How Bitcoin Works

A fairly low level description as to how Bitcoins actually work.

How Bitcoin Works Under the Hood - YouTube - https://www.youtube.com/watch?v=Lx9zgZCMqXE

Force a Kernel Panic

Have you ever wanted to generate a kernel panic, for kernel testing purposes?

A few ways to generate a kernel panic, a kernel oops and reboots:

# echo 1 > /proc/sys/kernel/panic

# dd if=/dev/random of=/dev/port

# cat /dev/port

# cat /dev/zero > /dev/mem

Forcing an Alt-SysReq-c command from the console:

# echo c > /proc/sysrq-trigger

To generate a kernel panic from source code, we will need to compile a kernel module from the kernel source: (source)

// source: force_panic.c
#ifdef __KERNEL__

/* Makefile :
obj-m := force_panic.o
KDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)

default:
 $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
*/

#include <linux/module.h>
#include <linux/kernel.h>

static int __init panic_init(void) {
  panic("force-panic");
  return 0;
}

static void __exit panic_exit(void) {
}

module_init(panic_init);
module_exit(panic_exit);

#endif

Generate a kernel oops: (source)

static int crash_module_init(void) {
     printf("crash module starting\n");
     int *p = 0;

     printk("%d\n", *p);

     return 0;
}

static void crash_module_exit(void) {
    printf("crash module exiting\n");
}

module_init(crash_module_init);
module_exit(crash_module_exit);

PSOD (Purple Screen of Death)

Example of a PSOD kernel stack trace screen

A PSOD (Purple Screen of Death) is the VMware ESX version of a Windows BSOD (Blue Screen of Death).  This occurs when the kernel panics and can no longer function.  There most common causes for a PSOD are:

• Hardware failure
• Out of memory
• Hung CPU conditions
• Misbehaving drivers (null pointers, invalid memory access, etc)
• NMI (Non Maskable Interrupts)

When a PSOD occurs, one should collect the following:

1. Screenshot of PSOD kernel stack trace screen (if possible)
2. Support logs from the vm-support command
3. Kernel log(should be included in vm-support, but better safe then sorry)
4. Kernel core dump (only needed if a developer asks for it)

If the cause of the PSOD isn't obvious from the PSOD kernel stack trace screen, then the kernel log is the second best place to look for the cause of a kernel panic. To manually collect the kernel log:

## Kernel log - will output: vmkernel-log.1
# esxcfg-dumppart -L /vmfs/devices/disks/$( \
  esxcfg-dumppart --get-active | awk '{print $1}' )

To manually collect the kernel core dump: (if developer asks for it)

## Kernel core dump - will output: vmkernel-zdump.1
## Note: ESXi 5.x will put kernel dump here:
##     /scratch/core/vmkernel-zdump.*
# esxcfg-dumppart -C -D /vmfs/devices/disks/$( \
  esxcfg-dumppart --get-active | awk '{print $1}' )

For testing purposes, one can manually trigger a PSOD:

# vsish -e set /reliability/crashMe/Panic

How To Disable ESXi Password Restrictions

The following instructions will allow you to change the "root" user's password to something simple like "password".

• ESXi 5.0, ESXi 5.1 and ESXi 5.5
• ESXi 4.1
• ESXi 4.0
• ESX 4.1
• ESX 4.0

ESXi 5.0, ESXi 5.1 and ESXi 5.5

1. Edit /etc/pam.d/passwd and add "enforce=none" to the end of the "password   requisite" line:

esx# vi /etc/pam.d/passwd
password  requisite  /lib/security/$ISA/pam_passwdqc.so \
        retry=3 min=8,8,8,7,6 enforce=none

2. Change password:

esx# passwd

ESXi 4.1

1. Edit /etc/pam.d/system-auth and add "enforce=none" to the end of the "password   requisite" line:

esx# vi /etc/pam.d/system-auth
password  requisite  /lib/security/$ISA/pam_passwdqc.so \
        retry=3 min=8,8,8,7,6 enforce=none

Note: You will need to force the write with an exclamation ":wq!".

2. Change password:

esx# passwd

Note: The enforce setting will be lost on the next reboot, so change your password now.

ESXi 4.0

1. Edit /etc/pam.d/common-password and add "enforce=none" to the end of the "password   requisite" line:

esx# vi /etc/pam.d/common-password
password  requisite  /lib/security/$ISA/pam_passwdqc.so \
        retry=3 min=8,8,8,7,6 enforce=none

2. Change password:

esx# passwd

ESX 4.1

1. Edit /etc/pam.d/system-auth and add "enforce=none" to the end of the "password   required" line:

esx# vi /etc/pam.d/system-auth
password  required  /lib/security/$ISA/pam_passwdqc.so \
        min=8,8,8,7,6 similar=deny match=0 enforce=none

2. Change password:

esx# passwd

ESX 4.0

1. Edit /etc/pam.d/system-auth-generic and add "enforce=none" to the end of the "password   required" line:

esx# vi /etc/pam.d/system-auth-generic
password  required  /lib/security/$ISA/pam_passwdqc.so \
        min=8,8,8,7,6 similar=deny match=0 enforce=none

