Troubleshooting External HTTP(S) Load Balancing

This guide describes how to troubleshoot configuration issues for a Google Cloud external HTTP(S) load balancer.


The types of issues discussed in this guide include the following:

  • Setup issues when backends have incompatible balancing modes
  • General connectivity issues
  • Issues with HTTP/2 connections to the backends
  • External backend and internet NEG issues
  • Serverless NEG issues

Before you begin

Before investigating issues, familiarize yourself with the following pages.

For general connectivity:

For logging and monitoring:

Backends have incompatible balancing modes

When creating a load balancer, you might see the error:

Validation failed for instance group INSTANCE_GROUP:

backend services 1 and 2 point to the same instance group
but the backends have incompatible balancing_mode. Values should be the same.

This happens when you try to use the same backend in two different load balancers, and the backends don't have compatible balancing modes.

For more information, see the following:

Troubleshooting general connectivity issues

Unexplained 502 errors

If 502 errors persist longer than a few minutes after you complete the load balancer configuration, it's likely that either:

Take the following steps to troubleshoot 502 responses:

  1. Verify that health check traffic reaches your backend VMs. To do this, enable health check logging and search for successful log entries.
    For new load balancers, the lack of successful health check log entries does not mean that health check traffic is not reaching your backends. It might mean that the backend's initial health state has not yet changed from UNHEALTHY to a different state. You see successful health check log entries only after the health check prober receives an HTTP 200 OK response from the backend.
  2. Check whether the 502 response is being served by the load balancer or if it is coming from the backends. Perform the following steps:

    1. Use Cloud Logging to view logs for the load balancer. You can create a query to look for 502 response codes only.
    2. Check the value of the statusDetails field.

      • If statusDetails returns a success message, such as response_sent_by_backend, then it is the backend that is serving HTTP 502 responses. Check logs on the backend and troubleshoot further depending on the service running on the backend.
      • If statusDetails returns a failure message, refer to the following list of solutions for some common failures related to 502 responses:
      statusDetail failure message Potential causes and solutions

      The load balancer failed to establish a connection with the backend. This could mean that the service running on the backend is not listening on the port defined in the backend service.


      • Set the health check's port to use the serving port. This means that the backend will be found unhealthy before it is eligible to serve real traffic.
      • Use the following command to make sure that there is a service running on the backend service's named port.
        $ netstat -tnl | grep PORT

      The load balancer could not pick a backend. This could mean all backends are unhealthy. Make sure that you configured the correct firewall rules for the health checks.

      backend_connection_closed_before_data_sent_to_client The backend unexpectedly closed its connection to the load balancer before the response was proxied to the client. This can happen if the load balancer is sending traffic to another entity. The other entity might be a third-party load balancer that has a TCP timeout that is shorter than the load balancer's timeout. For more details, see Timeouts and retries.
      backend_timeout The backend took too long to respond. The backend service timeout might be set too low for the given service to respond. Consider increasing the backend service timeout or look into why your service is taking so long to respond.

Load balanced traffic does not have the source address of the original client

Traffic from the load balancer to your instances has an IP address in the ranges of and When viewing logs on your load balanced instances, you will not see the source address of the original client. Instead, you will see source addresses from this range.

Getting a permission error when trying to view an object in my Cloud Storage bucket

In order to serve objects through load balancing, the Cloud Storage objects must be publicly accessible. Make sure to update the permissions of the objects being served so they are publicly readable.

URL doesn't serve expected Cloud Storage object

The Cloud Storage object to serve is determined based on your URL map and the URL that you request. If the request path maps to a backend bucket in your URL map, the Cloud Storage object is determined by appending the full request path onto the Cloud Storage bucket that the URL map specifies.

For example, if you map /static/* to gs://[EXAMPLE_BUCKET], the request to https://<GCLB IP or Host>/static/path/to/content.jpg will try to serve gs://[EXAMPLE_BUCKET]/static/path/to/content.jpg. If that object doesn't exist, you will get the following error message instead of the object:

The specified key does not exist.

Compression isn't working

HTTP(S) Load Balancing does not compress or decompress responses itself, but it can serve responses generated by your backend service that are compressed by using tools such as gzip or DEFLATE.

If responses served by HTTP(S) Load Balancing are not compressed but should be, check to be sure that the web server software running on your instances is configured to compress responses. By default, some web server software automatically disables compression for requests that include a Via header, which indicates that the request was forwarded by a proxy. Because it is a proxy, HTTP(S) Load Balancing adds a Via header to each request as required by the HTTP specification. To enable compression, you may have to override your web server's default configuration to tell it to compress responses even if the request had a Via header.

To configure nginx backends to serve compressed responses proxied through an external HTTP(S) load balancer:

To configure Apache backends to serve compressed responses proxied through an external HTTP(S) load balancer:

Resolving HTTP 408 errors

With HTTP traffic, the maximum amount of time for the client to complete sending its request is equal to the backend service timeout. If you see HTTP 408 responses with the jsonPayload.statusDetail client_timed_out, this means that there was insufficient progress while the request from the client was proxied or the response from the backend was proxied. If the problem is because of clients that are experiencing performance issues, you can resolve this issue by increasing the backend service timeout.

