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Open vSwitch with SSL

If you plan to configure Open vSwitch to connect across the network to an OpenFlow controller, then we recommend that you build Open vSwitch with OpenSSL. SSL support ensures integrity and confidentiality of the OpenFlow connections, increasing network security.

This document describes how to configure an Open vSwitch to connect to an OpenFlow controller over SSL. Refer to the general installation guide for instructions on building Open vSwitch with SSL support.

Open vSwitch uses TLS version 1.0 or later (TLSv1), as specified by RFC 2246, which is very similar to SSL version 3.0. TLSv1 was released in January 1999, so all current software and hardware should implement it.

This document assumes basic familiarity with public-key cryptography and public-key infrastructure.

SSL Concepts for OpenFlow

This section is an introduction to the public-key infrastructure architectures that Open vSwitch supports for SSL authentication.

To connect over SSL, every Open vSwitch must have a unique private/public key pair and a certificate that signs that public key. Typically, the Open vSwitch generates its own public/private key pair. There are two common ways to obtain a certificate for a switch:

  • Self-signed certificates: The Open vSwitch signs its certificate with its own private key. In this case, each switch must be individually approved by the OpenFlow controller(s), since there is no central authority.

    This is the only switch PKI model currently supported by NOX (http://noxrepo.org).

  • Switch certificate authority: A certificate authority (the "switch CA") signs each Open vSwitch's public key. The OpenFlow controllers then check that any connecting switches' certificates are signed by that certificate authority.

    This is the only switch PKI model supported by the simple OpenFlow controller included with Open vSwitch.

Each Open vSwitch must also have a copy of the CA certificate for the certificate authority that signs OpenFlow controllers' keys (the "controller CA" certificate). Typically, the same controller CA certificate is installed on all of the switches within a given administrative unit. There are two common ways for a switch to obtain the controller CA certificate:

  • Manually copy the certificate to the switch through some secure means, e.g. using a USB flash drive, or over the network with "scp", or even FTP or HTTP followed by manual verification.
  • Open vSwitch "bootstrap" mode, in which Open vSwitch accepts and saves the controller CA certificate that it obtains from the OpenFlow controller on its first connection. Thereafter the switch will only connect to controllers signed by the same CA certificate.

Establishing a Public Key Infrastructure

Open vSwitch can make use of your existing public key infrastructure. If you already have a PKI, you may skip forward to the next section. Otherwise, if you do not have a PKI, the ovs-pki script included with Open vSwitch can help. To create an initial PKI structure, invoke it as:

$ ovs-pki init

This will create and populate a new PKI directory. The default location for the PKI directory depends on how the Open vSwitch tree was configured (to see the configured default, look for the --dir option description in the output of ovs-pki --help).

The pki directory contains two important subdirectories. The controllerca subdirectory contains controller CA files, including the following:

cacert.pem
Root certificate for the controller certificate authority. Each Open vSwitch must have a copy of this file to allow it to authenticate valid controllers.
private/cakey.pem
Private signing key for the controller certificate authority. This file must be kept secret. There is no need for switches or controllers to have a copy of it.

The switchca subdirectory contains switch CA files, analogous to those in the controllerca subdirectory:

cacert.pem
Root certificate for the switch certificate authority. The OpenFlow controller must have this file to enable it to authenticate valid switches.
private/cakey.pem
Private signing key for the switch certificate authority. This file must be kept secret. There is no need for switches or controllers to have a copy of it.

After you create the initial structure, you can create keys and certificates for switches and controllers with ovs-pki. Refer to the ovs-pki(8) manage for complete details. A few examples of its use follow:

Controller Key Generation

To create a controller private key and certificate in files named ctl-privkey.pem and ctl-cert.pem, run the following on the machine that contains the PKI structure:

$ ovs-pki req+sign ctl controller

ctl-privkey.pem and ctl-cert.pem would need to be copied to the controller for its use at runtime. If, for testing purposes, you were to use ovs-testcontroller, the simple OpenFlow controller included with Open vSwitch, then the --private-key and --certificate options, respectively, would point to these files.

It is very important to make sure that no stray copies of ctl-privkey.pem are created, because they could be used to impersonate the controller.

Switch Key Generation with Self-Signed Certificates

If you are using self-signed certificates (see "SSL Concepts for OpenFlow"), this is one way to create an acceptable certificate for your controller to approve.

  1. Run the following command on the Open vSwitch itself:

    $ ovs-pki self-sign sc

    Note

    This command does not require a copy of any of the PKI files generated by ovs-pki init, and you should not copy them to the switch because some of them have contents that must remain secret for security.)

    The ovs-pki self-sign command has the following output:

    sc-privkey.pem

    the switch private key file. For security, the contents of this file must remain secret. There is ordinarily no need to copy this file off the Open vSwitch.

    sc-cert.pem

    the switch certificate, signed by the switch's own private key. Its contents are not a secret.

  2. Optionally, copy controllerca/cacert.pem from the machine that has the OpenFlow PKI structure and verify that it is correct. (Otherwise, you will have to use CA certificate bootstrapping when you configure Open vSwitch in the next step.)

