How to make nginx mail auth connection persistent?

[Prabhash Rathore]

Hello,

We are using nginx 1.7.3 as a reverse proxy for our Mail SMTP service. For
authentication of each SMTP connection, we have configured nginx to connect
with a http based service for authentication. Here is a snippet of our
nginx config:

mail {
    # auth_server
    auth_http auth_host:auth_port/authserver;

    # mail server
    server {
        protocol                 smtp;
        listen                   25;
        proxy                    on;
        xclient                  on;
        timeout                  15;
        starttls                 on;
        ... other configs...
    }
}

With above config, we notice that nginx closes the connection after every
auth request/response to Mail Authentication Server (auth_http
auth_host:auth_port/authserver;) based on tcpdump analysis. We would like
to make this connection persistent so that we could reuse connection for
multiple auth requests.

I looked at nginx mail auth module documentation (
http://nginx.org/en/docs/mail/ngx_mail_auth_http_module.html#auth_http_header
) but I don't see any directive to make mail auth connection persistent.

I also looked at ngx_http_upstream_module (
http://nginx.org/en/docs/http/ngx_http_upstream_module.html#keepalive)
which has "keepalive" directive but my understanding is this directive is
for http upstream server not for mail auth server.

Can someone please help? Thanks!
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