A backup script that runs on several Linux systems recently produced the following error:
ls: cannot access '/tmp/sortrq9hq8': No such file or directory
It happens every time the backup runs. Other than the above message, there seems to be no ill effect on the backup, which completes successfully. The ls command did not find a file that it expected to be there.
This article explains how the missing file was created by the Linux sort command as a temporary storage area, how error messages about these files are likely to crop up from time to time, how to reproduce the error, and some background about the behaviour of the sort command on Linux and Solaris. Continue reading →
An http website is not encrypted. That is to say, the data comprising the site is not encrypted as it flows from the web server to the device (pc, phone, tablet) on which the user is viewing the page. Anyone able to “listen in” on the network could read that data, which is a security risk. An https web site is different. Data is encrypted. The web server encrypts each web page before transmission, and the user’s browser decrypts it, providing end-to-end protection from eavesdropping.
This article explains how to convert an existing basic website to https by obtaining a free digital certificate from Let’s Encrypt. It is based on a Raspberry Pi running the “Apache” web server, but will also work on other Linux systems. It is intended for home users and people running small-scale web sites, and as a learning aid. Continue reading →
Ansibleprovides a rich pattern matching ability. Modules like lineinfile can match strings based on regular expressions. Similar expressions are used in Python, Perl and older tools such as egrep, grep, sed and awk.
When attempting to match a string containing awkward characters, an escape mechanism can be used. For example, the dollar character ($) has a special meaning within in a regular expression, being the match for end-of-line. So to match a literal dollar, an escape character, usually a backslash (\), is needed. For example, the regular expression “\$1.65” will successfully match $1.65, without treating $ as end of line.
When processing a string that contains many special characters, the escape syntax can become onerous. One solution is to just “blanket” match the special character, rather than trying to match it precisely. In other words: just use a dot. Continue reading →
Nextcloud is an open source software package providing remote file sharing services. It is similar to Dropbox. But with Nextcloud, you retain ownership, security and control of the shared data. This procedure describes how to build a working Nextcloud service using just 3 commands. It was tested successfully on a Raspberry Pi 4 running Raspbian/Raspberry Pi OS 10 (Buster). Article updated 21/6/21.
Note: If you would rather do the installation manually, step-by-step, without the help of a script, please see my previous article “Simple Nextcloud Installation on Raspberry Pi“. It explains how to do the installation in detail, and provides more background information on Nextcloud. Both procedures achieve the same overall result, however.
SSH allows secure connections from one host to another. All traffic is encrypted. Authentication is usually by means of a key pair, where the private key resides on your local machine, and the public key is imported to the remote system. SSH keys have become particularly important for cloud computing, where users need to access cloud servers over a potentially hostile Internet.
Sometimes, the requirement is to access one system via another. You “hop” through the first system to reach the second. For example, an AWS server through a gateway or “jump” system. The following article shows how to do that, in a secure way, either by proxying or agent forwarding, without having to place a private SSH key onto the middle system. Continue reading →
Bitbucket is a paid-for version of Github*. Along with Jira and Confluence, it forms the Atlassian framework, a suite of devops tools in widespread use.
Using the Bitbucket web interface, a repository can easily be renamed. However, this causes a change in the URL, which breaks the link from existing clones of the repo. They can be deleted and re-cloned, or renamed. This post explains how to do the rename. Continue reading →
Iptables is the name of the firewall built into the Linux kernel. It is also the tool used for firewall configuration. This post explains how to use iptables with a range of IP addresses and/or ports. It could be used, for example, to allow SSH traffic from a number of systems. Or to open up a range of ports with a single firewall rule.
Note: This article is not about blacklisting. If you are looking to set up a blacklist, perhaps to protect your server from a number of unrelated IP addresses, my related procedure on how to protect your webserver with IPset might be more appropriate.
The Linux firewall (part of the Netfilter project) is important on Internet facing systems, “edge” servers and “jump” boxes. Particularly when they do not sit behind another protective network element such as a load balancer or discrete firewall. For example, standaline cloud instances that are not part of a protected VPC infrastructure. Continue reading →
“Link shortening” happens when a short URL, such as http://bit.ly/2bo3XYY, points to the same web page as a longer link, such as https://en.wikipedia.org/wiki/BBC. Short links are often used where there are a limited number of characters available, such as an SMS text or a Twitter post. Short links are also quicker to type and neater than the associated full length links.
Two of the main providers of short links are Bitly and Google (Goo.gl). For example, I used Bitly to create the short link in the above paragraph. However, if you have a Raspberry Pi (or any kind of Linux server), you don’t need to use a provider. You can create your own short links. This article explains how. Continue reading →
In Perl, hash (associative array) sorting is a common and easy practice. Sorting values by key is easy. And so is sorting by value. But how do you sort the values of a hash by key? One answer is to use a hash slice. Continue reading →
The Raspberry Pi’s low power consumption makes it well suited to the role of always-on web server. This post describes how to use a domain name with your Pi-based web site. Setting up a web site on the Pi is very easy and was explained in an earlier post of mine, just here.
This article explains how to set up a domain name with your web site, so that you can surf to http://your.domain.name instead of http://your.ip.address. It assumes that you have already have an Apache web site running. If not, please read the above post, before coming back here. Continue reading →