exim in a docker container

aliases

# Default aliases file, installed by Exim. This file contains no real aliases.
# You should edit it to taste.


# The following alias is required by the mail RFCs 2821 and 2822.
# Set it to the address of a HUMAN who deals with this system's mail problems.

# postmaster: someone@your.domain

# It is also common to set the following alias so that if anybody replies to a
# bounce message from this host, the reply goes to the postmaster.

# mailer-daemon: postmaster


# You should also set up an alias for messages to root, because it is not
# usually a good idea to deliver mail as root.

# root: postmaster

# It is a good idea to redirect any messages sent to system accounts so that
# they don't just get ignored. Here are some common examples:

# bin: root
# daemon: root
# ftp: root
# nobody: root
# operator: root
# uucp: root

# You should check your /etc/passwd for any others.


# Other commonly enountered aliases are:
#
# abuse:       the person dealing with network and mail abuse
# hostmaster:  the person dealing with DNS problems
# webmaster:   the person dealing with your web site

####

Dockerfile

FROM alpine:3.3

RUN echo "http://dl-4.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories
RUN apk add --no-cache exim

COPY . /

RUN /usr/sbin/harden.sh

# Volumes must be created AFTER we harden.
VOLUME ["/var/log/exim", "/var/spool/exim"]

USER exim

# http://www.exim.org/exim-html-current/doc/html/spec_html/ch-the_exim_command_line.html
ENTRYPOINT ["exim"]
CMD ["-bd", "-v", "-oP", "/dev/null", "-C", "/etc/mail/local.conf"]

file layout

$ tree
.
├── Dockerfile
├── etc
│   └── mail
│       ├── aliases
│       └── local.conf
└── usr
    └── sbin
        └── harden.sh

4 directories, 4 files

harden.sh

#!/bin/sh
set -x
set -e
#
# Docker build calls this script to harden the image during build.
#
# NOTE: To build on CircleCI, you must take care to keep the `find`
# command out of the /proc filesystem to avoid errors like:
#
#    find: /proc/tty/driver: Permission denied
#    lxc-start: The container failed to start.
#    lxc-start: Additional information can be obtained by \
#        setting the --logfile and --logpriority options.

# Remove existing crontabs, if any.
rm -fr /var/spool/cron
rm -fr /etc/crontabs
rm -fr /etc/periodic

# Remove all but a handful of admin commands.
find /sbin /usr/sbin ! -type d \
  -a ! -name exim \
  -a ! -name nologin \
  -delete

# Remove world-writable permissions.
# This breaks apps that need to write to /tmp,
# such as ssh-agent.
find / -xdev -type d -perm +0002 -exec chmod o-w {} +
find / -xdev -type f -perm +0002 -exec chmod o-w {} +

# Remove unnecessary user accounts, including root.
sed -i -r '/^(exim)/!d' /etc/group
sed -i -r '/^(exim)/!d' /etc/passwd

# Disable interactive login for everybody.
sed -i -r 's#^(.*):[^:]*$#\1:/sbin/nologin#' /etc/passwd

sysdirs="
  /bin
  /etc
  /lib
  /sbin
  /usr
"

# Remove apk configs.
find $sysdirs -xdev -regex '.*apk.*' -exec rm -fr {} +

# Remove crufty...
#   /etc/shadow-
#   /etc/passwd-
#   /etc/group-
find $sysdirs -xdev -type f -regex '.*-$' -exec rm -f {} +

# Ensure system dirs are owned by root and not writable by anybody else.
find $sysdirs -xdev -type d \
  -exec chown 0:0 {} \; \
  -exec chmod 0755 {} \;

# Create spool dir so that exim user doesn't have to.
# Dockerfile treats this as a volume.
mkdir -p /var/spool/exim
chown -R exim:exim /var/spool/exim
chmod 0755 /var/spool/exim

# Remove suid bit from exim.
chmod u-s /usr/sbin/exim

# Remove all suid files.
find $sysdirs -xdev -type f -a -perm +4000 -delete

# Remove other programs that could be dangerous.
find $sysdirs -xdev \( \
  -name hexdump -o \
  -name chgrp -o \
  -name chmod -o \
  -name chown -o \
  -name ln -o \
  -name od -o \
  -name strings -o \
  -name su \
  \) -delete

