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Category: How-To Guides

The How-To Guides category is for posts that explain how to fix an issue, configure a device or a piece of software, or make something work.

Posted on by Arnon Erba in How-To Guides

Ubuntu has been using update-motd as a MOTD (Message of the Day) generator for several years. Some of the default messages — such as the number of available security patches — can be helpful, but not everyone likes being greeted by a barrage of text every time they log in to their server. In this article, we’ll explore how to adjust, disable, or replace the dynamic MOTD in Ubuntu.

Before You Begin

If you’d rather work with update-motd than turn it off, detailed documentation for changing its output is available in the man page for update-motd. Essentially, the dynamic MOTD is generated by a collection of executable scripts found in the /etc/update-motd.d/ directory. These scripts can be updated, removed, or reordered, and new scripts can be added.

Disabling the Dynamic MOTD

While Ubuntu does not provide a straightforward way to remove update-motd, it’s possible to disable it by adjusting a few PAM options. Two lines, found in both /etc/pam.d/login and /etc/pam.d/sshd, cause update-motd to run on login:

session optional pam_motd.so motd=/run/motd.dynamic
session optional pam_motd.so noupdate

Commenting out these lines in both files will prevent the pam_motd.so module from being loaded and will disable the dynamic MOTD.

Bonus Section: Enabling a Static MOTD

If you still want a message printed to the console on login, you can fall back to a static MOTD. Per the man page for sshd_config, OpenSSH can easily be configured to display a static MOTD:

PrintMotd
Specifies whether sshd should print /etc/motd when a user logs in interactively. (On some systems it is also printed by the shell, /etc/profile, or equivalent.) The default is “yes”.

Ubuntu disables this option by default and incorporates /etc/motd into its dynamic generator, but we can re-enable the option to make /etc/motd work again. Add or uncomment the following line in /etc/ssh/sshd_config and restart the OpenSSH daemon to have OpenSSH print /etc/motd on login:

PrintMotd yes

Sources

Posted on by Arnon Erba in How-To Guides

(Editor’s note: This post has been updated since publication to fix broken links.)

If you have a recent business-class Dell PC with TPM version 1.2, you may be able to upgrade it to TPM version 2.0. Several Dell models are capable of switching between TPM version 1.2 and 2.0 provided a few conditions are met.

Prerequisites

First, your PC must support switching to TPM 2.0. Most supported models are listed in the “Compatible Systems” section of the instructions for the Dell TPM 2.0 Firmware Update Utility itself. If you can’t find your system in that list, there’s a good chance it isn’t supported by this process.

Second, your PC should be configured in UEFI Boot Mode instead of Legacy Boot Mode. Switching boot modes generally requires a reinstallation of Windows, so it’s best to choose UEFI from the start.

Finally, while optional, it’s recommended that you update your BIOS to the latest version. You can get your serial number by running wmic bios get serialnumber from within PowerShell or Command Prompt. Then, you can provide this serial number to the Dell support website to find the latest drivers and downloads for your PC.

Once you’re ready, you can clear the TPM and run the firmware update utility. However, since Windows will automatically take ownership of a fresh TPM after a reboot by default, we have to take some additional steps to make sure the TPM stays deprovisioned throughout the upgrade process.

Step-By-Step Instructions

  1. First, launch a PowerShell window with administrative privileges. Then, run the following command to disable TPM auto-provisioning (we’ll turn it back on later):
    PS C:\> Disable-TpmAutoProvisioning 
  2. Next, reboot, and enter the BIOS settings. Navigate to “Security > TPM 1.2/2.0 Security”. If the TPM is turned off or disabled, enable it. Otherwise, click the “Clear” checkbox and select “Yes” to clear the TPM settings.
  3. Then, boot back to Windows, and download the TPM 2.0 Firmware Update Utility. Run the package, which will trigger a reboot similar to a BIOS update.
  4. When your PC boots back up, run the following command in another elevated PowerShell window:
    PS C:\> Enable-TpmAutoProvisioning 
  5. Reboot your PC again so that Windows can automatically provision the TPM. While you’re rebooting, you can take this opportunity to enter the BIOS and ensure that Secure Boot is enabled (Legacy Option ROMs under “General > Advanced Boot Options” must be disabled first).
  6. Finally, check tpm.msc or the Windows Security app to ensure that your TPM is active and provisioned.

