Provided by: mxallowd_1.9-2_amd64 
NAME
mxallowd - dynamically whitelist your Mail eXchanger
SYNOPSIS
mxallowd [-d] [-c configfile] [-t whitelist-time] [-p pflog-interface] [-l pcap-filter] [-F] [-s] [-q]
[-p] -f fake-mailserver -r real-mailserver -n queue-num
DESCRIPTION
mxallowd is a daemon which uses libnetfilter_queue (on Linux) or pf and pflog (on BSD) to allow (or deny)
connections to a mailserver (or similar application) if the remote host hasn't connected to a fake daemon
before.
This is an improved version of the so-called nolisting (see http://www.nolisting.org/). The assumption is
that spammers are not using RFC 2821-compatible SMTP-clients and are sending fire-and-forget spam
(directly to the first or second MX-entry without retrying on error). This direct access is blocked with
mxallowd, you'll only get a connection if you retry.
NOTE: It is highly recommended to install nscd (nameserver caching daemon) or a similar software in order
to speed-up DNS lookups. Since version 1.3, DNS lookups are done in a thread (so they don't block the
main process), however, on very-high-traffic-sites, mxallowd may show significantly better overall
performance in combination with nscd.
OPTIONS
-b, --no-rdns-whitelist
Disable whitelisting all IP-addresses that have the same RDNS as the connecting one (necessary for
google mail)
-c, --config
Specifies an alternative configuration file (instead of /etc/mxallowd.conf)
-t, --whitelist-time
Specify the amount of time (in seconds) until an IP-address will be removed from the whitelist
-s, --stdout
Log to stdout, not to syslog
-q, --quiet
Don't log anything but errors.
-f, --fake-mailserver
Specify which IP-address the fake mailserver has (connecting to it will whitelist you for the real
mailserver)
-r, --real-mailserver
Specify which IP-address the real mailserver has
-F, --foreground
Do not fork into background, stay on console
-n, --queue-num (only available when compiled for netfilter_queue)
Specify the queue number which will be used for the netfilter_queue-link. This has to be the same
which is specified in the iptables-rule and it has to be specified, there is no default.
-p, --pflog-interface (only available when compiled for pf)
Specify the pflog(4) interface which you configured in pf(4). The default is pflog0. Also see the
pcap-filter-option if you use an interface which does not only get smtp-traffic.
-l, --pcap-filter (only available when compiled for pf)
Specify the filter for pcap. The default is "port 25". See tcpdump(8) for more information on the
filters.
FILES
/etc/mxallowd.conf
System-wide configuration file. Use the long options without the beginning two dashes. For
example:
stdout
fake-mailserver 192.168.1.3
fake-mailserver 192.168.1.4
real-mailserver 192.168.1.5
queue-num 23
EXAMPLES FOR NETFILTER
The machine has two IP-addresses. The mailserver only listens on 192.168.1.4, the nameserver returns the
mx-records mx1.domain.com (192.168.1.3) with priority 5 and mx2.domain.com (192.168.1.4) with priority
10.
# modprobe nfnetlink_queue
# iptables -A INPUT -p tcp --dport 25 -m state --state NEW -j NFQUEUE --queue-num 23
# mxallowd -s -F -f 192.168.1.3 -r 192.168.1.4 -n 23
Then open a separate terminal and connect via telnet on your real mailserver. You'll see the connection
attempt being dropped. Now connect to the fake mailserver and watch mxallowd's output. Afterwards,
connect to the real mailserver to verify your mailserver is still working.
EXAMPLES FOR PF
The machine has two IP-addresses. The mailserver only listens on 192.168.1.4, the nameserver returns the
mx-records mx1.domain.com (192.168.1.3) with priority 5 and mx2.domain.com (192.168.1.4) with priority
10.
Create a pf.conf like this:
table <mx-white> persist
real_mailserver="192.168.1.4"
fake_mailserver="192.168.1.3"
real_mailserver6="2001:dead:beef::1"
fake_mailserver6="2001:dead:beef::2"
pass in quick log on fxp0 proto tcp from <mx-white> to $real_mailserver port smtp
pass in quick log on fxp0 inet6 proto tcp from <mx-white> to $real_mailserver6 port smtp
block in log on fxp0 proto tcp to { $fake_mailserver $real_mailserver } port smtp
block in log on fxp0 inet6 proto tcp to { $fake_mailserver6 $real_mailserver6 } port smtp
Afterwards, load it and start mxallowd using the following commands:
# pfctl -f /etc/pf.conf
# mxallowd -s -F -f 192.168.1.3 -r 192.168.1.4
Then open a separate terminal and connect via telnet on your real mailserver. You'll see the connection
attempt being dropped. Now connect to the fake mailserver and watch mxallowd's output. Afterwards,
connect to the real mailserver to verify your mailserver is still working.
The ruleset for pf is actually longer because pf does more than netfilter on linux -- netfilter passes
the packets and lets mxallowd decide whether to drop/accept whilst pf blocks/passes before even "passing"
to mxallowd.
SEE ALSO
iptables(8), pf(4), pflog(4), tcpdump(8)
AUTHOR
Michael Stapelberg <michael+mxallowd at stapelberg dot de>