Google Cloud provides health checks to determine if backends respond to traffic. This document discusses health checking concepts for Google Cloud load balancers and Traffic Director.
Health checks connect to backends on a configurable, periodic basis. Each connection attempt is called a probe. Google Cloud records the success or failure of each probe.
Based on a configurable number of sequential successful or failed probes, an overall health state is computed for each backend. Backends that respond successfully for the configured number of times are considered healthy. Backends that fail to respond successfully for a separately configurable number of times are unhealthy.
The overall health state of each backend determines eligibility to receive new requests or connections. You can configure the criteria that define a successful probe. This is discussed in detail in the section How health checks work.
Health checks use special routes that aren't defined in your Virtual Private Cloud (VPC) network. For complete information, see Load balancer return paths.
Health check categories, protocols, and ports
Health checks have a category and a protocol. The two categories are health checks and legacy health checks and their supported protocols are as follows:
Legacy health checks:
The protocol and port determine how health check probes are done. For example, a health check can use the HTTP protocol on TCP port 80, or it can use the TCP protocol for a named port in an instance group.
You cannot convert a legacy health check to a health check, and you cannot convert a health check to a legacy health check.
Selecting a health check
Health checks must be compatible with the type of load balancer (or Traffic Director) and the backend types. The factors to consider when you select a health check are as follows:
- Category: health check or legacy health check.
- Protocol: protocol that Google Cloud uses to probe the backends.
- Port specification: ports that Google Cloud uses with the protocol.
Category and protocol
It's a best practice to use the same protocol as the load balancer; however, this is not a requirement, nor is it always possible.
For example, target pool-based network load balancers require legacy health checks, and they require that the legacy health checks use the HTTP protocol, even though target pool-based network load balancers support TCP or UDP. For target pool-based network load balancers, you must run an HTTP server on your virtual machine (VM) instances so that they can respond to health check probes.
For almost all other load balancer types, you must use regular, non-legacy health checks where the protocol matches the load balancer's backend service protocol.
Category and port specification
You must specify a port for your health check. Health checks have two port
specification methods (
--use-serving-port). For legacy health
checks, there is one method (
Load balancer guide
This table shows the supported category, scope, and port specification for each load balancer and backend type.
|Load balancer||Backend type||Health check category and scope||Port specification|
Global external HTTP(S) load balancer
Global external HTTP(S) load balancer (classic) 1
TCP Proxy Load Balancing
SSL Proxy Load Balancing
|Zonal NEGs||Health check (global)||
|Instance groups||Health check (global)||
|Regional external HTTP(S) load balancer||Zonal NEGs||Health check (regional)||
|Instance groups||Health check (regional)||
|Internal HTTP(S) Load Balancing||Zonal NEGs||Health check (regional)||
|Instance groups||Health check (regional)||
|Network Load Balancing 2||Instance groups||Health check (regional)||
in target pools
|Legacy health check
(global with the HTTP protocol)
|Legacy health checks only support the port number
|Internal TCP/UDP Load Balancing 2||Zonal NEGs||Health check (global or regional)||
|Instance groups||Health check (global or regional)||
|Load balancer mode||Legacy health checks supported|
|Global external HTTP(S) load balancer||No|
|Regional external HTTP(S) load balancer||No|
|External HTTP(S) load balancer||Yes, if both of the following are true:
--use-serving-portflag because backend services used with Internal TCP/UDP Load Balancing and Network Load Balancing don't subscribe to any named port.
How health checks work
The following sections describe how health checks work.
When you create a health check or a legacy health check, you specify the following flags or accept their default values. Each health check or legacy health check that you create is implemented by multiple probes. These flags control how frequently each probe evaluates instances in instance groups or endpoints in zonal NEGs.
A health check's settings cannot be configured on a per-backend basis. Health checks are associated with an entire backend service. For a target pool-based Network Load Balancing, a legacy HTTP health check is associated with the entire target pool. Thus, the parameters for the probe are the same for all backends referenced by a given backend service or target pool.
|Configuration flag||Purpose||Default value|
||The check interval is the amount of time from the start of one probe issued by one prober to the start of the next probe issued by the same prober. Units are seconds.||
||The timeout is the amount of time that Google Cloud waits for a response to a probe. Its value must be less than or equal to the check interval. Units are seconds.||
Probe IP ranges and firewall rules
For health checks to work, you must create ingress
allow firewall rules so
that traffic from Google Cloud probers can connect to your backends.
