Backend service-based external passthrough Network Load Balancer overview

External passthrough Network Load Balancers are regional, layer 4 load balancers that distribute external traffic among backends (instance groups or network endpoint groups (NEGs)) in the same region as the load balancer. These backends must be in the same region and project but can be in different VPC networks. These load balancers are built on Maglev and the Andromeda network virtualization stack.

External passthrough Network Load Balancers can receive traffic from:

  • Any client on the internet
  • Google Cloud VMs with external IPs
  • Google Cloud VMs that have internet access through Cloud NAT or instance-based NAT

External passthrough Network Load Balancers are not proxies. The load balancer itself doesn't terminate user connections. Load-balanced packets are sent to the backend VMs with their source and destination IP addresses, protocol, and, if applicable, ports, unchanged. The backend VMs then terminate user connections. Responses from the backend VMs go directly to the clients, not back through the load balancer. This process is known as direct server return (DSR).

Backend service-based external passthrough Network Load Balancers support the following features:

  • Managed and unmanaged instance group backends. Backend service-based external passthrough Network Load Balancers support both managed and unmanaged instance groups as backends. Managed instance groups automate certain aspects of backend management and provide better scalability and reliability as compared to unmanaged instance groups.
  • Zonal NEG backends. Backend service-based external passthrough Network Load Balancers support using zonal NEGs with GCE_VM_IP endpoints. Zonal NEG GCE_VM_IP endpoints let you do the following:
    • Forward packets to any network interface, not just nic0.
    • Place the same GCE_VM_IP endpoint in two or more zonal NEGs connected to different backend services.
  • Support for multiple protocols. Backend service-based external passthrough Network Load Balancers can load-balance TCP, UDP, ESP, GRE, ICMP, and ICMPv6 traffic.
  • Support for IPv6 connectivity. Backend service-based external passthrough Network Load Balancers can handle both IPv4 and IPv6 traffic.
  • Fine-grained traffic distribution control. A backend service allows traffic to be distributed according to the configured session affinity, connection tracking policy, and weighted load balancing settings. The backend service can also be configured to enable connection draining and designate failover backends for the load balancer. Most of these settings have default values that let you get started quickly. For more information, see Traffic distribution for external passthrough Network Load Balancers.
  • Support for non-legacy health checks. Backend service-based external passthrough Network Load Balancers let you use health checks that match the type of traffic (TCP, SSL, HTTP, HTTPS, or HTTP/2) that they are distributing.
  • Google Cloud Armor integration. Google Cloud Armor supports advanced network DDoS protection for external passthrough Network Load Balancers. For more information, see Configure advanced network DDoS protection.

  • GKE integration. If you are building applications in GKE, we recommend that you use the built-in GKE Service controller, which deploys Google Cloud load balancers on behalf of GKE users. This is the same as the standalone load balancing architecture described on this page, except that its lifecycle is fully automated and controlled by GKE.

    Related GKE documentation:

Architecture

The following diagram illustrates the components of an external passthrough Network Load Balancer:

External passthrough Network Load Balancer with a regional backend service
External passthrough Network Load Balancer with a regional backend service

The load balancer is made up of several configuration components. A single load balancer can have the following:

  • One or more regional external IP addresses
  • One or more regional external forwarding rules
  • One regional external backend service
  • One or more backends: either all instance groups or all zonal NEG backends (GCE_VM_IP endpoints)
  • Health check associated with the backend service

Additionally, you must create firewall rules that allow your load balancing traffic and health check probes to reach the backend VMs.

IP address

An external passthrough Network Load Balancer requires at least one forwarding rule. The forwarding rule references a regional external IP address that is accessible anywhere on the internet.

Use a reserved IP address for the forwarding rule if you need to keep the address associated with your project for reuse after you delete a forwarding rule or if you need multiple forwarding rules to reference the same IP address.

External passthrough Network Load Balancers support both Standard Tier and Premium Tier for regional external IPv4 addresses. Both the IP address and the forwarding rule must use the same network tier. Regional external IPv6 addresses are only available in the Premium Tier.

