intermittent connectivity issues ngx_mail_pop3_module

[Maxim Dounin]

Hello!

On Tue, Oct 08, 2013 at 08:42:56PM +0100, Markus Gerstel wrote:

> Hi everyone,
> 
> I've recently installed nginx as a POP3/IMAP proxy, fronting for a
> single server. Everything works most of the time. But every once in
> a while, nginx fails to forward incoming connections with "-ERR
> internal server error".
> I've narrowed it down to the stage *after* the authorization, but
> need some help interpreting the nginx debug output.

[...]

> failure:
>  *3028 recv: fd:26 0 of 4096
>  *3028 close mail proxy connection: 26
>  *3028 event timer del: 26: 1381257081145
>  *3028 reusable connection: 0
>  *3028 SSL to write: 28
>  *3028 SSL_write: 28
>  *3028 close mail connection: 23
> 
> What does 'recv 0' mean? Does this mean that nginx has a problem in
> opening the connection to the actual pop3 server? (If I open
> connections from the nginx computer to the pop3 server directly they
> always work.)

It means the recv() syscall returned 0, which in turn means 
connection was closed by other side, i.e. by your backend server.  
Try looking into your backend's logs to find out why - there is no 
additional information available on nginx side.

-- 
Maxim Dounin
http://nginx.org/en/donation.html