Linux Bridges and Firewalls for Containers

Hubs and Repeaters work on bits on Layer1. Bridges and Switches work on frames, the protocol data unit for Layer2. A Hub supports a star configuration – like a splitter it simply forwards traffic received on one segment to all the others. A Repeater amplifies signal for longer distances. A Bridge is a smarter type of Hub, usually implemented in software, which looks at the destination MAC address of the packet before forwarding to the segment with MAC matching that destination, thereby avoiding unnecessary network traffic on all the other segments. It does not look at IP addresses. A Switch is a reincarnation of a Bridge in hardware with several ports.  The basic operation of a Bridge/Switch is L2 forwarding, which includes ARL (Address Resolution Logic) table learning and ageing.  An L3 Router switches packets based on the IP address instead of the frame address.

A linux bridge from a VM to the Host, does not need an IP address, but it is a good idea if you want to talk to the host for management or set firewall rules on it.

To create a bridge br0 between two interfaces eth0 and eth1, one runs commands: brctl addbr br0, brctl stp br0 on, brctl addif br0 eth0, brctl addif br0 eth1.

Since a bridge is an L2 construct, how can one filter IP addresses with it. A quote: An Ethernet interface cannot usefully have an IP address if it is also attached to a bridge. However it is possible to achieve the same effect by binding an address to the bridge itself:

ifconfig br0

An example of firewall configuration on a bridge.

Troubleshooting bridges:

brctl showmacs br0

Docker creates a few bridges by default. A primer on Docker Container networking and the bridges it creates are here and  here. Here are three common ones.

bridge: docker0 bridge . This is the default, allowing all containers to talk to each other

host: container listens to host, without any isolation.

Some problems with host iptables rules reveal themselves in working with docker.

A look at firewalld, ebtables, ipset, nftables at . The command line tools talk to the kernel over a netlink socket.

A more extensible approach is with Berkeley Packet Filter (BPF), the classical one now called cBPF, the extended successor called eBPF.   A very basic problem is filtering HTTP or DNS packets based on the destination hostname, rather than the IP address.  This is discussed in an example by Cloudflare here.

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