How do I set a dynamic variable in HAProxy?

Question

Is it possible to set a dynamic variable which will hold the content of HTTP header e.g. Host/X-Forwarded-Host and would be used later in ACLs?

frontend web1
  # ...
  set-var s1(Host)
  acl site1 hdr_end(host) -i %[s1]
  # ...
  use_backend %[s1] if site1

Solution

You have a mix of techniques here. You don't need variables at all to set ACLs based on the host address and select a backend using those ACLs. That would be something simple like:

frontend web1
  # ...
  acl site1 hdr(host) -i example.com
  acl site2 hdr(host) -i example.net
  # ...
  use_backend com if site1
  use_backend net if site2

Is that all you're trying to do, or are you trying to accomplish something more sophisticated?

UPDATE: Here's how to select a backend based on the Host header:

frontend web1
  # ..
  http-request set-var(req.s1) req.hdr(Host),field(1,:),lower,regsub(\.,_,g)
  use_backend %[var(req.s1)]

backend example_com
  # ..

backend example_net
  # ..

This sets a variable that is valid in the context of the request, using the value of the Host header lowercased and with periods replaced with underscores. Actually, you don't even need a variable:

frontend web1
  # ..
  use_backend %[req.hdr(Host),field(1,:),lower,regsub(\.,_,g)]

HAproxy will return 503 if a backend that matches the Host header cannot be found. You can set a default_backend if you want such requests to go somewhere else (I tested this and it works in 1.6.3, at least).