502 Bad Gateway error - upstream sent too big header while reading response header from upstream

[Muthuvel R]

I'm using wordpress in Google App Engine. When I make a POST API, getting an Error : "upstream sent too big header while reading response header from upstream". which returns 502, Bad Gateway, nginx

The data(JSON) i'm sending in POST API is around 4kb. If the Data is sent below 2kb API returns success.

I tried App Engine Standard and Flexible Environment, but facing the same issue.

As per this link: upstream sent too big header while reading response header from upstream

Modifying the nginx-app.config file with buffers will fix this issue. But it's not working.

In App Engine, the default nginx-app.config looks like.

location / {
    try_files $uri /index.php?q=$uri&$args;
}

location ~ ^/wp-admin {
    try_files $uri $uri/index.php?$args;
}

Not sure where I need to add these proxy buffers.

I tried adding the proxy buffers inside the default locations as shown below, but it didn't help.

location / {
    try_files $uri /index.php?q=$uri&$args;
    fastcgi_buffers 16 16k;
    fastcgi_buffer_size 32k;
    proxy_buffer_size 16k;
    proxy_buffers 4 32k;
    proxy_busy_buffers_size 32k;
}

Kindly help me to fix this issue.

[Julie (cloud platform support)]

As mentioned on the Stackoverflow post the issue is likely due to default proxy_buffer_size which is 4K and while there is an internal feature request to increase this the current workaround to to reduce proxy_buffer_size to 4K or lower.