The following table shows the source IP ranges to allow:
|Product||Probe source IP ranges||Firewall rule example|
||Firewall rules for all products except network load balancers|
|Network Load Balancing||
For IPv4 traffic to the backends:
For IPv6 traffic to the backends:
Target pool-based network load balancers support only IPv4 traffic and
might proxy health checks
through the metadata server. In this case, health check packet sources
match the IP address of the metadata server:
|Firewall rules for network load balancers|
Importance of firewall rules
Google Cloud requires that you create the necessary ingress
firewall rules to permit traffic from probers to your backends. As a best
practice, limit these rules to just the protocols and ports that
match those used by your health checks. For the source IP ranges, make sure to
use the documented probe IP ranges listed in the preceding section.
If you don't have ingress
allow firewall rules that permit the health check,
deny rule blocks
inbound traffic. When probers can't contact your backends, the
load balancer considers your backends to be unhealthy.
The behavior when all backends are unhealthy depends on the type of load
The global external HTTP(S) load balancer (classic) returns HTTP 502 responses to clients when all backends are unhealthy.
Global external HTTP(S) load balancers, regional external HTTP(S) load balancers, and internal HTTP(S) load balancers return HTTP 503 responses to clients when all backends are unhealthy.
SSL proxy load balancers and TCP proxy load balancers time out when all backends are unhealthy.
When all backends are unhealthy and failover is not configured, internal TCP/UDP load balancers and backend service-based network load balancers distribute traffic to all backend VMs as a last resort. For more details on this behavior, see Traffic distribution for internal TCP/UDP load balancers and Traffic distribution for backend service-based network load balancers.
Security considerations for probe IP ranges
Consider the following information when planning health checks and the necessary firewall rules:
The probe IP ranges belong to Google. Google Cloud uses special routes outside of your VPC network but within Google's production network to facilitate communication from probers.
Google uses the probe IP ranges to send health check probes for external HTTP(S) load balancers, SSL proxy load balancers, and TCP proxy load balancers. If a packet is received from the internet and the packet's source IP address is within a probe IP range, Google drops the packet. This includes the external IP address of a Compute Engine instance or a Google Kubernetes Engine (GKE) node.
The probe IP ranges are a complete set of possible IP addresses used by Google Cloud probers. If you use
tcpdumpor a similar tool, you might not observe traffic from all IP addresses in all probe IP ranges. As a best practice, create ingress firewall rules that allow all of the probe IP ranges as sources. Google Cloud can implement new probers automatically without notification.
Multiple probes and frequency
Google Cloud sends health check probes from multiple redundant systems called probers. Probers use specific source IP ranges. Google Cloud does not rely on just one prober to implement a health check—multiple probers simultaneously evaluate the instances in instance group backends or the endpoints in zonal NEG backends. If one prober fails, Google Cloud continues to track backend health states.
The interval and timeout settings that you configure for a health
check are applied to each prober. For a given backend, software access logs and
tcpdump show more frequent probes than your configured settings.
This is expected behavior, and you cannot configure the number of probers that Google Cloud uses for health checks. However, you can estimate the effect of multiple simultaneous probes by considering the following factors.
To estimate the probe frequency per backend service, consider the following:
Base frequency per backend service. Each health check has an associated check frequency, inversely proportional to the configured check interval:
When you associate a health check with a backend service, you establish a base frequency used by each prober for backends on that backend service.
Probe scale factor. The backend service's base frequency is multiplied by the number of simultaneous probers that Google Cloud uses. This number can vary, but is generally between 5 and 10.
Multiple forwarding rules for internal TCP/UDP load balancers. If you have configured multiple internal forwarding rules (each having a different IP address) pointing to the same regional internal backend service, Google Cloud uses multiple probers to check each IP address. The probe frequency per backend service is multiplied by the number of configured forwarding rules.
Multiple forwarding rules for network load balancers. If you have configured multiple forwarding rules that point to the same backend service or target pool, Google Cloud uses multiple probers to check each IP address. The probe frequency per backend VM, is multiplied by the number of configured forwarding rules.
