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#language en ~-[[DebianWiki/EditorGuide#translation|Translation(s)]]: English - [[fr/BridgeNetworkConnections|Français]] -~ ---- |
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<<TableOfContents()>> | |
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Bridging you network connection is a handy method for sharing your internet connection between two (or more) computers. It's useful if you can't buy a router with more than one ethernet port, or if you're a college student in a dorm room with limited ethernet jacks and no router. Basically, bridging is plugging one computer into another computer that already has a connection to a larger network (like the internet) and letting the bridged computer use the networked computer's connection. To do so though, the networked computer needs to have two ethernet ports, one for the big network, and one for the bridged computer. Make sure that before starting that the computer you're gonna bridge through has two ethernet ports, and that the hardware is capable of bridging ethernet connections (it probably should be). |
Bridging your network connection is a handy method for sharing your internet connection between two (or more) computers. It’s useful if you can’t buy a router with more than one ethernet port, or if you’re a college student in a dorm room with limited ethernet jacks and no router. Basically, bridging is plugging one computer into another computer that already has a connection to a larger network (like the internet) and letting the bridged computer use the networked computer’s connection. To do so though, the networked computer needs to have two ethernet ports, one for the big network, and one for the bridged computer. Make sure that before starting that the computer you’re gonna bridge through has two ethernet ports, and that the hardware is capable of bridging ethernet connections (it probably should be). Another example scenario for using bridging is to provide redundant networking capabilities. For example using two network interfaces to connect to two spanning tree enabled switches provides a redundant connection in the event of a cable, interface or switch failure. This requires spanning tree to be enabled on both the bridge interface and the switch. |
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The program you're going to need is called brctl. Find it in Synaptic, or install it using this command: {{{ # aptitude install brctl }}} This program will allow us to set up and use the bridge interface. The bridge interface appears as a new interface in ifconfig, much like eth0 or eth1. It doesn't physically exist on your computer, but instead it is a virtual interface that jsut takes the packets from one physical interface, and transparently routes them to the other. |
The program you’re going to need is called `brctl` and is included in DebianPkg:bridge-utils. Find it in Synaptic, or install it using this command: {{{ # aptitude install bridge-utils }}} This program will allow us to set up and use the bridge interface. The bridge interface appears as a new interface in `ip link`, much like `eth0` or `eth1`. It doesn’t physically exist on your computer, but instead it is a virtual interface that just takes the packets from one physical interface, and transparently routes them to the other. |
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'''Note:''' All these commands are to be issued on the computer with the existing network connection. To set up the computer that's going to be bridged, just set it up normally, as you would any other computer. You CAN use DHCP, or you can use a static address. It doesn't matter. The bridged computer won't even know it's being bridged. | === Manual bridge setup === '''Note:''' All these commands are to be issued on the computer with the existing network connection. To set up the computer that’s going to be bridged, just set it up normally, as you would any other computer. You CAN use DHCP, or you can use a static address. It doesn’t matter. '''Note:''' If, after trying to use the bridge interface, you find your network link becomes dead and refuses to work again, it might be that the router/switch upstream is blocking "unauthorized switches" in the network (for example, by detecting BPDU packets). You'll have to change its configuration to explicitly allow the host machine/network port as a "switch". |
