Meditation, The Art of Exploitation

Thinking? At last I have discovered it--thought; this alone is inseparable from me. I am, I exist--that is certain. But for how long? For as long as I am thinking. For it could be, that were I totally to cease from thinking, I should totally cease to exist....I am, then, in the strict sense only a thing that thinks.

Tuesday, April 01, 2008

Linux Networking 3: network bridge and bump in the wire

A Linux network bridge can be understood as a bump in the wire on steroid. In the physical world, a bridge is used to connect multiple landmass together. This notion is used in a similar meaning in networking. A Linux network bridge is virtual and it connects different 'Ethernet' network segments together, albeit transparently to the Ethernet packets going through it.

Ever wondered what the 'Bridged' networking means in VMWare? It's exactly the kind of network setup allowed by Linux network bridge (practically handled by bridge-utils). Except that VMWare has its own implementation of a virtual network bridge that connects the guest virtual network and the host network together, thus allowing the guest OS virtual network direct access to the external (relative to the host) network.

Because bridge works at Layer 2 for Ethernet packets, a bridge can often be considered functionally equivalent to a switch, albeit a virtual software solution. The simplest bridge acts as a bump in the wire, connecting different network segments just like a switch. However, bridge has the following distinctive advantages:

1) A bridge can act as a network interface and assume a binding IP address, allowing network access to the Linux host.

2) Network packets going through a bridge can be manipulated by iptables, thus allowing greater control such as mangling and filtering not present in switch and bump in the wire.

Because a virtual bridge only examines Ethernet header (layer 2), it's transparent to IP protocols . This has some implications:

1) It's important to take care of ARP tables that translate Ethernet address to IP address, no arp poisoning or other monkey business. The Linux host must forward arp packets properly.

2) It's important to avoid routing loops (cyclic routes through bridges), often requiring turning Spanning Tree Protocol (STP) in network bridges.

A bridge is most useful in the following scenarios:

1) Network transparency and redundancy is required for internal network users. Redundant virtual network bridges can be set up (use STP if necessary) to allow non-interrupted network traffic flow.

2) Administrator needs better packet filtering control over packets striding over network segments.

3) Simply replacing a hardware switch or act as a bump in the wire (connecting two hosts on same network for example)