# Intermittent connectivity issues ngx\_mail\_pop3\_module

**URL:** <https://www.ruby-forum.com/t/intermittent-connectivity-issues-ngx-mail-pop3-module/246392>\
**Category:** NGINX\
**Created:** [October 8, 2013, 9:43pm UTC](https://www.ruby-forum.com/t/intermittent-connectivity-issues-ngx-mail-pop3-module/246392 "2013-10-08T21:43:41Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Rafa\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/958977/32.png) [@Rafa\_F](https://www.ruby-forum.com/u/Rafa_F)\
**Post date:** [October 8, 2013, 9:43pm UTC](https://www.ruby-forum.com/t/intermittent-connectivity-issues-ngx-mail-pop3-module/246392/1 "2013-10-08T21:43:41Z")

</div>

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.

Here’s a failing session: [Failed session - Pastebin.com](http://pastebin.com/VtQKsb92)  
and a successful session: [Success session - Pastebin.com](http://pastebin.com/vae5YwAy)

They start to diverge at line 107:

success:  
\*3181 recv: fd:18 18 of 4096  
\*3181 mail proxy send user  
\*3181 send: fd:18 13 of 13  
\*3181 post event 0000000001CE83D0  
\*3181 post event 0000000001D1C3E0  
\*3181 delete posted event 0000000001D1C3E0  
\*3181 mail proxy dummy handler

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.)

Anyone have an idea how to get more information from nginx at this  
stage?

-Markus

tried with 1.2.1 and  
nginx version: nginx/1.4.1  
TLS SNI support enabled  
configure arguments: --prefix=/usr/share/nginx  
–conf-path=/etc/nginx/nginx.conf  
–error-log-path=/var/log/nginx/error.log  
–http-client-body-temp-path=/var/lib/nginx/body  
–http-fastcgi-temp-path=/var/lib/nginx/fastcgi  
–http-log-path=/var/log/nginx/access.log  
–http-proxy-temp-path=/var/lib/nginx/proxy  
–http-scgi-temp-path=/var/lib/nginx/scgi  
–http-uwsgi-temp-path=/var/lib/nginx/uwsgi  
–lock-path=/var/lock/nginx.lock --pid-path=/run/nginx.pid  
–with-pcre-jit --with-debug --with-http\_addition\_module  
–with-http\_dav\_module --with-http\_geoip\_module  
–with-http\_gzip\_static\_module --with-http\_image\_filter\_module  
–with-http\_realip\_module --with-http\_stub\_status\_module  
–with-http\_ssl\_module --with-http\_sub\_module --with-http\_xslt\_module  
–with-ipv6 --with-mail --with-mail\_ssl\_module  
–add-module=/build/nginx-WYjrxz/nginx-1.4.1/debian/modules/nginx-auth-pam  
–add-module=/build/nginx-WYjrxz/nginx-1.4.1/debian/modules/nginx-dav-ext-module  
–add-module=/build/nginx-WYjrxz/nginx-1.4.1/debian/modules/nginx-echo  
–add-module=/build/nginx-WYjrxz/nginx-1.4.1/debian/modules/nginx-upstream-fair  
–add-module=/build/nginx-WYjrxz/nginx-1.4.1/debian/modules/ngx\_http\_substitutions\_filter\_module

POP3 communication - success case:  
fetchmail: POP3\< +OK POP3 ready  
fetchmail: POP3\> CAPA  
fetchmail: POP3\< +OK Capability list follows  
fetchmail: POP3\< TOP  
fetchmail: POP3\< USER  
fetchmail: POP3\< SASL LOGIN PLAIN  
fetchmail: POP3\< STLS  
fetchmail: POP3\< .  
fetchmail: POP3\> STLS  
fetchmail: POP3\< +OK  
fetchmail: Server certificate:  
(…)  
fetchmail: POP3\> CAPA  
fetchmail: POP3\< +OK Capability list follows  
fetchmail: POP3\< TOP  
fetchmail: POP3\< USER  
fetchmail: POP3\< SASL LOGIN PLAIN  
fetchmail: POP3\< .  
fetchmail: upgrade to TLS succeeded.  
fetchmail: POP3\> USER web1p1  
fetchmail: POP3\< +OK  
fetchmail: POP3\> PASS \*  
fetchmail: POP3\< +OK logged in.  
fetchmail: POP3\> STAT  
fetchmail: POP3\< +OK 0 0  
fetchmail: No mail for web1p1  
fetchmail: POP3\> QUIT  
fetchmail: POP3\< +OK Bye-bye.

POP3 communication - failure case:  
fetchmail: POP3\< +OK POP3 ready  
fetchmail: POP3\> CAPA  
fetchmail: POP3\< +OK Capability list follows  
fetchmail: POP3\< TOP  
fetchmail: POP3\< USER  
fetchmail: POP3\< SASL LOGIN PLAIN  
fetchmail: POP3\< STLS  
fetchmail: POP3\< .  
fetchmail: POP3\> STLS  
fetchmail: POP3\< +OK  
fetchmail: Server certificate:  
(…)  
fetchmail: POP3\> CAPA  
fetchmail: POP3\< +OK Capability list follows  
fetchmail: POP3\< TOP  
fetchmail: POP3\< USER  
fetchmail: POP3\< SASL LOGIN PLAIN  
fetchmail: POP3\< .  
fetchmail: upgrade to TLS succeeded.  
fetchmail: POP3\> USER web1p1  
fetchmail: POP3\< +OK  
fetchmail: POP3\> PASS \*  
fetchmail: POP3\< -ERR internal server error  
fetchmail: POP3\> QUIT

POP3 communication - failure case:  
fetchmail: POP3\< +OK POP3 ready  
fetchmail: POP3\> CAPA  
fetchmail: POP3\< +OK Capability list follows  
fetchmail: POP3\< TOP  
fetchmail: POP3\< USER  
fetchmail: POP3\< SASL LOGIN PLAIN  
fetchmail: POP3\< STLS  
fetchmail: POP3\< .  
fetchmail: POP3\> STLS  
fetchmail: POP3\< +OK  
fetchmail: Server certificate:  
(…)  
fetchmail: POP3\> CAPA  
fetchmail: POP3\< +OK Capability list follows  
fetchmail: POP3\< TOP  
fetchmail: POP3\< USER  
fetchmail: POP3\< SASL LOGIN PLAIN  
fetchmail: POP3\< .  
fetchmail: upgrade to TLS succeeded.  
fetchmail: POP3\> USER web1p1  
fetchmail: POP3\< +OK  
fetchmail: POP3\> PASS \*  
fetchmail: POP3\< -ERR internal server error  
fetchmail: POP3\> QUIT

---

<div class="post-metadata">

**Author:** ![Markus\_Gerstel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/bc79bd/32.png) [@Markus\_Gerstel](https://www.ruby-forum.com/u/Markus_Gerstel)\
**Post date:** [October 9, 2013, 1:10pm UTC](https://www.ruby-forum.com/t/intermittent-connectivity-issues-ngx-mail-pop3-module/246392/2 "2013-10-09T13:10:05Z")

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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.

[…]

> 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 D.

> **[nginx: donation](http://nginx.org/en/donation.html)**