SSL Proxy Load Balancing overview

SSL Proxy Load Balancing is a reverse proxy load balancer that distributes SSL traffic coming from the internet to virtual machine (VM) instances in your Google Cloud VPC network.

When using SSL Proxy Load Balancing for your SSL traffic, user SSL (TLS) connections are terminated at the load balancing layer, and then proxied to the closest available backend instances by using either SSL (recommended) or TCP. For the types of backends that are supported, see Backends.

With the Premium Tier, SSL Proxy Load Balancing can be configured as a global load balancing service. With Standard Tier, the SSL proxy load balancer handles load balancing regionally. For details, see Load balancer behavior in Network Service Tiers.

In this example, traffic from users in Iowa and Boston is terminated at the load balancing layer, and a separate connection is established to the selected backend.

Cloud Load Balancing with SSL termination (click to enlarge)
Cloud Load Balancing with SSL termination (click to enlarge)

SSL Proxy Load Balancing is intended for non-HTTP(S) traffic. For HTTP(S) traffic, we recommend that you use HTTP(S) Load Balancing.

For information about how the Google Cloud load balancers differ from each other, see the following documents:


Following are some benefits of using SSL Proxy Load Balancing:

  • IPv6 termination. SSL Proxy Load Balancing supports both IPv4 and IPv6 addresses for client traffic. Client IPv6 requests are terminated at the load balancing layer, and then proxied over IPv4 to your VMs.

  • Intelligent routing. The load balancer can route requests to backend locations where there is capacity. In contrast, an L3/L4 load balancer must route to regional backends without considering capacity. The use of smarter routing allows provisioning at N+1 or N+2 instead of x*N.

  • Better utilization of backends. SSL processing can be very CPU-intensive if the ciphers used are not CPU efficient. To maximize CPU performance, use ECDSA SSL certificates and TLS 1.2, and prefer the ECDHE-ECDSA-AES128-GCM-SHA256 cipher suite for SSL between the load balancer and your backend instances.

  • Certificate management. Your customer-facing SSL certificates can be either certificates that you obtain and manage (self-managed certificates), or certificates that Google obtains and manages for you (Google-managed certificates). Google-managed SSL certificates each support up to 100 domains. Multiple domains are supported for Google-managed certificates. You only need to provision certificates on the load balancer. On your VMs, you can simplify management by using self-signed certificates.

  • Security patching. If vulnerabilities arise in the SSL or TCP stack, we apply patches at the load balancer automatically to keep your VMs safe.

  • Support for the following well-known ports: 25, 43, 110, 143, 195, 443, 465, 587, 700, 993, 995, 1883, 3389, 5222, 5432, 5671, 5672, 5900, 5901, 6379, 8085, 8099, 9092, 9200, and 9300. When you use Google-managed SSL certificates with SSL Proxy Load Balancing, the frontend port for traffic must be 443 to enable the Google-managed SSL certificates to be provisioned and renewed.

  • SSL policies. SSL policies give you the ability to control the features of SSL that your SSL proxy load balancer negotiates with clients.

  • Geographic control over where TLS is terminated. The SSL proxy load balancer terminates TLS in locations that are distributed globally, so as to minimize latency between clients and the load balancer. If you require geographic control over where TLS is terminated, you should use Network Load Balancing instead, and terminate TLS on backends that are located in regions appropriate to your needs.

Load balancer behavior in Network Service Tiers

TCP Proxy Load Balancing can be configured as a global load balancing service with Premium Tier, and as a regional service in the Standard Tier.

Premium Tier

You can have only one backend service, and the backend service can have backends in multiple regions. For global load balancing, you deploy your backends in multiple regions, and the load balancer automatically directs traffic to the region closest to the user. If a region is at capacity, the load balancer automatically directs new connections to another region with available capacity. Existing user connections remain in the current region.

Traffic is allocated to backends as follows:

  1. When a client sends a request, the load balancing service determines the approximate origin of the request from the source IP address.
  2. The load balancing service determines the locations of the backends owned by the backend service, their overall capacity, and their overall current usage.
  3. If the closest backend instances to the user have available capacity, the request is forwarded to that closest set of backends.
  4. Incoming requests to the given region are distributed evenly across all available backend instances in that region. However, at very small loads, the distribution might appear to be uneven.
  5. If there are no healthy backend instances with available capacity in a given region, the load balancer instead sends the request to the next closest region with available capacity.

