Cannot connect to RDS Postgres using the IAM Authentication

Question

Steps taken:

1. Created the IAM user rds-user which has the rds-db:connect permissions for the cluster.
```
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "rds-db:connect"
            ],
            "Resource": [
                "arn:aws:rds-db:eu-west-1:11111111111:dbuser:example-db/rds-test"
            ]
        }
    ]
}
```
2. Enabled RDS IAM Authentication on the Cluster.
3. Created the rds-test user inside postgres and granted the rds_iam role.
4. Generated an AWS authentication token
```
export RDSHOST="example-db.eu-west-1.rds.amazonaws.com"
export PGPASSWORD="$(aws rds generate-db-auth-token --hostname $RDSHOST --port 5432 --region us-west-1 --username rds-test)"
psql "host=$RDSHOST port=5432 sslmode=verify-full sslrootcert=eu-west-1-bundle.pem dbname=postgres user=rds-test password=$PGPASSWORD"
```
Output:
```
FATAL:  password authentication failed for user "rds-test"
```

The SSL certificate is not expired.

Answer

Hello.

I recommend that you first review all troubleshooting methods in the documentation below.  
https://repost.aws/knowledge-center/aurora-postgresql-connect-iam

By the way, is the ARN set in the IAM policy correct?  
If you try changing the ARN to "*" and are able to connect, the ARN may be incorrect.  
```
"arn:aws:rds-db:eu-west-1:11111111111:dbuser:example-db/rds-test"
```

Cannot connect to RDS Postgres using the IAM Authentication

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