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The name br0 is totally up to you, this is just an example name that I've chosen for the wiki article. Anyway, now that you have your bridge device, you have to add the interfaces that are gonna be bridged. Add both the interface with the second computer, and the interface that leads to the existing network. Do it with this command: | The name `br0` is totally up to you, this is just an example name that I’ve chosen for the wiki article. Anyway, now that you have your bridge device, you have to add the interfaces that are gonna be bridged. You can cross-check the enumeration of your ethernet devices with (`eth0`, `eth1`, etc. is common): {{{ # ip addr show }}} Add both the interface with the second computer, and the interface that leads to the existing network. Do it with this command: |
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This will add the two interfaces eth0 and eth1 to bridge br0. Simple enough. There's no distinction with how you add the bridges, or what order you do it, or any special commands you have to add to distinguish them. So don't worry about that. Well, now we have our bridges, so bring all the interfaces up, and you'll be set! == /etc/network/interfaces and bridging == To make your bridge a little more permanent, you're gonna need to edit /etc/network/interfaces. Using our example names, make it look like this and you're set (if you want to use DHCP): |
This will add the two interfaces `eth0` and `eth1` to bridge `br0`. Simple enough. There’s no distinction with how you add the bridges, or what order you do it, or any special commands you have to add to distinguish them. So don’t worry about that. Well, now we have our bridges, so bring all the interfaces up, and you’ll be set! === Configuring bridging in /etc/network/interfaces === To make your bridge a little more permanent, you’re gonna need to edit `/etc/network/interfaces`. Using our example names, make it look like this and you’re set (if you want to use DHCP): |
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# Set up interfaces manually, avoiding conflicts with, e.g., network manager iface eth0 inet manual iface eth1 inet manual |
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iface br0 inet dhcp | iface br0 inet dhcp |
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To bring up your bridge, you just have to issue {{{ # ifup br0 }}} and it'll bring up the other necessary interfaces without anything in your {{{ interfaces }}} file about the bridged interfaces. If you like static ip's, then you can just add the static IP options under the br0 interface setup. Kinda like this: |
To bring up your bridge, you just have to issue {{{ # ifup br0}}} and it’ll bring up the other necessary interfaces without anything in your {{{interfaces}}} file about the bridged interfaces. If you like static IP’s, then you can just add the static IP options under the `br0` interface setup. Kinda like this: |
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# Set up interfaces manually, avoiding conflicts with, e.g., network manager iface eth0 inet manual iface eth1 inet manual |
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==== Useful options for virtualised environments ==== Some other useful options to use in any stanza in a virtualised environment are: {{{ bridge_stp off # disable Spanning Tree Protocol bridge_waitport 0 # no delay before a port becomes available bridge_fd 0 # no forwarding delay bridge_ports none # if you do not want to bind to any ports bridge_ports regex eth* # use a regular expression to define ports }}} === Setting up bridge-related kernel variables === There are several kernel variables that affect bridge operation. In [[http://wiki.libvirt.org/page/Networking|some cases]] you may need to tweak these variables. There are two common options: * Add variables to ''/etc/sysctl.conf'' directly * Put them to a sysctl configuration file fragment (e.g. ''/etc/sysctl.d/bridge_local.conf'') In the latter case, the ''procps'' init script ''should'' take care of loading them during boot. However, on Squeeze it does not, and you need to restart it from ''/etc/rc.local'' (or similar): {{{ # /etc/rc.local # Load kernel variables from /etc/sysctl.d /etc/init.d/procps restart exit 0 }}} == Libvirt and bridging == [[http://libvirt.org/index.html|Libvirt]] is a virtualization API which supports KVM and various other virtualization techniques. In many cases, it's desirable to [[http://wiki.libvirt.org/page/Networking#Bridged_networking_.28aka_.22shared_physical_device.22.29|share a physical network