Forwarding rule

A regional external forwarding rule specifies the protocol and ports on which the load balancer accepts traffic. Because external passthrough Network Load Balancers are not proxies, they pass traffic to backends on the same protocol and ports, if the packet carries port information. The forwarding rule in combination with the IP address forms the frontend of the load balancer.

The load balancer preserves the source IP addresses of incoming packets. The destination IP address for incoming packets is an IP address associated with the load balancer's forwarding rule.

Incoming traffic is matched to a forwarding rule, which is a combination of a particular IP address (either an IPv4 address or an IPv6 address range), protocol, and if the protocol is port-based, one of port(s), a range of ports, or all ports. The forwarding rule then directs traffic to the load balancer's backend service.

  • If the forwarding rule references an IPv4 address, the forwarding rule is not associated with any subnet. That is, its IP address comes from outside of any Google Cloud subnet range.

  • If the forwarding rule references a /96 IPv6 address range, the forwarding rule must be associated with a subnet, and that subnet must be (a) dual-stack and (b) have an external IPv6 subnet range (--ipv6-access-type set to EXTERNAL). The subnet that the forwarding rule references can be the same subnet used by the backend instances; however, backend instances can use a separate subnet if chosen. When backend instances use a separate subnet, the following must be true:

An external passthrough Network Load Balancer requires at least one forwarding rule. Forwarding rules can be configured to direct traffic coming from a specific range of source IP addresses to a specific backend service (or target instance). For details, see traffic steering. You can define multiple forwarding rules for the same load balancer as described in Multiple forwarding rules.

If you want the load balancer to handle both IPv4 and IPv6 traffic, create two forwarding rules: one rule for IPv4 traffic that points to IPv4 (or dual-stack) backends, and one rule for IPv6 traffic that points only to dual-stack backends. It's possible to have an IPv4 and an IPv6 forwarding rule reference the same backend service, but the backend service must reference dual-stack backends.

Forwarding rule protocols

External passthrough Network Load Balancers support the following protocol options for each forwarding rule: TCP, UDP, and L3_DEFAULT.

Use the TCP and UDP options to configure TCP or UDP load balancing. The L3_DEFAULT protocol option enables an external passthrough Network Load Balancer to load balance TCP, UDP, ESP, GRE, ICMP, and ICMPv6 traffic.

In addition to supporting protocols other than TCP and UDP, L3_DEFAULT makes it possible for a single forwarding rule to serve multiple protocols. For example, IPsec services typically handle some combination of ESP and UDP-based IKE and NAT-T traffic. The L3_DEFAULT option allows a single forwarding rule to be configured to process all of those protocols.

Forwarding rules using the TCP or UDP protocols can reference a backend service using either the same protocol as the forwarding rule or a backend service whose protocol is UNSPECIFIED. L3_DEFAULT forwarding rules can only reference a backend service with protocol UNSPECIFIED.

If you're using the L3_DEFAULT protocol, you must configure the forwarding rule to accept traffic on all ports. To configure all ports, either set --ports=ALL by using the Google Cloud CLI, or set allPorts to True by using the API.

The following table summarizes how to use these settings for different protocols.

Traffic to be load balanced Forwarding rule protocol Backend service protocol
TCP TCP TCP or UNSPECIFIED
L3_DEFAULT UNSPECIFIED
UDP UDP UDP or UNSPECIFIED
L3_DEFAULT UNSPECIFIED
ESP, GRE, ICMP/ICMPv6 (echo request only) L3_DEFAULT UNSPECIFIED

Multiple forwarding rules

You can configure multiple regional external forwarding rules for the same external passthrough Network Load Balancer. Each forwarding rule can have a different regional external IP address, or multiple forwarding rules can have the same regional external IP address.

Configuring multiple regional external forwarding rules can be useful for these use cases:

  • You need to configure more than one external IP address for the same backend service.
  • You need to configure different protocols or non-overlapping ports or port ranges for the same external IP address.
  • You need to steer traffic from certain source IP addresses to specific load balancer backends.

