Internet sharing

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

Requirements

The machine acting as server should have an additional network device. That network device requires a functional data link layer to the machine(s) that are going to receive internet access:

To be able to share internet to several machines a switch can provide the data link layer connection.
A wireless device can share access to several machines as well, see Software access point first for this case.
If you are sharing to only one machine, a crossover cable is sufficient. In case one of the two computers' ethernet cards has MDI-X capability, a crossover cable is not necessary and a regular ethernet cable can be used. Executing ethtool interface | grep MDI as root helps to figure it.

Configuration

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, unless both interfaces have netmasks strictly greater than /24.

# ip link set up dev net0
# ip addr add 192.168.123.100/24 dev net0 # arbitrary address

To have your static IP assigned at boot, you can use a network manager.

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 systemd.network(5) 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:

/etc/sysctl.d/30-ipforward.conf

net.ipv4.ip_forward=1
net.ipv6.conf.default.forwarding=1
net.ipv6.conf.all.forwarding=1

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

Enable NAT

With iptables

Install the iptables package. 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).

If connected via PPPoE, clamp mss to pmtu in order to prevent fragmentation:

# iptables -t mangle -A FORWARD -o ppp0 -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

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.

With nftables

Install the nftables package. To enable NAT with nftables, you will have to create the postrouting chain in a new/existing table:

# nft add table inet nat
# nft add chain inet nat postrouting '{ type nat hook postrouting priority 100 ; }'

After that, you have to masquerade the net0 addresses for internet0:

# nft add rule inet nat postrouting oifname internet0 masquerade

You may want to add some more firewall restrictions on the forwarding (assuming the filter table already exists, like configured in nftables#Server):

# nft add chain inet filter forward '{ type filter hook forward priority 0; policy drop; }'
# nft add rule inet filter forward ct state related,established accept
# nft add rule inet filter forward iifname net0 oifname internet0 accept

You can find more information on NAT in nftables in the nftables Wiki. If you want to make these changes permanent, follow the instructions on nftables

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, such as dhcpd or dnsmasq. Then configure a DHCP client (e.g. dhcpcd) on every client PC.

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Reason: This is not an iptables guide. Expanding the chain with iptables -I might skip other important rules; if you need to script an ON/OFF switch for this, use custom chain with a jump placed carefully in the INPUT chain. (Discuss in Talk:Internet sharing)

Incoming connections to UDP port 67 has to be allowed for DHCP server. It also necessary to allow incoming connections to UDP/TCP port 53 for DNS requests.

# iptables -I INPUT -p udp --dport 67 -i net0 -j ACCEPT
# iptables -I INPUT -p udp --dport 53 -s 192.168.123.0/24 -j ACCEPT
# iptables -I INPUT -p tcp --dport 53 -s 192.168.123.0/24 -j ACCEPT

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

Manually adding an IP

Instead of using DHCP, a static IP address and a default route via 192.168.123.100 can also be configured manually. There are many tools available to configure the network accordingly. One prominent example of such a tool is ip(8), see Network configuration#Network management. Alternatively, one can use a .network file, see Systemd-networkd#Wired adapter using a static IP to setup a static IP.

Configure a DNS server for each client, see Domain name resolution for details.

That is it. The client PC should now have Internet.

Troubleshooting

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.