interface with guests]], i.e. setup a bridge they can use. This operation is composed of two parts: * Setup the bridge interface on host as described in this article and in [[http://wiki.libvirt.org/page/Networking#Altering_the_interface_config|here]] * Configure guest to use the newly-created bridge You can verify if bridging is working properly by looking at ''brctl'' output: {{{ root@server:/etc/libvirt/qemu# brctl show bridge name bridge id STP enabled interfaces br0 8000.001ec952d26b yes eth0 vnet0 vnet1 vnet2 virbr0 8000.000000000000 yes }}} As can be seen, guest network interfaces ''vnet0'', ''vnet1'' and ''vnet2'' are bound with the physical interface ''eth0'' in the bridge ''br0''. The ''virbr0'' interface only used for NAT connectivity with libvirt. == Bridging with a wireless NIC == Just like you can bridge two wired ethernet interfaces, you can bridge between an ethernet interface and a wireless interface. However, most Access Points (APs) will reject frames that have a source address that didn’t authenticate with the AP. Since Linux does ethernet bridging transparently (doesn’t modify outgoing or incoming frames), we have to set up some rules to do this with a program called DebianPkg:ebtables. For an alternative Layer 3 approach using proxy ARP and routing, see [[BridgeNetworkConnectionsProxyArp]]. === ebtables Overview === ebtables is essentially like DebianPkg:iptables, except it operates on the MAC sublayer of the data-link layer of the OSI model, instead of the network layer. In our case, this allows to change the source MAC address of all of our frames. This is handy because we fool our AP into thinking that all of our forwarded frames come from the machine which authenticated to the AP. === bridge-utils Modifications === Before this will work, you need to modify your `/etc/network/interfaces` file, and add this line to your bridge stanza: {{{ pre-up iwconfig wlan0 essid $YOUR_ESSID bridge_hw $MAC_ADDRESS_OF_YOUR_WIRELESS_CARD }}} Obviously replacing `$MAC_ADDRESS_OF_YOUR_WIRELESS_CARD` with the actual MAC address of your wireless card, and `$YOUR_ESSID` as the ESSID of your wireless network. If you don’t know your MAC address, you can find it by typing {{{ # ip link show wlan0 }}} Where `wlan0` is your wireless interface. Your MAC address is listed as the HWaddr. === Setting up the rules === First, install ebtables: {{{ # aptitude install ebtables }}} Now we can start setting up the rules. The syntax for ebtables is almost identical to that of iptables, so if you have experience with iptables, this will look pretty familiar to you. The first rule we’re going to set up will set the source MAC address to the MAC address of the bridge for all frames sent to the AP. {{{ # ebtables -t nat -A POSTROUTING -o wlan0 -j snat --to-src $MAC_OF_BRIDGE --snat-arp --snat-target ACCEPT }}} The next rules will require you to know the MAC and IP of each of the machines behind your bridge. Replace $MAC and $IP with these. {{{ # ebtables -t nat -A PREROUTING -p IPv4 -i wlan0 --ip-dst $IP -j dnat --to-dst $MAC --dnat-target ACCEPT # ebtables -t nat -A PREROUTING -p ARP -i wlan0 --arp-ip-dst $IP -j dnat --to-dst $MAC --dnat-target ACCEPT }}} This is tedious to have to type in everytime you add a new computer to a switch behind your bridge, so I wrote a script to do it for you {{{#!highlight bash #!/bin/bash # addcomputer # Will Orr - 2009 INIF="wlan0" function add_ebtables () { COMPIP=$1 COMPMAC=$2 ebtables -t nat -A PREROUTING -i $INIF -p IPv4 --ip-dst $COMPIP -j \ dnat --to-dst $COMPMAC --dnat-target ACCEPT ebtables -t nat -A PREROUTING -i $INIF -p ARP --arp-ip-dst $COMPIP \ -j dnat --to-dst $COMPMAC --dnat-target ACCEPT } if [[ $# -ne 2 ]]; then echo "Usage: $0 ip mac" elif [[ $(whoami) != "root" ]]; then echo "Error: must be root" else add_ebtables $1 $2 fi }}} === Saving your rules === After you have written your ebtables rules, you need to save them in an atomic file. Otherwise, your rules will not be preserved. Saving them is rather simple though. {{{ # EBTABLES_ATOMIC_FILE=/root/ebtables-atomic ebtables -t nat --atomic-save }}} And then load them like this: {{{ # EBTABLES_ATOMIC_FILE=/root/ebtables-atomic ebtables -t nat --atomic-commit }}} If you want to load your ebtables rules at boot time, a handy place to stick the commit command is in /etc/rc.local. Just pop it in there before the {{{exit 0}}} line. == Link Aggregation (LACP) with VLANs == Here is an example of the {{{/etc/network/interfaces}}} file for 2 interfaces LACP bonded together