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2002-09-19: links about ebtables have been updated in the "Related Topics" Section. Added note about "false positive" br-nf debugging output.
2002-10-08: Added section Actual configuration and hints about routing in Setting up the routing, Ping it, Jim! , resp.
Ethernet bridges connect two or more distinct ethernet segments transparently.
An ethernet bridge distributes ethernet frames coming in on one port to other ports associated to the bridge interface. This is accomplished with brain: Whenever the bridge knows on which port the MAC address to which the frame is to be delivered is located it forwards this frame only to this only port instead of polluting all ports together.
Ethernet interfaces can be added to an existing bridge interface and become then (logical) ports of the bridge interface.
Putting a netfilter structure on top of a bridge interface renders the bridge capable of servicing filtering mechanisms. This way, a transparent filtering instance can be created. It even needs no IP address assigned to work. Of course, you can assign an IP address to the bridge interface for maintenance purposes ( certainly, with ssh only ;-).
The advantage of this system is evident. Transparency alleviates the network administrator of the pain of restructuring the network topology. And users may not notice the existence of the bridge but their connection beeing blocked. Also, users are not disturbed while working (think of a company where network connection loss pays alot).
The other common case is a client beeing connected to the global web via a leased router. As the providers seldomly grant administration privileges on their leasing hardware, the client cannot change the interconnecting configuration. But, of course, the client has a network running, and wants to spend at least as possible, he does not want to reconfigure his entire network. And he does not need to if he uses a bridging device.
This software setup is needed on the ethernet bridge computer. According to our Testing grounds.
As of kernel version 2.4.18 there's already support for the Ethernet Bridge
capability built-in. No patches needed so far.
But if we intend to use netfilter capabilities, because we want to run iptables on our new Linux router/fw box, we still need to apply a patch. Any patches needed can be found and downloaded on the sourceforge Ethernet Bridge homepage.
root@bridge:~> cd /usr/src/ root@bridge:~> wget -c http://bridge.sourceforge.net/devel/bridge-nf/bridge-nf-0.0.7-against-2.4.18.diff root@bridge:~> cd /usr/src/linux/ root@bridge:~> patch -p1 -i ../bridge-nf/bridge-nf-0.0.7-against-2.4.18.diff
Supposedly we want netfilter support on our bridge interface and we have already patched the vanillal kernel we may now activate some necessary kernel configuration items. On how to build a private kernel image see the CD-Net-Install-HOWTO, Toolbox. Oh, yeah, it's still in German only. Hm, I have to fix this some time..
Nevertheless, we start by now: In
Code maturity level options
[*] Prompt for development and/or incomplete code/drivers
Loadable module support
Ok, so far so good. Now, we go to
[*] Enable loadable module support [*] Set version information on all module symbols [*] Kernel module loader
[*] Network packet filtering (replaces ipchains) [*] Network packet filtering debugging
we mark any item we need as module. Now the long awaited item: activate
IP: Netfilter Configuration --->
as well as
<M> 802.1d Ethernet Bridging
[*] netfilter (firewalling) support
The above entry is available only if we successfully patched our kernel!
Finally, we just need a successful
cycle and we're done. Don't forget to edit
root@bridge:~> make dep clean bzImage modules modules_install
root@bridge:~> lilo -t root@bridge:~> lilo root@bridge:~> reboot
Perhaps we might mark our new kernel as the bridge kernel? We
vi the toplevel Makefile in our kernel sources and edit the head
We may actually set it to, say bridge? ;-)
modules_install we find the fresh modules in
Once our kernel has the capabilities needed to perform Ethernet Bridge and netfilter
actions, we prepare the user space tool
brctl is the configuration
tool we use to
set up anything to suit our needs.
We download the source tarball, unpack it and change directory into it.
At this time, read the
root@bridge:~> wget -c http://bridge.sourceforge.net/bridge-utils/bridge-utils-0.9.5.tar.gz root@bridge:~> tar xvzf bridge-utils-0.9.5.tar.gz root@bridge:~> cd bridge-utils-0.9.5
READMEand the files in the
doc/subdirectory. Then do a simple make and copy the resulting
This is it. Go for Setup now.
root@bridge:~> make root@bridge:~> cp -vi brctl/brctl /sbin/
We need Linux to know about the bridge. First tell it that we want one virtual
ethernet bridge interface: (this is to be executed on host
bridge, of course.