References

Posted on by Arnon Erba in How-To Guides

Let’s Encrypt has steadily improved since its public debut in late 2015. Certbot, the most popular Let’s Encrypt client, is available for a wide variety of Linux distributions, making Let’s Encrypt an easy integration for most common web server configurations. However, because of this broad support, and because Certbot offers many internal options, there are several different ways to integrate Certbot with Nginx.

If you run Certbot with the --nginx flag, it will automatically make whatever changes are necessary to your Nginx configuration to enable SSL/TLS for your website. On the other hand, if you’d prefer to handle the Nginx configuration separately, you can run Certbot with the --webroot flag. In this mode, Certbot will still fetch a certificate, but it’s up to you to integrate it with Nginx.

Once you’ve obtained certificates from Let’s Encrypt, you’ll need to set up a method to automatically renew them, since they expire after just 90 days. On Ubuntu 18.04, the “certbot” package from the Ubuntu repositories includes an automatic renewal framework right out of the box. However, you’ll also need to force your web server to reload its configuration so it can actually serve the renewed certificates (more on this below). The packaged renewal scripts on Ubuntu won’t restart Nginx unless you used the --nginx flag to request certificates in the first place. If you’re using --webroot or some other method, there’s an additional important step to take.

Automatically Restarting Nginx

On Ubuntu 18.04, Certbot comes with two automated methods for renewing certificates: a cron job, located at /etc/cron.d/certbot, and a systemd timer. The cron job is set to run every 12 hours but only takes effect if systemd is not active. Instead, the systemd timer (visible in the output of systemctl list-timers) works in tandem with the certbot systemd service to handle certificate renewals.

Instead of modifying the cron job or the systemd service, we can change Certbot’s renewal behavior by editing a config file. Add the following line to /etc/letsencrypt/cli.ini:

renew-hook = systemctl reload nginx

This will cause Certbot to reload Nginx after it renews a certificate. With the renew-hook option, Certbot will only reload Nginx when a certificate is actually renewed, not every time the Certbot renewal check runs.

A Little Background Information

If you’re new to Let’s Encrypt, and you’re wondering why you need to automatically renew your certificates and restart your web server when you get new ones, it’s a good thing you’re here. While “traditional” SSL/TLS certificates are manually requested and can be valid for up to two years, certificates from Let’s Encrypt are only valid for 90 days. In their blog post, the Let’s Encrypt team explains their reasoning behind such short certificate lifetimes: they limit the time period for damage to be caused by stolen keys or mis-issued certificates, and they heavily encourage automation, which is key to the success of the Let’s Encrypt model.

This means that you’re going to need to automatically renew your certificates in order to take full advantage of Let’s Encrypt. Fortunately, since this is how Let’s Encrypt is designed to work, auto-renewal functionality is built directly into Certbot, the recommended ACME client for Let’s Encrypt.

A slightly less obvious question is why you’d want to automatically restart your web server as well. The answer is simple: web servers, such as Apache or Nginx, don’t read your SSL/TLS certificates directly from disk every time they need them. Instead, they load them into memory along with the rest of the web server configuration. This is great, and perfectly normal, since reading the certificates from disk would be horribly inefficient. However, it means that updating (or renewing) a certificate with Let’s Encrypt won’t directly change the certificate that Apache/Nginx serves when a page is requested. Instead, the web server must be restarted in order to load the new certificate into memory.

Sources

Posted on by Arnon Erba in How-To Guides

Julia, the fast-moving and popular open source programming language for scientific computing, allows for the usage of multiple BLAS implementations. Pre-built Julia binaries ship with OpenBLAS due to licensing restrictions surrounding the Intel Math Kernel Library, but by building Julia from source you can replace OpenBLAS with a free copy of MKL obtained from Intel’s Yum or Apt repositories. As of the time of writing, there are instructions for this process on the Julia GitHub repository.

