global
        log /dev/log local0 debug
        chroot /var/lib/haproxy
        stats socket /run/haproxy/admin.sock mode 660 level admin
        stats timeout 30s
        user haproxy
        group haproxy
        daemon

defaults
        log     global
        mode    http
        option  httplog
        option  dontlognull
        timeout connect 5000
        timeout client  50000
        timeout server  50000
        errorfile 400 /etc/haproxy/errors/400.http
        errorfile 403 /etc/haproxy/errors/403.http
        errorfile 408 /etc/haproxy/errors/408.http
        errorfile 500 /etc/haproxy/errors/500.http
        errorfile 502 /etc/haproxy/errors/502.http
        errorfile 503 /etc/haproxy/errors/503.http
        errorfile 504 /etc/haproxy/errors/504.http

frontend webfront
  option  forwardfor
  stats enable
  stats uri /haproxy?statis
  stats realm Haproxy Auth
  stats auth user:password
  bind *:80
  timeout client 86400000
  acl is_discourse  hdr_end(host) -i discourse.mydomain.com
  use_backend       discourse     if is_discourse
  use_backend       webserver     if !is_discourse

backend discourse
  balance source
  option forwardfor
  option httpclose
  server server1 127.0.0.1:3080 weight 1 maxconn 1024 check inter 10000

backend webserver
  balance source
  option forwardfor
  option httpclose
  server server2 127.0.0.1:4080 weight 1 maxconn 1024 check inter 10000

root@kayak:/var/log/haproxy# tail haproxy.log
Nov 26 21:25:25 kayak haproxy[21646]: Proxy webfront started.
Nov 26 21:25:25 kayak haproxy[21646]: Proxy webfront started.
Nov 26 21:25:25 kayak haproxy[21646]: Proxy discourse started.
Nov 26 21:25:25 kayak haproxy[21646]: Proxy webserver started.
Nov 26 21:28:10 kayak haproxy[21868]: Proxy webfront started.
Nov 26 21:28:10 kayak haproxy[21868]: Proxy discourse started.
Nov 26 21:28:10 kayak haproxy[21868]: Proxy webserver started.
Nov 26 21:30:31 kayak haproxy[22045]: Proxy webfront started.
Nov 26 21:30:31 kayak haproxy[22045]: Proxy discourse started.
Nov 26 21:30:31 kayak haproxy[22045]: Proxy webserver started.

frontend webfront
  log /dev/log local0 debug