Hi.

I'm the new admin of a website and its servers. I have a frontend nginx proxy server and an Apache backend.
I sometimes have errors :

2012/04/12 17:44:33 [error] 13442#0: *74193485 upstream timed out (110: Connection timed out) while reading response header from upstream, client: 98.87.76.65, server: www.mysite.tld, request: "POST /my/url HTTP/1.1", upstream: "http://12.23.34.45:80/my/url", host: "www.mysite.tld", referrer: "https://www.mysite.tld/another/url"
2012/04/12 17:44:33 [error] 13442#0: *74193485 no live upstreams while connecting to upstream, client: 98.87.76.65, server: www.mysite.tld, request: "POST /my/url HTTP/1.1", upstream: "http://backend/my/url", host: "www.mysite.tld", referrer: "https://www.goeland.fr/another/url"

(I've juste anonymized the urls and ip addresses)

Here are the configuration files :

conf.d/proxy.conf :

proxy_redirect off;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
client_max_body_size 32M;
client_body_buffer_size 256k;
client_header_buffer_size 128k;
proxy_connect_timeout 30;
proxy_send_timeout 30;
proxy_read_timeout 30;
proxy_buffer_size 32k;
proxy_buffers 64 32k;
proxy_busy_buffers_size 128k;


conf.d/loadbalancer.conf :

upstream backend {
ip_hash;
# web1
# server 127.0.0.1:8098 backup;
# web2
server 44.33.22.11;
#web3
server 11.22.33.44 down;
}


sites-enabled/mysite.conf :

server {
listen 80;
listen 443 ssl;
server_name www.mysite.tld www3.mysite.tld;

access_log off; #/home/nginx/log/mysite.access.log;
#error_log /dev/null crit;

# Possible level values : debug | info | notice | warn | error | crit
error_log /home/nginx/log/mysite.error.log notice;
log_not_found off;

#set $proto "http";
#if ( $server_port = "443" ) {
#set $proto "https";
#}

# serve static files directly
location ~* ^.+\.(jpg|jpeg|gif|css|png|js|ico|html|pdf|zip|tgz|gz|bz2|swf)$ {
expires 10d;
root /home/mysite/www;
}

location / {
#proxy_set_header X-Forwarded-Proto $proto;
proxy_pass http://backend;
add_header X-Whom candidature@mysite.tld;
}

error_page 405 = $uri;
error_page 500 502 503 504 /50x.html;

location = /50x.html {
root /home/mysitewww;
}

location = /favicon.ico { access_log off; log_not_found off; }
}


The problem seems to be recent and sometimes happens immediatly after clients have paid through the bank online credit card form which is integrated (we do not redirect to the bank website). It's a real problem for us since when it happens at that moment, clients think their order and paiement have not been saved and they order a second time (and also pay a second time).

I don't really know what i'm looking for : can it be a network connection problem between frontend and backend servers ? Is it because of the configuration of the proxy of the server ? or... ?

Thanks for any help.