# Connection Troubleshooting > Depth-first rabbithole dossier for Connection Troubleshooting; source-anchored research pack. Parent: [MongoDB Atlas AWS Networking](https://llms-explorer.com/tree/mongodb-atlas-aws-networking/) · 4 facets · 22 facts · page: https://llms-explorer.com/tree/connection-troubleshooting/ ## Definitions - [low] If you have a special character in your password, refer to the documentation for special characters in connection string passwords. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-connection.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` ## How it works - [low] When you use an optimized connection string, Atlas limits the number of connections per `mongos` between your application and your sharded cluster. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-connection.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] If one interface endpoint is unavailable, the next is used, which is opaque to the driver or other connection mechanism. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-private-endpoints.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] To enable the flow of traffic between VPCs using private IP addresses, the owner of each VPC in the VPC peering connection must manually add a route to one or more of their VPC route tables that points to the IP address range of the other VPC. Sources: https://docs.aws.amazon.com/vpc/latest/peering/vpc-peering-basics.html — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] If you delete the last endpoint, the entire endpoint service is automatically deleted, and you must restart the entire configuration process. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-private-endpoints.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` ## Problems, failure modes and limitations - [medium] If you try to connect when you are at the connection limit, MongoDB displays an error stating `connection refused because too many open connections`. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-connection.md, https://www.mongodb.com/docs/atlas/troubleshoot-private-endpoints.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] If you are using a `mongodb+srv://` connection string and your driver or shell can't find the DNS host of the Atlas cluster, the cluster might be paused or deleted. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-connection.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] To prevent connection limit issues in the future, consider using the `maxPoolSize` connection string option to limit the number of connections in the connection pool. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-connection.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] If your connections exceed the connection limits for your cluster service limit, you should increase the cluster tier. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-private-endpoints.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] If you can't access the required ports, check your system firewall settings and ensure that they are not blocking access to these ports. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-connection.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` ## Facts and statements - [low] Atlas automatically pauses idle Free clusters (formerly known as `M0`) after 30 days with no connections. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-connection.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] Atlas allows client connections only from IP addresses and CIDR (Classless Inter-Domain Routing) address ranges in the IP access list. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-connection.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] Atlas can generate an optimized SRV (DNS Service Record) connection string for sharded clusters using the load balancers from your private endpoint service. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-connection.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] Provisioning for free clusters takes less than 15 seconds, while provisioning for Flex and Dedicated clusters might take up to 10 minutes. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-connection.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] For each resource that needs to connect to your Atlas clusters using AWS PrivateLink, the resource's security group must allow outbound traffic to the interface endpoint's private IP addresses on all ports (1024-65535). Sources: https://www.mongodb.com/docs/atlas/troubleshoot-private-endpoints.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] Your interface endpoint security group must allow inbound traffic on all ports from each resource that needs to connect to your Atlas clusters using AWS PrivateLink. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-private-endpoints.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] Cross-region endpoints might take an additional 30 seconds to become discoverable by the Atlas endpoint service in a different region. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-private-endpoints.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] On AWS, for MongoDB 8.1 (v8.1.0+), 8.0 (v8.0.10+) and 7.0 (v7.0.22+), you can view the client source IP in the audit logs for both replica sets and sharded clusters connecting via Private Endpoints; the origin client IP address and port is indicated by the `sourceClient` field. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-private-endpoints.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] You can use the tools `nslookup` and `telnet` to test the connectivity from your application to your Private Endpoint in Atlas. Sources: https://www.mongodb.com/docs/atlas/troubleshoot-private-endpoints.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] If required, update the security group rules that are associated with your EC2 instance to ensure that traffic to and from the peer VPC is not restricted. Sources: https://docs.aws.amazon.com/vpc/latest/peering/vpc-peering-basics.html — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] If both VPCs are in the same Region, you can reference a security group from the peer VPC as a source or destination for inbound or outbound rules in your security group. Sources: https://docs.aws.amazon.com/vpc/latest/peering/vpc-peering-basics.html — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#troubleshooting` - [low] You can connect to your Atlas cluster over private networking using a Network Peering connection between your Atlas cluster and your AWS Lambda function. Sources: https://www.mongodb.com/docs/atlas/manage-connections-aws-lambda.md — source: `~/.global-ai-hub/research/.pipeline/mongodb-2026-10-14/frb/connection-troubleshooting-c9f0769084/rabbithole-synthesis.md#core` ## Related concepts - Troubleshooting — is a part of Connection Troubleshooting - Connection — is a part of Connection Troubleshooting