Steps to generate CSR for SAN certificate with openssl

In this tutorial we will learn about SAN certificates and steps to generate CSR for SAN certificates.

 

What are SAN (Subject Alternative name) Certificates

  • SAN is an acronym for Subject Alternative Name
  • These certificates generally cost a little bit more than single-name certs, because they have more capabilities.
  • When you request a SAN certificate, you have the option of defining multiple DNS names that the certificate can protect.
  • Once issued, the SAN certificate will contain a primary DNS name, which is typically the main name of the website, and, further inside the cert properties, you will find listed the additional DNS names that you specified during your request.
  • This single certificate can be installed on a web server and used to validate traffic for any of the DNS names that are contained in the certificate.
  • For example have a look at the certificate of facebook.com. It is using a Subject Alternative Name with multiple DNS defined in the certificate so it avoids creating multiple certificate for each sub domain.

Steps to generate CSR for SAN certificate with openssl

 

Lab Environment

We will need RootCA certificate and Private key to sign the certificates. I have already created these certificates to demonstrate this article. I will share the commands to create the same, but if you are interested then you should check other articles on similar topic from the LEFT Sidebar MENU:

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## create a directory structure for storing the rootca certificates
mkdir /root/tls/{private,certs}

## navigate inside your tls path
cd /root/tls

## generate rootca private key
openssl genrsa  -out private/cakey.pem 4096

## generate rootCA certificate
openssl req -new -x509 -days 3650  -config openssl.cnf  -key private/cakey.pem -out certs/cacert.pem

## Verify the rootCA certificate content and X.509 extensions
openssl x509 -noout -text -in certs/cacert.pem

You can collect my sample openssl.cnf from a different article.

 

Generate Private Key

First of all we need a private key. Now I could have combined the steps to generate private key and CSR for SAN but let's keep it simple. I have not assigned any passphrase to the private key, you can also use -des3 encryption algorithm to add a passphrase to your private key

# openssl genrsa -out server.key 4096
Generating RSA private key, 4096 bit long modulus (2 primes)
.......................................................++++
.................++++
e is 65537 (0x010001)
IMPORTANT NOTE:

You must keep your private key safely as this CSR will only work with this private key.

 

Generate CSR for SAN Certificate

We will not use the complete /etc/pki/tls/openssl.cnf instead we will create our own custom ssl configuration file with required parameters only.

To generate CSR for SAN we need distinguished_name and req_extensions

I have also added the value for individual distinguished_name parameters in this configuration file to avoid user prompt. If you are not familiar with these parameters then I suggest you to read Things to consider when creating CSR with OpenSSL

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# cat server_cert.cnf
[req]
distinguished_name = req_distinguished_name
req_extensions = req_ext
prompt = no

[req_distinguished_name]
C   = IN
ST  = Karnataka
L   = Bengaluru
O   = GoLinuxCloud
OU  = R&D
CN  = ban21.example.com

[req_ext]
subjectAltName = @alt_names

[alt_names]
IP.1 = 10.10.10.13
IP.2 = 10.10.10.14
IP.3 = 10.10.10.17
DNS.1 = centos8-2.example.com
DNS.2 = centos8-3.example.com

If you prefer to manually enter the CSR details such as Country, State, Common Name etc then you can use this configuration file

[req]
[req]
distinguished_name = req_distinguished_name
req_extensions = req_ext

[req_distinguished_name]
countryName                     = Country Name (2 letter code)
stateOrProvinceName             = State or Province Name (full name)
localityName                    = Locality Name (eg, city)
organizationalUnitName          = Organizational Unit Name (eg, section)
commonName                      = Common Name (eg, your name or your server\'s hostname)
emailAddress                    = Email Address

[req_ext]
subjectAltName = @alt_names

[alt_names]
IP.1 = 10.10.10.13
IP.2 = 10.10.10.14
IP.3 = 10.10.10.17
DNS.1 = centos8-2.example.com
DNS.2 = centos8-3.example.com

Next we will use openssl to generate our Certificate Signing Request for SAN certificate.

