Application Load Balancers - Elastic Load Balancing
Subnets for your load balancerLoad balancer security groupsLoad balancer stateLoad balancer attributesIP address typeLoad balancer connectionsCross-zone load balancingDNS name

Application Load Balancers

A load balancer serves as the single point of contact for clients. Clients send requests to the load balancer, and the load balancer sends them to targets, such as EC2 instances. To configure your load balancer, you create target groups, and then register targets with your target groups. You also create listeners to check for connection requests from clients, and listener rules to route requests from clients to the targets in one or more target groups.

For more information, see How Elastic Load Balancing works in the Elastic Load Balancing User Guide.

Contents

Subnets for your load balancer

When you create an Application Load Balancer, you must enable the zones that contain your targets. To enable a zone, specify a subnet in the zone. Elastic Load Balancing creates a load balancer node in each zone that you specify.

Considerations

  • Your load balancer is most effective when you ensure that each enabled zone has at least one registered target.

  • If you register targets in a zone but do not enable the zone, these registered targets do not receive traffic from the load balancer.

  • If you enable multiple zones for your load balancer, the zones must be of the same type. For example, you can't enable both an Availability Zone and a Local Zone.

  • You can specify a subnet that was shared with you.

Application Load Balancers support the following types of subnets.

Availability Zone subnets

You must select at least two Availability Zone subnets. The following restrictions apply:

  • Each subnet must be from a different Availability Zone.

  • To ensure that your load balancer can scale properly, verify that each Availability Zone subnet for your load balancer has a CIDR block with at least a /27 bitmask (for example, 10.0.0.0/27) and at least eight free IP addresses per subnet. These eight IP addresses are required to allow the load balancer to scale out if needed. Your load balancer uses these IP addresses to establish connections with the targets. Without them your Application Load Balancer could experience difficulties with node replacement attempts, causing it to enter a failed state.

    Note: If an Application Load Balancers subnet runs out of usable IP addresses while attempting to scale, the Application Load Balancer will run with insufficient capacity. During this time old nodes will continue to serve traffic, but the stalled scaling attempt may cause 5xx errors or timeouts when attempting to establish a connection.

Local Zone subnets

You can specify one or more Local Zone subnets. The following restrictions apply:

  • You cannot use AWS WAF with the load balancer.

  • You cannot use a Lambda function as a target.

  • You cannot use sticky sessions or application stickiness.

Outpost subnets

You can specify a single Outpost subnet. The following restrictions apply:

  • You must have installed and configured an Outpost in your on-premises data center. You must have a reliable network connection between your Outpost and its AWS Region. For more information, see the AWS Outposts User Guide.

  • The load balancer requires two large instances on the Outpost for the load balancer nodes. The supported instance types are shown in the following table. The load balancer scales as needed, resizing the nodes one size at a time (from large to xlarge, then xlarge to 2xlarge, and then 2xlarge to 4xlarge). After scaling the nodes to the largest instance size, if you need additional capacity, the load balancer adds 4xlarge instances as load balancer nodes. If you do not have sufficient instance capacity or available IP addresses to scale the load balancer, the load balancer reports an event to the AWS Health Dashboard and the load balancer state is active_impaired.

  • You can register targets by instance ID or IP address. If you register targets in the AWS Region for the Outpost, they are not used.

  • The following features are not available: Lambda functions as targets, AWS WAF integration, sticky sessions, authentication support, and integration with AWS Global Accelerator.

An Application Load Balancer can be deployed on c5/c5d, m5/m5d, or r5/r5d instances on an Outpost. The following table shows the size and EBS volume per instance type that the load balancer can use on an Outpost:

Instance type and size EBS volume (GB)
c5/c5d
large 50
xlarge 50
2xlarge 50
4xlarge 100
m5/m5d
large 50
xlarge 50
2xlarge 100
4xlarge 100
r5/r5d
large 50
xlarge 100
2xlarge 100
4xlarge 100

Load balancer security groups

A security group acts as a firewall that controls the traffic allowed to and from your load balancer. You can choose the ports and protocols to allow for both inbound and outbound traffic.

