Server Name Indication

Overview

Server Name Indication, or SNI, is a method of virtual hosting multiple domain names for an SSL enabled virtual IP. A single VIP is advertised for multiple virtual services. When a client connects to the VIP, Avi Vantage begins the SSL/TLS negotiation, but does not choose a virtual service, or an SSL certificate, until the client has requested the site by name via the TLS hello packet’s domain field. If the requested domain name is configured on the virtual IP, the appropriate certificate is returned to the client and the connection is bound to the proper virtual service.

Additional References

Configuration

Avi Vantage uses the concept of parent and child virtual services for SNI virtual hosting. When the option for virtual hosting virtual service is selected on the create (via advanced mode) or edit, the virtual service participates in the virtual hosting. The virtual hosting virtual service must be configured as either a parent or a child virtual service.

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Parent Virtual Service

The parent virtual service governs the networking properties used to negotiate TCP and SSL with the client. It may also be a catch-all if a client’s requested domain name does not exist or does not match one of the configured child virtual services.

Configure the following properties on the parent virtual service:

  • Network: The listener IP address, service port, network profile, and SSL profile. No networking properties are configured on the child virtual services.
  • Pool: Optionally specify a pool for the parent virtual service. The pool will only be used if no child virtual service matches a client’s requested domain name.
  • SSL Certificate: An SSL certificate may be configured which could either be a wildcard certificate or a specific domain name. The parent’s SSL certificate will only be used if the client’s request does not match a child virtual service domain. If an SSL certificate with specific domain name is returned to the client, as in the case of sending a friendly error message, the client will receive an SSL name mismatch message. So, it is advisable to use a wildcard on the parent.

The parent virtual service will receive all new client TCP connection handshakes, which will be reflected in the statistics. Once a child virtual service is selected, the connection is internally handed off to a child virtual service, so subsequent metrics such as packets, concurrent connections, throughput, requests, logs and other statistics will only be recorded on the child virtual service. Similarly the child virtual service will not have logs for the initial TCP or SSL handshakes, such as the SSL version mismatch errors, which are recorded at the parent virtual service.

Child Virtual Service

The child virtual service does not have an IP address or service port. Instead, it points to a parent virtual service, which must be created first. The domain name field is a fully qualified name requested by the SNI-enabled client within the SSL handshake. The parent matches the client request with the child’s configured domain name. It does not match against the configured SSL certificate. The child may use a wildcard or domain specific certificate.

If no child matches the client request, the parent’s SSL certificate and pool are used.

However, when you have a TLS SNI parent with a TLS/SSL profile that supports TLS versions 1, 1.1, and 1.2, and a TLS child which has only TLS 1.2 configured, the child will continue to use TLS 1.2.

In such a setup where the parent and child virtual services use different SSL profiles, the flow for SSL handshake is as follows:

  1. TCP handshake -> Parent virtual service
  2. Client Hello -> Parent virtual service The client Hello contains the SNI and so Avi Vantage will select the child virtual service.
  3. SSL profile of the child is used Child virtual service SSL profile is used to allow or deny based on the SSL/TLS version and select a cipher.
  4. Child virtual service responds with a server Hello that includes the cipher and the child certificate.