haproxy.conf

global
  daemon
  maxconn 32
  tune.ssl.default-dh-param 2048

defaults
  mode http
  option forwardfor
  option http-server-close
  timeout connect 2s
  timeout client 5s
  timeout server 5s

frontend http-in
  bind *:80
  bind *:443 ssl crt /usr/local/etc/ssl/snake-oil.pem ciphers AES128+EECDH:AES128+EDH force-tlsv12 no-sslv3
  mode http

  acl out-1 hdr_end(host) -i api.dev

  use_backend http-out-1 if out-1
  default_backend http-out

backend http-out
  mode http
  server app 127.0.0.1:3000
  http-response set-header Access-Control-Allow-Origin "*"

backend http-out-1
  mode http
  server app 127.0.0.1:3001
  http-response set-header Access-Control-Allow-Origin "*"

homebrew.mxcl.haproxy.plist

<!-- /usr/local/opt/haproxy/homebrew.mxcl.haproxy.plist -->

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
  <dict>
    <key>Label</key>
    <string>homebrew.mxcl.haproxy</string>
    <key>ProgramArguments</key>
    <array>
      <string>/usr/local/bin/haproxy</string>
      <string>-f</string>
      <string>/usr/local/etc/haproxy.conf</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
    <key>KeepAlive</key>
    <false/>
  </dict>
</plist>