Google Cloud requires that incoming packets match no more than one forwarding rule. Except for steering forwarding rules, which are discussed in the next section, two or more forwarding rules that use the same regional external IP address must have unique protocol and port combinations according to these constraints:

  • A forwarding rule configured for all ports of a protocol prevents the creation of other forwarding rules using the same protocol and IP address. Forwarding rules using TCP or UDP protocols can be configured to use all ports, or they can be configured for specific ports. For example, if you create a forwarding rule using IP address 198.51.100.1, the TCP protocol, and all ports, you cannot create any other forwarding rule using IP address 198.51.100.1and the TCP protocol. You can create two forwarding rules, both using the IP address 198.51.100.1 and the TCP protocol, if each one has unique ports or non-overlapping port ranges. For example, you can create two forwarding rules using IP address 198.51.100.1and the TCP protocol, where one forwarding rule's ports are 80,443 and the other uses the port range 81-442.
  • Only one L3_DEFAULT forwarding rule can be created per IP address. This is because the L3_DEFAULT protocol uses all ports by definition. In this context, the all ports term includes protocols without port information.

  • A single L3_DEFAULT forwarding rule can coexist with other forwarding rules that use specific protocols (TCP or UDP). The L3_DEFAULT forwarding rule can be used as a last resort when forwarding rules using the same IP address but more specific protocols exist. An L3_DEFAULT forwarding rule processes packets sent to its destination IP address if and only if the packet's destination IP address, protocol, and destination port don't match a protocol-specific forwarding rule.

    To illustrate this, consider these two scenarios. Forwarding rules in both scenarios use the same IP address 198.51.100.1.

    • Scenario 1. The first forwarding rule uses the L3_DEFAULT protocol. The second forwarding rule uses the TCP protocol and all ports. TCP packets sent to any destination port of 198.51.100.1 are processed by the second forwarding rule. Packets using different protocols are processed by the first forwarding rule.
    • Scenario 2. The first forwarding rule uses the L3_DEFAULT protocol. The second forwarding rule uses the TCP protocol and port 8080. TCP packets sent to 198.51.100.1:8080 are processed by the second forwarding rule. All other packets, including TCP packets sent to different destination ports, are processed by the first forwarding rule.

Forwarding rule selection

Google Cloud selects one or zero forwarding rules to process an incoming packet by using this elimination process, starting with the set of forwarding rule candidates which match the destination IP address of the packet:

  • Eliminate forwarding rules whose protocol doesn't match the packet's protocol, except for L3_DEFAULT forwarding rules. Forwarding rules using the L3_DEFAULT protocol are never eliminated by this step because L3_DEFAULT matches all protocols. For example, if the packet's protocol is TCP, only forwarding rules using the UDP protocol are eliminated.

  • Eliminate forwarding rules whose port doesn't match the packet's port. Forwarding rules configured for all ports are never eliminated by this step because an all ports forwarding rule matches any port.

  • If the remaining forwarding rule candidates include both L3_DEFAULT and protocol specific forwarding rules, eliminate the L3_DEFAULT forwarding rules. If the remaining forwarding rule candidates are all L3_DEFAULT forwarding rules, none are eliminated at this step.

  • At this point, either the remaining forwarding rule candidates fall into one of the following categories:

    • A single forwarding rule remains which matches the packet's destination IP address, protocol, and port, and is used to route the packet.
    • Two or more forwarding rule candidates remain which match the packet's destination IP address, protocol, and port. This means the remaining forwarding rule candidates include steering forwarding rules (discussed in the next section). Select the steering forwarding rule whose source range includes the most specific (longest prefix match) CIDR containing the packet's source IP address. If no steering forwarding rules have a source range including the packet's source IP address, select the parent forwarding rule.
    • Zero forwarding rule candidates remain and the packet is dropped.

When using multiple forwarding rules, make sure that you configure the software running on your backend VMs so that it binds to all the external IP address(es) of the load balancer's forwarding rule(s).

Traffic steering

Forwarding rules for external passthrough Network Load Balancers can be configured to direct traffic coming from a specific range of source IP addresses to a specific backend service (or target instance).

