Hi folks,

I have a handful of iOS 10.2.1 Mail app IMAP clients that intermittently break into this unexplained authenticate-then-immediately-disconnect behavior when connecting to a RHEL7 Dovecot (dovecot-2.2.10-7.el7) proxy, providing proxied connections to a backend Panda/UW-IMAP server. From talking to the users, the activity would appear to be spontaneous (ie: not caused by user interaction with the device).

The behavior doesn't seem to have any observable implications for the end user, other than momentarily hitting the Dovecot process_limit (which, if not raised to a rather large number, disrupts new IMAP proxy connections momentarily).

I reckon this is not an issue with Dovecot, but I'm curious to know if other folks have observed this behavior when dealing with iOS Mail app clients?

The log entries look like this:

iOS 10 device = 172.16.0.1 RHEL7 Dovecot proxy host = 192.168.0.1 ("proxyhost") Panda/UW-IMAP target = panda.imap.tld

Mar 6 12:11:00 proxyhost dovecot: imap-login: proxy(jdoe): started proxying to panda.imap.tld:993: user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS, session=<VvzqehVKhwBCol00> Mar 6 12:11:00 proxyhost dovecot: imap-login: proxy(jdoe): disconnecting 172.16.0.1 (Disconnected by client): user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS: Disconnected, session=<VvzqehVKhwBCol00> Mar 6 12:11:01 proxyhost dovecot: imap-login: proxy(jdoe): started proxying to panda.imap.tld:993: user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS, session=<jaXxehVKiABCol00> Mar 6 12:11:01 proxyhost dovecot: imap-login: proxy(jdoe): disconnecting 172.16.0.1 (Disconnected by server): user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS, session=<jaXxehVKiABCol00> Mar 6 12:11:01 proxyhost dovecot: imap-login: proxy(jdoe): started proxying to panda.imap.tld:993: user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS, session=<RmX4ehVKiQBCol00> Mar 6 12:11:01 proxyhost dovecot: imap-login: proxy(jdoe): disconnecting 172.16.0.1 (Disconnected by server): user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS, session=<RmX4ehVKiQBCol00> Mar 6 12:11:02 proxyhost dovecot: imap-login: proxy(jdoe): started proxying to panda.imap.tld:993: user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS, session=<inYBexVKigBCol00> Mar 6 12:11:02 proxyhost dovecot: imap-login: proxy(jdoe): disconnecting 172.16.0.1 (Disconnected by server): user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS, session=<inYBexVKigBCol00> Mar 6 12:11:02 proxyhost dovecot: imap-login: proxy(jdoe): started proxying to panda.imap.tld:993: user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS, session=<GwAJexVKiwBCol00> Mar 6 12:11:02 proxyhost dovecot: imap-login: proxy(jdoe): disconnecting 172.16.0.1 (Disconnected by server): user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS, session=<GwAJexVKiwBCol00> Mar 6 12:11:03 proxyhost dovecot: imap-login: proxy(jdoe): started proxying to panda.imap.tld:993: user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS, session=<asUPexVKswBCol00> Mar 6 12:11:03 proxyhost dovecot: imap-login: proxy(jdoe): disconnecting 172.16.0.1 (Disconnected by server): user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS, session=<asUPexVKswBCol00> Mar 6 12:11:03 proxyhost dovecot: imap-login: proxy(jdoe): started proxying to panda.imap.tld:993: user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS, session=<n9YYexVKjQBCol00> Mar 6 12:11:04 proxyhost dovecot: imap-login: proxy(jdoe): disconnecting 172.16.0.1 (Disconnected by server): user=<jdoe>, method=PLAIN, rip=172.16.0.1, lip=192.168.0.1, TLS, session=<n9YYexVKjQBCol00>

...and on and on, usually until the 'service imap-login' process_limit is reached. You could naturally apply some iptables rate-limiting to avoid hitting process_limit, but it'd be nice to have the iOS client simply behave properly instead.

dovecot -n:

# 2.2.10: /etc/dovecot/dovecot.conf # OS: Linux 3.10.0-514.6.2.el7.x86_64 x86_64 Red Hat Enterprise Linux Server release 7.3 (Maipo) auth_mechanisms = plain login auth_verbose = yes first_valid_uid = 1000 imap_capability = +I18NLEVEL=1 mbox_write_locks = fcntl passdb { args = nopassword=y default_fields = proxy=y ssl=any-cert host=panda.imap.tld driver = static } protocols = imap pop3 service imap-login { process_limit = 400-ish at the moment process_min_avail = 2 } service pop3-login { inet_listener pop3s { port = 995 ssl = yes } } ssl = required ssl_ca = </etc/pki/ca-trust/extracted/pem/tls-ca-bundle.pem ssl_cert = <proxyhost.crt ssl_dh_parameters_length = 2048 ssl_key = <proxyhost.key ssl_protocols = !SSLv3 !SSLv3 ssl_require_crl = no userdb { driver = static }

Thanks for any insight out there.

Robert Giles