Multiple target proxies for external HTTP(S) load balancers. If you have multiple target proxies that direct traffic to the same URL map, Google Cloud uses multiple probers to check the IP address associated with each target proxy. The probe frequency per backend service is multiplied by the number of configured target proxies.
Multiple target proxies for SSL proxy load balancers and TCP proxy load balancers. If you have configured multiple target proxies that direct traffic to the same backend service, Google Cloud uses multiple probers to check the IP address associated with each target proxy. The probe frequency per backend service is multiplied by the number of configured target proxies.
Sum over backend services. If a backend is used by multiple backend services, the backend instances are contacted as frequently as the sum of frequencies for each backend service's health check.
With zonal NEG backends, it's more difficult to determine the exact number of health check probes. For example, the same endpoint can be in multiple zonal NEGs. Those zonal NEGs don't necessarily have the same set of endpoints, and different endpoints can point to the same backend.
Destination for probe packets
The following table shows the network interface and destination IP addresses to which health check probers send packets, depending on the type of load balancer.
For network load balancers and internal TCP/UDP load balancers, the application must bind to
the load balancer's IP address (or any IP address
|Load balancer||Destination network interface||Destination IP address|
||Primary network interface (
|Network Load Balancing||Primary network interface (
||The IP address of the external forwarding rule.
If multiple forwarding rules point to the same backend service (for target-pool based Network Load Balancings, the same target pool), Google Cloud sends probes to each forwarding rule's IP address. This can result in an increase in the number of probes.
|Internal TCP/UDP Load Balancing||
The network interface of the instance located in the network specified
for the internal backend service. If not specified, the primary network
For more information, see Backend services and network interfaces.
|The IP address of the internal forwarding rule.
If multiple forwarding rules point to the same backend service, Google Cloud sends probes to each forwarding rule's IP address. This can result in an increase in the number of probes.
Success criteria for HTTP, HTTPS, and HTTP/2
When a health check uses the HTTP, HTTPS, or HTTP/2 protocol, each probe
HTTP 200 (OK) response code to be delivered before the probe
timeout. In addition, you can do the following:
You can configure Google Cloud probers to send HTTP requests to a specific request path. If you don't specify a request path,
If you configure a content-based health check by specifying an expected response string, Google Cloud must find the expected string within the first 1,024 bytes of the HTTP response body.
The following combinations of request path and response string flags are available for health checks that use HTTP, HTTPS, and HTTP/2 protocols.
|Configuration flag||Success criteria|
||Specify the URL path to which Google Cloud sends health check
If omitted, Google Cloud sends probe requests to the root path,
||The optional response flag allows you to configure a content-based
health check. The expected response string must be less than or equal
to 1,024 ASCII (single byte) characters. When configured,
Google Cloud expects this string within the first 1,024 bytes of the
response in addition to receiving
Success criteria for SSL and TCP
Unless you specify an expected response string, probes for health checks that use the SSL and TCP protocols are successful when both of the following base conditions are true:
- Each Google Cloud prober can successfully complete an SSL or TCP handshake before the configured probe timeout.
- For TCP health checks, the TCP session is terminated gracefully either by:
- The backend, or
- The Google Cloud prober sending a TCP RST (reset) packet while the TCP session to the prober is still established
If the backend sends a TCP RST (reset) packet to close a TCP session for a TCP health check, the probe might be considered unsuccessful. This happens when the Google Cloud prober has already initiated a graceful TCP termination.
You can create a content-based health check if you provide a request string and an expected response string, each up to 1,024 ASCII (single byte) characters in length. When an expected response string is configured, Google Cloud considers a probe successful only if the base conditions are satisfied and the response string returned exactly matches the expected response string.
