Configuration of a Gateway to the Internet

Our iptables.rules-Gateway.sh script sets up an iptables configuration with various services that can be enabled (e.g. for masquerading, IPTV, different kinds of servers) as an example that we hope is secure enough [1].

[1]	Please email us if you found otherwise! Refer to the top of this document for that.

This script expects a settings file to be available as /etc/iptables.settings-Gateway.sh (this file is meant to be sourced, not executed).

An example thereof:

# Local firewall settings.
#
# Meant to be sourced by the iptables.rules-Gateway.sh script.

# Where firewall-related outputs will be written:
log_file=/root/.last-gateway-firewall-activation

# Local (LAN) interface, the one we trust:
#lan_if=eth1
lan_if=enp2s0

# Internet (WAN) interface, the one we distrust:

# For PPP ADSL connections:
#net_if=ppp0

# For direct connection to a set-top (telecom) box from your provider:
#net_if=eth0
net_if=enp4s0

ban_file="/etc/ban-rules.iptables"

# As the IPs banned through the ban file above are quite minimal:
use_ban_rules="true"
#use_ban_rules="false"

# IP of a test client (to avoid too many logs, selecting only related events):
#test_client_ip="xxx"

# Enabled input TCP port range for traffic from LAN to gateway:
enable_unfiltered_tcp_range="true"

# TCP unfiltered window (e.g. for passive FTP and BEAM port ranges):
tcp_unfiltered_low_port=50000
tcp_unfiltered_high_port=55000

# Tells whether IPTV (TV on the Internet thanks to a box) should be allowed:
enable_iptv=false

# Tells whether a SMTP server can be used:
enable_smtp=false

# Typically a set-top box from one's ISP (defined as a possibly log match
# criteria):

# Classical example:
telecom_box="192.168.0.254"

# DHT subsection, for P2P exchanges:
# More infos: https://github.com/rakshasa/rtorrent/wiki/Using-DHT

dht_udp_port=7881

#use_dht="true"
use_dht="false"

# One may use a non-standard port:
#ssh_port=22
ssh_port=22320

smtp_port=25

# SMTPS is obsolete:
smtp_secure_port=465

# STARTTLS over SMTP is the proper way of securing SMTP:
msa_port=587

pop3_port=110

# POP3S:
pop3_secure_port=995

imap_port=143
imap_secure_port=993

A script to configure iptables is best integrated to systemd, see the iptables.rules-Gateway.service file for that (typically to be placed in /etc/systemd/system). Then one may test with:

$ systemctl start iptables.rules-Gateway.service

and enable it for good with:

$ systemctl enable iptables.rules-Gateway.service

Note that often these scripts are setup remotely, while being connected thanks to SSH from another host. Care must be taken in order not to lock oneself out of the target server, notably when updating rules (this happens quite easily). We advise to prefer the restart option of our iptables script in order to reduce the risk of "bricking" one's server.

Firewall-related Troubleshooting

Use iptables-inspect.sh to list the currently-used firewall rules for the chains of the main tables. Like iptables -nL --line-numbers, it displays the number of each rule of a given chain, which allows to add/remove rules more easily, like in:

# Deletes the first rule of the FORWARD chain (of the 'filter' table):
# (note that all the next rules will bear a decremented number afterwards!)
$ iptables -D FORWARD 1

Setting environment variables (either through files such as /etc/iptables.settings-Gateway.sh or directly in the shell) is less error-prone; e.g.

[...]
$ lan_if=enp2s0
$ net_if=enp4s0
$ iptables -I FORWARD -i ${lan_if} -o ${net_if} -d ${telecom_box} -j LOG
$ journalctl -kf --grep="IN=.*OUT=.*" | grep -v "SRC=${telecom_box}"

To further match packets, one may specify log prefixes, like in:

$ iptables -A INPUT -i lan.foobar -j LOG --log-prefix "[VLAN INP FOO]"

Note that the LOG target does not intercept a packet, which thus continues to flow in the next rule(s). so log targets are better defined as first rules (and thus could be inserted lastly).

As a reminder, for a given table (filter by default), rules may be:

• appended at the end of the selected chain with -A (then of course any previous rule may eclipse it)
• inserted either at the beginning of the selected chain with -I, or at its position N with -I N

Adding/removing rules "safely" from the command-line may be done more easily by rule specification rather than by rule position.

For example removing a given rule can be done:

• by first listing all rules thanks to iptables -S, to pick the one of interest
• by removing the prefix (typically -A) of the spec, and pasting the rest of that spec after iptables -D in order to remove the corresponding rule

Putting back this rule at the first position on its chain instead is just a matter of using iptables -I and the same shortened spec.

So, for example if wanting to silence an untrusted device (e.g. some "smart" box, a netcam of dubious origin), an approach is to:

• determine its MAC address and associate a static IP to it; for example, if using dnsmasq, in /etc/dnsmasq.conf one may define dhcp-host=30:ef:50:36:54:68,10.0.17.203, so that the IP 10.0.17.203 is always assigned to the device of MAC address 30:ef:50:36:54:68
• define a firewall rule so that one's gateway never forwards outgoing packets from that (local) IP, for example with: iptables -I FORWARD -s 10.0.17.203/32 -i my_lan_interface -j DROP

See also:

• our filtered_local_hosts section of iptables.rules-Gateway.sh, the hosts being filtered out based on the ones specified in the setting_file configuration file
• the iptables section in the Arch wiki.