  3. Configure Open vSwitch to use the keys and certificates (see "Configuring SSL Support", below).

Switch Key Generation with a Switch PKI (Easy Method)

If you are using a switch PKI (see "SSL Concepts for OpenFlow", above), this method of switch key generation is a little easier than the alternate method described below, but it is also a little less secure because it requires copying a sensitive private key from file from the machine hosting the PKI to the switch.

  1. Run the following on the machine that contains the PKI structure:

    $ ovs-pki req+sign sc switch

    This command has the following output:

    sc-privkey.pem

    the switch private key file. For security, the contents of this file must remain secret.

    sc-cert.pem

    the switch certificate. Its contents are not a secret.

  2. Copy sc-privkey.pem and sc-cert.pem, plus controllerca/cacert.pem, to the Open vSwitch.

  3. Delete the copies of sc-privkey.pem and sc-cert.pem on the PKI machine and any other copies that may have been made in transit. It is very important to make sure that there are no stray copies of sc-privkey.pem, because they could be used to impersonate the switch.

    Warning

    Don't delete controllerca/cacert.pem! It is not security-sensitive and you will need it to configure additional switches.

  4. Configure Open vSwitch to use the keys and certificates (see "Configuring SSL Support", below).

Switch Key Generation with a Switch PKI (More Secure)

If you are using a switch PKI (see "SSL Concepts for OpenFlow", above), then, compared to the previous method, the method described here takes a little more work, but it does not involve copying the private key from one machine to another, so it may also be a little more secure.

  1. Run the following command on the Open vSwitch itself:

    $ ovs-pki req sc

    Note

    This command does not require a copy of any of the PKI files generated by "ovs-pki init", and you should not copy them to the switch because some of them have contents that must remain secret for security.

    The "ovs-pki req" command has the following output:

    sc-privkey.pem

    the switch private key file. For security, the contents of this file must remain secret. There is ordinarily no need to copy this file off the Open vSwitch.

    sc-req.pem

    the switch "certificate request", which is essentially the switch's public key. Its contents are not a secret.

    a fingerprint

    this is output on stdout.

  2. Write the fingerprint down on a slip of paper and copy sc-req.pem to the machine that contains the PKI structure.

  3. On the machine that contains the PKI structure, run:

    $ ovs-pki sign sc switch

    This command will output a fingerprint to stdout and request that you verify it. Check that it is the same as the fingerprint that you wrote down on the slip of paper before you answer "yes".

    ovs-pki sign creates a file named sc-cert.pem, which is the switch certificate. Its contents are not a secret.

  4. Copy the generated sc-cert.pem, plus controllerca/cacert.pem from the PKI structure, to the Open vSwitch, and verify that they were copied correctly.

    You may delete sc-cert.pem from the machine that hosts the PKI structure now, although it is not important that you do so.

    Warning

    Don't delete controllerca/cacert.pem! It is not security-sensitive and you will need it to configure additional switches.

  5. Configure Open vSwitch to use the keys and certificates (see "Configuring SSL Support", below).

Configuring SSL Support

SSL configuration requires three additional configuration files. The first two of these are unique to each Open vSwitch. If you used the instructions above to build your PKI, then these files will be named sc-privkey.pem and sc-cert.pem, respectively:

  • A private key file, which contains the private half of an RSA or DSA key.

    This file can be generated on the Open vSwitch itself, for the greatest security, or it can be generated elsewhere and copied to the Open vSwitch.

    The contents of the private key file are secret and must not be exposed.

  • A certificate file, which certifies that the private key is that of a trustworthy Open vSwitch.

    This file has to be generated on a machine that has the private key for the switch certification authority, which should not be an Open vSwitch; ideally, it should be a machine that is not networked at all.

    The certificate file itself is not a secret.

The third configuration file is typically the same across all the switches in a given administrative unit. If you used the instructions above to build your PKI, then this file will be named cacert.pem:

  • The root certificate for the controller certificate authority. The Open vSwitch verifies it that is authorized to connect to an OpenFlow controller by verifying a signature against this CA certificate.

Once you have these files, configure ovs-vswitchd to use them using the ovs-vsctl set-ssl command, e.g.:

$ ovs-vsctl set-ssl /etc/openvswitch/sc-privkey.pem \
    /etc/openvswitch/sc-cert.pem /etc/openvswitch/cacert.pem

Substitute the correct file names, of course, if they differ from the ones used above. You should use absolute file names (ones that begin with /), because ovs-vswitchd's current directory is unrelated to the one from which you run ovs-vsctl.

If you are using self-signed certificates (see "SSL Concepts for OpenFlow") and you did not copy controllerca/cacert.pem from the PKI machine to the Open vSwitch, then add the --bootstrap option, e.g.:

$ ovs-vsctl -- --bootstrap set-ssl /etc/openvswitch/sc-privkey.pem \
    /etc/openvswitch/sc-cert.pem /etc/openvswitch/cacert.pem

After you have added all of these configuration keys, you may specify ssl: connection methods elsewhere in the configuration database. tcp: connection methods are still allowed even after SSL has been configured, so for security you should use only ssl: connections.

Reporting Bugs

Report problems to [email protected].