with VLANs defined on top of the bond. Notes: Tested on Debian Jessie 8.0 rc1 on 3/11/2015 (AMD64 arch) Required debian packages for vlan and bonding: {{{ aptitude install vlan ifenslave echo "8021q" >> /etc/modules }}} The {{{/etc/network/interfaces}}} could be: {{{ auto lo iface lo inet loopback # there is one other thing I am forgetting here... see bonding setup above ###### Bonded 10Gig setup - LACP Aggregation ####### bond eth4 and eth5 #### (set up from switch as trunk links with LACP) then split out VLANS ## View status with: cat /proc/net/bonding/bond0 auto bond0 iface bond0 inet manual up ifconfig bond0 0.0.0.0 up slaves eth4 eth5 # bond-mode 4 = 802.3ad bond-mode 4 bond-miimon 100 bond-downdelay 200 bond-updelay 200 bond-lacp-rate 1 bond-xmit-hash-policy layer2+3 auto vlan1023 iface vlan1023 inet static address 18.28.4.123 netmask 255.255.255.0 gateway 18.28.4.1 vlan-raw-device bond0 auto vlan1024 iface vlan1024 inet static address 10.15.160.1 netmask 255.255.255.0 gateway 10.15.160.100 vlan-raw-device bond0 }}} |
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---- CategoryNetwork |
Translation(s): English - Français
Bridging Network Connections
Contents
Introduction
Bridging your network connection is a handy method for sharing your internet connection between two (or more) computers. It’s useful if you can’t buy a router with more than one ethernet port, or if you’re a college student in a dorm room with limited ethernet jacks and no router.
Basically, bridging is plugging one computer into another computer that already has a connection to a larger network (like the internet) and letting the bridged computer use the networked computer’s connection. To do so though, the networked computer needs to have two ethernet ports, one for the big network, and one for the bridged computer. Make sure that before starting that the computer you’re gonna bridge through has two ethernet ports, and that the hardware is capable of bridging ethernet connections (it probably should be).
Another example scenario for using bridging is to provide redundant networking capabilities. For example using two network interfaces to connect to two spanning tree enabled switches provides a redundant connection in the event of a cable, interface or switch failure. This requires spanning tree to be enabled on both the bridge interface and the switch.
Installing the software
The program you’re going to need is called brctl and is included in bridge-utils. Find it in Synaptic, or install it using this command:
# aptitude install bridge-utils
This program will allow us to set up and use the bridge interface. The bridge interface appears as a new interface in ip link, much like eth0 or eth1. It doesn’t physically exist on your computer, but instead it is a virtual interface that just takes the packets from one physical interface, and transparently routes them to the other.
Setting up your Bridge
Manual bridge setup
Note: All these commands are to be issued on the computer with the existing network connection. To set up the computer that’s going to be bridged, just set it up normally, as you would any other computer. You CAN use DHCP, or you can use a static address. It doesn’t matter.
Note: If, after trying to use the bridge interface, you find your network link becomes dead and refuses to work again, it might be that the router/switch upstream is blocking "unauthorized switches" in the network (for example, by detecting BPDU packets). You'll have to change its configuration to explicitly allow the host machine/network port as a "switch".
First step to creating the bridge network is actually creating it. Issue this command to get the ball rolling and create the new interface.
# brctl addbr br0
The name br0 is totally up to you, this is just an example name that I’ve chosen for the wiki article. Anyway, now that you have your bridge device, you have to add the interfaces that are gonna be bridged. You can cross-check the enumeration of your ethernet devices with (eth0, eth1, etc. is common):
# ip addr show
Add both the interface with the second computer, and the interface that leads to the existing network. Do it with this command:
# brctl addif br0 eth0 eth1
This will add the two interfaces eth0 and eth1 to bridge br0. Simple enough. There’s no distinction with how you add the bridges, or what order you do it, or any special commands you have to add to distinguish them. So don’t worry about that.
Well, now we have our bridges, so bring all the interfaces up, and you’ll be set!
