Troubleshooting and Support

This guide can help you solve common issues with Cloud NAT.

Troubleshooting

VMs can reach the Internet unexpectedly, without Cloud NAT

If your VM or container instances can reach the Internet without Cloud NAT, but you don't want them to, check for the following issues:

  • Check to see if the VM's network interface has an external IP address. If the network interface has an external IP address assigned to it, Google Cloud automatically performs one-to-one NAT for packets whose sources match the interface's primary internal IP address. See Cloud NAT specifications for more details.

    To determine if a VM has an external IP address, refer to changing or assigning an external IP address to an existing instance.

  • Make sure your GKE cluster is a private cluster. Each node VM in a non-private cluster has an external IP addresses, so each node can use routes in your VPC network whose next hop is the default internet gateway without relying on Cloud NAT. Refer to GKE interaction for additional details, including how non-private clusters interact with Cloud NAT gateways.

  • List routes in your VPC network, looking for ones that could provide internet connectivity through a next hop different than the default internet gateway. As examples:

    • Custom static routes whose next hops are VMs, internal TCP/UDP load balancers, or Cloud VPN tunnels might indirectly provide internet connectivity. For example, the next hop VMs or backend VMs for an internal TCP/UDP load balancer might have external IP addresses themselves, or a Cloud VPN tunnel might connect to a network that offers internet access.

    • Custom dynamic routes learned from on-premises networks by Cloud Routers in your VPC might connect to a network that offers internet access.

  • Keep in mind that other custom routes in your VPC network might have higher priorities than routes whose next hops are default internet gateway. Carefully review routing applicability and order for details about how Google Cloud evaluates routes.

Support

Get help with questions.

FAQs

Regional restriction for Cloud NAT

Question: Can I use the same Cloud NAT gateway in more than one region?

Answer: No. Cloud NAT gateways are regional resources, associated with a single region, VPC network, and Cloud Router.

You can create additional Cloud NAT gateways in other regions or other VPC networks. Refer to subnet IP address range applicability for how to determine if you can create more than one gateway in a given region and VPC network.

Question: Are the external NAT IP addresses used by Cloud NAT gateways global or regional?

Answer: Cloud NAT gateways use regional external IP addresses as NAT IP addresses. Even though they are regional, they are publicly routable. See NAT IP addresses for details about different ways that NAT IP addresses can be allocated or assigned.

When Cloud NAT can and cannot be used

Question: Does Cloud NAT apply to instances, including GKE node VMs, that have external IP addresses?

Answer: Generally, no. If the network interface of a VM has an external IP address, Google Cloud always performs 1-to-1 NAT for packets sent from the primary internal IP address of the network interface without using Cloud NAT. However, a Cloud NAT could still provide NAT services to packets sent from alias IP address ranges of that same network interface. For additional details, refer to Cloud NAT specifications and GKE interaction.

Question: Can I use Cloud NAT for communication between VMs in a VPC network?

Answer: No, Cloud NAT is designed to provide connectivity to the internet only.

Question: Can I use Cloud NAT to connect a VPC network to another network in order to work around overlapping IP addresses?

Answer: No, Cloud NAT cannot apply to any custom route whose next hop is not the default Internet gateway. For example, Cloud NAT cannot apply to traffic sent to a next hop Cloud VPN tunnel, even if the destination is a publicly routable IP address.

Question: Does Cloud NAT allow a source VM, whose network interface lacks an external IP address, to send traffic to a destination VM or load balancer that has an external IP address, even when the source and destination are in the same VPC network?

Answer: Yes. The network path involves sending traffic out of the VPC network through a default internet gateway, then receiving it in the same network — hairpinning.

When the source VM sends a packet to the destination, Cloud NAT performs source NAT (SNAT) before delivering the packet to the second instance. Cloud NAT performs destination NAT (DNAT) for responses from the second instance to the first. See NAT flow for a step-by-step example.

Unsolicited incoming connections not supported

Question: Does Cloud NAT allow for inbound connections (for example, SSH) to instances without external IP addresses?

Answer: No, Cloud NAT does not support unsolicited incoming connections. See Cloud NAT specifications.

If you need to connect to a VM that doesn't have an external IP address, see connecting to instances that do not have external IP addresses. For example, as part of the Cloud NAT Example Compute Engine Setup, you connect to a VM without an external IP address using Identity-Aware Proxy.

Cloud NAT and ports

Question: Why does a VM have a fixed number of ports (64 by default)?

Answer: When a Cloud NAT gateway provides NAT for a VM, it reserves source address and source port tuples according to the port reservation procedure.

See port reservation examples for more details.

Question: Can I change the minimum number of ports reserved for a VM?

Answer: Yes. You can increase or decrease the minimum number of ports per VM when you create a new Cloud NAT gateway or by editing it later. Each Cloud NAT gateway reserves source address and source port tuples according to the port reservation procedure.

For additional information about decreasing the minimum number of ports, refer to the next question.

Question: Can I decrease the minimum number of ports per VM after creating the Cloud NAT gateway?

Answer: Yes — however, decreasing the minimum number of ports could result in the port reservation procedure reserving a smaller number of ports per VM. When this happens, existing TCP connections are reset and must be re-established.

Cloud NAT and other Google services

Question: Does Cloud NAT enable access to Google APIs and services?

Answer: When you enable Cloud NAT for a subnet's primary IP range, Google Cloud automatically enables Private Google Access. See Private Google Access interaction for additional information.

What's next

  • See Using Cloud NAT for instructions on configuring and maintaining Cloud NAT.