Standard Tier

With Standard Tier, TCP Proxy Load Balancing is a regional service. Its backends must all be located in the region used by the load balancer's external IP address and forwarding rule.


The following are components of SSL proxy load balancers.

Forwarding rules and IP addresses

Forwarding rules route traffic by IP address, port, and protocol to a load balancing configuration consisting of a target proxy and a backend service.

Each forwarding rule provides a single IP address that you can use in DNS records for your application. No DNS-based load balancing is required. You can either reserve a static IP address that you can use or let Cloud Load Balancing assign one for you. We recommend that you reserve a static IP address; otherwise, you must update your DNS record with the newly- assigned ephemeral IP address whenever you delete a forwarding rule and create a new one.

Each external forwarding rule that you use in an SSL proxy load balancer can reference exactly one of the ports listed in: Port specifications for forwarding rules.

Target proxies

SSL Proxy Load Balancing terminates SSL connections from the client and creates new connections to the backends. The target proxy routes incoming requests directly to the backend service.

By default, the original client IP address and port information is not preserved. You can preserve this information by using the PROXY protocol.

SSL certificates

You must install one or more SSL certificates on the target SSL proxy.

These certificates are used by target SSL proxies to secure communications between a Google Front End (GFE) and the client. These can be self-managed or Google-managed SSL certificates.

For information about SSL certificate limits and quotas, see SSL certificates on the load balancing quotas page.

For the best security, use end-to-end encryption for your SSL proxy load balancer deployment. For more information, see Encryption from the load balancer to the backends.

For general information about how Google encrypts user traffic, see the Encryption in Transit in Google Cloud white paper.

Backend services

Backend services direct incoming traffic to one or more attached backends. Each backend is composed of an instance group or network endpoint group, and information about the backend's serving capacity. Backend serving capacity can be based on CPU or requests per second (RPS).

Each backend service specifies the health checks to perform for the available backends.

To ensure minimal interruptions to your users, you can enable connection draining on backend services. Such interruptions might happen when a backend is terminated, removed manually, or removed by an autoscaler. To learn more about using connection draining to minimize service interruptions, see Enabling connection draining.

Firewall rules

The backend instances must allow connections from the load balancer GFE/health check ranges. This means that you must create an ingress allow firewall rule for traffic from and to your backend instances or endpoints. These IP address ranges are used as sources for health check packets and for all load-balanced packets sent to your backends.

The ports you configure for this firewall rule must:

  • Allow traffic to the destination port needed by each backend service's configured health check.

  • For instance group backends: Allow traffic to the destination port matching the port number(s) to which the backend service's named port subscribes.

  • For GCE_VM_IP_PORT NEG backends: Allow traffic to the port(s) of the endpoints in the NEGs.

Firewall rules are implemented at the VM instance level, not on Google Front End (GFE) proxies. You cannot use Google Cloud firewall rules to prevent traffic from reaching the load balancer.

For more information about health check probes and why it's necessary to allow traffic from and, see Probe IP ranges and firewall rules.

Source IP addresses

The source IP addresses for packets, as seen by each backend virtual machine (VM) instance or container, is an IP address from these ranges:


The source IP address for actual load-balanced traffic is the same as the health checks probe IP ranges.

The source IP addresses for traffic, as seen by the backends, is not the Google Cloud external IP address of the load balancer. In other words, there are two HTTP, SSL, or TCP sessions:

  • Session 1, from original client to the load balancer (GFE):

    • Source IP address: the original client (or external IP address if the client is behind NAT).
    • Destination IP address: your load balancer's IP address.
  • Session 2, from the load balancer (GFE) to the backend VM or container:

    • Source IP address: an IP address in one of these ranges: or

      You cannot predict the actual source address.

    • Destination IP address: the internal IP address of the backend VM or container in the Virtual Private Cloud (VPC) network.

Preserving client source IP addresses

To preserve the original source IP addresses of incoming connections to the load balancer, you can configure the load balancer to prepend a PROXY protocol version 1 header to retain the original connection information. For more information, see Update proxy protocol header for the proxy.

Open ports

The SSL proxy load balancers are reverse proxy load balancers. The load balancer terminates incoming connections, and then opens new connections from the load balancer to the backends. These load balancers are implemented using Google Front End (GFE) proxies worldwide.

GFEs have several open ports to support other Google services that run on the same architecture. To see a list of some of the ports likely to be open on GFEs, see Forwarding rule: Port specifications. There might be other open ports for other Google services running on GFEs.

