HAproxy

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HAProxy is a free, very fast and reliable solution offering high availability, load balancing, and proxying for TCP and HTTP-based applications. It is particularly suited for very high traffic web sites and powers quite a number of the world's most visited ones. Over the years it has become the de-facto standard opensource load balancer, is now shipped with most mainstream Linux distributions, and is often deployed by default in cloud platforms.

Installation

Install the haproxy package.

Running

Enable haproxy.service. HAProxy's configuration can be reloaded live by reloading haproxy.service as root.

Configuration

An example configuration is available in /etc/haproxy/haproxy.cfg. Edit it to suit your needs, and then start haproxy.service.

General configuration

ACLs

HAProxy supports ACLs, which can be used to test conditions and perform a given action based on the results of those tests. A typical ACL would be written as follows: 

/etc/haproxy/haproxy.cfg
acl photo_page path_beg /photos

In this case, the ACL is matched if the user's request path begins with /photos.

Backends

In HAProxy terminology, backends are a server or set of servers that will receive forwarded requests. Backends can balance load based on several load balancing algorithms, including:

  • Round-robin
  • Static round-robin (also known as weighted round-robin)
  • Least connections

An example backend may be written as follows: 

/etc/haproxy/haproxy.cfg
backend http-in
   balance roundrobin
   server s1 web1.example.com:80 check
   server s2 web2.example.com:80 check

Frontends

Frontends are used to define how requests should be forwarded to backends. They consist of the following:

  • IP addresses and ports
  • ACLs
  • use_backend rules

Health checks

When a backend is declared with the check option, HAProxy will check on startup and on scheduled intervals if the backend is available to process forwarded requests. If a backend fails the health check, it will be removed from rotation until it is deemed to be healthy again, i.e. it passes the health check.

By default, HAProxy will attempt to establish a TCP connection to the backend to determine healthiness.

If a large number of backends are declared with the check option, HAProxy will query all of them on startup, which may delay startup time.

Logging with systemd

To configure HAproxy to use systemd /dev/log compatibility socket add the following to your configuration file under the global section. 

log /dev/log local0 info

If you use the chroot option in your global configuration, you need to bind the socket into the chroot.

For this we will use a systemd.mount(5) unit. Generate the unit name with: 

# systemd-escape --suffix=mount --path /var/lib/haproxy/dev/log

and create a fresh unit with 

# systemctl edit --force --full var-lib-haproxy-dev-log.mount

We want mount /dev/log into the chroot, but just after journald came up. 

[Unit]
Requires=systemd-journald.service
Description=/Expose Systemd Log for HAProxy

[Mount]
What=/dev/log
Where=/var/lib/haproxy/dev/log
Type=none
Options=bind

With the mount file created it is time to expand the orignal service unit so it mounts up everything correctly. Edit haproxy.service and add 

[Unit]
Requires=var-lib-haproxy-dev-log.mount

Performing TLS/SSL termination

In order to use haproxy as a TLS terminator you have to set inside your frontend section 

bind :80
bind :443 ssl crt <path-to-combined-cert>
Note: To generate the cert you can run cat certificate-full-chain certificate-private-key > combined-cert.
Tip: To add support for HTTP/2 add alpn h2,http/1.1 at the end of the bind :443 line.
Redirecting HTTP to HTTPS

Set in your frontend section 

redirect scheme https code 301 if !{ ssl_fc }

Virtual host like configuration

Suppose you have two backends: foo and bar and each should handle requests only for a specific domain. In order to perform this in your frontend section you can configure 

use_backend foo-backend if { hdr(host) -i foo.example.com || hdr(host) -i www.foo.example.com }
use_backend bar-backend if { hdr(host) -i bar.example.com || hdr(host) -i www.bar.example.com }

See also