How to generate a CSR with OpenSSL

One openssl req -new -newkey command creates both the private key and the certificate signing request (CSR), with the names in -addext "subjectAltName=...". Check the CSR with openssl req -verify, then paste it into the CA's order form.

The commands apply to OpenSSL 3.x on Linux, macOS and Windows; notes cover OpenSSL 1.1.1 where options differ.

Prerequisites

You need OpenSSL and the exact host names the certificate must cover.

• OpenSSL 3.x (openssl version). OpenSSL 1.1.1 works with -nodes instead of -noenc; 1.0.2 lacks -addext and should be upgraded.
• The list of names: every host name visitors use, for example example.com and www.example.com.
• A secure place to keep the private key; generate it on the server that will use it when you can.

Step 1: Generate an ECDSA key and CSR

This command writes an unencrypted P-256 key and a CSR for example.com and www.example.com:

openssl req -new -newkey ec -pkeyopt ec_paramgen_curve:prime256v1 -noenc \
  -keyout example.com.key -out example.com.csr \
  -subj "/CN=example.com" \
  -addext "subjectAltName=DNS:example.com,DNS:www.example.com"
chmod 600 example.com.key

Step 2 (alternative): Generate an RSA key and CSR

An RSA request uses the same command with an rsa:2048 key instead of the EC options:

openssl req -new -newkey rsa:2048 -noenc \
  -keyout example.com.key -out example.com.csr \
  -subj "/CN=example.com" \
  -addext "subjectAltName=DNS:example.com,DNS:www.example.com"
chmod 600 example.com.key

Use rsa:3072 if your policy requires it. For a wildcard, list DNS:*.example.com,DNS:example.com.

Step 3: Use a config file for many names

A config file keeps long SAN lists and repeatable requests out of the command line:

# example.com.cnf
[req]
prompt = no
distinguished_name = dn
req_extensions = ext

[dn]
CN = example.com

[ext]
subjectAltName = DNS:example.com,DNS:www.example.com,DNS:api.example.com

openssl req -new -newkey ec -pkeyopt ec_paramgen_curve:prime256v1 -noenc \
  -keyout example.com.key -out example.com.csr -config example.com.cnf

Step 4: Submit the CSR

Copy the whole file, including the -----BEGIN CERTIFICATE REQUEST----- and -----END CERTIFICATE REQUEST----- lines, into your CA's order form:

cat example.com.csr

Keep example.com.key on the server. The CA never needs it. If you prefer a form, the CSR generator creates the key and CSR in your browser without uploading the key.

Verify

A valid CSR passes the signature check and lists every name you need under Subject Alternative Name. Check the signature, subject and SANs:

openssl req -in example.com.csr -noout -verify -subject
openssl req -in example.com.csr -noout -text | grep -A1 "Subject Alternative Name"

Confirm the key and CSR belong together (the two hashes must match):

openssl pkey -in example.com.key -pubout | openssl sha256
openssl req -in example.com.csr -noout -pubkey | openssl sha256

The CSR decoder and key matcher do the same checks in the browser. Once the certificate is installed, verify the live site with the SSL checker or curl ssl.sh/example.com.

Automatic renewal

A CSR is a one-off artefact; renewal means a new CSR. Public certificates are limited to 200 days since March 15, 2026, 100 days from March 15, 2027 and 47 days from March 15, 2029, so a manual "generate CSR, paste, wait, install" cycle will soon run every few weeks. Where possible, let an ACME client generate keys and CSRs for you: Certbot, acme.sh and lego create a fresh key on every renewal, and many commercial CAs accept ACME with External Account Binding. If you must stay manual, script Steps 1 to 4 and add the host to ssl.sh monitoring (in development).

Troubleshooting

• req: Unknown option or cipher: noenc: you are on OpenSSL 1.1.1; use -nodes.
• CA says the CSR has no SAN or the wrong names: the -addext was dropped or mistyped; decode the CSR and regenerate.
• Browser reports a name mismatch after install: a host name was missing from the SAN list; see NET::ERR_CERT_COMMON_NAME_INVALID.
• Server reports key values mismatch: the certificate was issued for a different CSR/key; see nginx key values mismatch.
• CA rejects the key: RSA keys below 2048 bits and unsupported curves are refused; use P-256 or RSA 2048+.