Configuring bridging in /etc/network/interfaces
To make your bridge a little more permanent, you’re gonna need to edit /etc/network/interfaces. Using our example names, make it look like this and you’re set (if you want to use DHCP):
# This file describes the network interfaces available on your system # and how to activate them. For more information, see interfaces(5). # The loopback network interface auto lo br0 iface lo inet loopback # Set up interfaces manually, avoiding conflicts with, e.g., network manager iface eth0 inet manual iface eth1 inet manual # Bridge setup iface br0 inet dhcp bridge_ports eth0 eth1
To bring up your bridge, you just have to issue # ifup br0 and it’ll bring up the other necessary interfaces without anything in your interfaces file about the bridged interfaces.
If you like static IP’s, then you can just add the static IP options under the br0 interface setup. Kinda like this:
# This file describes the network interfaces available on your system # and how to activate them. For more information, see interfaces(5). # The loopback network interface auto lo br0 iface lo inet loopback # Set up interfaces manually, avoiding conflicts with, e.g., network manager iface eth0 inet manual iface eth1 inet manual # Bridge setup iface br0 inet static bridge_ports eth0 eth1 address 192.168.1.2 broadcast 192.168.1.255 netmask 255.255.255.0 gateway 192.168.1.1
Useful options for virtualised environments
Some other useful options to use in any stanza in a virtualised environment are:
bridge_stp off # disable Spanning Tree Protocol bridge_waitport 0 # no delay before a port becomes available bridge_fd 0 # no forwarding delay bridge_ports none # if you do not want to bind to any ports bridge_ports regex eth* # use a regular expression to define ports
Setting up bridge-related kernel variables
There are several kernel variables that affect bridge operation. In some cases you may need to tweak these variables. There are two common options:
Add variables to /etc/sysctl.conf directly
Put them to a sysctl configuration file fragment (e.g. /etc/sysctl.d/bridge_local.conf)
In the latter case, the procps init script should take care of loading them during boot. However, on Squeeze it does not, and you need to restart it from /etc/rc.local (or similar):
# /etc/rc.local # Load kernel variables from /etc/sysctl.d /etc/init.d/procps restart exit 0
Libvirt and bridging
Libvirt is a virtualization API which supports KVM and various other virtualization techniques. In many cases, it's desirable to share a physical network interface with guests, i.e. setup a bridge they can use. This operation is composed of two parts:
Setup the bridge interface on host as described in this article and in here
- Configure guest to use the newly-created bridge
You can verify if bridging is working properly by looking at brctl output:
root@server:/etc/libvirt/qemu# brctl show bridge name bridge id STP enabled interfaces br0 8000.001ec952d26b yes eth0 vnet0 vnet1 vnet2 virbr0 8000.000000000000 yes
As can be seen, guest network interfaces vnet0, vnet1 and vnet2 are bound with the physical interface eth0 in the bridge br0. The virbr0 interface only used for NAT connectivity with libvirt.
Bridging with a wireless NIC
Just like you can bridge two wired ethernet interfaces, you can bridge between an ethernet interface and a wireless interface. However, most Access Points (APs) will reject frames that have a source address that didn’t authenticate with the AP. Since Linux does ethernet bridging transparently (doesn’t modify outgoing or incoming frames), we have to set up some rules to do this with a program called ebtables.
For an alternative Layer 3 approach using proxy ARP and routing, see BridgeNetworkConnectionsProxyArp.
ebtables Overview
ebtables is essentially like iptables, except it operates on the MAC sublayer of the data-link layer of the OSI model, instead of the network layer. In our case, this allows to change the source MAC address of all of our frames. This is handy because we fool our AP into thinking that all of our forwarded frames come from the machine which authenticated to the AP.
bridge-utils Modifications
Before this will work, you need to modify your /etc/network/interfaces file, and add this line to your bridge stanza:
pre-up iwconfig wlan0 essid $YOUR_ESSID bridge_hw $MAC_ADDRESS_OF_YOUR_WIRELESS_CARD
Obviously replacing $MAC_ADDRESS_OF_YOUR_WIRELESS_CARD with the actual MAC address of your wireless card, and $YOUR_ESSID as the ESSID of your wireless network. If you don’t know your MAC address, you can find it by typing
# ip link show wlan0
Where wlan0 is your wireless interface. Your MAC address is listed as the HWaddr.
