Datanasov: New howto: router forwarding, networkd interfaces, minimal iptables with DMZ DNAT

2026-08-09T21:57:35Z

New howto: router forwarding, networkd interfaces, minimal iptables with DMZ DNAT

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Turning a Linux box into a router: enabling IP forwarding, addressing the three
interfaces with systemd-networkd, and a minimal iptables policy that publishes a
mail server and a web server from the DMZ.

{{Box|1=Interfaces and addresses used throughout: '''eth0''' WAN, '''eth1''' LAN <code>10.0.1.0/24</code>, '''eth2''' DMZ <code>10.0.0.0/24</code>. The router holds the <code>.1</code> in each subnet — <code>10.0.1.1</code> on the LAN, <code>10.0.0.1</code> on the DMZ.}}

{| class="wikitable"
! Role !! Interface !! Address
|-
| Router, WAN || eth0 || from the ISP (DHCP or static)
|-
| Router, LAN || eth1 || <code>10.0.1.1/24</code>
|-
| Router, DMZ || eth2 || <code>10.0.0.1/24</code>
|-
| Mail server || — || <code>10.0.0.10</code>
|-
| Web server || — || <code>10.0.0.20</code>
|}

== Enable IP forwarding ==
Without this the kernel drops anything not addressed to itself, and no amount of
firewall rules will make traffic cross between interfaces.

Immediately, until reboot:
<syntaxhighlight lang="bash">
sysctl -w net.ipv4.ip_forward=1
</syntaxhighlight>

Permanently:
<syntaxhighlight lang="bash">
cat >/etc/sysctl.d/99-router.conf <<'EOF'
net.ipv4.ip_forward = 1
# Uncomment if the router also routes IPv6:
# net.ipv6.conf.all.forwarding = 1
EOF

sysctl --system
</syntaxhighlight>

Check the value in force:
<syntaxhighlight lang="bash">
sysctl net.ipv4.ip_forward
cat /proc/sys/net/ipv4/ip_forward
</syntaxhighlight>

=== Disable it again ===
<syntaxhighlight lang="bash">
sysctl -w net.ipv4.ip_forward=0
</syntaxhighlight>

To make that stick, set the value to <code>0</code> in
<code>/etc/sysctl.d/99-router.conf</code> and run <code>sysctl --system</code>
— deleting the file only reverts to the kernel default, which is also
<code>0</code>, but leaves nothing documenting the intent.

Turning forwarding off stops routing but leaves the NAT and filter rules loaded.
To stand the router down completely, flush them as well — see
[[#Tearing it down|Tearing it down]].

{{Box|1=systemd-networkd can set forwarding itself, but the option was renamed: <code>IPForward=</code> in <code>[Network]</code> is deprecated from systemd 256 in favour of <code>IPv4Forwarding=</code> / <code>IPv6Forwarding=</code>. <code>sysctl</code> behaves the same on every version, so the examples above use it.}}

== Interfaces with systemd-networkd ==
One file per interface in <code>/etc/systemd/network/</code>. The numeric prefix
only controls the order the files are read.

WAN — <code>/etc/systemd/network/10-wan.network</code>:
<syntaxhighlight lang="ini">
[Match]
Name=eth0

[Network]
DHCP=ipv4
# Static instead? Drop DHCP= and use:
# Address=203.0.113.10/24
# Gateway=203.0.113.1
# DNS=1.1.1.1

[DHCPv4]
UseDNS=yes
UseNTP=yes
</syntaxhighlight>

LAN — <code>/etc/systemd/network/20-lan.network</code>:
<syntaxhighlight lang="ini">
[Match]
Name=eth1

[Network]
Address=10.0.1.1/24
ConfigureWithoutCarrier=yes
</syntaxhighlight>

DMZ — <code>/etc/systemd/network/30-dmz.network</code>:
<syntaxhighlight lang="ini">
[Match]
Name=eth2

[Network]
Address=10.0.0.1/24
ConfigureWithoutCarrier=yes
</syntaxhighlight>

Apply and inspect:
<syntaxhighlight lang="bash">
systemctl enable --now systemd-networkd
networkctl reload
networkctl status eth1
</syntaxhighlight>

Handing out addresses on the LAN is a separate job — see
[[DHCP and NTP for the LAN with systemd-networkd]].

== Minimal firewall ==
Default deny inbound and forwarded, allow outbound, and let conntrack carry
return traffic. Everything published from the DMZ is opened explicitly.

=== Policies and baseline ===
<syntaxhighlight lang="bash">
# Start from a clean slate
iptables -F
iptables -t nat -F
iptables -X

iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT ACCEPT

# Loopback, and anything conntrack already knows about
iptables -A INPUT -i lo -j ACCEPT
iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -m conntrack --ctstate INVALID -j DROP
</syntaxhighlight>

=== Services on the router itself ===
Only from the LAN. Nothing on the router listens to the WAN.
<syntaxhighlight lang="bash">
iptables -A INPUT -i eth1 -p tcp --dport 22 -j ACCEPT # ssh
iptables -A INPUT -i eth1 -p udp --dport 67 -j ACCEPT # dhcp server
iptables -A INPUT -i eth1 -p udp --dport 123 -j ACCEPT # ntp server
iptables -A INPUT -i eth1 -p udp --dport 53 -j ACCEPT # dns, if the router resolves
iptables -A INPUT -i eth1 -p tcp --dport 53 -j ACCEPT

# Ping, rate limited, from anywhere
iptables -A INPUT -p icmp --icmp-type echo-request -m limit --limit 5/s -j ACCEPT
</syntaxhighlight>

=== Publish the DMZ servers (DNAT) ===
<code>25</code> is plain SMTP with STARTTLS, <code>587</code> is the submission
port (also STARTTLS) and <code>465</code> is implicit TLS submission. Drop
whichever of the two submission ports your mail server does not offer.

