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So You Want A FreeBSD Debugging Kernel…

September 25, 2012 1 comment

Installing a debugging kernel in FreeBSD

The Use Case

There are numerous reasons for needing a debugging kernel. For example, during a kernel panic. This was the case with us. Therefore, we installed a kernel with DDB enabled. In the event of a kernel panic, the system would drop to the debugger and the software engineers were then able to begin diagnosing the problem.

Preparing To Build The Kernel

Prerequisites

FreeBSD Sources

Operators can use cvs or Subversion to install the sources. Information on FreeBSD source trees can be found here.

The Kernel Config

Create a kernel config by opening /usr/src/sys/${arch}/conf/DEBUG. Where ${arch} is the machine architecture the kernel is to be built for. In this case, we build an amd64 kernel on an amd64 machine. The kernel config should appear as follows:

include GENERIC
options KDB_UNATTENDED
options DDB

Compiling and Installing The Kernel

Compiling and installing the debugging kernel is as simple as executing:

# cd /usr/src
# make buildkernel KERNCONF=DEBUG INSTKERNNAME=DEBUG
# make installkernel KERCONF=DEBUG INSTKERNNAME=DEBUG

Executing/Using The Kernel

From the loader menu

At the loader menu, press 6 to escape to the loader(8) prompt and type:

boot DEBUG

From the running system

The DEBUG kernel can be loaded by default by adding/modifying the kernel directive in loader.conf as follows. Each subsequent boot will load the DEBUG kernel.

kernel=DEBUG

Dropping into the debugger

During a kernel panic, it will simply reboot unless debug.debugger_on_panic is enabled. To enable this execute:

sysctl debug.debugger_on_panic=1

To induce a panic:

sysctl debug.kdb.panic=1

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Categories: FreeBSD

Installing FreeBSD 8.x Distributions After Installation

September 17, 2012 Leave a comment

Installing Distributions After The Fact

While I did not end up needing to perform this procedure, I found the command line option intriguing…

FreeBSD 8.x [ and earlier ] media contain distributions one can select to install via sysinstall(8). This is accomplished via GUI or automated scripting requiring an install.cfg.

This image shows a typical FreeBSD installation where a kernel/distribution is extracted onto disk, What is an admin to do when they later find that they need to install a distribution after the fact? There are two methods to installing media distributions after the fact.

Using the Command Line

This is my preferred method as it does not require using sysinstall‘s graphical interface, but I am sure both have their merits. In this example, we install lib32. First, download the distribution from FreeBSD.org

echo CHECKSUM.SHA256 CHECKSUM.MD5 install.sh lib32.a{a,b,c,d,e,f,g,h,i,j,k,l,m} lib32.inf lib32.mtree | xargs -n 1 -I%s fetch ftp://ftp.freebsd.org/pub/FreeBSD/releases/amd64/8.2-RELEASE/lib32/%s

The next step is to install the distribution…

./install.sh

and you’re done…

Using sysinstall(8)

Start the procedure by executing sysinstall. It opens an interactive GUI interface. The Following procedure installs the distribution…

Select “Configure”…

Select “Distributions”…

Select the distribution to install…

Select install media (this example uses ftp)…

Select ftp location (This step may differ depending on the media selected above)…

If networking is configured, select yet. Otherwise, select no and sysinstall will prompt for network configuration…

The distribution installs…

…and when completed, exit sysinstall.

References

* The command line used can be referenced in the FreeBSD Forums

Categories: FreeBSD

Encrypting User Password Strings

September 10, 2012 Leave a comment

Encrypting User Password Strings

The Use Case

User account passwords can be set during systems provisioning in multiple ways. Setting a host’s root password is a common use case. During a FreeBSD 8.x install, one can set root’s password (or other user account password) executing the following within the install.cfg:

/bin/sh -c ‘echo passwd |pw mod user username -h 0’

Unfortunately, doing so exposes the user’s account password. Therefore, the recommendation is to use an encrypted hash as follows:

/usr/bin/chpass -p $1$nPUexUs5$O4JuN.Ed/LqWHJKmf8K0h0 root

Encrypting The Password

The use case begs the question: How does one ascertain an encrypted hash given a string?

On FreeBSD, there are varying methods to ascertain an encrypted hash. Here are just two of the many.

openssl

# openssl passwd -1 MySecret
$1$YRth1v3T$MvGupL8n.VBjvM12JhR4G0

openssl(1) returns the entire encrypted hash. The character between the first and second dollar signs identifies the crypt mechanism. The following eight characters are the salt. The remaining characters following the third dollar sign are the encrypted string.

md5

# md5 -s MySecret
MD5 (“MySecret”) = 4132d75e6cb04073cc7756707057027f

md5(1) returns a partial encrypted hash. In the key-value pair, the key is the crypt mechanism. The first 8 characters of the value are the salt. The remaining characters are the encrypted string.

In this method, one must prepend $1$ to the value before feeding it into chpass(1).

Related Documentation

* Chapter 15, Security, of the FreeBSD handbook explains identifying a host’s crypt mechanism.