The following combinations of request and response flags are available for health checks that use the SSL and TCP protocols.
|Configuration flags||Success criteria|
|Neither request nor response specified
Neither flag specified:
|Google Cloud considers the probe successful when the base conditions are satisfied.|
|Both request and response specified
Both flags specified:
|Google Cloud sends your configured request string and waits for the expected response string. Google Cloud considers the probe successful when the base conditions are satisfied and the response string returned exactly matches the expected response string.|
|Only response specified
Flags specified: only
|Google Cloud waits for the expected response string, and considers
the probe successful when the base conditions are satisfied and the
response string returned exactly matches the expected response
You should only use
|Only request specified
Flags specified: only
|Google Cloud sends your configured request string and considers the probe successful when the base conditions are satisfied. The response, if any, is not checked.|
Success criteria for gRPC
If you are using gRPC health checks, make sure that the gRPC service sends the
RPC response with the status
OK and the status field set to
Note the following:
- gRPC health checks are used only with gRPC applications and Traffic Director.
- gRPC health checks don't support TLS.
For more information, see the following:
Google Cloud uses the following configuration flags to determine the overall health state of each backend to which traffic is load balanced.
|Configuration flag||Purpose||Default value|
||The healthy threshold specifies the number of sequential successful probe results for a backend to be considered healthy.||A threshold of
||The unhealthy threshold specifies the number of sequential failed probe results for a backend to be considered unhealthy.||A threshold of
Google Cloud considers backends to be healthy after this healthy threshold has been met. Healthy backends are eligible to receive new connections.
Google Cloud considers backends to be unhealthy when the unhealthy threshold has been met. Unhealthy backends are not eligible to receive new connections; however, existing connections are not immediately terminated. Instead, the connection remains open until a timeout occurs or until traffic is dropped. The specific behavior differs depending on the type of load balancer that you're using.
Existing connections might fail to return responses, depending on the cause for failing the probe. An unhealthy backend can become healthy if it is able to meet the healthy threshold again.
Content-based health checks
A content-based health check is one whose success criteria depends on evaluation of an expected response string. Use a content-based health check to instruct Google Cloud health check probes to more completely validate your backend's response.
You configure an HTTP, HTTPS, or HTTP/2 content-based health check by specifying an expected response string, and optionally by defining a request path. For more details, see Success criteria for HTTP, HTTPS, and HTTP/2.
You configure an SSL or TCP content-based health check by specifying an expected response string, and optionally by defining a request string. For more details, see Success criteria for SSL and TCP.
Certificates and health checks
Google Cloud health check probers do not perform certificate validation, even for protocols that require that your backends use certificates (SSL, HTTPS, and HTTP/2)—for example:
- You can use self-signed certificates or certificates signed by any certificate authority (CA).
- Certificates that have expired or that are not yet valid are acceptable.
- Neither the
subjectAlternativeNameattributes need to match a
Hostheader or DNS PTR record.
Health checks that use any protocol, but not legacy health checks, allow you to
set a proxy header by using the
Health checks that use HTTP, HTTPS, or HTTP/2 protocols and legacy health
checks allow you to specify an HTTP
Host header by using the
Example health check
Suppose you set up a health check with the following settings:
- Interval: 30 seconds
- Timeout: 5 seconds
- Protocol: HTTP
- Unhealthy threshold: 2 (default)
- Healthy threshold: 2 (default)
With these settings, the health check behaves as follows:
- Multiple redundant systems are simultaneously configured with the health check parameters. Interval and timeout settings are applied to each system. For more information, see Multiple probes and frequency.
Each health check prober does the following:
A backend is considered unhealthy when at least one health check probe system does the following:
- Does not receive an
HTTP 200 (OK)response code for two consecutive probes. For example, the connection might be refused, or there might be a connection or socket timeout.
- Receives two consecutive responses that don't match the protocol-specific success criteria.
- Does not receive an
A backend is considered healthy when at least one health check probe system receives two consecutive responses that match the protocol-specific success criteria.
In this example, each prober initiates a connection every 30 seconds. Thirty seconds elapses between a prober's connection attempts regardless of the duration of the timeout (whether or not the connection timed out). In other words, the timeout must always be less than or equal to the interval, and the timeout never increases the interval.
In this example, each prober's timing looks like the following, in seconds:
- t=0: Start probe A.
- t=5: Stop probe A.
- t=30: Start probe B.
- t=35: Stop probe B.
- t=60: Start probe C.
- t=65: Stop probe C.
- To create, modify, and use health checks, see Creating health checks.
- To troubleshoot health checks, enable health check logging.