Determining the BLAS Vendor

Regardless of which BLAS implementation you choose, it is nice to check that Julia is actually using the one you want, especially if you are building Julia from source. In recent versions of Julia, you can run the following two commands in the Julia REPL to find your BLAS vendor:

julia> using LinearAlgebra
julia> LinearAlgebra.BLAS.vendor()

The second command should output a string indicating which BLAS implementation your Julia installation is currently built with.

Posted on by Arnon Erba in How-To Guides

As a blogger, getting your content in front of as many people as possible is crucial to growing your reader base. One way to broaden your reach is to take advantage of Apple News as a publishing platform so that iOS users can easily discover your content. Fortunately, getting an established blog on Apple News takes nothing but an Apple ID and a little patience.

Two Ways to Publish

Apple News offers two different formats for displaying your content in the News app. If you have some extra time, you can publish your articles in Apple News Format, which allows you to create a custom visual theme and gives you access to more advanced features like analytics. There are third-party Apple News Format plugins available for popular content management systems such as WordPress and Drupal, and you can find the instructions for setting up Apple News Format in the News Publisher documentation.

However, if you’re in a hurry, you can simply add an RSS or Atom feed to your Apple News account. We’ll be covering that method in this post. If you’re using WordPress, you can take advantage of its built-in RSS and Atom feeds for this tutorial.

Publishing Using an RSS/Atom Feed

Adding a blog to Apple News with just an RSS or Atom feed can be accomplished in a few quick steps:

  1. First, head to the Apple News sign-up portal. The sign-up process for a News Publisher account is handled through iCloud, so you’ll need an Apple ID for this step.
  2. Next, enter your publisher information. Apple News requires a name, email address, and mailing address to proceed.
  3. After setting up your personal News Publisher account, you’ll be able set up a “channel” for your blog. This is separated from the previous step, and requires you to provide a phone number.
  4. Next, you can break off and set up Apple News Format, or you can skip directly to adding an RSS/Atom feed. You can even add multiple feeds if you have different streams of content.
  5. Once you add your feed, Apple News will ask if you want to upload branding for your channel. Apple has clear guidelines for this optional branding in the News Publisher documentation, but you can skip this step too if you’re short on time.

Finally, your new News channel containing your blog’s feed will be submitted to Apple for review. The waiting period can vary, and it took about three weeks for Apple to approve the Atom feed for my blog. You’ll get an email from Apple if your submission is approved, and you should be able to find your channel in the News app immediately.

Posted on by Arnon Erba in How-To Guides

The Fibonacci sequence (or series) is a classic example of a problem that can be solved by using recursion. As such, it’s often used to teach the concept of recursion in introductory programming courses. In this post, I’m going to explain how to write a simple recursive Fibonacci sequence calculator in C++ and how to optimize it so that it is capable of computing large Fibonacci numbers more efficiently.

Some Background: What is Recursion?

If you’re already familiar with the concept of recursion, you’re welcome to skip to the next section, where I’ll jump right into the code. Otherwise, you may want to review this section before continuing.

An Informal Definition

Put most simply, a recursive problem is one that is defined in terms of itself. In computer science, recursive procedures (functions) are used to break these kinds of problems down into simpler, easier-to-solve versions of themselves. Recursive functions, like the problems they solve, are also defined in terms of themselves.

Recursion in Code

In practice, this means a recursively defined function must call itself when it runs. To prevent an infinite loop, a correctly written recursive function will stop calling itself once it reaches a base case, which is a problem it can solve without having to call itself again. This is the idea behind recursion: A complex problem is eventually reduced to an easy-to-solve base case with a known solution. Recursive functions achieve this goal by re-using the same logic with different parameters every time the function is recursively called.

Recursion and the Fibonacci Sequence

With this in mind, it’s easy to see why the Fibonacci sequence is a good example of recursion. In the Fibonacci sequence, each number is recursively defined as the sum of the two previous numbers. For example, to find the fifth Fibonacci number, you first have to find the third and fourth numbers. However, you can’t find the fourth number without finding the second and third numbers, and so on. In fact, the whole sequence is constructed from the first two numbers, 0 and 1, which are the base cases. Some recursive problems only have one base case, but the Fibonacci sequence has two.