# openssl req -new -key server.key -out server.csr -config server_cert.cnf

Since we have used prompt=no and have also provided the CSR information, there is no output for this command but our CSR is generated

# ls -l server.csr
-rw-r--r-- 1 root root 1842 Aug 10 15:55 server.csr

 

Verify Subject Alternative Name value in CSR

Next verify the content of your Certificate Signing Request to make sure it contains Subject Alternative Name section under "Requested Extensions"

# openssl req -noout -text -in server.csr | grep -A 1 "Subject Alternative Name"
            X509v3 Subject Alternative Name:
                IP Address:10.10.10.13, IP Address:10.10.10.14, IP Address:10.10.10.17, DNS:centos8-2.example.com, DNS:centos8-3.example.com

So our CSR contains all the IP Address and DNS value which we provided while generating the CSR for SAN.

 

Generate SAN certificate

Next we will use this CSR to generate our SAN certificate:

# openssl x509 -req -days 365 -in server.csr -CA /root/tls/certs/cacert.pem -CAkey /root/tls/private/cakey.pem -CAcreateserial -out server.crt
Signature ok
subject=C = IN, ST = Karnataka, L = Bengaluru, O = GoLinuxCloud, OU = R&D, CN = ban21.example.com
Getting CA Private Key

 

Verify SAN Extensions in the certificate

Once the certificate is generated, let's verify if our SAN fields are retained inside the certificate:

# openssl x509 -text -noout -in server.crt | grep -A 1 "Subject Alternative Name"

We get an empty output. The SAN Extensions are missing from our certificate.

 

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Missing SAN Extensions from the Certificate

This is an expected behaviour. As per official documentation from openssl

Extensions in certificates are not transferred to certificate requests and vice versa.

Due to this, the extensions which we added in our CSR were not transferred by default to the certificate. So these extensions must be added to the certificate explicitly.

 

Generate SAN Certificates with X509 extensions

We need to use -extensions argument with the name of the extension to be used from the configuration file. We have defined our SAN fields under req_ext extension so we will use the same and re-generate the certificate:

# openssl x509 -req -days 365 -in server.csr -CA /root/tls/certs/cacert.pem -CAkey /root/tls/private/cakey.pem -CAcreateserial -out server.crt -extensions req_ext -extfile server_cert.cnf
Signature ok
subject=C = IN, ST = Karnataka, L = Bengaluru, O = GoLinuxCloud, OU = R&D, CN = ban21.example.com
Getting CA Private Key

 

Re-verify the SAN Extensions in the certificate

Now let us re-verify the SAN extension in our certificate:

# openssl x509 -text -noout -in server.crt | grep -A 1 "Subject Alternative Name"                            
X509v3 Subject Alternative Name:
                IP Address:10.10.10.13, IP Address:10.10.10.14, IP Address:10.10.10.17, DNS:centos8-2.example.com, DNS:centos8-3.example.com

So this time our certificates are properly generated with the SAN fields.

 

What's Next

Now since you have your Certificate Signing Request, you can send it to Certificate Authority to generate SAN certificates. If this was created for intranet then you can also create your own CA certificate or CA certificate chain and use these CA to sign and generate your server certificates

 

Conclusion

In this tutorial I gave you an overview on SAN certificates, and the steps to create Certificate Signing Request for SAN certificates using openssl in Linux. SAN certificates have gained alot of popularity with major domains across world choose for this option as this saves money because it avoids creating individual certificates for respective domains.

Lastly I hope the steps from the article to generate csr for SAN on Linux using openssl was helpful. So, let me know your suggestions and feedback using the comment section.

 

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4 thoughts on “Steps to generate CSR for SAN certificate with openssl”

  1. hello,
    my csr output shows three SAN entries as you show in your last screenshot. After generating a certificate out of it, the certificat doesn’t show any of these entries (like in your first screenshot)
    Where I’m wrong?

    My Code
    openssl req -new -key wikiCERT-key.pem -out certificate.csr -config opensslWiki.cnf
    openssl x509 -req -in certificate.csr -CA servoCA-root.pem -CAkey servoCA-key.pem -CAcreateserial -out wikiCERT-pub.pem -days 365 -sha512

    Reply
    • If your CSR shows all the hostnames then that should be sufficient for creating a SAN certificate. The first screenshot is just an example to understand how companies like Facebook is also using SAN for their certificates.

      Reply
        • Apologies for not finding this earlier. I am sure you must have found a solution by now. But since I recently encountered this myself, I have updated the article with the solution.

          This seems to be a known “BUG” or “expected behaviour” from OpenSSL

          Reply

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