The rules for the security groups that are associated with your load balancer must allow traffic in both directions on both the listener and the health check ports. Whenever you add a listener to a load balancer or update the health check port for a target group, you must review your security group rules to ensure that they allow traffic on the new port in both directions. For more information, see Recommended rules.

Load balancer state

A load balancer can be in one of the following states:

provisioning

The load balancer is being set up.

active

The load balancer is fully set up and ready to route traffic.

active_impaired

The load balancer is routing traffic but does not have the resources it needs to scale.

failed

The load balancer could not be set up.

Load balancer attributes

You can configure your Application Load Balancer by editing its attributes. For more information, see Edit load balancer attributes.

The following are the load balancer attributes:

access_logs.s3.enabled

Indicates whether access logs stored in Amazon S3 are enabled. The default is false.

access_logs.s3.bucket

The name of the Amazon S3 bucket for the access logs. This attribute is required if access logs are enabled. For more information, see Enable access logs.

access_logs.s3.prefix

The prefix for the location in the Amazon S3 bucket.

client_keep_alive.seconds

The client keepalive value, in seconds. The default is 3600 seconds.

deletion_protection.enabled

Indicates whether deletion protection is enabled. The default is false.

idle_timeout.timeout_seconds

The idle timeout value, in seconds. The default is 60 seconds.

ipv6.deny_all_igw_traffic

Blocks internet gateway (IGW) access to the load balancer, preventing unintended access to your internal load balancer through an internet gateway. It is set to false for internet-facing load balancers and true for internal load balancers. This attribute does not prevent non-IGW internet access (such as, through peering, Transit Gateway, AWS Direct Connect, or AWS VPN).

routing.http.desync_mitigation_mode

Determines how the load balancer handles requests that might pose a security risk to your application. The possible values are monitor, defensive, and strictest. The default is defensive.

routing.http.drop_invalid_header_fields.enabled

Indicates whether HTTP headers with header fields that are not valid are removed by the load balancer (true), or routed to targets (false). The default is false. Elastic Load Balancing requires that valid HTTP header names conform to the regular expression [-A-Za-z0-9]+, as described in the HTTP Field Name Registry. Each name consists of alphanumeric characters or hyphens. Select true if you want HTTP headers that do not conform to this pattern, to be removed from requests.

routing.http.preserve_host_header.enabled

Indicates whether the Application Load Balancer should preserve the Host header in the HTTP request and send it to targets without any change. The possible values are true and false. The default is false.

routing.http.x_amzn_tls_version_and_cipher_suite.enabled

Indicates whether the two headers (x-amzn-tls-version and x-amzn-tls-cipher-suite), which contain information about the negotiated TLS version and cipher suite, are added to the client request before sending it to the target. The x-amzn-tls-version header has information about the TLS protocol version negotiated with the client, and the x-amzn-tls-cipher-suite header has information about the cipher suite negotiated with the client. Both headers are in OpenSSL format. The possible values for the attribute are true and false. The default is false.

routing.http.xff_client_port.enabled

Indicates whether the X-Forwarded-For header should preserve the source port that the client used to connect to the load balancer. The possible values are true and false. The default is false.

routing.http.xff_header_processing.mode

Enables you to modify, preserve, or remove the X-Forwarded-For header in the HTTP request before the Application Load Balancer sends the request to the target. The possible values are append, preserve, and remove. The default is append.

  • If the value is append, the Application Load Balancer adds the client IP address (of the last hop) to the X-Forwarded-For header in the HTTP request before it sends it to targets.

  • If the value is preserve, the Application Load Balancer preserves the X-Forwarded-For header in the HTTP request, and sends it to targets without any change.

  • If the value is remove, the Application Load Balancer removes the X-Forwarded-For header in the HTTP request before it sends it to targets.

routing.http2.enabled

Indicates whether HTTP/2 is enabled. The default is true.

waf.fail_open.enabled

Indicates whether to allow a AWS WAF-enabled load balancer to route requests to targets if it is unable to forward the request to AWS WAF. The possible values are true and false. The default is false.