# Remove init scripts since we do not use them.
rm -fr /etc/init.d
rm -fr /lib/rc
rm -fr /etc/conf.d
rm -fr /etc/inittab
rm -fr /etc/runlevels
rm -fr /etc/rc.conf

# Remove kernel tunables since we do not need them.
rm -fr /etc/sysctl*
rm -fr /etc/modprobe.d
rm -fr /etc/modules
rm -fr /etc/mdev.conf
rm -fr /etc/acpi

# Remove root homedir since we do not need it.
rm -fr /root

# Remove fstab since we do not need it.
rm -f /etc/fstab

# Remove broken symlinks (because we removed the targets above).
find $sysdirs -xdev -type l -exec test ! -e {} \; -delete

local.conf

######################################################################
#                  Runtime configuration file for Exim               #
######################################################################


# This is a default configuration file which will operate correctly in
# uncomplicated installations. Please see the manual for a complete list
# of all the runtime configuration options that can be included in a
# configuration file. There are many more than are mentioned here. The
# manual is in the file doc/spec.txt in the Exim distribution as a plain
# ASCII file. Other formats (PostScript, Texinfo, HTML, PDF) are available
# from the Exim ftp sites. The manual is also online at the Exim web sites.


# This file is divided into several parts, all but the first of which are
# headed by a line starting with the word "begin". Only those parts that
# are required need to be present. Blank lines, and lines starting with #
# are ignored.


########### IMPORTANT ########## IMPORTANT ########### IMPORTANT ###########
#                                                                          #
# Whenever you change Exim's configuration file, you *must* remember to    #
# HUP the Exim daemon, because it will not pick up the new configuration   #
# until you do. However, any other Exim processes that are started, for    #
# example, a process started by an MUA in order to send a message, will    #
# see the new configuration as soon as it is in place.                     #
#                                                                          #
# You do not need to HUP the daemon for changes in auxiliary files that    #
# are referenced from this file. They are read every time they are used.   #
#                                                                          #
# It is usually a good idea to test a new configuration for syntactic      #
# correctness before installing it (for example, by running the command    #
# "exim -C /config/file.new -bV").                                         #
#                                                                          #
########### IMPORTANT ########## IMPORTANT ########### IMPORTANT ###########


# Avoid any local log files because we start in Dockerfile to stdout.
log_file_path = syslog


# The next three settings create two lists of domains and one list of hosts.
# These lists are referred to later in this configuration using the syntax
# +local_domains, +relay_to_domains, and +relay_from_hosts, respectively. They
# are all colon-separated lists:

# We do not accept local delivery. We only relay.
domainlist local_domains =

# We relay TO these domains.
domainlist relay_to_domains = *.example.com

# We relay FROM these IP ranges.
# The "<;" means to use semicolon as separator.
hostlist   relay_from_hosts = <; 127.0.0.1 ; ::1 ; 10.8.0.0/16 ; 10.188.0.0/16

# The lists above are used in the ACLs for checking incoming messages.
# The names of these ACLs are defined here:

acl_smtp_rcpt = acl_check_rcpt
acl_smtp_data = acl_check_data



# Start daemon on high port so we can start unprivileged.
# Docker maps the port to 25 on the host to enable clients
# to use plain old port 25.
daemon_smtp_ports = 2525


# If somebody sends email to "pmorgan",
# change the address to "pmorgan@example.com".
qualify_domain = example.com


# Disable RFC1413 (ident) callbacks for incoming smtp calls.
rfc1413_query_timeout = 0s


# Unfreeze bounce messages after one day, then try again to deliver.
# Remove frozen messages that are older than two days.
# See also the retry configuration elsewhere in this file.
ignore_bounce_errors_after = 1d
timeout_frozen_after = 2d


# By default, messages that are waiting on Exim's queue are all held in a
# single directory called "input" which it itself within Exim's spool
# directory. (The default spool directory is specified when Exim is built, and
# is often /var/spool/exim/.) Exim works best when its queue is kept short, but
# there are circumstances where this is not always possible. If you uncomment
# the setting below, messages on the queue are held in 62 subdirectories of
# "input" instead of all in the same directory. The subdirectories are called
# 0, 1, ... A, B, ... a, b, ... z. This has two benefits: (1) If your file
# system degrades with many files in one directory, this is less likely to
# happen; (2) Exim can process the queue one subdirectory at a time instead of
# all at once, which can give better performance with large queues.

