How to setup HTTP check for TCP backend in HAProxy?

@kaizenCoder wrote:

The documentation suggests that I can setup a HTTP check for a TCP backend.

The liveness of my backend is determined by a 405 Method Not allowed response when hitting a-app.com/ap_service

My config looks like this:

    frontend app-api
        bind *:443
        mode tcp
    
        option tcplog
        default_backend app-api_backend
    
    
    backend app-api_backend
        mode tcp
        option httpchk GET /app_service HTTP/1.1
        http-check expect status 405
        server a a-app.com:443  resolvers dns verify none inter 1000  check
        server b b-app.com:443  resolvers dns verify none inter 1000  check

However, in the logs I get:

    Server app-api_backend/a is DOWN, reason: Layer7 invalid response, check duration: 1ms. 1 active and 0 backup servers left. 0 sessions active, 0 requeued, 0 remaining in queue.
    Server app-api_backend/b is DOWN, reason: Layer7 invalid response, check duration: 1ms. 1 active and 0 backup servers left. 0 sessions active, 0 requeued, 0 remaining in queue.

SOLVED:

@lukastribus pointed me in the right direction. Working config looks like the following:

 backend app-api_backend
        mode tcp
        option httpchk OPTIONS /app_service HTTP/1.1
        server a a-app.com:443  resolvers dns verify none inter 1000  check check-ssl
        server b b-app.com:443  resolvers dns verify none inter 1000  check check-ssl

check-ssl was the missing piece. I also discovered the API endpoint supports the OPTIONS method which returns a 200 OK. so substituted that instead.

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