Creates SAN certificate requests with OpenSSL without any file/configuration changes

openssl-san.conf

########################################
# Usage:
#
# SAN Certificate:
# > export OPENSSL_SAN=DNS:a.example.com,DNS:b.example.com,DNS:c.example.com
# > openssl req -reqexts v3_req_san -new -newkey rsa:2048 -out tmp.csr -keyout tmp.key (-subj "<subject>")
#
# Non-SAN Certificate:
# > openssl req -new -newkey rsa:2048 -out tmp.csr -keyout tmp.key (-subj "<subject>")
########################################

# ... all other OpenSSL configuration goes here ...

[ v3_req ]
basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment

[ v3_req_san ]
basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment
subjectAltName = ${ENV::OPENSSL_SAN}