This assumes you are already familiar with the basic setup of HAProxy with Cookies and Sticky Sessions. If not, check out my article FreeKB - HAProxy (Load Balance) - Load Balance with Cookies and Sticky Sessions.

Let's say you are load balancing multiple subdomains that are part of the same domain, such as foo.example.com and bar.example.com.

frontend main
    bind *:80
    balance roundrobin
    option prefer-last-server

    acl foo hdr(host) -i foo.example.com
    use_backend foo if foo

    acl bar hdr(host) -i bar.example.com
    use_backend bar if bar

    default_backend foo

backend foo
    cookie sticky insert indirect nocache
    server server1 10.0.0.1:11111 check cookie server1
    server server2 10.0.0.2:11111 check cookie server2

backend bar
    cookie sticky insert indirect nocache
    server server1 10.0.0.1:22222 check cookie server1
    server server2 10.0.0.2:22222 check cookie server2

And when foo.example.com is requested perhaps they hit server1.

And when bar.example.com is requested perhaps they hit server2.

And you want to instead have it so that when any subdomain is request, subsequent requests are sticky to the same backend server. One option would be to use the domain option in the cookie directive. Notice in this example that the cookie directive contains "domain .example.com".

• cookie sticky means the name of the cookie will be "sticky"
• insert is used to create the cookie
• indirect removes the cookie on each incoming request before forwarding the message to the server
• nocache sets the Cache-Control: private HTTP header so that cache servers between HAProxy and the user won’t cache the response
• domain .example.com means a single cookie will be used for all subdomains in the .example.com domain

• In this example, the following log-format options were used

  • %ci logs the client IP address
  • %cp logs the client port
  • %b is the name of the backend that was requested
  • %CC cookie request (e.g. my_cookie=foo)
  • %s is the server name in the backend
  • %ST is the HTTP response code (e.g. 200 OK or 404 Not Found or 500 Internal Server Error)
  • %r logs HTTP requests (e.g. GET /index.html HTTP/1.1)

   

frontend main
    bind *:80
    balance roundrobin
    option prefer-last-server
    
    capture cookie sticky len 32
    log-format %[date]\ %ci:%cp\ %b\ %CC\ %s\ %ST\ %r

    acl foo hdr(host) -i foo.example.com
    use_backend foo if foo

    acl bar hdr(host) -i bar.example.com
    use_backend bar if bar

    default_backend foo

backend foo
    cookie sticky insert indirect nocache domain .example.com
    server server1 10.0.0.1:11111 check cookie server1
    server server2 10.0.0.2:11111 check cookie server2

backend bar
    cookie sticky insert indirect nocache domain .example.com
    server server1 10.0.0.1:22222 check cookie server1
    server server2 10.0.0.2:22222 check cookie server2

And when going to any domain in the example.com domain there will be a single cookie. For example, when requesting foo.example.com if you hit server1 and you then went to bar.example.com, you'll remain on server1.

And the haproxy.log should show that both foo.example.com and bar.example.com were sticky to the same server.

~]$ sudo tail /var/log/haproxy.log -f
Jun  3 05:24:17 localhost haproxy[1517]: 135.111.212.50:57242 foo sticky=server1 server1 GET / HTTP/1.1
Jun  3 05:24:20 localhost haproxy[1517]: 135.111.212.50:57263 bar sticky=server1 server1 GET / HTTP/1.1