Note

The routing.http.drop_invalid_header_fields.enabled attribute was introduced to offer HTTP desync protection. The routing.http.desync_mitigation_mode attribute was added to provide more comprehensive protection from HTTP desync for your applications. You aren't required to use both attributes and may choose either, depending on your application's requirements.

IP address type

You can set the types of IP addresses that clients can use to access your internet-facing and internal load balancers.

Application Load Balancers support the following IP address types:

ipv4

Clients must connect to the load balancer using IPv4 addresses (for example, 192.0.2.1)

dualstack

Clients can connect to the load balancer using both IPv4 addresses (for example, 192.0.2.1) and IPv6 addresses (for example, 2001:0db8:85a3:0:0:8a2e:0370:7334).

Considerations

  • The load balancer communicates with targets based on the IP address type of the target group.

  • When you enable dualstack mode for the load balancer, Elastic Load Balancing provides an AAAA DNS record for the load balancer. Clients that communicate with the load balancer using IPv4 addresses resolve the A DNS record. Clients that communicate with the load balancer using IPv6 addresses resolve the AAAA DNS record.

  • Access to your internal dualstack load balancers through the internet gateway is blocked to prevent unintended internet access. However, this does not prevent non-IGW internet access (such as, through peering, Transit Gateway, AWS Direct Connect, or AWS VPN).

dualstack-without-public-ipv4

Clients must connect to the load balancer using IPv6 addresses (for example, 2001:0db8:85a3:0:0:8a2e:0370:7334).

Considerations

  • Application Load Balancer authentication only supports IPv4 when connecting to an Identity Provider (IdP) or Amazon Cognito endpoint. Without a public IPv4 address the load balancer cannot complete the authentication process, resulting in HTTP 500 errors.

For more information about IP address types, see Update the IP address types for your Application Load Balancer.

Load balancer connections

When processing a request, the load balancer maintains two connections: one connection with the client and one connection with a target. The connection between the load balancer and the client is also referred to as the front-end connection. The connection between the load balancer and the target is also referred to as the back-end connection.

Cross-zone load balancing

With Application Load Balancers, cross-zone load balancing is on by default and cannot be changed at the load balancer level. For more information, see the Cross-zone load balancing section in the Elastic Load Balancing User Guide.

Turning off cross-zone load balancing is possible at the target group level. For more information, see Turn off cross-zone load balancing.

DNS name

Each Application Load Balancer receives a default Domain Name System (DNS) name with the following syntax: name-id.elb.region.amazonaws.com. For example, my-load-balancer-1234567890abcdef.elb.us-east-2.amazonaws.com.

If you'd prefer to use a DNS name that is easier to remember, you can create a custom domain name and associate it with the DNS name for your Application Load Balancer. When a client makes a request using this custom domain name, the DNS server resolves it to the DNS name for your Application Load Balancer.

First, register a domain name with an accredited domain name registrar. Next, use your DNS service, such as your domain registrar, to create a DNS record to route requests to your Application Load Balancer. For more information, see the documentation for your DNS service. For example, if you use Amazon Route 53 as your DNS service, you create an alias record that points to your Application Load Balancer. For more information, see Routing traffic to an ELB load balancer in the Amazon Route 53 Developer Guide.

The Application Load Balancer has one IP address per enabled Availability Zone. These are the IP addresses of the Application Load Balancer nodes. The DNS name of the Application Load Balancer resolves to these addresses. For example, suppose that the custom domain name for your Application Load Balancer is example.applicationloadbalancer.com. Use the following dig or nslookup command to determine the IP addresses of the Application Load Balancer nodes.

Linux or Mac

$ dig +short example.applicationloadbalancer.com

Windows

C:\> nslookup example.applicationloadbalancer.com

The Application Load Balancer has DNS records for its nodes. You can use DNS names with the following syntax to determine the IP addresses of the Application Load Balancer nodes: az.name-id.elb.region.amazonaws.com.

Linux or Mac

$ dig +short us-east-2b.my-load-balancer-1234567890abcdef.elb.us-east-2.amazonaws.com

Windows

C:\> nslookup us-east-2b.my-load-balancer-1234567890abcdef.elb.us-east-2.amazonaws.com