2. Change password:

esx# passwd

How To Enable ESXi SSH Access

The following instructions will allow you to enable root ssh access to an ESXi server:

• ESXi 5.0 and ESXi 5.1
• ESXi 4.1
• ESXi 4.0
• ESX 4.1 and ESX 4.0

ESXi 5.0 and ESXi 5.1

1. From the DCUI (Direct Console User Interface), press "[F2]" and login.

2. Navigate down to "Troubleshooting Options", and press enter.

3. Navigate down to "Enable SSH" and press enter.

4. To disable the configuration warning "Configuration Issues: SSH for the host has been enabled" in the vSphere Client, change the following:

• Configuration -> Software (section) -> Advanced Settings -> UserVars -> SuppressShellWarning = 0

ESXi 4.1

1. From the DCUI (Direct Console User Interface), press "[F2]" and login.

2. Navigate down to "Troubleshooting Options", and press enter.

3. Navigate down to "Enable Remote Tech Support (SSH)" and press enter.

ESXi 4.0

1. From the DCUI (Direct Console User Interface), switch to the hidden shell login screen with "[Alt]+[F1]".

2.  Type in "unsupported" (the characters will not be echoed back) and press enter.  Now enter in the root user's password and press enter.

3. Edit /etc/inetd.conf and uncomment the "ssh stream tcp" line:

esx# vi /etc/inetd.conf
ssh  stream tcp nowait root /sbin/dropbearmulti dropbear  ...

4. Reboot the server:

esx# reboot

ESX 4.1 and ESX 4.0

1. From the terminal screen, switch to the shell login screen with "[Alt]+[F1]" and login as root.

2. Edit /etc/ssh/sshd_config and change the 'PermitRootLogin' line to allow root login:

esx# vi /etc/ssh/sshd_config
PermitRootLogin yes

3. Restart the SSH service:

esx# service sshd restart

How To Manage Kernel Module Load Settings on ESXi

The following are instructions for managing kernel modules (drivers) on ESX(i).

List all loaded kernel modules

# vmkload_mod -l    # --list

Show kernel module information

# vmkload_mod -s some_module    # --showinfo

Note: This works in the same manner as modinfo on Linux

List module load parameters for on boot

# esxcfg-module -g some_module    # --get-options

Set module load parameters for on boot

# esxcfg-module -s 'parameter_a=1 parameter_b=test' some_module

Note: This does not affect parameters on manual module load

Clear module load parameters for on boot

# esxcfg-module -s '' some_module    # --set-options

Disable module on boot

# esxcfg-module -d some_module    # --disable

## write boot config, extra step for ESX 4.x only 
# esxcfg-boot -b

Enable module on boot

# esxcfg-module -e some_module    # --enable

NOTE: On ESXi 5.1 you will need to re-enable the module to even manually load the driver.

Manually load module

# vmkload_mod some_module
# vmkload_mod some_module parameter_a=1 parameter_b=test

Note: This works in the same manner as modprobe/insmod on Linux

Manually unload module

# vmkload_mod -u some_module    # --unload

Note: This works in the same manner as modprobe/rmmod on Linux

How To Reset ESXi Trial License

WARNING: This is for education/informational testing/development purposes only, and should not be used on a production server.

WARNING: This trick will only work with an ESX(i) stand alone server.  It will not work if the ESX(i) server is connected to a vCenter Server, as the vCenter Server knows better than to let you do this.  (you can always remove and readd the ESX(i) server to vCenter.)

To reset your ESX 4.x, ESXi 4.x and ESXi 5.x 60 day evaluation license:

1. Login to the TSM through SSH or Shell
2. Remove the following two files:
1. /etc/vmware/vmware.lic
2. /etc/vmware/license.cfg
3. Reboot server

If your ESX server is connected to a vCenter server, please remove the ESX server first.  Once the steps above are completed, you can add it back to the vCenter server.

Sample commands:

rm -f /etc/vmware/vmware.lic /etc/vmware/license.cfg
reboot

For ESXi 5.1 and ESXi 5.5, you may need to continually remove the license files as the server reboots for this to work.  The following should do this quite nicely:

rm -f /etc/vmware/vmware.lic /etc/vmware/license.cfg
reboot ; while true ; do
    rm -f /etc/vmware/vmware.lic /etc/vmware/license.cfg
done

---

An alternative shows that restarting the services should works just as well as rebooting the server:

# For ESXi 5.0
rm -f /etc/vmware/vmware.lic /etc/vmware/license.cfg
services.sh restart

# For ESXi 5.1
rm -r /etc/vmware/license.cfg
cp /etc/vmware/.#license.cfg /etc/vmware/license.cfg
/etc/init.d/vpxa restart

The alternative also shows a method for resetting the trial license while connected to vCenter server.  I still think removing and re-adding the ESXi server is cleaner.