Traffic steering is useful for troubleshooting and for advanced configurations. With traffic steering, you can direct certain clients to a different set of backends, a different backend service configuration, or both. For example:

  • Traffic steering lets you create two forwarding rules which direct traffic to the same backend (instance group or NEG) by way of two backend services. The two backend services can be configured with different health checks, different session affinities, or different traffic distribution control policies (connection tracking, connection draining, and failover).
  • Traffic steering lets you create a forwarding rule to redirect traffic from a low-bandwidth backend service to a high-bandwidth backend service. Both backend services contain the same set of backend VMs or endpoints, but load-balanced with different weights using weighted load balancing.
  • Traffic steering lets you create two forwarding rules which direct traffic to different backend services, with different backends (instance groups or NEGs). For example, one backend could be configured using different machine types in order to better process traffic from a certain set of source IP addresses.

Traffic steering is configured with a forwarding rule API parameter called sourceIPRanges. Forwarding rules that have at least one source IP range configured are called steering forwarding rules.

A steering forwarding rule can have a list of up to 64 source IP ranges. You can update the list of source IP ranges configured for a steering forwarding rule at any time.

Each steering forwarding rule requires that you first create a parent forwarding rule. The parent and steering forwarding rules share the same regional external IP address, IP protocol, and port information; however, the parent forwarding rule does not have any source IP address information. For example:

  • Parent forwarding rule: IP address: 198.51.100.1, IP protocol: TCP, ports: 80
  • Steering forwarding rule: IP address: 198.51.100.1, IP protocol: TCP, ports: 80, sourceIPRanges: 203.0.113.0/24

A parent forwarding rule that points to a backend service can be associated with a steering forwarding rule that points to a backend service or a target instance.

For a given parent forwarding rule, two or more steering forwarding rules can have overlapping, but not identical, source IP ranges. As an example, one steering forwarding rule can have the source IP range 203.0.113.0/24 and another steering forwarding rule for the same parent can have the source IP range 203.0.113.0.

You must delete all steering forwarding rules before you can delete the parent forwarding rule upon which they depend.

To learn how incoming packets are processed when steering forwarding rules are used, see Forwarding rule selection.

Session affinity behavior across steering changes

This section describes the conditions under which session affinity might break when the source IP ranges for steering forwarding rules are updated:

  • If an existing connection continues to match the same forwarding rule after you change the source IP ranges for a steering forwarding rule, session affinity doesn't break. If your change results in an existing connection matching a different forwarding rule, then:
  • Session affinity always breaks under these circumstances:
    • The newly matched forwarding rule directs an established connection to a backend service (or target instance) which doesn't reference the previously selected backend VM.
    • The newly matched forwarding rule directs an established connection to a backend service which does reference the previously selected backend VM, but the backend service is not configured to persist connections when backends are unhealthy, and the backend VM fails the backend service's health check.
  • Session affinity might break when the newly matched forwarding rule directs an established connection to a backend service, and the backend service does reference the previously selected VM, but the backend service's combination of session affinity and connection tracking mode results in a different connection tracking hash.

Preserving session affinity across steering changes

This section describes how to avoid breaking session affinity when the source IP ranges for steering forwarding rules are updated:

  • Steering forwarding rules pointing to backend services. If both the parent and the steering forwarding rule point to backend services, you'll need to manually make sure that the session affinity and connection tracking policy settings are identical. Google Cloud does not automatically reject configurations if they are not identical.
  • Steering forwarding rules pointing to target instances. A parent forwarding rule that points to a backend service can be associated with a steering forwarding rule that points to a target instance. In this case, the steering forwarding rule inherits session affinity and connection tracking policy settings from the parent forwarding rule.

For instructions on how to configure traffic steering, see Configure traffic steering.

Regional backend service

Each external passthrough Network Load Balancer has one regional backend service that defines the behavior of the load balancer and how traffic is distributed to its backends. The name of the backend service is the name of the external passthrough Network Load Balancer shown in the Google Cloud console.

Each backend service defines the following backend parameters:

  • Protocol. A backend service accepts traffic on the IP address and ports (if configured) specified by one or more regional external forwarding rules. The backend service passes packets to backend VMs while preserving the packet's source and destination IP addresses, protocol, and, if the protocol is port-based, the source and destination ports.

    Backend services used with external passthrough Network Load Balancers support the following protocol options: TCP, UDP, or UNSPECIFIED.

