Can I use one SSL Certificate on multiple sub-domains?

Question

We have these separate sites: a.example.com, b.example.com, and c.example.com. Now we want to add an SSL Certificate to these sites.

There is a pretty good tutorial for this, but I have a couple questions:

1. Can I use one SSL certificate on these three sites?

2. If the answer of 1 is yes, do I need to change the CSR generating commands for this?

list the commands below

#Must do these as root.
openssl genrsa -des3 -out example.com.key 2048
#you will be prompted to enter password
openssl rsa -in example.com.key -out example.com.key.nopass
#you will be prompted to enter password you created in last step
openssl req -new -key example.com.key.nopass -out example.com.csr

Answer 1

If the a, b and c resolves to the same ip, you can create a certificate with alias names.

E.g. add the folowing to your request:

# Instead the additional names are form entries on web
# pages where one requests the certificate...
subjectAltName          = @alt_names
[alt_names]
DNS.1_default           = a.example.com
DNS.2_default           = b.example.com
DNS.3_default           = c.example.com