Difference between revisions of "Bridge doesn't forward packets"
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| − | Sometimes bridge can mysteriously drop | + | Sometimes a bridge can mysteriously drop packets and not forward them. |
e.g. eyck user experienced a problem when some of the broadcasts were not | e.g. eyck user experienced a problem when some of the broadcasts were not | ||
delivered to container via the bridge. | delivered to container via the bridge. | ||
| Line 23: | Line 23: | ||
BOOTP/DHCP, Reply, length 300 | BOOTP/DHCP, Reply, length 300 | ||
| − | However, eth0 inside container | + | However, eth0 inside the container received only 2nd packet with a BOOTP/DHCP reply and doesn't see the 1st one with the request itself: |
15:21:52.291145 00:08:02:ac:36:20 > ff:ff:ff:ff:ff:ff, ethertype IPv4 (0x0800), length 342: 172.17.8.254.67 > 255.255.255.255.68: | 15:21:52.291145 00:08:02:ac:36:20 > ff:ff:ff:ff:ff:ff, ethertype IPv4 (0x0800), length 342: 172.17.8.254.67 > 255.255.255.255.68: | ||
BOOTP/DHCP, Reply, length 300 | BOOTP/DHCP, Reply, length 300 | ||
| Line 29: | Line 29: | ||
== Resolution == | == Resolution == | ||
| − | It is not obvious at all, but bridges (though have own ebtables filters) do also call iptables FORWARD chain when forwarding packets between interfaces. | + | It is not obvious at all, but bridges (though they have their own ebtables filters) do also call iptables FORWARD chain when forwarding packets between interfaces. |
Thus your FORWARD iptables rules should allow all the packets which are supposed to go through. | Thus your FORWARD iptables rules should allow all the packets which are supposed to go through. | ||
| Line 35: | Line 35: | ||
iptables -A FORWARD -d 255.255.255.255 -j ACCEPT | iptables -A FORWARD -d 255.255.255.255 -j ACCEPT | ||
to fix the issue. | to fix the issue. | ||
| + | |||
| + | == Another Problem Case == | ||
| + | I had setup a bridge and got the same problem, but iptables was setup well. In my case the problem was lying in /proc/sys/net/bridge/. | ||
| + | Everything inside had value "1". Changing them to "0" solved the problem. This stopped ARP and bridge packets from being | ||
| + | passed through the FORWARD chain. These settings can be placed inside /etc/sysctl.conf (Debian) so that they are persistent. | ||
== Credits == | == Credits == | ||
Latest revision as of 05:02, 19 February 2012
Sometimes a bridge can mysteriously drop packets and not forward them. e.g. eyck user experienced a problem when some of the broadcasts were not delivered to container via the bridge.
Original report and the thread: forum thread
Simplest configuration[edit]
Container #101 with veth interface (veth101.0) connected to eth0 physical interface via bridge.
Problem statement[edit]
We faced a situation when some of the broadcast packets were not delivered to the container. Actually it could happen with any packets, not with the broadcasts only. But broadcasts are simpler and obviously should have been delivered to all the networking interfaces with no doubt.
Using tcpdump we see that BOOTP/DHCP request is visible on br0 interface in the host system (CT0):
15:21:52.258220 00:1b:d5:2c:bf:38 > ff:ff:ff:ff:ff:ff, ethertype IPv4 (0x0800), length 350: 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:1b:d5:2c:bf:38, length 308 15:21:52.287269 00:08:02:ac:36:20 > ff:ff:ff:ff:ff:ff, ethertype IPv4 (0x0800), length 342: 172.17.8.254.67 > 255.255.255.255.68: BOOTP/DHCP, Reply, length 300
However, eth0 inside the container received only 2nd packet with a BOOTP/DHCP reply and doesn't see the 1st one with the request itself:
15:21:52.291145 00:08:02:ac:36:20 > ff:ff:ff:ff:ff:ff, ethertype IPv4 (0x0800), length 342: 172.17.8.254.67 > 255.255.255.255.68: BOOTP/DHCP, Reply, length 300
Resolution[edit]
It is not obvious at all, but bridges (though they have their own ebtables filters) do also call iptables FORWARD chain when forwarding packets between interfaces. Thus your FORWARD iptables rules should allow all the packets which are supposed to go through.
in our case eyck had a default DROP policy on FORWARD and had to add:
iptables -A FORWARD -d 255.255.255.255 -j ACCEPT
to fix the issue.
Another Problem Case[edit]
I had setup a bridge and got the same problem, but iptables was setup well. In my case the problem was lying in /proc/sys/net/bridge/. Everything inside had value "1". Changing them to "0" solved the problem. This stopped ARP and bridge packets from being passed through the FORWARD chain. These settings can be placed inside /etc/sysctl.conf (Debian) so that they are persistent.
Credits[edit]
Many credits to Dariush Pietrzak, who patiently helped to debug this.
