250 -U root 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

Grub

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
default=0
timeout=5
hiddenmenu
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 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, and the RHEL5 distribution will automatically add /dev/xvc0 to /etc/securetty as well (so that root can log in).


login prompt

If you use the Xen kernel, and use xencons=

Useful Links

Retrieved from "https://wiki.nikhef.nl/grid/index.php?title=Serial_Consoles&oldid=1370"