Difference between revisions of "Internet sharing"

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[[Category:Networking (English)]]
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[[Category:Networking]]
{{i18n|Internet_Share}}
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[[cs:Internet Share]]
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[[es:Conexion a Internet compartida]]
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[[fr:Partage de connexion]]
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[[it:Internet Share]]
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[[ru:Internet Share]]
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This article explains how to share the internet connection from one machine to other(s).
  
==Preface==
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==Requirements==
Let's assume you have an internet connection and you want to share it. There are two main ways to do that.
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* The machine acting as server should have an additional network device
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* 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 [[Wikipedia:Network switch|switch]] is required. If you are sharing to only one machine, a [[Wikipedia:Ethernet crossover cable|crossover cable]] is sufficient
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{{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}}
  
<pre>
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==Configuration==
  internet                           pc1
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Using [[Udev#Setting static device names]], name the network device connected to the other computer(s) as '''''net0''''' and the network device connected to the internet as '''''internet0'''''.
1. ----> |router| ---> |switch| --->-<
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===Static IP address===
                                      pc2 ..etc
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Assign an 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.
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{{bc|
  internet
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# ip link set up dev net0
2. ------> |pc1 (router)| --> pc2..etc
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# ip addr add 139.96.30.100/24 dev net0 # arbitrary address
</pre>
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}}
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To have your static ip assigned at boot, you can use [[netcfg]].
  
==Instructions==
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===Enable packet forwarding===
I'll explain the second way (it is easier and requires one less machine).
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Enter this command to temporaly enable packet forwarding:
<ol>
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{{bc|<nowiki>sysctl net.ipv4.ip_forward=1</nowiki>}}
<li>Install a second network card to the first PC.</li>
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Edit {{ic|/etc/sysctl.conf}} and add this line, which will make the previous change persistent after a reboot.
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{{bc|<nowiki>net.ipv4.ip_forward=1</nowiki>}}
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If you are using ipv6, use these lines:
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{{bc|<nowiki>
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net.ipv6.conf.default.forwarding=1
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net.ipv6.conf.all.forwarding=1
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</nowiki>}}
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===Enable NAT===
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[[pacman|Install]] the package {{Pkg|iptables}} from the [[Official Repositories|official repositories]].
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Use iptables to enable NAT:{{bc|<nowiki>
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# iptables -t nat -A POSTROUTING -o internet0 -j MASQUERADE
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# iptables-save > /etc/iptables/iptables.rules
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# systemctl start iptables
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</nowiki>}}{{Note| Of course, this also works with a mobile broadband connection (usually called ppp0 on PC1)}}
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You can set {{ic|iptables.service}} to [[Daemon|auto start a boot]].
  
<li>Connect the two PCs (using [http://en.wikipedia.org/wiki/Ethernet_crossover_cable crossover cable] or a [http://en.wikipedia.org/wiki/Network_switch switch]).</li>
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Read the [[iptables]] article for more information.
  
<li>Let's assume that the first card (with the internet) is called '''''eth0''''' and the other one (for the sharing) is called '''''eth1'''''. (If those two keep switching at every boot read [http://wiki.archlinux.org/index.php/Udev#Mixed_Up_Devices.2C_Sound.2FNetwork_Cards_Changing_Order_Each_Boot this] ).</li>
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===Assigning ip addresses to the client pc(s)===
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If you are planning to regularly have several machines using the internet shared by this machine, then is a good idea to install a [[Wikipedia:dhcp|dhcp server]].
  
<li>Configure the second network card with:
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You can read the [[dhcpd]] wiki article, to add a dhcp server. Then, install the [[dhcpcd]] client on every client pc.
:'''IP:''' 192.168.0.1
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:'''Netmask:''' 255.255.255.0
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or enter in a console (as root)
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<pre>ifconfig eth1 192.168.0.1 netmask 255.255.255.0
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ifconfig eth1 up</pre></li>
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<li>Enter that same information in your '''/etc/rc.conf''' so that this card is set up correctly earch time your computer starts.
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If you are not planing to use this setup regularly, you can manually add an ip to each client instead.
'''Note''': If you use '''Wicd''', you don't do this.
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====Manually adding an ip====
<pre>eth1="eth1 192.168.0.1 netmask 255.255.255.0"
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INTERFACES=(lo eth0 eth1)</pre></li>
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<li>Enable packet forwarding. To do so, write a "'''1'''" to '''/proc/sys/net/ipv4/ip_forward''' with:
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Instead of using dhcp, on each client pc, add an ip address and the default route:
<pre>echo 1 > /proc/sys/net/ipv4/ip_forward</pre></li>
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{{bc|<nowiki>
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ip addr add 139.96.30.120/24 dev eth0
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ip link set up dev eth0
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ip route add default via 139.96.30.100 dev eth0
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</nowiki>}}
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Add a nameserver:
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{{bc|<nowiki>
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echo "nameserver <nameserver ip>" >> /etc/resolv.conf
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</nowiki>}}
  