    Backend services with the UNSPECIFIED protocol can be used with any forwarding rule regardless of the forwarding rule protocol. Backend services with a specific protocol (TCP or UDP) can only be referenced by forwarding rules with the same protocol (TCP or UDP). Forwarding rules with the L3_DEFAULT protocol can only refer to backend services with the UNSPECIFIED protocol.

    See Forwarding rule protocol specification for a table with possible forwarding rule and backend service protocol combinations.

  • Traffic distribution. A backend service allows traffic to be distributed according to the configured session affinity, connection tracking policy, and weighted load balancing settings. The backend service can also be configured to enable connection draining and designate failover backends for the load balancer. Most of these settings have default values that let you get started quickly. For more information, see Traffic distribution for external passthrough Network Load Balancers.

  • Health check. A backend service must have an associated regional health check.

  • Backends: each backend service operates in a single region and distributes traffic to either instance groups or zonal NEGs in the same region. You can use either instance groups or zonal NEGs, but not a combination of both, as backends for an external passthrough Network Load Balancer:

    • If you choose instance groups, you can use unmanaged instance groups, zonal managed instance groups, regional managed instance groups, or a combination of instance group types.
    • If you choose zonal NEGs, you must use GCE_VM_IP zonal NEGs.

    Each instance group or NEG backend has an associated VPC network, even if that instance group or NEG hasn't been connected to a backend service yet. For more information about how a network is associated with each type of backend, see Instance group backends and network interfaces and Zonal NEG backends and network interfaces.

Instance groups

An external passthrough Network Load Balancer distributes connections among backend VMs contained within managed or unmanaged instance groups. Instance groups can be regional or zonal in scope.

An external passthrough Network Load Balancer only distributes traffic to the first network interface (nic0) of backend VMs. The load balancer supports instance groups whose member instances use any VPC network in the same region, as long as the VPC network is in the same project as the backend service. (All VMs within a given instance group must use the same VPC network.)

Each instance group has an associated VPC network, even if that instance group hasn't been connected to a backend service yet. For more information about how a network is associated with instance groups, see Instance group backends and network interfaces.

The external passthrough Network Load Balancer is highly available by design. There are no special steps needed to make the load balancer highly available because the mechanism doesn't rely on a single device or VM instance. You only need to make sure that your backend VM instances are deployed to multiple zones so that the load balancer can work around potential issues in any given zone.

  • Regional managed instance groups. Use regional managed instance groups if you can deploy your software by using instance templates. Regional managed instance groups automatically distribute traffic among multiple zones, providing the best option to avoid potential issues in any given zone.

    An example deployment using a regional managed instance group is shown here. The instance group has an instance template that defines how instances should be provisioned, and each group deploys instances within three zones of the us-central1 region.

    External passthrough Network Load Balancer with a regional managed instance group
    External passthrough Network Load Balancer with a regional managed instance group

  • Zonal managed or unmanaged instance groups. Use zonal instance groups in different zones (in the same region) to protect against potential issues in any given zone.

    An example deployment using zonal instance groups is shown here. This load balancer provides availability across two zones.

    External passthrough Network Load Balancer with zonal instance groups
    External passthrough Network Load Balancer with zonal instance groups

Zonal NEGs

An external passthrough Network Load Balancer distributes connections among GCE_VM_IP endpoints contained within zonal network endpoint groups. These endpoints must be located in the same region as the load balancer. For some recommended zonal NEG use cases, see Zonal network endpoint groups overview.

Endpoints in the NEG must be primary internal IPv4 addresses of VM network interfaces that are in the same subnet and zone as the zonal NEG. The primary internal IPv4 address from any network interface of a multi-NIC VM instance can be added to a NEG as long as it is in the NEG's subnet.

Zonal NEGs support both IPv4 and dual-stack (IPv4 and IPv6) VMs. For both IPv4 and dual-stack VMs, it is sufficient to specify only the VM instance when attaching an endpoint to a NEG. You don't need to specify the endpoint's IP address. The VM instance must always be in the same zone as the NEG.

Each zonal NEG has an associated VPC network and a subnet, even if that zonal NEG hasn't been connected to a backend service yet. For more information about how a network is associated with zonal NEGs, see Zonal NEG backends and network interfaces.

