Internet sharing

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This article explains how to share the internet connection from one machine to other(s).


  • The machine acting as server should have an additional network device.
  • That network device should be connected to the machines that are going to receive internet access. They can be one or more machines. To be able to share internet to several machines a switch is required. If you are sharing to only one machine, a crossover cable is sufficient.
Note: If one of the two computers has a gigabit ethernet card, a crossover cable is not necessary and a regular ethernet cable should be enough


This section assumes, that the network device connected to the client computer(s) is named net0 and the network device connected to the internet as internet0.

Tip: You can rename your devices to this scheme using Udev#Setting static device names.

All configuration is done on the server computer, except for the final step of #Assigning ip addresses to the client pc(s).

Static IP address

On the server computer, assign a static IPv4 address to the interface connected to the other machines. The first 3 bytes of this address cannot be exactly the same as those of another interface.

# ip link set up dev net0
# ip addr add dev net0 # arbitrary address

To have your static ip assigned at boot, you can use netctl.

Enable packet forwarding

Check the current packet forwarding settings:

# sysctl -a | grep forward

You will note that options exist for controlling forwarding per default, per interface, as well as separate options for IPv4/IPv6 per interface.

Enter this command to temporarily enable packet forwarding at runtime:

# sysctl net.ipv4.ip_forward=1
Tip: To enable packet forwarding selectively for a specific interface, use sysctl net.ipv4.conf.interface_name.forwarding=1 instead.
Warning: If the system uses systemd-networkd to control the network interfaces, a per-interface setting for IPv4 is not possible, i.e. systemd logic propagates any configured forwarding into a global (for all interfaces) setting for IPv4. The advised work-around is to use a firewall to forbid forwarding again on selective interfaces. See the manual page for more information. The IPForward=kernel semantics introduced in a previous systemd release 220/221 to honor kernel settings does not apply anymore.[1] [2]

Edit /etc/sysctl.d/30-ipforward.conf to make the previous change persistent after a reboot for all interfaces:


Afterwards it is advisable to double-check forwarding is enabled as required after a reboot.

Enable NAT

Install the package iptables from the official repositories. Use iptables to enable NAT:

# iptables -t nat -A POSTROUTING -o internet0 -j MASQUERADE
# iptables -A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
# iptables -A FORWARD -i net0 -o internet0 -j ACCEPT
Note: Of course, this also works with a mobile broadband connection (usually called ppp0 on routing PC).

Read the iptables article for more information (especially saving the rule and applying it automatically on boot). There is also an excellent guide on iptables Simple stateful firewall.

Assigning ip addresses to the client pc(s)

If you are planning to regularly have several machines using the internet shared by this machine, then is a good idea to install a dhcp server.

You can read the dhcpd wiki article, to add a dhcp server. Then, install the dhcpcd client on every client pc.

If you are not planing to use this setup regularly, you can manually add an ip to each client instead.

Manually adding an ip

Instead of using dhcp, on each client pc, add an ip address and the default route:

# ip addr add dev eth0  # arbitrary address, first three blocks must match the address from above
# ip link set up dev eth0
# ip route add default via dev eth0   # same address as in the beginning

Configure a DNS server for each client, see resolv.conf for details.

That's it. The client PC should now have Internet.


If you are able to connect the two PCs but cannot send data (for example, if the client PC makes a DHCP request to the server PC, the server PC receives the request and offers an IP to the client, but the client does not accept it, timing out instead), check that you do not have other Iptables rules interfering.

See also