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Encrypting Hosts with IPsec

Overview

IPsec protects traffic in an OpenShift Container Platform cluster by encrypting the communication between all master and node hosts that communicate using the Internet Protocol (IP).

This topic shows how to secure communication of an entire IP subnet from which the OpenShift Container Platform hosts receive their IP addresses, including all cluster management and pod data traffic.

Because OpenShift Container Platform management traffic uses HTTPS, enabling IPsec encrypts management traffic a second time.

This procedure should be repeated on each master host, then node host, in your cluster. Hosts that do not have IPsec enabled will not be able to communicate with a host that does.

Encrypting Hosts

Step 1: Prerequisites

At this time, libreswan version 3.15 is the latest version supported on Red Hat Enterprise Linux 7. Ensure that libreswan 3.15 or later is installed on cluster hosts. If opportunistic group functionality is required, then libreswan version 3.19 or later is required.

Configure the SDN MTU to allow space for the IPSec header. In the configuration described here IPSec requires 62 bytes. If the cluster is operating on an ethernet network with an MTU of 1500 then the SDN MTU should be 1388, to allow for the overhead of IPSec and the SDN encapsulation.

Step 2: Certificates

By default, OpenShift Container Platform secures cluster management communication with mutually authenticated HTTPS communication. This means that both the client (for example, an OpenShift Container Platform node) and the server (for example, an OpenShift Container Platform api-server) send each other their certificates, which are checked against a known certificate authority (CA). These certificates are generated at cluster set up time, and typically live on each host.

These certificates can also be used to secure pod communications with IPsec. You need three files on each host:

  • Cluster CA file

  • Host client certificate file

  • Host private key file

    1. Determine what the certificate’s nickname will be after it has been imported into the libreswan certificate database. The nickname is taken directly from the certificate’s subject’s Common Name (CN):

      # openssl x509 \
  -in /path/to/client-certificate -subject -noout | \
  sed -n 's/.*CN=\(.*\)/\1/p'

    2. Use openssl to combine the client certificate, CA certificate, and private key files into a PKCS#12 file, which is a common file format for multiple certificates and keys:

      # openssl pkcs12 -export \
  -in /path/to/client-certificate \
  -inkey /path/to/private-key \
  -certfile /path/to/certificate-authority \
  -passout pass: \
  -out certs.p12

    3. Import the PKCS#12 file into the libreswan certificate database. The -W option is left empty, because no password is assigned to the PKCS#12 file, as it is only temporary.

      # ipsec initnss
# pk12util -i certs.p12 -d sql:/etc/ipsec.d -W ""
# rm certs.p12

Step 3: libreswan IPsec Policy

Now that the necessary certificates are imported into the libreswan certificate database, create a policy that uses them to secure communication between hosts in your cluster.

If you are using libreswan 3.19 or later, then opportunistic group configuration is recommended. Otherwise, explicit connections are required.

Opportunistic Group Configuration

The following configuration creates two libreswan connections. The first encrypts traffic using the OpenShift Container Platform certificates, while the second creates exceptions to the encryption for cluster-external traffic.

  1. Place the following into the /etc/ipsec.d/openshift-cluster.conf file:

    conn private
	left=%defaultroute
	leftid=%fromcert
	# our certificate
	leftcert="NSS Certificate DB:<cert_nickname>" (1)
	right=%opportunisticgroup
	rightid=%fromcert
	# their certificate transmitted via IKE
	rightca=%same
	ikev2=insist
	authby=rsasig
	failureshunt=drop
	negotiationshunt=hold
	auto=ondemand

conn clear
	left=%defaultroute
	right=%group
	authby=never
	type=passthrough
	auto=route
	priority=100
    1 Replace <cert_nickname> with the certificate nickname from step one.

  2. Tell libreswan which IP subnets and hosts to apply each policy using policy files in /etc/ipsec.d/policies/, where each configured connection has a corresponding policy file. So, in the example above, the two connections, private and clear, each have a file in /etc/ipsec.d/policies/.

    /etc/ipsec.d/policies/private should contain the IP subnet of your cluster, which your hosts receive IP addresses from. By default, this causes all communication between hosts in the cluster subnet to be encrypted if the remote host’s client certificate authenticates against the local host’s Certificate Authority certificate. If the remote host’s certificate does not authenticate, all traffic between the two hosts will be blocked.

    For example, if all hosts are configured to use addresses in the 172.16.0.0/16 address space, your private policy file would contain 172.16.0.0/16. Any number of additional subnets to encrypt may be added to this file, which results in all traffic to those subnets using IPsec as well.

  3. Unencrypt the encryption between all hosts and the subnet gateway to ensure that traffic can enter and exit the cluster. Add the gateway to the /etc/ipsec.d/policies/clear file:

    172.16.0.1/32

    Additional hosts and subnets may be added to this file, which will result in all traffic to these hosts and subnets being unencrypted.

Explicit Connection Configuration

In this configuration, each IPSec node configuration must explicitly list the configuration of every other node in the cluster. Using a configuration management tool such as Ansible to generate this file on each host is recommended.

  1. Place the following lines into the /etc/ipsec.d/openshift-cluster.conf file on each node for every other node in the cluster:

    conn <other_node_hostname>
        left=<this_node_ip> (1)
        leftid="CN=<this_node_cert_nickname>" (2)
        leftrsasigkey=%cert
        leftcert=<this_node_cert_nickname> (2)
        right=<other_node_ip> (3)
        rightid="CN=<other_node_cert_nickname>" (4)
        rightrsasigkey=%cert
        auto=start
        keyingtries=%forever
    1 Replace <this_node_ip> with the cluster IP address of this node.
    2 Replace <this_node_cert_nickname> with the node certificate nickname from step one.
    3 Replace <other_node_ip> with the cluster IP address of the other node.
    4 Replace <other_node_cert_nickname> with the other node certificate nickname from step one.

  2. Place the following in the /etc/ipsec.d/openshift-cluster.secrets file on each node:

    : RSA "<this_node_cert_nickname>" (1)
    1 Replace <this_node_cert_nickname> with the node certificate nickname from step one.

IPSec Firewall Configuration

All nodes within the cluster need to allow IPSec related network traffic. This includes IP protocol numbers 50 and 51 as well as UDP port 500.

For example, if the cluster nodes communicate over interface eth0:

-A OS_FIREWALL_ALLOW -i eth0 -p 50 -j ACCEPT
-A OS_FIREWALL_ALLOW -i eth0 -p 51 -j ACCEPT
-A OS_FIREWALL_ALLOW -i eth0 -p udp --dport 500 -j ACCEPT

IPSec also uses UDP port 4500 for NAT traversal, though this should not apply to normal cluster deployments.

Starting and Enabling IPSec

  1. Start the ipsec service to load the new configuration and policies, and begin encrypting:

    # systemctl start ipsec

  2. Enable the ipsec service to start on boot:

    # systemctl enable ipsec

Troubleshooting

When authentication cannot be completed between two hosts, you will not be able to ping between them, because all IP traffic will be rejected. If the clear policy is not configured correctly, you will also not be able to SSH to the host from another host in the cluster.

You can use the ipsec status command to check that the clear and private policies have been loaded.