# split_spool_directory = true


# If you're in a part of the world where ASCII is not sufficient for most
# text, then you're probably familiar with RFC2047 message header extensions.
# By default, Exim adheres to the specification, including a limit of 76
# characters to a line, with encoded words fitting within a line.
# If you wish to use decoded headers in message filters in such a way
# that successful decoding of malformed messages matters, you may wish to
# configure Exim to be more lenient.
#
# check_rfc2047_length = false
#
# In particular, the Exim maintainers have had multiple reports of problems
# from Russian administrators of issues until they disable this check,
# because of some popular, yet buggy, mail composition software.


# If you wish to be strictly RFC compliant, or if you know you'll be
# exchanging email with systems that are not 8-bit clean, then you may
# wish to disable advertising 8BITMIME.  Uncomment this option to do so.

# accept_8bitmime = false


######################################################################
#                       ACL CONFIGURATION                            #
#         Specifies access control lists for incoming SMTP mail      #
######################################################################

begin acl

# This access control list is used for every RCPT command in an incoming
# SMTP message. The tests are run in order until the address is either
# accepted or denied.

acl_check_rcpt:

  # Accept if the source is local SMTP (i.e. not over TCP/IP). We do this by
  # testing for an empty sending host field.

  accept  hosts = :
          control = dkim_disable_verify

  #############################################################################
  # The following section of the ACL is concerned with local parts that contain
  # @ or % or ! or / or | or dots in unusual places.
  #
  # The characters other than dots are rarely found in genuine local parts, but
  # are often tried by people looking to circumvent relaying restrictions.
  # Therefore, although they are valid in local parts, these rules lock them
  # out, as a precaution.
  #
  # Empty components (two dots in a row) are not valid in RFC 2822, but Exim
  # allows them because they have been encountered. (Consider local parts
  # constructed as "firstinitial.secondinitial.familyname" when applied to
  # someone like me, who has no second initial.) However, a local part starting
  # with a dot or containing /../ can cause trouble if it is used as part of a
  # file name (e.g. for a mailing list). This is also true for local parts that
  # contain slashes. A pipe symbol can also be troublesome if the local part is
  # incorporated unthinkingly into a shell command line.
  #
  # Two different rules are used. The first one is stricter, and is applied to
  # messages that are addressed to one of the local domains handled by this
  # host. The line "domains = +local_domains" restricts it to domains that are
  # defined by the "domainlist local_domains" setting above. The rule  blocks
  # local parts that begin with a dot or contain @ % ! / or |. If you have
  # local accounts that include these characters, you will have to modify this
  # rule.

  deny    message       = Restricted characters in address
          domains       = +local_domains
          local_parts   = ^[.] : ^.*[@%!/|]

  # The second rule applies to all other domains, and is less strict. The line
  # "domains = !+local_domains" restricts it to domains that are NOT defined by
  # the "domainlist local_domains" setting above. The exclamation mark is a
  # negating operator. This rule allows your own users to send outgoing
  # messages to sites that use slashes and vertical bars in their local parts.
  # It blocks local parts that begin with a dot, slash, or vertical bar, but
  # allows these characters within the local part. However, the sequence /../
  # is barred. The use of @ % and ! is blocked, as before. The motivation here
  # is to prevent your users (or your users' viruses) from mounting certain
  # kinds of attack on remote sites.

  deny    message       = Restricted characters in address
          domains       = !+local_domains
          local_parts   = ^[./|] : ^.*[@%!] : ^.*/\\.\\./
  #############################################################################

  # Accept mail to postmaster in any local domain, regardless of the source,
  # and without verifying the sender.

  accept  local_parts   = postmaster
          domains       = +local_domains

  # Deny unless the sender address can be verified.

  require verify        = sender

  # Accept if the message comes from one of the hosts for which we are an
  # outgoing relay. It is assumed that such hosts are most likely to be MUAs,
  # so we set control=submission to make Exim treat the message as a
  # submission. It will fix up various errors in the message, for example, the
  # lack of a Date: header line. If you are actually relaying out out from
  # MTAs, you may want to disable this. If you are handling both relaying from
  # MTAs and submissions from MUAs you should probably split them into two
  # lists, and handle them differently.

  # Recipient verification is omitted here, because in many cases the clients
  # are dumb MUAs that don't cope well with SMTP error responses. If you are
  # actually relaying out from MTAs, you should probably add recipient
  # verification here.

