How to Generate a Self-Signed SSL Certificate on Linux

how to generate a self signed ssl certificate on linux

In today’s guide, we will discuss how to generate a self-signed SSL certificate on Linux as well as how to implement them in Apache. SSL is becoming more and more important as the internet becomes more popular. With free Let’s Encrypt certificate alternatives becoming a commodity that anyone can use, there’s no reason for anyone to not use SSL – not to mention the search ranking benefits, and the fact that browsers and search engines will trust your site. Here’s a guide on why you should use SSL on your website.

However, you can also generate your own self-signed SSL certificate for private use on your server. One big reason to do this is encryption. While your personal certificate won’t mean anything to browsers, and visitors will still get a warning message if they visit your site directly, you can at least be sure that you’re protected against “man-in-the-middle” attacks. A self-signed certificate is a good first step when you’re just testing things out on your server, and perhaps don’t even have a domain name yet.

Let’s start with our step-by-step procedure on how to create a self-signed SSL certificate on Linux. On a related note, check out our list of affordable and fast SSL certificates by brand.

Step 1: Create an RSA Keypair

The first step in generating your own self-signed SSL certificate is to use the “openssl” package on Linux/CentOS to create an RSA key pair. To do this, make sure that you have the package installed. If not, install it with this command:

sudo yum install openssl

Chances are that you already have it available on your system – it should now be installed regardless. Once the package is confirmed to be installed on your system, generate the keypair using the following command:

openssl genrsa -des3 -passout pass:x -out keypair.key 2048

This command uses 2048 bit encryption and outputs a file called keypair.key, as shown here:

generating a self signed ssl certificate on linux

As you can see, the key has been generated and placed in the current directory.

Step 2: Extract the Private Key into the “httpd” Folder

The /etc/httpd folder is where the operating system keeps all important SSL-related items. First, let’s create a new folder to hold all of our files related to our private key:

sudo mkdir /etc/httpd/httpscertificate

We called the folder httpscertificate and will refer to it by that name for all of the other command line examples. You can name the folder anything you want.

To extract the private key from the keypair file that we just created, type in the following:

openssl rsa -passin pass:x -in keypair.key -out /etc/httpd/httpscertificate/012.345.678.90.key

Replace the section in bold with the IP address of your own server. Or if you’re able to access your site with a domain name, you can use that as well.

This will create a .key file in the folder that we just created. When this process is done, we can delete the original keypair file:

rm keypair.key

Step 3: Creating a “Certificate Signing Request” (CSR) File

With the key, we can create a special .csr file that we can either sign ourselves or submit to a “Certificate Authority”. It’s in a standardized format and can be easily generated with our key from the previous step. To create it, type the following command:

openssl req -new -key /etc/httpd/httpscertificate/012.345.678.90.key -out /etc/httpd/httpscertificate/012.345.678.90.csr

Again, replace the items in bold with the IP address or domain name that you settled on in step 2. When you run this command, the tool will ask you for some of your personal information, such as your location and organization name:

set up a self signed ssl certificate on linux

A CA (short for Certificate Authority) can use these details to verify that you are indeed who you say you are. Try to populate the fields with as much information as you can.

Once you’ve finished entering these details, the tool will finish with its work and place a .csr file in the directory that we created for just this purpose.

Step 4: Creating the Certificate “.crt” File

With the CSR, we can create the final certificate file as follows. We will now use our .csr and .key files to create our .crt file:

openssl x509 -req -days 365 -in /etc/httpd/httpscertificate/012.345.678.90.csr -signkey /etc/httpd/httpscertificate/012.345.678.90.key -out /etc/httpd/httpscertificate/012.345.678.90.crt

This creates a .crt file in the location with all of our other files. We now know how to generate our self-signed SSL certificate. Here’s a screenshot of the final files in our security folder:

configuring a self signed ssl certificate on linux

Now we need to tell Apache where these files are.

Step 5: Configuring Apache to Use the Files

All that we need to do now is show Apache where our generated self-signed certificates are. First, we need to install the mod_ssl package with the command:

sudo yum install mod_ssl

Once done, this will place a ssl.conf file inside the /etc/httpd/conf.d/ folder. We need to modify this default file. Use your preferred text editor:

sudo vi /etc/httpd/conf.d/ssl.conf

Now scroll down till you find the lines starting with:

SSL CertificateKeyFile

Change the default paths with the paths to the certificate file and key file respectively, as shown here:

how to configure a self signed ssl certificate on linux

Save your changes. Now just restart Apache with:

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sudo apachectl restart

And you’re done! When Apache restarts, it will be configured to allow SSL connections by using the generated self-signed SSL certificates.

When you connect to your IP address via HTTPS the next time, you’ll be warned that it’s not a trusted certificate:

how to se up a self signed ssl certificate on linux

That’s ok. We know this since we signed it ourselves! Just proceed and it’ll take you to the actual website:

Here you can see that it’s using the certificate that we created. It’s not much use for anyone else visiting your site since they can’t verify your identity. But you know it’s safe, and moreover that it’s encrypted. No man in the middle attacks!

You now know how to generate your own self-signed SSL certificates and implement them on your Apache web server.

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6 thoughts on “How to Generate a Self-Signed SSL Certificate on Linux”

  1. I have been trying to create a self-signed certificate but I keep getting an error related to the random number generator. I entered this in the terminal:

    openssl req -x509 -days 365 -sha256 -newkey rsa:4096 -keyout mycert.pem -out mycert.pem

    I then get the prompt to enter my information. I get through that just fine but when I press enter, I get the following error:

    Cannot write random bytes:

    139680915939776:error:2407007A:random number generator:RAND_write_file:Not a regular file:crypto/rand/randfile.c:163:Filename=/home/user/.rnd

    Can you provide any help?

    • If there is a RANDFILE line in your openssl.cnf configuration file (/etc/pki/tls/openssl.cnf) you should be able to safely delete or comment the line by placing # in front, for example:

      # RANDFILE = $ENV::HOME/.rnd

      After you do this, try to run the command again.

  2. For anyone running in to the following error, there has been a deliberate change of behavior from SSL 1.0.1 to SSL 1.1.x to the -passout pass:x password being at least 4 characters. Running the first command documented in the article will result in:

    # openssl genrsa -des3 -passout pass:x -out keypair.key 2048
    Generating RSA private key, 2048 bit long modulus (2 primes)
    e is 65537 (0x010001)
    140657812918720:error:28078065:UI routines:UI_set_result_ex:result too small:../crypto/ui/ui_lib.c:903:You must type in 4 to 1023 characters
    140657812918720:error:28078065:UI routines:UI_set_result_ex:result too small:../crypto/ui/ui_lib.c:903:You must type in 4 to 1023 characters
    140657812918720:error:0906906F:PEM routines:PEM_ASN1_write_bio:read key:../crypto/pem/pem_lib.c:357:

    To remedy this on SSL 1.1.0 run:

    openssl genrsa -des3 -passout pass:xxxx -out keypair.key 2048

    or run command:

    openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out keypair.key

    as a substitute.

    documentation on the ‘not a bug’ here:

    Test system: Ubuntu 18.04.4 LTS
    Library: openssl-1.1.1


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