Odd reverse proxy behavior

Hi, all!

I’ve stumbled upon a problem I can’t find a solution for many hours in a
row. I use nginx as a reverse proxy to python/spawning web-server. I’ve
done it for 1000 times already and all worked fine before this day.

Nginx on a server I’m talking about serves 3 separate projects
(hostnames) as a reverse proxy. I’ve added fourth. Three existing
projects continued to work perfectly. But the new one sometimes works
and sometimes returns 502.

The observation is: once it began to return 502 it would return it on
every browser refresh. However if I’ll throttle F5 fast, after 5-10
refreshes it starts to work properly. And vice versa: once it works like
it should, slow refreshes keep it working, but if I’d throttle F5 again
or if I’d wait for a minute or so nginx declines back to 502-state.

There is nothing recorded in neither log (access/error nginx/backend) if
it is in 502-state. The deal is not in a backend. I’ve tried to set
proxy_pass to the same value as in a sibling well-working project conf
and the same story repeats. Problem exists for static resources as well.
Is this conf cursed?

user www-data;
worker_processes 1;

error_log /var/log/nginx/error.log debug;
pid /var/run/nginx.pid;

events {
worker_connections 1024;
# multi_accept on;

http {
include /etc/nginx/mime.types;

access_log  /var/log/nginx/access.log;

sendfile        on;
#tcp_nopush     on;

#keepalive_timeout  0;
keepalive_timeout  65;
tcp_nodelay        on;

gzip  on;
gzip_disable "MSIE [1-6]\.(?!.*SV1)";

include /etc/nginx/conf.d/*.conf;
include /etc/nginx/sites-enabled/*;



server {
listen 80;
server_name hostname.iam.listening.to;

# Main location
location / {
    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;

location /static/ { alias /srv/some/path/static/; }



I feel I’ve tried all combinations of buffer-tweaking and none of them
helped. The system has > 700 Mb or RAM free and mostly idle CPU. Could
someone guess what the hell is going on?

Posted at Nginx Forum:

Got it!

The problem was absolutely not related to nginx configuration. I found
it when I continued to get 502s even with nginx process down O_o

I’ve accidentally added two A-records to the DNS-zone with same name but
different IPs. Looks like the resulting IP was chosen in
semi-round-robin fashion. And when a not configured server was chosen,
I’ve got those 502s.

Whew!… Sorry for disturbing.

Posted at Nginx Forum: