Debugging connection and session issues - Haproxy TCP Mode


Hi there,

We are having some troubles to debug a problem with our haproxy and our backend server.

Scenario: haproxy server are a Ubuntu 14.04 LTS. Haproxy was installed using apt-get and are currently in version 1.5.14. Right now, we have only one haproxy server and only one backend server with a php application whose can server 120 connections max. The problem happened when we received a huge traffic and the backend could not server all connections.

In this scenario HAproxy started queue process and timeout the packet in the queue and that’s expected but, after the timeout a empty message was send to the client and we get a lot of erros in the stats page. They showed in the nodes session, backend line. error column, conn column. No other erros are showed to request or response in stats page, in other words, Req and Resp Erros column are 0.

I researched about and did not find anything that explain what is this Conn Erros in stats page, however I think that is related to packets timeout in the queue, is that right?

Our haproxy are in TCP Mode and below are the config file.

        log /dev/log    local0
        log /dev/log    local1 notice
        chroot /var/lib/haproxy
        stats socket /run/haproxy/admin.sock mode 660 level admin
        stats timeout 30s
        user haproxy
        group haproxy

        maxconn 12288

        ca-base /etc/ssl/certs
        crt-base /etc/ssl/private

        ssl-default-bind-options no-sslv3

        log     global
        mode    http
        option  httplog
        option  dontlognull
        option abortonclose
        option httpclose
        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

listen stats :9090
        mode http
        stats enable
        stats auth edited:edited

frontend localnodes
        bind *:80
        option tcplog
        mode tcp
        default_backend nodes

backend nodes
        mode tcp
        balance roundrobin

        server php-app maxconn 120 check

Below are the kernel configuration we did to haproxy:

fs.file-max = 999999
net.ipv4.ip_local_port_range = 1024     65000
net.ipv4.tcp_tw_reuse = 1
net.ipv4.tcp_fin_timeout = 10
net.core.somaxconn = 65535
net.ipv4.tcp_max_syn_backlog = 65535
net.ipv4.tcp_max_tw_buckets = 409600

Trying to reproduce this problem and identify if the issue are in haproxy or in the backend server, I create a staging scenario with one haproxy and one server (with a simple html file) and change the haproxy backend configuration to:

 backend nodes
        mode tcp
        balance roundrobin
        fullconn 4
        server php-app maxconn check minconn 1 maxconn 1

I stressed the connection on this server but, even with packet timing out of the queue and a lot of conn errors, I received HTTP 200 ok and the html message from the server. To do this I used looped curl requisitions in a script in 4 servers. Every server made 100 requisitions.

But, when we use a browser (Chrome) to create concurrent connections in this scenario, only one computer received the html message the others received “connection timeout”. Even reloading the page, this behavior remained.

So, we did two tests with different conclusions and we could not realize why.

Then, anyone knows another way or more detailed way to debug what could happened? Everything I saw are about http mode and not apply to my case.

If someone knows what could be will help a lot too.

Thanks and regards,


Why using tcp mode on this service?
If I were you, I would turn on http mode in HAProxy, it will make it aware of the protocol and only forward to the server the connections where there is a request inside.
With your current configuration, if a browser open 5 connections as its pre-connect feature teach it, then all of them are forwarded to the server, regardless of HTTP protocol. This is perfectly emphasized by your curl / chrome test: curl open only 1 connection to send its request whil your browser may have opened 5 (or more) of them, virtually saturating the number of connection slots available.



Hi Baptiste,

I will make test with http mode and see what will happened. Also, I had forget about the pre-connect feature of browsers.

I will update this thread with what I found about.