Instance group backends and network interfaces

The VPC network associated with an instance group is the VPC network used by every member VM's nic0 network interface.

  • For managed instance groups (MIGs), the VPC network for the instance group is defined in the instance template.
  • For unmanaged instance groups, the VPC network for the instance group is defined as the VPC network used by the nic0 network interface of the first VM instance that you add to the unmanaged instance group.

Within a given (managed or unmanaged) instance group, all VM instances must have their nic0 network interfaces in the same VPC network. Each member VM can optionally have additional network interfaces (nic1 through nic7), but each network interface must attach to a different VPC network. These networks must also be different from the VPC network associated with the instance group.

Backend services can't distribute traffic to instance group member VMs on non-nic0 interfaces. If you want to receive traffic on a non-default network interface (nic1 through nic7), you must use zonal NEGs with GCE_VM_IP endpoints.

Zonal NEG backends and network interfaces

When you create a new zonal NEG with GCE_VM_IP endpoints, you must explicitly associate the NEG with a subnetwork of a VPC network before you can add any endpoints to the NEG. Neither the subnet nor the VPC network can be changed after the NEG is created.

Within a given NEG, each GCE_VM_IP endpoint actually represents a network interface. The network interface must be in the subnetwork associated with the NEG. From the perspective of a Compute Engine instance, the network interface can use any identifier (nic0 through nic7). From the perspective of being an endpoint in a NEG, the network interface is identified by using its primary IPv4 address.

There are two ways to add a GCE_VM_IP endpoint to a NEG:

  • If you specify only a VM name (without any IP address) when adding an endpoint, Google Cloud requires that the VM has a network interface in the subnetwork associated with the NEG. The IP address that Google Cloud chooses for the endpoint is the primary internal IPv4 address of the VM's network interface in the subnetwork associated with the NEG.
  • If you specify both a VM name and an IP address when adding an endpoint, the IP address that you provide must be a primary internal IPv4 address for one of the VM's network interfaces. That network interface must be in the subnetwork associated with the NEG. Note that specifying an IP address is redundant because there can only be a single network interface that is in the subnetwork associated with the NEG.

Backend services and VPC networks

The backend service is not associated with any VPC network; however, each backend instance group or zonal NEG is associated with a VPC network, as noted previously. As long as all backends are located in the same region and project, and as long as all backends are of the same type (instance groups or zonal NEGs), you can add backends that use either the same or different VPC networks.

To distribute packets to nic1 through nic7, you must use zonal NEG backends (with GCE_VM_IP endpoints) because an instance group's associated VPC network always matches the VPC network used by the nic0 interface of all member instances.

Dual-stack backends (IPv4 and IPv6)

If you want the load balancer to use dual-stack backends that handle both IPv4 and IPv6 traffic, note the following requirements:

  • Backends must be configured in dual-stack subnets that are in the same region as the load balancer's IPv6 forwarding rule. For the backends, you can use a subnet with the ipv6-access-type set to either EXTERNAL or INTERNAL. If the backend subnet's ipv6-access-type is set to INTERNAL, you must use a different IPv6-only subnet with ipv6-access-type set to EXTERNAL for the load balancer's external forwarding rule.
  • Backends must be configured to be dual-stack with stack-type set to IPv4_IPv6. If the backend subnet's ipv6-access-type is set to EXTERNAL, you must also set the --ipv6-network-tier to PREMIUM. For instructions, see Create an instance template with IPv6 addresses.

IPv6-only backends

If you want the load balancer to use IPv6-only backends, note the following requirements:

  • IPv6-only instances are only supported in unmanaged instance groups. No other backend type is supported.
  • Backends must be configured in either dual-stack or IPv6-only subnets that are in the same region as the load balancer's IPv6 forwarding rule. For the backends, you can use a subnet with the ipv6-access-type set to either INTERNAL or EXTERNAL. If the backend subnet's ipv6-access-type is set to INTERNAL, you must use a different IPv6-only subnet with ipv6-access-type set to EXTERNAL for the load balancer's external forwarding rule.
  • Backends must be configured to be IPv6-only with the VM stack-type set to IPv6_ONLY. If the backend subnet's ipv6-access-type is set to EXTERNAL, you must also set the --ipv6-network-tier to PREMIUM. For instructions, see Create an instance template with IPv6 addresses.