Setting up the rules
First, install ebtables:
# aptitude install ebtables
Now we can start setting up the rules. The syntax for ebtables is almost identical to that of iptables, so if you have experience with iptables, this will look pretty familiar to you.
The first rule we’re going to set up will set the source MAC address to the MAC address of the bridge for all frames sent to the AP.
# ebtables -t nat -A POSTROUTING -o wlan0 -j snat --to-src $MAC_OF_BRIDGE --snat-arp --snat-target ACCEPT
The next rules will require you to know the MAC and IP of each of the machines behind your bridge. Replace $MAC and $IP with these.
# ebtables -t nat -A PREROUTING -p IPv4 -i wlan0 --ip-dst $IP -j dnat --to-dst $MAC --dnat-target ACCEPT # ebtables -t nat -A PREROUTING -p ARP -i wlan0 --arp-ip-dst $IP -j dnat --to-dst $MAC --dnat-target ACCEPT
This is tedious to have to type in everytime you add a new computer to a switch behind your bridge, so I wrote a script to do it for you
1 #!/bin/bash
2 # addcomputer
3 # Will Orr - 2009
4
5 INIF="wlan0"
6
7 function add_ebtables () {
8 COMPIP=$1
9 COMPMAC=$2
10
11 ebtables -t nat -A PREROUTING -i $INIF -p IPv4 --ip-dst $COMPIP -j \
12 dnat --to-dst $COMPMAC --dnat-target ACCEPT
13 ebtables -t nat -A PREROUTING -i $INIF -p ARP --arp-ip-dst $COMPIP \
14 -j dnat --to-dst $COMPMAC --dnat-target ACCEPT
15
16 }
17
18 if [[ $# -ne 2 ]]; then
19 echo "Usage: $0 ip mac"
20 elif [[ $(whoami) != "root" ]]; then
21 echo "Error: must be root"
22 else
23 add_ebtables $1 $2
24 fi
Saving your rules
After you have written your ebtables rules, you need to save them in an atomic file. Otherwise, your rules will not be preserved. Saving them is rather simple though.
# EBTABLES_ATOMIC_FILE=/root/ebtables-atomic ebtables -t nat --atomic-save
And then load them like this:
# EBTABLES_ATOMIC_FILE=/root/ebtables-atomic ebtables -t nat --atomic-commit
If you want to load your ebtables rules at boot time, a handy place to stick the commit command is in /etc/rc.local. Just pop it in there before the exit 0 line.
Link Aggregation (LACP) with VLANs
Here is an example of the /etc/network/interfaces file for 2 interfaces LACP bonded together with VLANs defined on top of the bond.
Notes: Tested on Debian Jessie 8.0 rc1 on 3/11/2015 (AMD64 arch)
Required debian packages for vlan and bonding:
aptitude install vlan ifenslave echo "8021q" >> /etc/modules
The /etc/network/interfaces could be:
auto lo iface lo inet loopback # there is one other thing I am forgetting here... see bonding setup above ###### Bonded 10Gig setup - LACP Aggregation ####### bond eth4 and eth5 #### (set up from switch as trunk links with LACP) then split out VLANS ## View status with: cat /proc/net/bonding/bond0 auto bond0 iface bond0 inet manual up ifconfig bond0 0.0.0.0 up slaves eth4 eth5 # bond-mode 4 = 802.3ad bond-mode 4 bond-miimon 100 bond-downdelay 200 bond-updelay 200 bond-lacp-rate 1 bond-xmit-hash-policy layer2+3 auto vlan1023 iface vlan1023 inet static address 18.28.4.123 netmask 255.255.255.0 gateway 18.28.4.1 vlan-raw-device bond0 auto vlan1024 iface vlan1024 inet static address 10.15.160.1 netmask 255.255.255.0 gateway 10.15.160.100 vlan-raw-device bond0