  # Note that, by putting this test before any DNS black list checks, you will
  # always accept from these hosts, even if they end up on a black list. The
  # assumption is that they are your friends, and if they get onto a black
  # list, it is a mistake.

  accept  hosts         = +relay_from_hosts
          control       = submission
          control       = dkim_disable_verify

  # Accept if the message arrived over an authenticated connection, from
  # any host. Again, these messages are usually from MUAs, so recipient
  # verification is omitted, and submission mode is set. And again, we do this
  # check before any black list tests.

  accept  authenticated = *
          control       = submission
          control       = dkim_disable_verify

  # Insist that any other recipient address that we accept is either in one of
  # our local domains, or is in a domain for which we explicitly allow
  # relaying. Any other domain is rejected as being unacceptable for relaying.

  require message = relay not permitted
          domains = +local_domains : +relay_to_domains

  # We also require all accepted addresses to be verifiable. This check will
  # do local part verification for local domains, but only check the domain
  # for remote domains. The only way to check local parts for the remote
  # relay domains is to use a callout (add /callout), but please read the
  # documentation about callouts before doing this.

  require verify = recipient

  #############################################################################
  # There are no default checks on DNS black lists because the domains that
  # contain these lists are changing all the time. However, here are two
  # examples of how you can get Exim to perform a DNS black list lookup at this
  # point. The first one denies, whereas the second just warns.
  #
  # deny    message       = rejected because $sender_host_address is in a black list at $dnslist_domain\n$dnslist_text
  #         dnslists      = black.list.example
  #
  # warn    dnslists      = black.list.example
  #         add_header    = X-Warning: $sender_host_address is in a black list at $dnslist_domain
  #         log_message   = found in $dnslist_domain
  #############################################################################

  #############################################################################
  # This check is commented out because it is recognized that not every
  # sysadmin will want to do it. If you enable it, the check performs
  # Client SMTP Authorization (csa) checks on the sending host. These checks
  # do DNS lookups for SRV records. The CSA proposal is currently (May 2005)
  # an Internet draft. You can, of course, add additional conditions to this
  # ACL statement to restrict the CSA checks to certain hosts only.
  #
  # require verify = csa
  #############################################################################

  # At this point, the address has passed all the checks that have been
  # configured, so we accept it unconditionally.

  accept


# This ACL is used after the contents of a message have been received. This
# is the ACL in which you can test a message's headers or body, and in
# particular, this is where you can invoke external virus or spam scanners.
# Some suggested ways of configuring these tests are shown below, commented
# out. Without any tests, this ACL accepts all messages. If you want to use
# such tests, you must ensure that Exim is compiled with the content-scanning
# extension (WITH_CONTENT_SCAN=yes in Local/Makefile).

acl_check_data:

  # Deny if the message contains a virus. Before enabling this check, you
  # must install a virus scanner and set the av_scanner option above.
  #
  # deny    malware    = *
  #         message    = This message contains a virus ($malware_name).

  # Add headers to a message if it is judged to be spam. Before enabling this,
  # you must install SpamAssassin. You may also need to set the spamd_address
  # option above.
  #
  # warn    spam       = nobody
  #         add_header = X-Spam_score: $spam_score\n\
  #                      X-Spam_score_int: $spam_score_int\n\
  #                      X-Spam_bar: $spam_bar\n\
  #                      X-Spam_report: $spam_report

  # Accept the message.

  accept



######################################################################
#                      ROUTERS CONFIGURATION                         #
#               Specifies how addresses are handled                  #
######################################################################
#     THE ORDER IN WHICH THE ROUTERS ARE DEFINED IS IMPORTANT!       #
# An address is passed to each router in turn until it is accepted.  #
######################################################################

begin routers

# This alternative router can be used when you want to send all mail to a
# server which handles DNS lookups for you; an ISP will typically run such
# a server for their customers.  If you uncomment "smarthost" then you
# should comment out "dnslookup" above.  Setting a real hostname in route_data
# wouldn't hurt either.

smarthost:
  driver = manualroute
  domains = ! +local_domains
  transport = remote_smtp
  # This is the public IP of our Threat Mgmt Gateway (TMG).
  route_data = 74.120.84.40
  ignore_target_hosts = <; 0.0.0.0 ; 127.0.0.0/8 ; ::1
  no_more


######################################################################
#                      TRANSPORTS CONFIGURATION                      #
######################################################################
#                       ORDER DOES NOT MATTER                        #
#     Only one appropriate transport is called for each delivery.    #
######################################################################

# A transport is used only when referenced from a router that successfully
# handles an address.

begin transports


# This transport is used for delivering messages over SMTP connections.

remote_smtp:
  driver = smtp


######################################################################
#                      RETRY CONFIGURATION                           #
######################################################################

begin retry

# This single retry rule applies to all domains and all errors. It specifies
# retries every 15 minutes for 2 hours, then increasing retry intervals,
# starting at 1 hour and increasing each time by a factor of 1.5, up to 16
# hours, then retries every 6 hours until 4 days have passed since the first
# failed delivery.

# WARNING: If you do not have any retry rules at all (this section of the
# configuration is non-existent or empty), Exim will not do any retries of
# messages that fail to get delivered at the first attempt. The effect will
# be to treat temporary errors as permanent. Therefore, DO NOT remove this
# retry rule unless you really don't want any retries.

# Address or Domain    Error       Retries
# -----------------    -----       -------

*                      *           F,2h,15m; G,16h,1h,1.5; F,4d,6h



######################################################################
#                      REWRITE CONFIGURATION                         #
######################################################################

# There are no rewriting specifications in this default configuration file.

begin rewrite



######################################################################
#                   AUTHENTICATION CONFIGURATION                     #
######################################################################

# The following authenticators support plaintext username/password
# authentication using the standard PLAIN mechanism and the traditional
# but non-standard LOGIN mechanism, with Exim acting as the server.
# PLAIN and LOGIN are enough to support most MUA software.
#
# These authenticators are not complete: you need to change the
# server_condition settings to specify how passwords are verified.
# They are set up to offer authentication to the client only if the
# connection is encrypted with TLS, so you also need to add support
# for TLS. See the global configuration options section at the start
# of this file for more about TLS.
#
# The default RCPT ACL checks for successful authentication, and will accept
# messages from authenticated users from anywhere on the Internet.

begin authenticators

# PLAIN authentication has no server prompts. The client sends its
# credentials in one lump, containing an authorization ID (which we do not
# use), an authentication ID, and a password. The latter two appear as
# $auth2 and $auth3 in the configuration and should be checked against a
# valid username and password. In a real configuration you would typically
# use $auth2 as a lookup key, and compare $auth3 against the result of the
# lookup, perhaps using the crypteq{}{} condition.

#PLAIN:
#  driver                     = plaintext
#  server_set_id              = $auth2
#  server_prompts             = :
#  server_condition           = Authentication is not yet configured
#  server_advertise_condition = ${if def:tls_in_cipher }

# LOGIN authentication has traditional prompts and responses. There is no
# authorization ID in this mechanism, so unlike PLAIN the username and
# password are $auth1 and $auth2. Apart from that you can use the same
# server_condition setting for both authenticators.

#LOGIN:
#  driver                     = plaintext
#  server_set_id              = $auth1
#  server_prompts             = <| Username: | Password:
#  server_condition           = Authentication is not yet configured
#  server_advertise_condition = ${if def:tls_in_cipher }


######################################################################
#                   CONFIGURATION FOR local_scan()                   #
######################################################################

# If you have built Exim to include a local_scan() function that contains
# tables for private options, you can define those options here. Remember to
# uncomment the "begin" line. It is commented by default because it provokes
# an error with Exim binaries that are not built with LOCAL_SCAN_HAS_OPTIONS
# set in the Local/Makefile.

# begin local_scan


# End of Exim configuration file

mail.service

[Unit]
Description=Site-local Mail Relay
After=docker.service
Requires=dns.service

[Service]
ExecStartPre=-/usr/bin/docker rm -f %n
ExecStart=/usr/bin/docker run \
  --name %n \
  -c 1023 \
  -m 20m \
  -p 25:2525 \
  -h ix-smtp01.inf.example.com \
  --dns 10.188.2.254 \
  --dns-search prod.example.com \
  --dns-search inf.example.com \
  --read-only \
  --cap-drop all \
  --cap-add setgid \
  --cap-add setuid \
  quay.io/example/mail_relay:latest
ExecStop=/usr/bin/docker stop %n
RestartSec=5s
Restart=always

[Install]
WantedBy=multi-user.target