Troubleshooting issues with HTTP/2 to the backends

Invalid value for field resource.loadBalancingScheme: 'EXTERNAL'

This could happen if you create a backend service without selecting the global option. When you issue a gcloud command as follows, you are prompted to designate a region or designate the load balancer as global:

gcloud beta compute backend-services create service-test \
    --health-checks=hc-test \
    --project=test1 \

For the following backend service:

- [service-test] choose a region or global:
[1] global
[2] region: [REGION_A_NAME]
[3] region: [REGION_B_NAME]
Please enter your numeric choice:

For the HTTP(S) load balancer, the backend services must be global, so you must choose option 1 or issue the gcloud command with the --global option:

gcloud beta compute backend-services create service-test \
    --health-checks=hc-test \
    --project=test \
    --protocol=http2 \

Unexplained 502 errors

Make sure that your backend instance is healthy and supports HTTP/2 protocol. You can verify this by testing connectivity to the backend instance using HTTP/2. Ensure that the VM uses HTTP/2 spec-compliant cipher suites. For example, certain TLS 1.2 cipher suites are disallowed by HTTP/2. Refer to the TLS 1.2 Cipher Suite Black List.

After you verify that the VM uses the HTTP/2 protocol, make sure your firewall setup allows the health checker and load balancer to pass through.

If there are no problems with the firewall setup, ensure that the load balancer is configured to talk to the correct port on the VM.

Troubleshooting external backend and internet NEG issues

Before investigating issues, familiarize yourself with the following pages:

Traffic does not reach the endpoints

After you configure a service, the new endpoint becomes reachable through the external HTTP(S) load balancer when:

  • The endpoint is attached to the internet NEG.
  • The associated FQDN can be DNS resolved successfully (if you are using FQDN endpoint type).
  • The endpoint is accessible over the internet.

If traffic cannot reach the endpoint, which results in a 502 error code for HTTP(s), check the following:

  • Query the DNS TXT record using a tool like dig or nslookup. Note the CIDRs (following ip4:) and ensure these ranges are allowed by your firewall or cloud Access Control List (ACL).

After configuring an external backend, requests to external backend failed with a 5xx error

  • Check Logging.
  • Verify that the network endpoint group is configured with the correct IP:Port or FQDN:Port for your external backend.
  • If you are using FQDN, make sure that it is resolvable through Google Public DNS. You can verify that the FQDN is resolvable through Google Public DNS using these steps or the web interface directly.
  • If you are accessing the HTTP(S) load balancer on its external IP only, and your origin web-server is expecting a hostname, ensure that you are sending a valid HTTP Host header to your backend by configuring a custom request header.
  • If communicating to a backend over HTTPS or HTTP2 (as set in the protocol field of the backend service) configured as an INTERNET_FQDN_PORT external backend endpoint, ensure that your origin is presenting a valid TLS (SSL) certificate and the configured FQDN matches a SAN (Subject Alternative Name) in the certificates' list of SANs. A valid certificate is defined as one signed by a public Certificate Authority and that has not expired.
  • When using INTERNET_FQDN_PORT external backend endpoints, self-signed certificates are not accepted by the HTTPS load balancer, and are rejected.
  • When using HTTPS or HTTP/2 with INTERNET_IP_PORT type endpoints, no SSL certificate validation/SAN check is performed. This means one can use self-signed certificates. When using SSL, our recommendation is to use INTERNET_FQDN_PORT endpoints to make sure server certificates and SANs can be validated.

Responses from my external backend are not cached by Cloud CDN

Ensure that:

  • You have enabled Cloud CDN on the backend service containing the NEG that points to your external backend by setting enableCDN to true.
  • Responses served by your external backend meet Cloud CDN caching requirements. For example, you are sending Cache-Control: public, max-age=3600 response headers from the origin.

Troubleshooting serverless NEG issues

Before investigating issues, familiarize yourself with the following pages:

Requests fail with a 404 error

Ensure that the underlying serverless resource (such as an App Engine, Cloud Functions, or Cloud Run service) is still running. If the serverless resource is deleted but the serverless NEG still exists, the external HTTP(S) load balancer will continue to attempt to route requests to the non-existence service. This results in a 404 response.

In general, an external HTTP(S) load balancer cannot detect if the underlying serverless resource is working as expected. This means that if your service in one region is returning errors but the overall Cloud Run, Cloud Functions, or App Engine infrastructure in that region is operating normally, your external HTTP(S) load balancer will not automatically direct traffic away to other regions. Make sure you thoroughly test new versions of your services before routing user traffic to them.

Handling URL mask mismatches

If applying the configured URL mask to a user request URL doesn't result in a service name, or if it results in a service name that does not exist, the load balancer might handle these mismatches differently depending on the serverless compute platform in use.

Cloud Run: In case of a URL mask mismatch, the load balancer returns an HTTP error 404 (Not Found).

Cloud Functions: In case of a URL mask mismatch, the load balancer returns an HTTP error 404 (Not Found).

App Engine: In case of a URL mask mismatch, App Engine uses dispatch.yaml and App Engine's default routing logic to determine which service to send the request to.