Note that IPv6-only VMs can be created under both dual-stack and IPv6-only subnets, but dual-stack VMs can't be created under IPv6-only subnets.

Health checks

External passthrough Network Load Balancers use regional health checks to determine which instances can receive new connections. Each external passthrough Network Load Balancer's backend service must be associated with a regional health check. Load balancers use health check status to determine how to route new connections to backend instances.

For more details about how Google Cloud health checks work, see How health checks work.

External passthrough Network Load Balancers support the following types of health checks:

Health checks for other protocol traffic

Google Cloud doesn't offer any protocol-specific health checks beyond the ones listed in the Health checks section, earlier on this page. When you use an external passthrough Network Load Balancer to load balance a protocol other than TCP, you must still run a TCP-based service on your backend VMs to provide the required health check information.

For example, if you are load balancing UDP traffic, client requests are load balanced by using the UDP protocol, and you must run a TCP service to provide information to Google Cloud health check probers. To achieve this, you can run an HTTP server on each backend VM that returns an HTTP 200 response to health check probers. You should use your own logic running on the backend VM to ensure that the HTTP server returns 200 only if the UDP service is properly configured and running.

Firewall rules

Because external passthrough Network Load Balancers are passthrough load balancers, you control access to the load balancer's backends using Google Cloud firewall rules. You must create ingress allow firewall rules or an ingress allow hierarchical firewall policy to permit health checks and the traffic that you're load balancing.

Forwarding rules and ingress allow firewall rules or Hierarchical firewall policies work together in the following way: a forwarding rule specifies the protocol and, if defined, port requirements that a packet must meet to be forwarded to a backend VM. Ingress allow firewall rules control whether the forwarded packets are delivered to the VM or dropped. All VPC networks have an implied deny ingress firewall rule that blocks incoming packets from any source. The Google Cloud default VPC network includes a limited set of pre-populated ingress allow firewall rules.

  • To accept traffic from any IP address on the internet, you must create an ingress allow firewall rule with the 0.0.0.0/0 source range. To only allow traffic from certain IP address ranges, use more restrictive source ranges.

  • As a security best practice, your ingress allow firewall rules should only permit the IP protocols and ports that you need. Restricting the protocol (and, if possible, port) configuration is especially important when using forwarding rules whose protocol is set to L3_DEFAULT. L3_DEFAULT forwarding rules forward packets for all supported IP protocols (on all ports if the protocol and packet have port information).

  • External passthrough Network Load Balancers use Google Cloud health checks. Therefore, you must always allow traffic from the health check IP address ranges. These ingress allow firewall rules can be made specific to the protocol and ports of the load balancer's health check.

IP addresses for request and return packets

When a backend VM receives a load-balanced packet from a client, the packet's source and destination are:

  • Source: the external IP address associated with a Google Cloud VM or internet-routable IP address of a system connecting to the load balancer.
  • Destination: the IP address of the load balancer's forwarding rule.

Because the load balancer is a pass-through load balancer (not a proxy), packets arrive bearing the destination IP address of the load balancer's forwarding rule. Software running on backend VMs should be configured to do the following:

  • Listen on (bind to) the load balancer's forwarding rule IP address or any IP address (0.0.0.0 or ::)
  • If the load balancer forwarding rule's protocol supports ports: Listen on (bind to) a port that's included in the load balancer's forwarding rule

Return packets are sent directly from the load balancer's backend VMs to the client. The return packet's source and destination IP addresses depend on the protocol:

  • TCP is connection-oriented so backend VMs must reply with packets whose source IP addresses match the forwarding rule's IP address so that the client can associate the response packets with the appropriate TCP connection.
  • UDP, ESP, GRE, and ICMP are connectionless. Backend VMs can send response packets whose source IP addresses either match the forwarding rule's IP address or match any assigned external IP address for the VM. Practically speaking, most clients expect the response to come from the same IP address to which they sent packets.