Second, we do not need the STP (Spanning Tree Protocol). I.e. we do only have one single router, so a loop is highly improbable. We may then deactivate this feature. (Results in less polluted networking environment, too):
root@bridge:~> brctl addbr br0
After these preparations, we now do finally some effective commands. We add our two (or even more) physical ethernet interfaces. That means, we attach them to the just born logical (virtual) bridge interface
root@bridge:~> brctl stp br0 off
Now, our two previously physical ethernet interfaces became a logical bridge port each. Erm, ok, there were and will be the physical devices. They are still there, go have a look ;-) But now they became part of the logical bridge device and therefore need no IP configuration any longer. So release the IPs:
root@bridge:~> brctl addif br0 eth0 root@bridge:~> brctl addif br0 eth1
Great! We now have a box w/o any IP attached. So if you were configuring your future fw/router via TP, go for your local console now ;-)) You have a serial console? Happy one :-)
root@bridge:~> ifconfig eth0 down root@bridge:~> ifconfig eth1 down root@bridge:~> ifconfig eth0 0.0.0.0 up root@bridge:~> ifconfig eth1 0.0.0.0 up
We tell Linux the new (logical) interface and associate one single IP with it:
root@bridge:~> ifconfig br0 10.0.3.129 up
In case we are configuring a gateway we enable the forwarding in the linux kernel.
Our box already has an IP assigned but no default route. We solve this now:
root@bridge:~> echo "1" > /proc/sys/net/ipv4/ip_forward
Finally, we should have a working net from, to and through the gateway.
root@bridge:~> route add default gw 10.0.3.129
We imagine this scenario or similar:
Our administrative power includes only machines marked with
/\ Ethernet Ethernet ATM /-/ \ --------- --------- --------- /-/ | | Box |----------|Bridge |----------|Router |-----| Inter- \ --------- --------- --------- \ net ---| ^ ^ ^ ^ \ / | | | | \---/ eth0 eth0 eth1 if0 ^ | | | | | 10.0.3.2 none/10.0.3.1 220.127.116.11 anything else \ / \ / ^ \-br0-/ | ^ ^ | ^ | | | | | | own own foreign hostile
own, the Router is completely off-limits and so is the Internet, of course.
We will configure the Box' eth0 as usual. The bridge's interfaces
are configured as described in
If we are to use forwarding we might perhaps do this one: ;-)
Optionally, we set up a default route:
root@bridge:~> echo "1" > /proc/sys/net/ipv4/ip_forward
Then we set up some iptables rules on host
root@bridge:~> route add default gw 10.0.3.129
The last line gives us the following output:
root@bridge:~> iptables -P FORWARD DROP root@bridge:~> iptables -F FORWARD root@bridge:~> iptables -I FORWARD -j ACCEPT root@bridge:~> iptables -I FORWARD -j LOG root@bridge:~> iptables -I FORWARD -j DROP root@bridge:~> iptables -A FORWARD -j DROP root@bridge:~> iptables -x -v --line-numbers -L FORWARD
Chain FORWARD (policy DROP 0 packets, 0 bytes) num pkts bytes target prot opt in out source destination 1 0 0 DROP all -- any any anywhere anywhere 2 0 0 LOG all -- any any anywhere anywhere LOG level warning 3 0 0 ACCEPT all -- any any anywhere anywhere 4 0 0 DROP all -- any any anywhere anywhere
LOGtarget logs every packet via
syslogd. Beware, this is intended for testing purposes only, remove in production environment. Else you end up either with filled logs and harddisk partitions by you yourself or anyone else does this Denial of Service to you. You've been warned.
