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> Chapter 26 - Server Load Balancing > SSL/TLS load balancing > SSL offloading

SSL/TLS offloading

Use SSL offloading to accelerate clients’ SSL or HTTPS connections to real servers by using the FortiGate unit to perform SSL/TLS operations (offloading them from the real servers using the FortiGate unit’s SSL acceleration hardware). FortiGate units can offload most versions of SSL/TLS, including SSL 3.0, TLS 1.0 and TLS 1.2. SSL/TLS offloading is available on FortiGate units that support SSL acceleration.

To configure SSL offloading from the web-based manager go to Policy & Objects > Virtual Servers. Add a virtual server and set the type to HTTPS or SSL and select the SSL offloading type (Client <-> FortiGate or Client <-> FortiGate <-> Server).

Select Client <-> FortiGate to apply hardware accelerated SSL/TLS processing only to the part of the connection between the client and the FortiGate unit. This mode is called half mode SSL offloading. The segment between the FortiGate unit and the server will use clear text communications. This results in best performance, but cannot be used in failover configurations where the failover path does not have an SSL accelerator.

Select Client <-> FortiGate <->Server to apply hardware accelerated SSL processing to both parts of the connection: the segment between client and the FortiGate unit, and the segment between the FortiGate unit and the server. This mode is called full mode SSL offloading. The segment between the FortiGate unit and the server will use encrypted communications, but the handshakes will be abbreviated. This is not as efficient as half mode SSL offloading, but still improves performance. As well full-mode SSL offloading can be used in failover configurations where the failover path does not have an SSL accelerator. If the server is already configured to use SSL, this also enables SSL acceleration without requiring changes to the server’s configuration.

SSL Offloading modes (Half Mode and Full Mode)

Configuring SSL offloading also requires selecting a certificate to use for the SSL offloading sessions. SSL offloading supports key sizes up to 4096. FortiGate models with CP9 processors support 3072 and 4096 DH bit sizes in hardware. All FortiGate models up to and including those with CP8 processors only support offloading DH bit sizes up to 2048 so any sizes larger than that are done in software and thus are relatively resource intensive

The following CLI command shows an example half mode HTTPS SSL offloading configuration. In the example the ssl-mode option sets the SSL offload mode to half (which is the default mode).

config firewall vip

edit Vserver-ssl-offload

set type server-load-balance

set server-type https

set ldb-method round-robin

set extip 172.20.120.30

set extintf wan1

set extport 443

set persistence ssl-session-id

set ssl-mode half

set ssl-certificate my-cert

set monitor tcp-mon-1

config realservers

edit 1

set ip 10.31.101.30

set port 443

next

edit 2

set ip 10.31.101.40

set port 443

end

end