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HAProxy ACL based on hostname

Access Control Lists in HAProxy let us shape traffic flow based on various criteria, offering control over our application’s routing.

Today, we are going to take a closer look at a powerful use case of HAProxy ACLs—host-based routing.

Understanding HAProxy ACLs

HAProxy ACLs act as a set of rules defining conditions for processing incoming requests. Furthermore, these rules are key when making decisions about how to route or handle requests, providing flexibility and control over our application’s traffic.

Hostname as a Criterion

One common use of ACLs in HAProxy is routing traffic based on the hostname in the HTTP request. This is often called “host-based routing”. So, it lets us direct requests to different backend servers based on the domain requested.

Configuring ACLs for Hostname Matching

In our HAProxy configuration file, ACLs are defined using the `acl` keyword.

Furthermore, for hostname-based ACLs, the `hdr` (header) or `hdr_dom` (domain) options are used.

acl acl_example hdr(host) -i example.com

So, the ACL named `acl_example` checks if the Host header in the HTTP request matches “example.com.”

Using ACLs in Configuration

Also, after defining ACLs, we can integrate them into our backend or frontend configuration to influence routing decisions.

use_backend example_backend if acl_example

This line tells HAProxy to route traffic to the `example_backend` if the condition specified in the ACL `acl_example` is true.

Consider the following simplified HAProxy configuration demonstrating hostname-based ACLs:

Hence, requests with the hostname “example.com” are directed to `example_backend`, while requests with the hostname “anotherdomain.com” are directed to `another_backend`.

Wildcard Matching

HAProxy also supports wildcard matching. So, this enables us to match multiple subdomains with a single ACL. For instance:

acl is_subdomain hdr(host) -i subdomain.*

So, this ACL would match any subdomain under “subdomain.”

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Conclusion

Today, our Support Engineers introduced us to HAProxy ACL based on hostname.