By default, we
root@box:~> ping -c 3 18.104.22.168 PING router.provider.net (22.214.171.124) from 10.0.3.2 : 56(84) bytes of data. --- router.provider.net ping statistics --- 3 packets transmitted, 0 received, 100% loss, time 2020ms ^C root@box:~>
DROPeverything. No response, no logged packet. This netfilter setup is designed to
DROPall packets unless we delete the rule that drops every packet (rule no. 1 above) before the
Now, the rules are:
root@bridge:~> iptables -D FORWARD 1 root@bridge:~> iptables -x -v --line-numbers -L FORWARD
And any packet may pass through. Test it with a ping on host
Chain FORWARD (policy DROP 0 packets, 0 bytes) num pkts bytes target prot opt in out source destination 2 0 0 LOG all -- any any anywhere anywhere LOG level warning 3 0 0 ACCEPT all -- any any anywhere anywhere 4 0 0 DROP all -- any any anywhere anywhere
Yippeah! The router is alive, up and running. (Well it has been all day long.. ;-)
root@box:~> ping -c 3 126.96.36.199 PING router.provider.net (188.8.131.52) from 10.0.3.2 : 56(84) bytes of data. 64 bytes from router.provider.net (184.108.40.206): icmp_seq=1 ttl=255 time=0.103 ms 64 bytes from router.provider.net (220.127.116.11): icmp_seq=2 ttl=255 time=0.082 ms 64 bytes from router.provider.net (18.104.22.168): icmp_seq=3 ttl=255 time=0.083 ms --- router.provider.net ping statistics --- 3 packets transmitted, 3 received, 0% loss, time 2002ms rtt min/avg/max/mdev = 0.082/0.089/0.103/0.012 ms root@box:~>
When we just fired up the bridge interface it takes about roughly 30 seconds
until the bridge is fully operational. This is due the 30-seconds-learning phase
of the bridge interface. During this phase, the bridge ports are learning what
MAC addresses exist on what port. The bridge author, Lennert, tells us in his
TODO file, the 30-seconds-learning phase is subjected to some improvement in a
timely manner some time.
During the test phase, no packet will we forwarded. No ping be answered. Remind this!
This section is intended to give you, dear reader, some hints about how your system should look and feel after having processed this howto successfully.
The output of your
ifconfig command might look similar to
root@bridge:~> ifconfig br0 Link encap:Ethernet HWaddr 00:04:75:81:D2:1D inet addr:10.0.3.129 Bcast:22.214.171.124 Mask:255.255.255.128 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:826 errors:0 dropped:0 overruns:0 frame:0 TX packets:737 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:161180 (157.4 Kb) TX bytes:66708 (65.1 Kb) eth0 Link encap:Ethernet HWaddr 00:04:75:81:ED:B7 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:5729 errors:0 dropped:0 overruns:0 frame:0 TX packets:3115 errors:0 dropped:0 overruns:0 carrier:656 collisions:0 txqueuelen:100 RX bytes:1922290 (1.8 Mb) TX bytes:298837 (291.8 Kb) Interrupt:11 Base address:0xe400 eth1 Link encap:Ethernet HWaddr 00:04:75:81:D2:1D UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:1 frame:0 TX packets:243 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:100 RX bytes:342 (342.0 b) TX bytes:48379 (47.2 Kb) Interrupt:7 Base address:0xe800 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:1034 errors:0 dropped:0 overruns:0 frame:0 TX packets:1034 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:82068 (80.1 Kb) TX bytes:82068 (80.1 Kb)
The output of your
route command might look similar to
root@bridge:~> route -n Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 10.0.3.129 0.0.0.0 255.255.255.128 U 0 0 0 br0 0.0.0.0 10.0.3.129 0.0.0.0 UG 0 0 0 br0 root@bridge:~>
Please have a look at the Ping it, Jim! section.
Apparently, there must have been a bug in the br-nf code:
From: Bart De Schuymer <bart.de.schuymer_@_pandora.be> Date: Sun, 1 Sep 2002 21:52:46 +0200 To: Nils Radtke <Nils.Radtke_@_Think-Future.de> Subject: Re: Ethernet-Brigde-netfilter-HOWTO Hello Nils, [...] Also, network packet filtering debugging is generally a bad idea with the br-nf patch. It can gives a lot of false warnings (about bugs) in the logs. [...]
Personally, I never had false positives in my log. Maybe, that bug has been fixed. This mailed to Bart, he wrote:
But (as of writing this 2002-09-19) I haven't found an official announcement, this particular bug has been closed. So have a constant look at this topic on the ethernet bridge mailinglist , if you are interested in it's cure.
From: Bart De Schuymer <bart.de.schuymer_@_pandora.be> Date: Mon, 2 Sep 2002 18:30:25 +0200 To: Nils Radtke <Nils.Radtke_@_Think-Future.de> Subject: Re: Ethernet-Brigde-netfilter-HOWTO On Monday 02 September 2002 00:39, Nils Radtke wrote: > Will the revision of the nf-debug code in br-nf be subject of improvement? I must admit I haven't been running any kernel with netfilter debugging lately. It sure used to give false positives a few months ago (the bridge mailing list has posts about that), I've been lacking time to see why and if it is still the case. It's on my todo list. [...]
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