Generate self-signed certificates

    If you build Flatcar Container Linux cluster on top of public networks it is recommended to enable encryption for Flatcar Container Linux services to prevent traffic interception and man-in-the-middle attacks. For these purposes you have to use Certificate Authority (CA), private keys and certificates signed by CA. Let’s use cfssl and walk through the whole process to create all these components.

    NOTE: We will use basic procedure here. If your configuration requires advanced security options, please refer to official cfssl documentation.

    Download cfssl

    CloudFlare’s distributes cfssl source code on github page and binaries on cfssl website .

    Our documentation assumes that you will run cfssl on your local x86_64 Linux host.

    mkdir ~/bin
    curl -s -L -o ~/bin/cfssl https://pkg.cfssl.org/R1.2/cfssl_linux-amd64
    curl -s -L -o ~/bin/cfssljson https://pkg.cfssl.org/R1.2/cfssljson_linux-amd64
    chmod +x ~/bin/{cfssl,cfssljson}
    export PATH=$PATH:~/bin
    

    Initialize a certificate authority

    First of all we have to save default cfssl options for future substitutions:

    mkdir ~/cfssl
    cd ~/cfssl
    cfssl print-defaults config > ca-config.json
    cfssl print-defaults csr > ca-csr.json
    

    Certificate types which are used inside Flatcar Container Linux

    • client certificate is used to authenticate client by server. For example etcdctl, etcd proxy, or docker clients.
    • server certificate is used by server and verified by client for server identity. For example docker server or kube-apiserver.
    • peer certificate is used by etcd cluster members as they communicate with each other in both ways.

    Configure CA options

    Now we can configure signing options inside ca-config.json config file. Default options contain following preconfigured fields:

    • profiles: www with server auth (TLS Web Server Authentication) X509 V3 extension and client with client auth (TLS Web Client Authentication) X509 V3 extension.
    • expiry: with 8760h default value (or 365 days)

    For compliance let’s rename www profile into server, create additional peer profile with both server auth and client auth extensions, and set expiry to 43800h (5 years):

    {
        "signing": {
            "default": {
                "expiry": "43800h"
            },
            "profiles": {
                "server": {
                    "expiry": "43800h",
                    "usages": [
                        "signing",
                        "key encipherment",
                        "server auth"
                    ]
                },
                "client": {
                    "expiry": "43800h",
                    "usages": [
                        "signing",
                        "key encipherment",
                        "client auth"
                    ]
                },
                "peer": {
                    "expiry": "43800h",
                    "usages": [
                        "signing",
                        "key encipherment",
                        "server auth",
                        "client auth"
                    ]
                }
            }
        }
    }
    

    You can also modify ca-csr.json Certificate Signing Request (CSR):

    {
        "CN": "My own CA",
        "key": {
            "algo": "rsa",
            "size": 2048
        },
        "names": [
            {
                "C": "US",
                "L": "CA",
                "O": "My Company Name",
                "ST": "San Francisco",
                "OU": "Org Unit 1",
                "OU": "Org Unit 2"
            }
        ]
    }
    

    And generate CA with defined options:

    cfssl gencert -initca ca-csr.json | cfssljson -bare ca -
    

    You’ll get following files:

    ca-key.pem
    ca.csr
    ca.pem
    
    • Please keep ca-key.pem file in safe. This key allows to create any kind of certificates within your CA.
    • *.csr files are not used in our example.

    Generate server certificate

    cfssl print-defaults csr > server.json
    

    Most important values for server certificate are Common Name (CN) and hosts. We have to substitute them, for example:

    ...
        "CN": "coreos1",
        "hosts": [
            "192.168.122.68",
            "ext.example.com",
            "coreos1.local",
            "coreos1"
        ],
    ...
    

