Use different backend depending on access-request hostname with HAProxy

frontend frontend_main
...
use_backend %[req.hdr(host),lower,map(/usr/local/etc/haproxy/subdomains.map,backend_default)]

he content of /usr/local/etc/haproxy/subdomains.map would look like this:

#domainname  backendname
one.example.com backend_one
two.example.com backend_two
...
etc.domain1.com backend_etc