Mysterious proxy timeout at 2 min

[Thomas Love]

Hello

I am configuring nginx to proxy to a node.js server which I am using
for long-polling. I want nginx to hold the connection open just over 5
minutes so that the node server can respond with content, or at least
a 204, when it's good and ready.

But nginx/1.1.0 keeps canceling with a 502 after exactly 2 minutes. I
can't find a a 2-minute timeout anywhere in my config.

I have the following:

upstream nodesub {
    server 127.0.0.1:8001;
}

location /sub {
    send_timeout 330s;
    proxy_read_timeout 330s;
    proxy_connect_timeout 30s;
    proxy_pass http://nodesub;
}

What am I missing, good people of the list?

Thomas