Load Balancing Multiple Web Servers with Separate SSL Certificates


With SSL certificates, there is the concept of a public key and a private key.  When looking at traffic sent from the client to the web server, the public key is used to encrypt data and the private key is used to decrypt data.  The client gets the public key from the SSL certificate.  The private key is a highly protected value stored on the web server.  If someone was able to obtain the private key, they could pretend to be the web server and successfully decrypt traffic that was supposed to be sent to the web server.  For this reason, the private key needs to be carefully protected.

When load balancing web servers that use SSL, the natural thought is that the same private key must exist on each load balanced web server since the web server functionality should be identical.  This can create a security concern because, ideally, the private key should never leave the server that it was configured on.  There is a method for allowing each web server to have its own SSL certificate and associated public/private key pair.  This method uses the “subject alternative name” field of an SSL certificate.  This field allows multiple SSL certificates to handle the same fully qualified domain name(FQDN).  For example if the website https://www.netcraftsmen.net was load balanced by three web servers, each web server could have its own SSL certificate that was populated with a unique CN name and associated public/private key pair.  Each of the SSL certificates would have the same subject alternative name.  This field would be populated with the name “www.netcraftsmen.net”.  End users would access https://www.netcraftsmen.net and each web server would be able to handle the SSL requests as long as the subject alternative name had “www.netcraftsmen.net” populated in the field.

Leave a Reply


Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

CCDP, CCNA V, CCNP, Cisco IPS Express Security for AM/EE
Field Solutions Architect, Tech Data

Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


John Cavanaugh

CCIE #1066, CCDE #20070002, CCAr
Chief Technology Officer, Practice Lead Security Services, NetCraftsmen

John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.