Configure log sinks with user-managed service accounts

This page describes how to route logs by creating sinks that are configured with user-managed service accounts. By default, Logging uses a Logging service account for all sinks in a resource. However, if your log sinks are in different projects, then you can create and manage your own user-managed service account, which lets you centrally manage Identity and Access Management permissions from the project that contains your user-managed service account.

You can only create a sink that uses a user-managed service account when the sink destination is a log bucket or a Google Cloud project. The example in this document illustrates how to set up a sink that uses a user-managed service account where the destination is a log bucket.

Before you begin

In the Google Cloud console, activate Cloud Shell.

Activate Cloud Shell

At the bottom of the Google Cloud console, a Cloud Shell session starts and displays a command-line prompt. Cloud Shell is a shell environment with the Google Cloud CLI already installed and with values already set for your current project. It can take a few seconds for the session to initialize.
Ensure that you have a user-managed service account and then set the following variables to appropriate values for your user-managed service account:
- CUSTOM_SA_PROJECT_ID: The project ID of the project that contains your user-managed service account.
- CUSTOM_SA: The email address of your user-managed service account.
For information about how to create a service account, see Create service accounts.
Ensure that you have a log bucket that can serve as the destination of a log sink, and then set the following variables to values that are appropriate for your log bucket. If necessary, create a log bucket:
- LOG_BUCKET_PROJECT_ID: The project ID of the project containing your log bucket.
- LOCATION: The location of your log bucket.
- BUCKET_NAME: The name of your log bucket.
Identify the name of the Logging service account which exists in the project in which you plan to create the log sink, and then set the following variables to their appropriate values:
- SINK_PROJECT_ID: The project ID of the project where you plan to create the log sink.
- LOGGING_SA: The email address of the default Logging service account. To get this address, run the following command:
```
gcloud logging settings describe --project=SINK_PROJECT_ID
```
  In the response, the line beginning with loggingServiceAccountId lists the email address of your service account.
In the project containing your user-managed service account, ensure the organization policy boolean constraint iam.disableCrossProjectServiceAccountUsage isn't enforced. By default, this constraint is enforced. To disable this constraint so that you can attach a service account to a resource in another project, run the following command:
```
gcloud resource-manager org-policies disable-enforce \
iam.disableCrossProjectServiceAccountUsage \
--project=CUSTOM_SA_PROJECT_ID
```
For more information about enabling service accounts across projects, see Enable service accounts to be attached across projects.

Grant IAM roles

This section describes the prerequisites for creating a sink that uses a user-managed service account.

Let user-managed service account write log entries to the sink destination

Give the user-managed service account the permissions it needs to write log entries to the destination of the sink that you will create in a subsequent step. The destination of the sink will be a log bucket stored in the project named LOG_BUCKET_PROJECT_ID.

To give the required permissions to the user-managed service account, grant it the Logs Bucket Writer role (roles/logging.bucketWriter) on the project containing the log bucket:

gcloud projects add-iam-policy-binding LOG_BUCKET_PROJECT_ID \
--member='serviceAccount:CUSTOM_SA' \
--role='roles/logging.bucketWriter'

For more information about the previous command, see gcloud projects add-iam-policy-binding.

Configure service account impersonation

Configure the default Cloud Logging service account, LOGGING_SA, so that it can impersonate the user-managed service account, CUSTOM_SA. The default Cloud Logging service account exists in the Google Cloud project that you want to create log sinks that use the user-managed service account.

To configure service account impersonation, grant the Service Account Token Creator role (roles/iam.serviceAccountTokenCreator) to the Cloud Logging service account on the user-managed service account:

gcloud iam service-accounts add-iam-policy-binding CUSTOM_SA \
--project=CUSTOM_SA_PROJECT_ID \
--member='serviceAccount:LOGGING_SA' \
--role='roles/iam.serviceAccountTokenCreator'

Service account impersonation involves two principals: the service account that lacks permissions to access a resource, and the privilege-bearing service account that has the permissions to access a resource. In this case, the user-managed service account is the privilege-bearing account because it has the ability to write log entries to the sink destination, which is a log bucket in the project named LOG_BUCKET_PROJECT_ID. The Logging service account has the privileges to route logs.

For more information about the Service Account Token Creator role, see Roles for managing and impersonating service accounts: Service Account Token Creator Role.

For more information about service account impersonation, see About service account impersonation.

Let principal run operations as the user-managed service account

Give the principal who will create the sink the permissions they need to run operations as the user-managed service account.

To give the required permissions, grant the principal the Service Account User role (roles/iam.serviceAccountUser) on the Google Cloud project that stores the user-managed service account, CUSTOM_SA_PROJECT_ID.

Before you run the following command, make the following replacements:

PRINCIPAL: An identifier for the principal that you want to grant the role to. Principal identifiers usually have the following form: PRINCIPAL-TYPE:ID. For example, user:my-user@example.com. For a full list of the formats that PRINCIPAL can have, see Principal identifiers.

Execute the gcloud iam service-accounts add-iam-policy-binding command:

gcloud iam service-accounts add-iam-policy-binding CUSTOM_SA \
--project=CUSTOM_SA_PROJECT_ID \
--member='PRINCIPAL' \
--role='roles/iam.serviceAccountUser'

If you use custom roles, then the principal needs the iam.serviceAccounts.actAs permission.

For more information about the Service Account User role, see Roles for managing and impersonating service accounts: Service Account User role.

Create a log sink that uses a user-managed service account

To create a sink with a user-managed service account, run the gcloud logging sinks create command and include the --custom-writer-identity option.

Before you run the following command, make the following replacements:

SINK_NAME: The name of the log sink.

Execute the gcloud logging sinks create command:

gcloud logging sinks create SINK_NAME \
logging.googleapis.com/projects/LOG_BUCKET_PROJECT_ID/locations/LOCATION/buckets/BUCKET_NAME \
--custom-writer-identity=serviceAccount:CUSTOM_SA \
--project=SINK_PROJECT_ID

Verify that your sink is routing logs

In this section, you use the gcloud CLI to write and read a log entry to verify that your sink is routing logs correctly.

To verify that your sink is routing logs correctly, do the following:

Run the gcloud logging write command:

Before you run the following command, make the following replacements:
- LOG_NAME: The name of the log. For example, you might set this field to mylog.
Execute the gcloud logging write command:
```
gcloud logging write LOG_NAME "Test log entry" --project=SINK_PROJECT_ID
```
The previous command returns the following message: Created log entry.

To read the log entry you just wrote, run the following command:

gcloud logging read 'textPayload="Test log entry"' \
--bucket=BUCKET_NAME --location=LOCATION \
--view=_AllLogs --project=SINK_PROJECT_ID

What's next

To learn about routing your logs to supported destinations, see Route logs to supported destinations.
For an overview about how Logging routes and stores your logs, see Routing and storage overview.
If you encounter issues as you use sinks to route logs, see Troubleshoot routing logs.
To learn how to view your routed logs in their destinations, as well as how the logs are formatted and organized, see View logs in sink destinations.