HAproxy 503 Service Unavailable No server is available to handle this request

Solution 1:

I've never used HAproxy but a quick search leads me to think you need to add default_backend app immediately below frontend main *:80. I see nowhere in that configuration connecting the backend and frontend together.

Solution 2:

Problem is in your HAProxy configuration. When I remove all comments from your config, I will get this:

global
  log         127.0.0.1 local2

  chroot      /var/lib/haproxy
  pidfile     /var/run/haproxy.pid
  maxconn     4000
  user        haproxy
  group       haproxy
  daemon

  stats socket /var/lib/haproxy/stats

defaults
  mode                    http
  log                     global
  option                  httplog
  option                  dontlognull
  option http-server-close
  option forwardfor       except 127.0.0.0/8
  option                  redispatch
  retries                 3
  timeout http-request    10s
  timeout queue           1m
  timeout connect         10s
  timeout client          1m
  timeout server          1m
  timeout http-keep-alive 10s
  timeout check           10s
  maxconn                 3000

frontend  main *:80

backend app
  mode tcp
  balance roundrobin
  server  server1 192.168.1.12:80 check inter 2000 rise 2 fall 5
  server  server2 192.168.1.13:80 check inter 2000 rise 2 fall 5

And now you can clearly see that there is no configuration of frontend at all. Requests comes to HAProxy via frontend main but HAProxy doesn't know which servers are reliable to handle it, so will return 503.

You have to link backend to frontend with default_backend or with acl.

You should use stats too, not only with socket but with protected web interface too. I can shows you information about clusters behind haproxy, which servers are offline, which has any problems, about response times and so on. Very usefull for debugging.

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Solution 3:

I got a similar error because HAProxy thought my backend was down due to the default health check it does. I disabled the health check and the 503 went away.

I'm using pfsense GUI: