> From: M2C
> Is there a way to limit outgoing emails for each authenticated user (LOGIN
> AUTH) in exim?. For example, 100 mails per day for User1 and 200 mails per
> day for User2.
What is the goal of the limit - impeding outgoing spam?
Simple ratelimits are better than nothing but constrain honest users
while still allowing spam to trickle through,
your relays still end up in blacklists.
Each relay needs automatic detection and blocking of compromised accounts
(stolen passwords). I implemented Andrew Hearn's idea
to check not rate of messages or all recipients, but
rate of attempts to send to nonexistent recipient email addresses.
Vast majority of spammers never try to validate every recipient address.
Spammers harvest strings looking like email addresses from
webpages and disks of trojaned Windowses, then sell huge lists of
email addresses to each other. These lists contain very much
email addresses which don't exist anymore or never existed:
Message-Ids, corrupted strings in memory and files.
In short, spammers' lists of email addresses are much dirtier
than lists honest users send to. Honest users are very unlikely
to attempt to send to 100 nonexistent email addresses in one hour.
My implementation is for Exim (version 4.67 or higher).
Below I explain in detail (for novices at Exim) what to change in
Exim config for automatic blocking of compromised and spammers' accounts,
with automatic email notification to abuse or support staff.
This code also blocks brute force password cracking via SMTP
(it's not as important but a little useful).
1. Replace the paragraph with the line "accept authenticated = *"
with three paragraphs:
accept authenticated = *
set acl_m_user = $authenticated_id
condition = ${if exists{$spool_directory/blocked_authenticated_users}}
condition = ${lookup{$acl_m_user}lsearch\
{$spool_directory/blocked_authenticated_users}{1}{0}}
control = freeze/no_tell
control = submission/domain=
add_header = X-Authenticated-As: $acl_m_user
accept authenticated = *
!verify = recipient/defer_ok/callout=10s,defer_ok,use_sender
ratelimit = LIM / PERIOD / per_rcpt / user-$acl_m_user
continue = ${run{SHELL -c "echo $acl_m_user \
>>$spool_directory/blocked_authenticated_users; \
\N{\N echo Subject: user $acl_m_user blocked; echo; echo because \
has sent mail to LIM invalid recipients during PERIOD.; \
\N}\N | EXIMBINARY WARNTO"}}
control = freeze/no_tell
control = submission/domain=
add_header = X-Authenticated-As: $acl_m_user
accept authenticated = *
control = submission/domain=
2. If the line "hostlist relay_from_hosts ="
contains something besides "localhost" or "127.0.0.1"
or this server's IP-address, i.e. your server is a relay for
your LAN or users on your company's IP-addresses,
then replace the paragraph with the line
"accept hosts = +relay_from_hosts"
with three paragraphs:
accept hosts = !@[] : +relay_from_hosts
set acl_m_user = $sender_host_address
# or username from RADIUS
condition = ${if exists{$spool_directory/blocked_relay_users}}
condition = ${lookup{$acl_m_user}lsearch\
{$spool_directory/blocked_relay_users}{1}{0}}
control = freeze/no_tell
control = submission/domain=
add_header = X-Relayed-From: $acl_m_user
accept hosts = !@[] : +relay_from_hosts
!verify = recipient/defer_ok/callout=10s,defer_ok,use_sender
ratelimit = LIM / PERIOD / per_rcpt / relayuser-$acl_m_user
continue = ${run{SHELL -c "echo $acl_m_user \
>>$spool_directory/blocked_relay_users; \
\N{\N echo Subject: relay user $acl_m_user blocked; echo; echo \
because has sent mail to LIM invalid recipients during PERIOD.; \
\N}\N | EXIMBINARY WARNTO"}}
control = freeze/no_tell
control = submission/domain=
add_header = X-Relayed-From: $acl_m_user
accept hosts = +relay_from_hosts
control = submission/domain=
3. Insert into beginning of config:
acl_smtp_connect = acl_check_connect
acl_smtp_auth = acl_check_auth
acl_smtp_mail = acl_check_mail
acl_smtp_quit = acl_check_quit
acl_smtp_notquit = acl_check_notquit
LIM = 100
PERIOD = 1h
WARNTO = abuse@???
EXIMBINARY = /usr/local/sbin/exim -f root
SHELL = /bin/sh
In the WARNTO line replace "abuse@???" with your
abuse or support or sysadmin email address;
I specified paths in EXIMBINARY and SHELL lines for FreeBSD,
adjust for your operating system.
