4. Test your new bridged environment!
4.1 Testing Grounds
We imagine this scenario or similar:
/\
Ethernet Ethernet ATM /-/ \
--------- --------- --------- /-/ |
| Box |----------|Bridge |----------|Router |-----| Inter- \
--------- --------- --------- \ net ---|
^ ^ ^ ^ \ /
| | | | \---/
eth0 eth0 eth1 if0 ^
| | | | |
10.0.3.2 none/10.0.3.1 195.137.15.7 anything else
\ /
\ /
^ \-br0-/
| ^ ^
| ^ | |
| | | |
own own foreign hostile
own, the Router is
completely off-limits and so is the Internet, of course.That means, if we want to control the flying bits'n'bytes on the ethernet wire we can chose to integrate a common firewall or file in a bridge.
Drawback of the standard way is you have to change the default gateway route on every and any single host in your net. And this is really a heavy weighting drawback, nobody wants to change more than 5 default routes on 5 different hosts more than one time. Keep the time in mind, this will consume, also! Not to forget, this is a error-prone way to handle the more about security..
The other way is clean, less time-consuming, more secure and less error-prone. More secure in that we won't have the need to assign any IP address. No IP, no danger. So far the theory, we hope, our stacks are safe. (Although this hope should better not relied on..) The overall advantage is, this bridge-setup is completely transparent, no IP, MAC, .. changes at all.
So it's up to you to chose your preferred method. But we will handle just the fancy one here ;-)
4.2 Ping it, Jim!
We will configure the Box' eth0 as usual. The bridge's interfaces
are configured as described in
Setup.
If we are to use forwarding we might perhaps do this one: ;-)
root@bridge:~> echo "1" > /proc/sys/net/ipv4/ip_forward
root@bridge:~> route add default gw 10.0.3.129
bridge:
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
LOG target 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.Test this ruleset now. Ping the router interface's IP (195.137.15.7) on host
box:
root@box:~> ping -c 3 195.137.15.7
PING router.provider.net (195.137.15.7) 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:~>
DROP everything. No response, no logged packet. This netfilter
setup is designed to DROP all packets unless we delete the rule that drops every
packet (rule no. 1 above) before the LOG target matches:
root@bridge:~> iptables -D FORWARD 1
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
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
box:
root@box:~> ping -c 3 195.137.15.7
PING router.provider.net (195.137.15.7) from 10.0.3.2 : 56(84) bytes of data.
64 bytes from router.provider.net (195.137.15.7): icmp_seq=1 ttl=255 time=0.103 ms
64 bytes from router.provider.net (195.137.15.7): icmp_seq=2 ttl=255 time=0.082 ms
64 bytes from router.provider.net (195.137.15.7): 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:~>
- Important Note:
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!
4.3 Actual configuration
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.
Interface configuration
The output of your ifconfig command might look similar to
this:
root@bridge:~> ifconfig
br0 Link encap:Ethernet HWaddr 00:04:75:81:D2:1D
inet addr:10.0.3.129 Bcast:195.30.198.255 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)
Routing configuration
The output of your route command might look similar to
this:
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:~>
Iptables configuration
Please have a look at the Ping it, Jim! section.
4.4 Final note (Important!)
I'd like to hear from you! :-)
Did you enjoy the trip?
Do you miss anything?
Need help? (Call you local assistant ;-) or rtfm.
You are still online? Then drop me a msg via email. I'd be really glad.
Wanna send me a cheque? Pitty, Don't accept these.. (Just kidding;)
Make it worth my time, just send me some nice words, that's enough.
Nothing motivates more than happy participants giving you valuable feedback.
So, go on, invest a minute and hack me a mail!
Thank you!
Nils
4.5 Bug-Notes
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:
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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