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We have a load balancer facing the Internet.

It's balancing load towards two systems behind it in the back-end running apache.

How many SSL certificates do I need to buy? - One for the load-balancer only - One for the load-balancer and for each server instance one (3 certs)+ - Only for the server instances (2 certs) - The certificates can be re-used

Also, can I re-use the server certificate as a client certificate to make calls to other backends from the apache servers?

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    Terminate SSL at the load balancers. If you don't trust the network, use mutually-authenticated TLS with self-signed certs between the load balancers and app servers. – Stephen Touset Apr 25 '14 at 0:22
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The point of a load-balancer is that all your servers appear to the client as one. So no matter where you handle the TLS (on the load-balancer or on the application servers), you will only need one certificate to ensure that no web browsers will show any HTTPS warnings.

When clients communicate only with the load-balancer (which handles TLS) and you control the network between the load-balancer and the backend servers, there is little reason to encrypt that connection too. But when you do not trust the network and want to use encryption between load-balancer and backend servers, you can use self-signed certificates for that. When you put the certificate on the servers yourself, you know they are trustworthy. So paying a 3rd party to sign them would be pretty superfluous.

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There are Virtual IP addresses used by Load Balancers, performing internal routing to the appropriate pools and nodes on the back end. Every Virtual IP address will need a SSL/TLS certificate, meaning that you may have a virtual IP address on the Load balancer pointing to www.gojack.com -this requires one certificate.

For www.gobrian.com, another cert will be needed.

Virtual IP(gojack.com)---> Jack Pool (Load balancing) --to--> Node1, Node2, Node3

The point is you do not need to have another certificate for every server at the back-end which is node1, node2, node3 in this context. However, a new certificate will be needed for every new website (virtual IP in this case).

Rgds.

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