4. Immediately after the "begin acl" line insert:
acl_check_auth:
drop message = authentication is allowed only once per message in order \
to slow down bruteforce cracking
set acl_m_auth = ${eval10:0$acl_m_auth+1}
condition = ${if >{$acl_m_auth}{2}}
delay = 22s
drop message = blacklisted for bruteforce cracking attempt
set acl_c_authnomail = ${eval10:0$acl_c_authnomail+1}
condition = ${if >{$acl_c_authnomail}{4}}
continue = ${run{SHELL -c "echo $sender_host_address \
>>$spool_directory/blocked_IPs; \
\N{\N echo Subject: $sender_host_address blocked; echo; echo \
for bruteforce auth cracking attempt.; \
\N}\N | EXIMBINARY WARNTO"}}
accept
acl_check_quit:
warn condition = ${if def:authentication_failed}
condition = $authentication_failed
logwrite = :reject: quit after authentication failed: \
${sg{$sender_rcvhost}{\N[\n\t]+\N}{\040}}
ratelimit = 7 / 5m / strict / per_conn
continue = ${run{SHELL -c "echo $sender_host_address \
>>$spool_directory/blocked_IPs; \
\N{\N echo Subject: $sender_host_address blocked; echo; echo \
for bruteforce auth cracking attempt.; \
\N}\N | EXIMBINARY WARNTO"}}
acl_check_notquit:
warn condition = ${if def:authentication_failed}
condition = $authentication_failed
logwrite = :reject: $smtp_notquit_reason after authentication failed: \
${sg{$sender_rcvhost}{\N[\n\t]+\N}{\040}}
condition = ${if eq{$smtp_notquit_reason}{connection-lost}}
ratelimit = 7 / 5m / strict / per_conn
continue = ${run{SHELL -c "echo $sender_host_address \
>>$spool_directory/blocked_IPs; \
\N{\N echo Subject: $sender_host_address blocked; echo; echo \
for bruteforce auth cracking attempt.; \
\N}\N | EXIMBINARY WARNTO"}}
acl_check_mail:
accept set acl_c_authnomail = 0
acl_check_connect:
drop message = $sender_host_address locally blacklisted for a bruteforce \
auth (username+password) cracking attempt
condition = ${if exists{$spool_directory/blocked_IPs}}
condition = ${lookup{$sender_host_address}lsearch\
{$spool_directory/blocked_IPs}{1}{0}}
accept
When your staff receives a message with Subject like
"115.150.81.95 blocked", just check that the IP-address is
unknown for you (in this example China) - it's who attempted
to crack passwords.
When your staff receives a message with Subject like
"user johndoe@??? blocked", it means that using this user's
password multiple messages were sent through your server,
and during last hour 100 of recipient email addresses were rejected
(5xx) by recipient MXs. This is very unlikely with honest users,
but typical for spammers. You can look in your logs
which recipient email addresses were rejected by your Exim:
cd /var/log/exim; zcat mainlog*.* | fgrep 'Unknown user' | fgrep -v 'sender verify fail' | sed -E -e 's/^.+<.*<//' -e 's/>.+$//' | sort | uniq -c | sort -nr | less
Besides role accounts nonexistent on your domains like
webmaster@, sales@, office@, you'll see Message-IDs,
pieces of email addresses, pieces glued with random pieces of words.
For example, there were 540 attempts to spam a never existing address
on my domain x-originating-ipa@???
Spammers will try to send through your relay (using passwords
stolen from your users) to multiple nonexistent addresses too.
When your staff receives notification with "Subject: user ... blocked",
using `exipick` command (comes with Exim) look at content of messages
frozen in the queue sent with the username specified in the notification,
for example:
exipick -zi '$h_X-Authenticated-As eq johndoe@???' | xargs -n 1 exim -Mvc | less
If you see not spam (very unlikely) then using a text editor
delete the line with that username from the file blocked_authenticated_users
in Exim spool directory, for example /var/spool/exim/ in FreeBSD
(if the file contains only one line which is likely
then you can just delete the file instead)
and unfreeze detained messages:
exipick -zi '$h_X-Authenticated-As eq johndoe@???' | xargs exim -Mt
If you see spam then change the user's password, notify the user
and delete the line or file (see previous paragraph).
In order for the user to really get it,
provide a clause in the user agreement or contract beforehand:
if the user's password was used for spam (no matter who spammed -
the user or somebody else) then the user pays a fine.
Spam frozen in the queue is evidence.
When the evidence is not needed anymore, delete the frozen spam:
exipick -zi '$h_X-Authenticated-As eq johndoe@???' | xargs exim -Mrm
Similarly with users on LAN or the ISP's IP-ranges
(file blocked_relay_users,
notifications with Subject like "relay user 192.168.12.34 blocked").
Lena