The following table summarizes sources and destinations for response packets:

Traffic type Source Destination
TCP The IP address of the load balancer's forwarding rule The requesting packet's source
UDP, ESP, GRE, ICMP For most use cases, the IP address of the load balancer's forwarding rule † The requesting packet's source.

† When a VM has an external IP address or when you are using Cloud NAT, it is also possible to set the response packet's source IP address to the VM NIC's primary internal IPv4 address. Google Cloud or Cloud NAT changes the response packet's source IP address to either the NIC's external IPv4 address or a Cloud NAT external IPv4 address in order to send the response packet to the client's external IP address. Not using the forwarding rule's IP address as a source is an advanced scenario because the client receives a response packet from an external IP address that does not match the IP address to which it sent a request packet.

Return path

External passthrough Network Load Balancers use special routes outside of your VPC network to direct incoming requests and health check probes to each backend VM.

The load balancer preserves the source IP addresses of packets. Responses from the backend VMs go directly to the clients, not back through the load balancer. The industry term for this is direct server return.

Outbound internet connectivity from backends

VM instances configured as an external passthrough Network Load Balancer's backend endpoints can initiate connections to the internet using the load balancer's forwarding rule IP address as the source IP address of the outbound connection.

Generally, a VM instance always uses its own external IP address or Cloud NAT to initiate connections. You use the forwarding rule IP address to initiate connections from backend endpoints only in special scenarios such as when you need VM instances to originate and receive connections at the same external IP address, and you also need the backend redundancy provided by the external passthrough Network Load Balancer for inbound connections.

Outbound packets sent from backend VMs directly to the internet have no restrictions on traffic protocols and ports. Even if an outbound packet is using the forwarding rule's IP address as the source, the packet's protocol and source port don't have to match the forwarding rule's protocol and port specification. However, inbound response packets must match the forwarding rule IP address, protocol, and destination port of the forwarding rule. For more information, see Paths for external passthrough Network Load Balancers and external protocol forwarding.

Additionally, any responses to the VM's outbound connections are subject to load balancing, just like all the other incoming packets meant for the load balancer. This means that responses might not arrive on the same backend VM that initiated the connection to the internet. If the outbound connections and load balanced inbound connections share common protocols and ports, then you can try one of the following suggestions:

  • Synchronize outbound connection state across backend VMs, so that connections can be served even if responses arrive at a backend VM other than the one that has initiated the connection.

  • Use a failover configuration, with a single primary VM and a single backup VM. Then, the active backend VM that initiates the outbound connections always receives the response packets.

This path to internet connectivity from an external passthrough Network Load Balancer's backends is the default intended behavior according to Google Cloud's implied firewall rules. However, if you have security concerns about leaving this path open, you can use targeted egress firewall rules to block unsolicited outbound traffic to the internet.

Shared VPC architecture

Except for the IP address, all of the components of an external passthrough Network Load Balancer must exist in the same project. The following table summarizes Shared VPC components for external passthrough Network Load Balancers:

IP address Forwarding rule Backend components
A regional external IP address must be defined in either the same project as the load balancer or the Shared VPC host project. A regional external forwarding rule must be defined in the same project as the instances in the backend service.

The regional backend service must be defined in the same project and same region where the backends (instance group or zonal NEG) exist.

Health checks associated with the backend service must be defined in the same project and the same region as the backend service.

Traffic distribution

External passthrough Network Load Balancers support a variety of traffic distribution customization options, including session affinity, connection tracking, weighted load balancing, and failover. For details about how external passthrough Network Load Balancers distribute traffic, and how these options interact with each other, see Traffic distribution for external passthrough Network Load Balancers.

Limitations

  • You cannot use the Google Cloud console to do the following tasks:

    • Create or modify an external passthrough Network Load Balancer whose forwarding rule uses the L3_DEFAULT protocol.
    • Create or modify an external passthrough Network Load Balancer whose backend service protocol is set to UNSPECIFIED.
    • Create or modify an external passthrough Network Load Balancer that configures a connection tracking policy.
    • Create or modify source IP-based traffic steering for a forwarding rule.

    Use either the Google Cloud CLI or the REST API instead.

  • External passthrough Network Load Balancers don't support VPC Network Peering.

Pricing

For pricing information, see Pricing.

What's next