Linux for internet sharing

Share wireless Internet connection through ethernet

I want to share the wireless Internet connection on my desktop with my old laptop, through ethernet. I have ticked the option «Make available to others» on both connections, on my desktop, but my laptop doesn’t connect. Am I doing something wrong or is it a bug?

I have done the exact same thing on 12.04 before. I am thinking there is a problem with network manager since i have Gnome 3.10 and it got updated today.

@Rmano: According to that article, modern Ethernet hardware auto-negotiates so crossover cables are not needed. I’ve also done this before with ordinary Ethernet cables.

@Mechanical snail: thanks, I didn’t noticed that. So I suppose I can dispose my precious manually connected cross-over cable 🙂

2 Answers 2

On the server computer:

1. On the computer, which is connected to the Internet, click the network icon in the panel and go to «Edit Connections. « at the bottom of the menu.
2. Double click your Wired Connection (Leave your wireless connection untouched, the one connected to Internet and the one you want to share, as I understand).
3. On the «IPv4 Settings tab», select Method: «Shared to other computers»
4. Reconnect by clicking on the Wired Network, so it gets a new IP address. (The two computers must be connected by an ethernet cable for this step, so connect them now if you haven’t already.)
5. Click on «Connection Information» in the network menu and write down the IP address and network mask (in my case it was assigned 10.42.0.1/255.255.255.0 but I do not know if that will always be the case).

On the client computer:

1. Go to «Edit Connections. » and assign a «Manual» Method. Assign an IP address on the same subnetwork (10.42.0.69 for example) and put the IP and network mask you wrote down in «Netmask» «Gateway» and «DNS servers»
2. Reconnect to the network to let the new settings be assigned.
3. Pat yourself on your back and surf away.

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Раздача Интернета с одного компьютера на другой (домашний вариант)

Инструкций уже много, в которых много всего написано, да так, что новички увидев это «многабукаф» закроют в ужасе. Попробую показать короткую инструкцию для раздачи Интернета для самого популярного случая с одного компьютера на другой, будь то Интернет через 3g-модем или PPPoE — не важно.

Исходные данные

Итак, что мы обычно имеем на дому?

Если у вас один из этих случаев, то инструкция для вас.

Считаем что инет висит на ppp0. Локальная сеть 192.168.0.0/24

Вместо ppp0 может быть eth0/1/2, wlan0 и т.п. Важно то, что мы знаем, что Инет приходит через этот интерфейс. Локальная сеть мб другой, поправьте под свой вариант или настройте свою сеть из 2-х компов как предложено здесь.

Настройка iptables

sudo su iptables -t nat -A POSTROUTING -o ppp0 ! -d 192.168.0.0/24 -j MASQUERADE iptables-save > /etc/iptables.up.rules echo "pre-up iptables-restore < /etc/iptables.up.rules" >> /etc/network/interfaces apt-get install bind9

Включили NAT, указав, что интерфейс с Инетом ppp0 и ему предназначены пакеты, не адресованные локальной сети 192.168.0.0/24

Можно обойтись без bind9. В этом случае на 2-м компе нужно будет указать те же IP DNS -серверов, что и на 1-м, либо просто указать гугловский DNS -сервер — 8.8.8.8

Включаем форвардинг (перенаправление пакетов)

находим и раскоментируем эту строку:

Чтобы изменения вступили в силу без перезагрузки ОС.

Всё, компьютер является шлюзом в Интернет. Для того, чтобы 2-й комп получил доступ в Инет нужно настроить сеть, указав шлюзом данный компьютер, в роли DNS -сервера выступает он же.

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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 .

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:

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).

Use -I DOCKER-USER instead of -A FORWARD if you installed docker. [3]

# iptables -t nat -A POSTROUTING -o internet0 -j MASQUERADE # iptables -I DOCKER-USER 1 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT # iptables -I DOCKER-USER 2 -i net0 -o internet0 -j ACCEPT

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

With firewalld

Install the firewalld package. firewalld is a firewall daemon which relies on nftables or iptables. First change the firewalld zones of network interfaces:

# firewall-cmd --zone=external --change-interface=internet0 --permanent # firewall-cmd --zone=internal --change-interface=net0 --permanent

Then add a new policy to let traffic flow between the internal and external zone:

# firewall-cmd --permanent --new-policy int2ext # firewall-cmd --permanent --policy int2ext --add-ingress-zone internal # firewall-cmd --permanent --policy int2ext --add-egress-zone external # firewall-cmd --permanent --policy int2ext --set-target ACCEPT # firewall-cmd --reload

Tip: You can use stricter policy rules than bare ACCEPT as illustrated in the Firewall Rules section of the firewalld concept page[4]

For example, to allow only nodes in 192.168.2.0/24 to access the internet, do:

firewall-cmd —permanent —policy int2ext —add-rich-rule=’rule family=ipv4 source address=192.168.2.0/24 accept’ Do not forget to reload rules afterwards:

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.

This article or section needs language, wiki syntax or style improvements. See Help:Style for reference.

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

# firewall-cmd --zone=internal --permanent --add-service dns # firewall-cmd --zone=internal --permanent --add-service dhcp # firewall-cmd --zone=internal --permanent --add-service dhcpv6

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.

Clients cannot access the internet or cannot connect

Symptoms might also include: Clients get host is down when pinging host, gets no route to host when pinging devices outside the LAN (that should be forwarded by NAT), DHCP offers not crossing a bridge, .

It is known that docker may cause these problems. Simply disabling docker.service and docker.socket solves this problem.

See also

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