intermittent connectivity issues ngx_mail_pop3_module

Tue Oct 8 19:42:56 UTC 2013

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: http://pastebin.com/VtQKsb92
and a successful session: 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