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NOTE: Commands to be updated according to the application you are using to generate the certificate. In this example we are using openssl to generate the certificates.

The Root Certificate (aka CA File) will be used to sign and identify your certificate. To generate it, run the command below.

$ openssl req -nodes -out ca.pem -new -x509 -keyout ca.key

Keep the root certificate and its key carefully, both will be used to sign your certificates. The root certificate might be used by your client as well.

When generating the Certificate Signing Request (aka CSR), input the exact hostname (or IP) of your node in the Common Name (aka CN) field. The others fields must have exactly the same value. You might need to modify your /etc/hosts file.

The commands below will generate the CSR files and the RSA Private Keys (4096 bits).

$ openssl req -nodes -newkey rsa:4096 -sha256 -keyout mongodb_node_1.key -out mongodb_node_1.csr
$ openssl req -nodes -newkey rsa:4096 -sha256 -keyout mongodb_node_2.key -out mongodb_node_2.csr
$ openssl req -nodes -newkey rsa:4096 -sha256 -keyout mongodb_node_3.key -out mongodb_node_3.csr

You must generate one CSR for each node of your ReplicaSet. Remember that the Common Name is not the same from one node to another. Don't base multiple CSRs on the same Private Key.

You must now have 3 CSRs and 3 Private Keys.

$ ls
mongodb_node_1.key - mongodb_node_2.key - mongodb_node_3.key
mongodb_node_1.csr - mongodb_node_2.csr - mongodb_node_3.csr

Use the CA File (ca.pem) and its Private Key (ca.key) generated previously to sign each Certificate Request by running the commands below.

$ openssl x509 -req -in mongodb_node_1.csr -CA ca.pem -CAkey ca.key -set_serial 00 -out mongodb_node_1.crt
$ openssl x509 -req -in mongodb_node_2.csr -CA ca.pem -CAkey ca.key -set_serial 00 -out mongodb_node_2.crt
$ openssl x509 -req -in mongodb_node_3.csr -CA ca.pem -CAkey ca.key -set_serial 00 -out mongodb_node_3.crt

You must sign each CSR.

Your must now have 3 CSRs, 3 Private Keys and 3 self-signed Certificates. Only the Private Keys and the Certificates will be used by MongoDB.

$ ls
mongodb_node_1.key - mongodb_node_2.key - mongodb_node_3.key
mongodb_node_1.csr - mongodb_node_2.csr - mongodb_node_3.csr
mongodb_node_1.crt - mongodb_node_2.crt - mongodb_node_3.crt

Each certificate corresponds to one node. Remember carefully which CN / hostname your gave to each CSR.

Run the commands below to concat each Node Certificate with its key in one file (MongoDB requirement).

$ cat mongodb_node_1.key mongodb_node_1.crt > mongodb_node_1.pem
$ cat mongodb_node_2.key mongodb_node_2.crt > mongodb_node_2.pem
$ cat mongodb_node_3.key mongodb_node_3.crt > mongodb_node_3.pem

Your must now have 3 PEM files.

$ ls
mongodb_node_1.pem - mongodb_node_2.pem - mongodb_node_3.pem

We will assume that your pem files are located in your current folder as well as data/data1, data/data2 and data/data3. Run the commands below to deploy your 3 Nodes ReplicaSet listening on port 27017, 27018 and 27019.

$ mongod --dbpath data/data_1 --replSet rs0 --port 27017 --sslMode requireSSL --sslPEMKeyFile mongodb_node_1.pem
$ mongod --dbpath data/data_2 --replSet rs0 --port 27018 --sslMode requireSSL --sslPEMKeyFile mongodb_node_2.pem
$ mongod --dbpath data/data_3 --replSet rs0 --port 27019 --sslMode requireSSL --sslPEMKeyFile mongodb_node_3.pem

You now have a 3 Nodes ReplicaSet deployed on your local environment and all their transactions are encrypted. You cannot connect to this ReplicaSet without using TLS.

To force your client to provide a Client Certificate (Mutual SSL), you must add the CA File when running your instances.

$ mongod --dbpath data/data_1 --replSet rs0 --port 27017 --sslMode requireSSL --sslPEMKeyFile mongodb_node_1.pem --sslCAFile ca.pem
$ mongod --dbpath data/data_2 --replSet rs0 --port 27018 --sslMode requireSSL --sslPEMKeyFile mongodb_node_2.pem --sslCAFile ca.pem
$ mongod --dbpath data/data_3 --replSet rs0 --port 27019 --sslMode requireSSL --sslPEMKeyFile mongodb_node_3.pem --sslCAFile ca.pem

You now have a 3 Nodes ReplicaSet deployed on your local environment and all their transactions are encrypted. You cannot connect to this ReplicaSet without using TLS or without providing a Client Certificate trusted by your CA.

In this example, we might use the CA File (ca.pem) that you generated during the "How to configure a ReplicaSet to support TLS/SSL?" section. We will assume that the CA file is located in your current folder.

We will assume that your 3 nodes are running on mongo01:27017, mongo02:27018 and mongo03:27019. (You might need to modify your /etc/hosts file.)

From MongoDB 3.2.6, if your CA File is registered in your Operating System Trust Store, you can connect to your ReplicaSet without providing the CA File.

$ mongo --ssl --host rs0/mongo01:27017,mongo02:27018,mongo03:27019

Otherwise you must provide the CA File.

$ mongo --ssl --sslCAFile ca.pem --host rs0/mongo01:27017,mongo02:27018,mongo03:27019

You are now connected to your ReplicaSet and all the transactions between your Mongo Shell and your ReplicaSet are encrypted.

If your ReplicaSet asks for a Client Certificate, you must provide one signed by the CA used by the ReplicaSet Deployment.
The steps to generate the Client Certificate are almost the same as the ones to generate the Server Certificate.

Indeed, you just need to modify the Common Name Field during the CSR creation.
Instead of providing 1 Node Hostname in the Common Name Field, you need to provide all the ReplicaSet Hostnames separated by a comma.

$ openssl req -nodes -newkey rsa:4096 -sha256 -keyout mongodb_client.key -out mongodb_client.csr
...
Common Name (e.g. server FQDN or YOUR name) []: mongo01,mongo02,mongo03
...

You might face the Common Name size limitation if the Common Name field is too long (more than 64 bytes long). To bypass this limitation, you must use the SubjectAltName when generating the CSR.

$ openssl req -nodes -newkey rsa:4096 -sha256 -keyout mongodb_client.key -out mongodb_client.csr -config <(
cat <<-EOF
[req]
default_bits = 4096
prompt = no
default_md = sha256
req_extensions = req_ext
distinguished_name = dn

[ dn ]
CN = .

[ req_ext ]
subjectAltName = @alt_names

[ alt_names ]
DNS.1 = mongo01
DNS.2 = mongo02
DNS.3 = mongo03
EOF
)

Then you sign the CSR using the CA certificate and key.

$ openssl x509 -req -in mongodb_client.csr -CA ca.pem -CAkey ca.key -set_serial 00 -out mongodb_client.crt

Finally, you concat the key and the signed certificate.

$ cat mongodb_client.key mongodb_client.crt > mongodb_client.pem

To connect to your ReplicaSet, you can now provide the newly generated Client Certificate.

$ mongo --ssl --sslCAFile ca.pem --host rs0/mongo01:27017,mongo02:27018,mongo03:27019 --sslPEMKeyFile mongodb_client.pem

You are now connected to your ReplicaSet and all the transactions between your Mongo Shell and your ReplicaSet are encrypted.