    Now we are ready to generate server certificate and private key:

    cfssl gencert -ca=ca.pem -ca-key=ca-key.pem -config=ca-config.json -profile=server server.json | cfssljson -bare server
    

    Or without CSR json file:

    echo '{"CN":"coreos1","hosts":[""],"key":{"algo":"rsa","size":2048}}' | cfssl gencert -ca=ca.pem -ca-key=ca-key.pem -config=ca-config.json -profile=server -hostname="192.168.122.68,ext.example.com,coreos1.local,coreos1" - | cfssljson -bare server
    

    You’ll get following files:

    server-key.pem
    server.csr
    server.pem
    

    Generate peer certificate

    cfssl print-defaults csr > member1.json
    

    Substitute CN and hosts values, for example:

    ...
        "CN": "member1",
        "hosts": [
            "192.168.122.101",
            "ext.example.com",
            "member1.local",
            "member1"
        ],
    ...
    

    Now we are ready to generate member1 certificate and private key:

    cfssl gencert -ca=ca.pem -ca-key=ca-key.pem -config=ca-config.json -profile=peer member1.json | cfssljson -bare member1
    

    Or without CSR json file:

    echo '{"CN":"member1","hosts":[""],"key":{"algo":"rsa","size":2048}}' | cfssl gencert -ca=ca.pem -ca-key=ca-key.pem -config=ca-config.json -profile=peer -hostname="192.168.122.101,ext.example.com,member1.local,member1" - | cfssljson -bare member1
    

    You’ll get following files:

    member1-key.pem
    member1.csr
    member1.pem
    

    Repeat these steps for each etcd member hostname.

    Generate client certificate

    cfssl print-defaults csr > client.json
    

    For client certificate we can ignore hosts values and set only Common Name (CN) to client value:

    ...
        "CN": "client",
        "hosts": [""],
    ...
    

    Generate client certificate:

    cfssl gencert -ca=ca.pem -ca-key=ca-key.pem -config=ca-config.json -profile=client client.json | cfssljson -bare client
    

    Or without CSR json file:

    echo '{"CN":"client","hosts":[""],"key":{"algo":"rsa","size":2048}}' | cfssl gencert -ca=ca.pem -ca-key=ca-key.pem -config=ca-config.json -profile=client - | cfssljson -bare client
    

    You’ll get following files:

    client-key.pem
    client.csr
    client.pem
    

    TLDR

    Download binaries

    mkdir ~/bin
    curl -s -L -o ~/bin/cfssl https://pkg.cfssl.org/R1.2/cfssl_linux-amd64
    curl -s -L -o ~/bin/cfssljson https://pkg.cfssl.org/R1.2/cfssljson_linux-amd64
    chmod +x ~/bin/{cfssl,cfssljson}
    export PATH=$PATH:~/bin
    

    Create directory to store certificates

    mkdir ~/cfssl
    cd ~/cfssl
    

    Generate CA and certificates

    echo '{"CN":"CA","key":{"algo":"rsa","size":2048}}' | cfssl gencert -initca - | cfssljson -bare ca -
    echo '{"signing":{"default":{"expiry":"43800h","usages":["signing","key encipherment","server auth","client auth"]}}}' > ca-config.json
    export ADDRESS=192.168.122.68,ext1.example.com,coreos1.local,coreos1
    export NAME=server
    echo '{"CN":"'$NAME'","hosts":[""],"key":{"algo":"rsa","size":2048}}' | cfssl gencert -config=ca-config.json -ca=ca.pem -ca-key=ca-key.pem -hostname="$ADDRESS" - | cfssljson -bare $NAME
    export ADDRESS=
    export NAME=client
    echo '{"CN":"'$NAME'","hosts":[""],"key":{"algo":"rsa","size":2048}}' | cfssl gencert -config=ca-config.json -ca=ca.pem -ca-key=ca-key.pem -hostname="$ADDRESS" - | cfssljson -bare $NAME
    

    Verify data

    openssl x509 -in ca.pem -text -noout
    openssl x509 -in server.pem -text -noout
    openssl x509 -in client.pem -text -noout
    

    Things to know

    • Don’t put your ca-key.pem into a Container Linux Config, it is recommended to store it in safe place. This key allows to generate as much certificates as possible.
    • Keep key files in safe. Don’t forget to set proper file permissions, i.e. chmod 0600 server-key.pem.
    • Certificates in this TLDR example have both server auth and client auth X509 V3 extensions and you can use them with servers and clients’ authentication.
    • You are free to generate keys and certificates for wildcard * address as well. They will work on any machine. It will simplify certificates routine but increase security risks.

    More information

    For more examples, check out these documents: