Implementing a stack in bash

I needed to convert simplified HTML to wiki markup. For that I first needed to check the syntax of the HTML. A stack is perfect for that, but how to do it?

declare -a stack
# push
stack+=($tag)
#pop
unset stack[${#stack[@]}-1]

And that is all there is to it.

Best laptops for Linux

Having owned a number of laptops over the years in my efforts to find the ideal one to run Linux I would recommend to anyone engaged in the same expensive and frustrating pursuit the following two machines/brands that firstly run Linux well, and secondly last more than 12 months before they fall apart.

Clevo L140MU or later models (also called Metabox, System 76 etc)

Advantages
• Comes without any operating system, so you can avoid the "Microsoft tax"
• Runs Linux well
• Is very light -- has magnesium body under 1kg
• Very durable. I have had mine for three years still works mostly OK (see below)
• Great 72 Wh battery
• Great keyboard with backlight
• Great trackpad though simple like Apple one
• 180 degree lid
• Can be charged from USB-C port using a generic 65W charger

Disadvantages
• The rubber feet eventually come off -- stuck them back on with superglue
• Power supply hard to source and the connector failed after 12 months, had to use USB-C
• Fan got a bit noisy after 3 years
• Function keys only work when pressing Fn button as well

Lenovo T480, T14, P14s

Advantages
• The toughest laptops I have ever used. Still work perfectly after 5 years
• P14s comes without Microsoft tax
• Supports Linux well
• Function keys work without pressing Fn
• USB-C charging only
• Splill-resistant keyboard
• Rubber feet never come off

Disadvantages
• Heaver than the Clevo (1.6kg)
• Battery not as good (50Wh)
• P14s requires a patch from Radeon to support suspend/resume
• Microsoft tax with T14
• T480, T490 keyboard usually not backlit

The only major brand I haven't tried is Asus. The others I wouldn't reccommend.

Migrating an openldap database to a new installation

If you are familiar with Openldap you may have become frustrated with the difficulty of migrating a database from one installation to a fresh instance of Openldap. There is an easy way to do this, using a combination of slapcat and slapadd. The only drawback is that it destroys the database at the destination. However, this is usually a new copy of Openldap, so it doesn't matter.

Ldap is a tree

It is important to realise that an ldap database is a tree. That is, there is only one node at the root, and every descendant node must have only one parent. When you install Openldap the tree consists of one node called "nodomain", which, if not destroyed, will likely conflict with anything you import. What you really want is for the root to consist of a "suffix" of your choice, such as mycompany.com, which ldap breaks down to dc=mycompany,dc=com.

Exporting your ldap tree

On your old Openldap installation you must first export the data. For this I recommend you use slapcat:

slapcat [-b suffix] -l output.ldif

You can specify a suffix, which will then export only that portion of the tree. This is useful when your old installation contains a sub-tree you want installed in standalone form on the new instance of Openldap. Or just omit it, and the entire tree will be exported to the file export.ldif.

Importing the exported tree to a new Openldap instance

slapadd is a simple utility that copies the exported database into an empty database. I provide here a script for doing this. Notice that it first deletes the old database (the one defining "nodomain" as the root) to avoid conflicts. To do this it must first stop the slapd service and restart it again afterwards.

#!/bin/bash read -p "ldif file to install: " LDIF if [ "$USER" != "root" ]; then echo "you must use sudo i.e. sudo ./install_ldif.sh" exit fi if [ -z "$LDIF" ] || [ ! -e "$LDIF" ]; then echo "Invalid or empty LDIF file: $LDIF" exit fi read -p "This will destroy your existing LDAP database! OK? (y/n) " YES if [ -z "$YES" ] || [ "${YES:0:1}" == "n" ] || [ "${YES:0:1}" == "N" ]; then exit fi # customise if not in this location LOCK="/var/lib/ldap/lock.mdb" DATA="/var/lib/ldap/data.mdb" systemctl stop slapd # remove existing database if [ -e "$LOCK" ]; then rm "$LOCK" fi if [ -e "$DATA" ]; then rm "$DATA" fi slapadd -l "$LDIF" > /dev/null 2>&1 if [ "$?" == 0 ]; then sudo chown openldap:openldap "$LOCK" sudo chown openldap:openldap "$DATA" if [ $? -eq 0 ]; then systemctl start slapd if [ $? -eq 0 ]; then echo "restarted openldap daemon successfully" fi else echo "Import failed!" fi else echo "Not restarting openldap daemon due to errors" fi

If you install ldap-account-manager (in Debian/Ubuntu) you can see the tree it creates in Tools->Tree view, which should be identical to the one you exported.

Adding SSL/TLS certificates to Openldap on localhost using Ubuntu Linux 22.04.2

I wanted to install ssl/tls certificates on my openldap installation that I had running on localhost as a demo, but the information I found on the Web was mostly out of date. Another approach is to use letsencrypt, but that is for registered domain names and won't work with localhost, so I will be using self-signed certificates in this post.

Requirements

You will need to have installed openldap and openssl. If you haven't then do so now:

sudo apt install openssl slapd

Create a certificate authority (CA) to sign your certificates

There is a script in openssl for doing this. Nowadays it is called CA.pl. I found it in /usr/lib/ssl/misc/, but it might be somewhere else on your machine. You can find it with:

sudo find / -name CA.pl

Once you have the path to it, create a directory to hold the CA. I created one in /var/myca:

sudo mkdir -p /var/myca
cd /var/myca

Now invoke the script:

sudo /usr/lib/ssl/misc/CA.pl -newca

When it prompts for a certifcate filename just hit return. It will ask you some certificate type questions, including a passphrase. Give it something simple so you can remember it. (Remember this is just for testing. On production you'll need a secure passphrase.) It will create a directory called demoCA inside /var/myca.

Create a certificate for openldap using the CA

sudo   openssl req -new -nodes -keyout newreq.pem -out newreq.pem

Again, you have to answer all the tedious certificate questions. When it has finished you should have newreq.pem in the /var/myca directory. Now sign the request:

sudo /usr/lib/ssl/misc/CA.pl -sign

This creates newcert.pem

Copy the certificates and key to where openldap can find them

Create a directory to hold them for openldap. I chose /etc/openldap/certs. Create it if it is not there:

sudo mkdir -p /etc/openldap/certs

Now copy the certificates over:

cd /etc/openldap/certs
sudo cp /var/myca/demoCA/cacert.pem .
sudo mv /var/myca/newcert.pem servercrt.pem
sudo mv /var/myca/newreq.pem serverkey.pem
sudo chmod 600 serverkey.pem 
sudo chown openldap:openldap *
ls -l

This produces:

-rw-r--r-- 1 openldap openldap 4730 May 25 11:49 cacert.pem
-rw-r--r-- 1 openldap openldap 4751 May 25 11:50 servercrt.pem
-rw------- 1 openldap openldap 2843 May 25 10:22 serverkey.pem

Allow Openldap to read this directory

Nowadays Openldap is controlled by apparmor, which restricts which directories openldap, aka slapd, can access. So we have to tell it about this new directory of certificates we just created. In /etc/apparmor.d you should find a file called usr.sbin.slapd, which is divided into sections, one of which is called "# ldap files". After that are a couple of lines. Add this line in bold using your favourite editor to tell it about all the files in /etc/openldap/certs:

  # ldap files
  /etc/ldap/** kr,
  /etc/ldap/slapd.d/** rw,
  /etc/openldap/certs/* r,

Now restart slapd to get it to see the directory:

sudo systemctl restart slapd

Update the openldap config to point to the certificates and key

Create a file add_ssl.ldif using nano or vi, with this content:

dn: cn=config
changetype: modify
replace: olcTLSCACertificateFile
olcTLSCACertificateFile: /etc/openldap/certs/cacert.pem
-
replace: olcTLSCertificateKeyFile
olcTLSCertificateKeyFile: /etc/openldap/certs/serverkey.pem
-
replace: olcTLSCertificateFile
olcTLSCertificateFile: /etc/openldap/certs/servercrt.pem

Now load it:

sudo ldapmodify -Q -Y EXTERNAL -H ldapi:/// -f add_ssl.ldif -W

It will respond with:

[sudo] password for username: 
Enter LDAP Password: 
modifying entry "cn=config"

If it says:

ldap_modify: Other (e.g., implementation specific) error (80)

check in the last lines of syslog (tail /var/log/syslog) to see if apparmor is denying access. If so, check your steps in "Allow Openldap to read this directory" above.

Tell Openldap where the CA certificate is

Edit /etc/ldap/ldap.conf and set the variable TLS_CACERT to /etc/openldap/certs/cacert.pem, then restart slapd:

sudo systemctl restart slapd

Your Openldap installation should now be ready to use SSL!

Ubuntu 22.04.1 on Lenovo Thinkpad P14s (Ryzen R7 Pro 6850U)

I was a bit nervous about this as the Canonical site says that the latest version of Ubuntu may not run on the Thinkpad P14s:

Pre-installed in some regions with a custom Ubuntu image that takes advantage of the system’s hardware features and may include additional software. Standard images of Ubuntu may not work well, or at all.

Indeed, on booting up the laptop is indeed has a custom kernel 5.14.0-1047-oem and Ubuntu 20.04 (Focal Fossal). But I was determined to upgrade to 22.04. After upgrading the hard drive to 2TB PCIe 4.0 I noticed that in this model (21J5) there are NO slots for memory expansion. So you're stuck with the soldered 32GB it comes with. However, the installation went smoothly. I chose "Install 3rd party software for graphics and wifi". After installation I rebooted and tested wifi, sleep, suspend, closing the lid and connecting to a dock with two screens, keyboard and mouse. Also tested the function keys. All was fine. So I'd recommend this model for those of us who want a Linux laptop and don't want to pay the Microsoft tax.