Administrators and sometimes end users can configure whether client connections are rejected if any or some server certificate checks fail.

Certificate checking occurs for SSL connections between Connection Server and Horizon Client. Certificate verification includes the following checks:

  • Is the certificate intended for a purpose other than verifying the identity of the sender and encrypting server communications? That is, is it the correct type of certificate?
  • Has the certificate expired, or is it valid only in the future? That is, is the certificate valid according to the computer clock?
  • Does the common name on the certificate match the host name of the server that sends it? A mismatch can occur if a load balancer redirects Horizon Client to a server that has a certificate that does not match the host name entered in Horizon Client. Another reason a mismatch can occur is if you enter an IP address rather than a host name in the client.
  • Is the certificate signed by an unknown or untrusted certificate authority (CA)? Self-signed certificates are one type of untrusted CA.

    To pass this check, the certificate's chain of trust must be rooted in the device's local certificate store.

Note: For information about distributing a self-signed root certificate that users can install on their Linux client systems, see the Ubuntu documentation.

Horizon Client uses the PEM-formatted certificates stored in the /etc/ssl/certs directory on the client system. For information about importing a root certificate stored in this location, see "Importing a Certificate into the System-Wide Certificate Authority Database" in the document at

In addition to presenting a server certificate, Connection Server also sends a certificate thumbprint to Horizon Client. The thumbprint is a hash of the certificate public key and is used as an abbreviation of the public key. If Connection Server does not send a thumbprint, you see a warning that the connection is untrusted.

If your administrator has allowed it, you can set the certificate checking mode. Select File > Preferences from the menu bar. You have three choices:

  • Never connect to untrusted servers. If any of the certificate checks fails, the client cannot connect to the server. An error message lists the checks that failed.
  • Warn before connecting to untrusted servers. If a certificate check fails because the server uses a self-signed certificate, you can click Continue to ignore the warning. For self-signed certificates, the certificate name is not required to match the server name you entered in Horizon Client.
  • Do not verify server identity certificates. This setting means that no certificate checking occurs.