A CSR with multiple SAN would require you to install openssl which you can download from here
Create the below files and save it as “req.conf”
[ req ]
default_bits = 2048
default_keyfile = privatekey.pem
distinguished_name = req_distinguished_name
req_extensions = req_ext # The extentions to add to the self signed cert
[ req_distinguished_name ]
countryName = Country Name (2 letter code)
countryName_default = SG
stateOrProvinceName = State or Province Name (full name)
stateOrProvinceName_default = Singapore
localityName = Locality Name (eg, city)
localityName_default = Singapore
organizationName = Organization Name (eg, company)
organizationName_default = Fortiedge Pte. Ltd.
commonName = Common Name (eg, YOUR name)
commonName_max = 128
[ req_ext ]
subjectAltName = @alt_names
DNS.1 = server1.fortiedge.com
DNS.2 = server2.fortiedge.com
DNS.3 = www.otherdomain.com
You will need to set your alt_names section to the FQDNs you wish to use. You do not need to repeat the CN here, but input other DNS name. If you need more simply add “DNS.4 = domain3.com” and so on. Once you have done that, save the file as “req.conf” and save the file in opensslbin and run the below commands.
openssl req -new -nodes -out myreq.csr -config req.conf
Generating a 2048 bit RSA private key
writing new private key to ‘privatekey.pem’
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter ‘.’, the field will be left blank.
Country Name (2 letter code) [SG]:SG
State or Province Name (full name) [Singapore]: Singapore
Locality Name (eg, city) [Singapore]:Singapore
Organization Name (eg, company) [Company]:Fortiedge Pte. Ltd
Common Name (eg, YOUR name) :www.fortiedge.com
You now have a “myreq.csr” and a “privatekey.pem” associated with the CSR. You can now submit this CSR to a CA.
You can find the detailed syntax here