<li>Then edit '''/etc/sysctl.conf''' and set 1 to the net.ipv4.ip_forward (It should be '''net.ipv4.ip_forward=1'''). This will make that change persistant after a reboot.</li>
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You can figure out the address of the nameserver by looking into the {{ic|/etc/resolv.conf}} of the server, if its Internet connection is already established. If you don't have a nameserver, you can use [https://code.google.com/speed/public-dns/ Google Public DNS] which is relatively fast. Its addresses are '''8.8.8.8''' and '''8.8.4.4'''.
  
<li>(If you haven't already) Install iptables and enter this rule (for the forwarding of the internet to the second PC) and save iptables.
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That's it. The client PC should now have Internet.
<pre>pacman -S iptables
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iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
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/etc/rc.d/iptables save</pre></li>
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<li>Edit '''/etc/conf.d/iptables''' and enable IP forwarding there (IPTABLES_FORWARD=1).</li>
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== Troubleshooting ==
  
<li>Start iptables:<pre>/etc/rc.d/iptables start</pre></li>
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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 don't have other [[Iptables]] rules [https://bbs.archlinux.org/viewtopic.php?pid=1093208 interfering].
 
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<li>Add iptables in your DAEMONS array in your /etc/rc.conf so that it is started each time.</li>
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<li>Go to the second PC and set:
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:'''IP:''' 192.168.0.2
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:'''Netmask:''' 255.255.255.0
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:'''Gateway:''' 192.168.0.1
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:'''DNS:''' The same DNS as the first PC
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Proceed like this (eth0 is assumed to be your network interface on PC2 with which you are connected to PC1):
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<pre>ifconfig eth0 192.168.0.2 netmask 255.255.255.0
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ifconfig eth0 up
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route add default gw 192.168.0.1 eth0
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echo "nameserver <adr of nameserver>" >> /etc/resolv.conf
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</pre>
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You can figure out the address of the nameserver by looking into the /etc/resolv.conf of PC1, if its internet connection is already established.</li></ol>
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{{Note| Of course, this also works with a mobile broadband connection (usually called ppp0 on PC1)}}
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That's it. The second PC should now have internet.
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==See also==
 
==See also==

Revision as of 15:55, 7 May 2013

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 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

Configuration

Using Udev#Setting static device names, name the network device connected to the other computer(s) as net0 and the network device connected to the internet as internet0.

Static IP address

Assign an 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 139.96.30.100/24 dev net0 # arbitrary address

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

Enable packet forwarding

Enter this command to temporaly enable packet forwarding:

sysctl net.ipv4.ip_forward=1

Edit /etc/sysctl.conf and add this line, which will make the previous change persistent after a reboot.

net.ipv4.ip_forward=1

If you are using ipv6, use these lines:

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

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-save > /etc/iptables/iptables.rules
# systemctl start iptables
Note: Of course, this also works with a mobile broadband connection (usually called ppp0 on PC1)

You can set iptables.service to auto start a boot.

Read the iptables article for more information.

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 139.96.30.120/24 dev eth0
ip link set up dev eth0
ip route add default via 139.96.30.100 dev eth0

Add a nameserver:

echo "nameserver <nameserver ip>" >> /etc/resolv.conf

You can figure out the address of the nameserver by looking into the /etc/resolv.conf of the server, if its Internet connection is already established. If you don't have a nameserver, you can use Google Public DNS which is relatively fast. Its addresses are 8.8.8.8 and 8.8.4.4.

That's 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 don't have other Iptables rules interfering.

See also