<syntaxhighlight lang="bash">
# Mail -> 10.0.0.10
iptables -t nat -A PREROUTING -i eth0 -p tcp -m multiport \
--dports 25,465,587,993 -j DNAT --to-destination 10.0.0.10

# Web -> 10.0.0.20
iptables -t nat -A PREROUTING -i eth0 -p tcp -m multiport \
--dports 80,443 -j DNAT --to-destination 10.0.0.20
</syntaxhighlight>

DNAT only rewrites the destination. The packet still has to survive
<code>FORWARD</code>, which is where the matching accepts belong:

<syntaxhighlight lang="bash">
iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -m conntrack --ctstate INVALID -j DROP

iptables -A FORWARD -i eth0 -o eth2 -d 10.0.0.10 -p tcp -m multiport \
--dports 25,465,587,993 -j ACCEPT
iptables -A FORWARD -i eth0 -o eth2 -d 10.0.0.20 -p tcp -m multiport \
--dports 80,443 -j ACCEPT
</syntaxhighlight>

=== Egress and inter-zone policy ===
<syntaxhighlight lang="bash">
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT # LAN -> internet
iptables -A FORWARD -i eth2 -o eth0 -j ACCEPT # DMZ -> internet (updates, outbound mail)
iptables -A FORWARD -i eth1 -o eth2 -j ACCEPT # LAN -> DMZ (administration)

# DMZ -> LAN is deliberately absent. That asymmetry is the whole point of a
# DMZ: if a published server is compromised it must not reach the LAN.
# Return traffic for LAN-initiated connections is already covered by the
# ESTABLISHED,RELATED rule above.
</syntaxhighlight>

=== Source NAT ===
<syntaxhighlight lang="bash">
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
</syntaxhighlight>

<code>MASQUERADE</code> takes the WAN address as it finds it, which is what you
want on a dynamic ISP address. On a static WAN address <code>SNAT</code> is
marginally cheaper because it does not look the address up per connection:

<syntaxhighlight lang="bash">
iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to-source 203.0.113.10
</syntaxhighlight>

=== Reaching the servers from the LAN ===
A LAN client resolving the public name gets the WAN address, sends the packet to
the router, and the reply comes back from <code>10.0.0.10</code> directly —
so the connection never completes. Either give the LAN a split-horizon DNS view
that answers with the internal address, or add hairpin NAT:

<syntaxhighlight lang="bash">
iptables -t nat -A POSTROUTING -s 10.0.1.0/24 -d 10.0.0.10 -p tcp -m multiport \
--dports 25,465,587,993 -j MASQUERADE
iptables -t nat -A POSTROUTING -s 10.0.1.0/24 -d 10.0.0.20 -p tcp -m multiport \
--dports 80,443 -j MASQUERADE
</syntaxhighlight>

Split-horizon DNS is the tidier of the two — hairpin NAT hides the client's real
address from the server, so its logs show the router for every internal visitor.

== Make the rules survive a reboot ==
Debian and Ubuntu:
<syntaxhighlight lang="bash">
apt install iptables-persistent
iptables-save > /etc/iptables/rules.v4
</syntaxhighlight>

RHEL, CentOS, Rocky:
<syntaxhighlight lang="bash">
dnf install iptables-services
systemctl enable --now iptables
iptables-save > /etc/sysconfig/iptables
</syntaxhighlight>

{{Box|type=warning|1=Save only after you have confirmed you can still reach the box. A default-DROP <code>INPUT</code> policy saved with no working ssh rule locks you out until you get physical or console access.}}

== Verify ==
<syntaxhighlight lang="bash">
# Rules, with counters, as they are actually loaded
iptables -L -v -n --line-numbers
iptables -t nat -L -v -n

# Watch a DNAT entry appear while testing from outside
conntrack -L -d 10.0.0.20

# From an external host
nc -vz <wan-address> 443
nc -vz <wan-address> 993
</syntaxhighlight>

If a published port does not answer, check the <code>FORWARD</code> counters
first. Traffic that was DNATed but has no matching <code>FORWARD</code> accept is
the usual cause, and it shows up as a rising policy-drop count rather than an
error anywhere.

== Tearing it down ==
<syntaxhighlight lang="bash">
sysctl -w net.ipv4.ip_forward=0

iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT
iptables -P OUTPUT ACCEPT
iptables -F
iptables -t nat -F
iptables -X
</syntaxhighlight>

{{Box|1=On current distributions <code>iptables</code> is usually a front end to nftables (<code>iptables-nft</code>). The syntax above is unchanged; <code>iptables -V</code> reports which backend is in use, and rules created this way are visible with <code>nft list ruleset</code>.}}

[[Category:Networking]]
[[Category:Linux]]
[[Category:Howto]]

Linux router with WAN, LAN and DMZ - Revision history

Datanasov: New howto: router forwarding, networkd interfaces, minimal iptables with DMZ DNAT