A user reported that she was unable to check her email today; she had also
reported the problem yesterday. When I checked
Sendmail,
which would handle her outgoing email, by using
Telnet to
connect to the well-known port for
Simple Mail Transfer Protocol (SMTP) on the server with
telnet mail.example.com 25, I saw the Sendmail banner as expected,
so I presumed her problem was likely with
Dovecot, the software on the system that would allow her to receive her
incoming email. I tried connecting to port 110, the well-known port
for Post Office Protocol version 3 (POP3) connections using
Telnet. When I saw the "Connected to" and "Escape character is" messages, I
entered the POP3 user command followed by the user's name,
but I would shortly thereafter see a "Connection closed" message every
time I tried the connection with Telnet. I never saw the "Dovecot ready" prompt
appear.
# telnet 127.0.0.1 110 Trying 127.0.0.1... Connected to 127.0.0.1. Escape character is '^]'. user nell Connection closed by foreign host. You have new mail in /var/spool/mail/root #
I've encountered the problem on several occasions in the past, e.g.,
Dovecot - client
connections are being dropped. I restarted Dovecot, as I've done
previously, with service dovecot restart, which resolved the
problem yesterday. Before restarting the service today, I checked the
number of pop3-login processes and saw many, as I've seen in
the past when the problem occurred. There were 100 of the
pop3-login processes being run under the dovenull account.
# ps -aux | grep dovecot root 1407 0.0 0.0 21928 1048 ? Ss May22 0:00 /usr/sbin/dovec ot -F dovecot 3243 0.0 0.0 9444 656 ? S May22 0:00 dovecot/anvil root 10693 0.0 0.0 112644 928 pts/0 S+ 20:38 0:00 grep --color=au to dovecot dovenull 15092 0.0 0.1 45164 2776 ? S 00:44 0:00 dovecot/pop3-lo gin root 15093 0.0 0.1 13384 2424 ? S 00:44 0:00 dovecot/config dovecot 15094 0.0 0.1 31368 2436 ? S 00:44 0:00 dovecot/auth dovenull 15141 0.0 0.1 45164 2772 ? S 00:46 0:00 dovecot/pop3-lo gin dovenull 15177 0.0 0.1 45164 2776 ? S 00:48 0:00 dovecot/pop3-lo gin dovenull 15200 0.0 0.1 45164 2776 ? S 00:49 0:00 dovecot/pop3-lo gin dovenull 15745 0.0 0.1 45164 2772 ? S 00:56 0:00 dovecot/pop3-lo gin dovenull 15755 0.0 0.1 45164 2780 ? S 00:57 0:00 dovecot/pop3-lo gin dovenull 15770 0.0 0.1 45164 2776 ? S 00:57 0:00 dovecot/pop3-lo gin dovenull 16551 0.0 0.1 45164 2772 ? S 01:03 0:00 dovecot/pop3-lo gin dovenull 16566 0.0 0.1 45164 2772 ? S 01:04 0:00 dovecot/pop3-lo gin dovenull 16654 0.0 0.1 45164 2776 ? S 01:08 0:00 dovecot/pop3-lo gin dovenull 16685 0.0 0.1 45164 2772 ? S 01:09 0:00 dovecot/pop3-lo gin dovenull 16894 0.0 0.1 45164 2772 ? S 01:17 0:00 dovecot/pop3-lo gin dovenull 17591 0.0 0.1 45164 2780 ? S 01:27 0:00 dovecot/pop3-lo gin dovenull 18758 0.0 0.1 45164 2776 ? S 01:42 0:00 dovecot/pop3-lo gin dovenull 18761 0.0 0.1 45164 2776 ? S 01:42 0:00 dovecot/pop3-lo gin dovenull 18765 0.0 0.1 45164 2780 ? S 01:42 0:00 dovecot/pop3-lo gin dovenull 18768 0.0 0.1 45164 2776 ? S 01:42 0:00 dovecot/pop3-lo gin <text snipped> # ps -aux | grep dovecot | grep -v grep | wc -l 104 # ps -aux | grep pop3-login | grep -v grep | wc -l 100 #
Dovecot uses the pop3-login process to allow the user to login - see Login processes. The maximum number of processes that can run simultaneously can be configured in the Dovecot configuration file. You can view that number using the command shown below, which was run from the root account on the Linux server.
# doveconf -d | grep "default_process_limit" default_process_limit = 100 #
You can view the
man page for the
Dovecot configuration file, dovecot.conf, with man
dovecot.conf. You can view other configuration
information for Dovecot with doveadm config.
# doveadm config | more # 2.2.10: /etc/dovecot/dovecot.conf # OS: Linux 3.10.0-229.7.2.el7.x86_64 x86_64 CentOS Linux release 7.1.1503 (Core ) # NOTE: Send doveconf -n output instead when asking for help. auth_anonymous_username = anonymous auth_cache_negative_ttl = 1 hours auth_cache_size = 0 auth_cache_ttl = 1 hours auth_debug = no auth_debug_passwords = no auth_default_realm = auth_failure_delay = 2 secs auth_gssapi_hostname = <text snipped>
In this case, I can see the configuration file is at
/etc/dovecot/dovecot.conf.
I tried killing all of the pop3-login processes to see what would happen
then - I had to use kill --signal HUP pop3-login to terminate
all of them. That didn't help. If I then used Telnet to connect to the pop3
port, port 110, I could enter the user's name and password, but I never
received any response from Dovecot; I would simply not get the "connection
closed by foreign host" message then. Instead, I would wait interminably for
a response until I hit Ctrl-] (the "ctrl" and "]" keys) to
get back to the telnet prompt.
# telnet 127.0.0.1 110 Trying 127.0.0.1... Connected to 127.0.0.1. Escape character is '^]'. user nell pass thePassword ^] telnet> quit Connection closed. # ps -aux | grep pop3-login | grep -v grep | wc -l 2 #
If I tried the telnet process repeatedly, I would see the process count
for the pop3-login processes increase for each attempt I made. I restarted
Dovecot as I had yesterday to resolve the problem. When I tested with
Telnet again afterwards, I saw the "+OK Dovecot ready" prompt. I could then
enter the user's userid and password to login at which point I could
enter the stat command for statistics showing the user
had 244 messages.
# service dovecot restart Redirecting to /bin/systemctl restart dovecot.service # telnet 127.0.0.1 110 Trying 127.0.0.1... Connected to 127.0.0.1. Escape character is '^]'. +OK Dovecot ready. user nell +OK pass thePassword +OK Logged in. stat +OK 244 34408213 quit +OK Logging out. Connection closed by foreign host. #