Running a port scan on the IP address of a GFE-based load balancer is not useful from an auditing perspective for the following reasons:

  • A port scan (for example, with nmap) generally expects no response packet or a TCP RST packet when performing TCP SYN probing. GFEs will send SYN-ACK packets in response to SYN probes for a variety of ports if your load balancer uses a Premium Tier IP address. However, GFEs only send packets to your backends in response to packets sent to your load balancer's IP address and the destination port configured on its forwarding rule. Packets sent to different load balancer IP addresses or your load balancer's IP address on a port not configured in your forwarding rule do not result in packets being sent to your load balancer's backends. Even without any special configuration, Google infrastructure and GFEs provide defense-in-depth for DDoS attacks and SYN floods.

  • Packets sent to the IP address of your load balancer could be answered by any GFE in Google's fleet; however, scanning a load balancer IP address and destination port combination only interrogates a single GFE per TCP connection. The IP address of your load balancer is not assigned to a single device or system. Thus, scanning the IP address of a GFE-based load balancer does not scan all the GFEs in Google's fleet.

With that in mind, the following are some more effective ways to audit the security of your backend instances:

  • A security auditor should inspect the forwarding rules configuration for the load balancer's configuration. The forwarding rules define the destination port for which your load balancer accepts packets and forwards them to the backends. For GFE-based load balancers, each external forwarding rule can only reference a single destination TCP port.

  • A security auditor should inspect the firewall rule configuration applicable to backend VMs. The firewall rules that you set block traffic from the GFEs to the backend VMs, but do not block incoming traffic to the GFEs. For best practices, see the firewall rules section.

Traffic distribution

The way a SSL proxy load balancer distributes traffic to its backends depends on the balancing mode and the hashing method selected to choose a backend (session affinity).

Balancing mode

When you add a backend to the backend service, you set a load balancing mode.

For SSL Proxy Load Balancing, the balancing mode can be CONNECTION or UTILIZATION.

If the load balancing mode is CONNECTION, the load is spread based on how many concurrent connections the backend can handle. You must also specify exactly one of the following parameters: maxConnections (except for regional managed instance groups), maxConnectionsPerInstance, or maxConnectionsPerEndpoint.

If the load balancing mode is UTILIZATION, the load is spread based on the utilization of instances in an instance group.

For information about comparing the load balancer types and the supported balancing modes, see Load balancing methods.

Session affinity

Session affinity sends all requests from the same client to the same backend, if the backend is healthy and has capacity.

SSL Proxy Load Balancing offers client IP affinity, which forwards all requests from the same client IP address to the same backend.


If a backend becomes unhealthy, traffic is automatically redirected to healthy backends within the same region. If all backends within a region are unhealthy, traffic is distributed to healthy backends in other regions (Premium Tier only). If all backends are unhealthy, the load balancer drops traffic.

Usage Notes and limitations

  • Sending traffic over unencrypted TCP between the load balancing layer and backend instances enables you to offload SSL processing from your backends; however, it also reduces security. Therefore, we do not recommend it.

  • You can create SSL policies by using the gcloud command-line tool.

  • SSL proxy load balancers each have a single backend service resource. Changes to the backend service are not instantaneous. It can take several minutes for changes to propagate to Google Front Ends (GFEs).

  • SSL proxy load balancers do not support client certificate-based authentication, also known as mutual TLS authentication.

  • Although SSL Proxy Load Balancing can handle HTTPS traffic, we don't recommend this. You should instead use HTTP(S) Load Balancing for HTTPS traffic. HTTP(S) Load Balancing also does the following, which makes it a better choice in most cases:

    • Negotiates HTTP/2 and SPDY/3.1.
    • Rejects invalid HTTP requests or responses.
    • Forwards requests to different VMs based on URL host and path.
    • Integrates with Cloud CDN.
    • Spreads the request load more evenly among backend instances, providing better backend utilization. HTTPS load balances each request separately, whereas SSL Proxy Load Balancing sends all bytes from the same SSL or TCP connection to the same backend instance.
  • For SSL proxy load balancers with Google-managed SSL certificates, the frontend ports must include 443 for the certificates to be provisioned and renewed successfully.

    SSL Proxy Load Balancing can be used for other protocols that use SSL, such as WebSockets and IMAP over SSL.

  • SSL proxy load balancers support only lowercase characters in domains in a common name (CN) attribute or a subject alternative name (SAN) attribute of the certificate. Certificates with uppercase characters in domains are returned only when set as the primary certificate in the target proxy.

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