mxallowd(1) 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.

AUTHOR

Michael Stapelberg <michael+mxallowd at stapelberg dot de>

SEE ALSO

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