The Simple Function (And Why It’s So Slow)

All of this can be expressed more clearly with code:

 1 int fibonacci(int n) {
 2     if (n == 0) {
 3         return 0;
 4     } else if (n == 1) {
 5         return 1;
 6     } else {
 7         return fibonacci(n-1) + fibonacci(n-2);
 8     }
 9 }

This basic fibonacci function takes a single integer n as a parameter. This integer is the index value of the desired Fibonacci number. The first two if statements (lines 2 and 4) are simple: If the 0th Fibonacci number is requested, return 0. If the 1st Fibonacci number is requested, return 1.

However, if anything beyond the 1st Fibonacci number is requested, the function jumps to the statement in the last else block (line 7):

return fibonacci(n-1) + fibonacci(n-2);

In this case, the function will first compute the previous two values of the sequence, and then add them together and return the answer. This is because the value of fibonacci(2) depends on the values of fibonacci(1) and fibonacci(0), and so on.

This quickly becomes complex. Calling fibonacci(5) results in calls to fibonacci(4) and fibonacci(3). However, calling fibonacci(4) results in a second call to fibonacci(3), since we need to know the value of the 3rd number in the sequence before we can find the value of the 4th. Likewise, both calls to fibonacci(3) will result in duplicate calls to fibonacci(2) (and eventually fibonacci(1) and fibonacci(0)).

Re-computing values we’ve already calculated is a big performance drain, and it only gets worse as we go farther into the sequence. Wouldn’t it be great if we could store the value of fibonacci(3) somewhere so we didn’t have to re-compute it later?

The Optimized Function

 1 unsigned long fibonacci(unsigned long n) {
 2         static std::vector<unsigned long> fibovector = {0, 1};
 3         unsigned long temp;
 4         if (n < fibovector.size()) {
 5                 return fibovector[n]; 
 6         } else {
 7                 temp = fibonacci(n-1) + fibonacci(n-2);
 8                 fibovector.push_back(temp);
 9                 return temp;
10         } 
11 }               

In this optimized version of the fibonacci function, I’ve added a vector (called fibovector) that caches the computed values. I’ve also changed the base case: The function no longer checks to see if the requested value is 0 or 1 but instead checks to see if the value is cached in the vector (line 4). If the requested value is already stored in the vector, the function uses the cached value and skips having to re-compute it. If it’s not already cached, the function recursively computes it (line 7) and then adds it to the vector for later use.

The Vector

There’s a few important points worth noting regarding the vector. The vector is declared with the static keyword so that its contents can persist between function calls. To get the program started, the vector is “preloaded” with the first two values in the Fibonacci sequence, 0 and 1. These are the initial base cases for the function.

Because the vector gets populated in order, there’s an easy way to check if the desired value is already cached or not. Since the index values of the numbers in the vector correspond to the actual index values of the numbers in the Fibonacci sequence, you can simply compare the desired index value with the size of the vector. If the requested index value is less than the size of the vector (line 4), the function has already cached that value. It’s important to keep in mind that this works because the largest index value in a vector is always one less than the total number of elements it contains (the size) since index values start at 0.

Why unsigned long?

Fibonacci numbers get very large very quickly, and due to the nature of how computers store numerical data it’s easy to overflow your chosen data type before getting very far. There’s a section about integer overflow at the end of this article about email servers, but the bottom line is that the 46th number in the Fibonacci sequence is the largest Fibonacci number that can be stored in a 32-bit integer. With unsigned long as the data type, we’re able to store up to the 93rd Fibonacci number before overflowing that data type as well. To keep the function simple, I’ve opted to use native C++ data types instead of writing or importing a custom “big integer” class that would allow for the storage of even larger Fibonacci numbers.

Checking Your Work

To test if your program is working correctly, you can compare its output to the list of the first 2000 Fibonacci numbers published by the On-Line Encyclopedia of Integer Sequences (OEIS).

Then, try timing each program and comparing how long they both take to reach the 46th Fibonacci number. The results might surprise you.