== Configuring VLANs on Fedora Core

Interactive VLAN configuration is done with the _vconfig_ program. The
basic usage is '_vconfig add eth0 6_'; this makes a new Ethernet device
called _eth0.6_ (by default; _vconfig_ can change this, but you probably
don't want to). '_vconfig rem eth0.6_' will then remove the VLAN.

A configured VLAN is up enough so that you can receive traffic on it. If
all you're interested in is doing things like bridging virtual machines
onto the VLAN's network, you don't need to do anything more at the host
level; otherwise, you're going to need to give the VLAN interface an
IP address somehow. I don't recommend using DHCP, because as far as I
know there's no way to tell the Fedora DHCP clients to *not* helpfully
rewrite your _/etc/resolv.conf_ for the new network.

(Really what one wants is a 'shut up and get me an IP address, JUST
an IP address, no routes, no nothing' option for some DHCP client.
But this is kind of an obscure thing, so I can understand why it's
not there.)

For permanent configuration, you can create ifcfg scripts in
_/etc/sysconfig/network-scripts_. The minimum contents are:

>  DEVICE=eth0.6
>  VLAN=yes
>  ONBOOT=yes

(You can say '_ONBOOT=no_' if you really want to; I suppose '_ifup
whatever_' is marginally less typing than doing the _vconfig_ by
hand.)

The '_VLAN=yes_' bit is the important magic. With this, Fedora cracks
open the device name to conclude that this is VLAN ID 6 on eth0, and
sets it up appropriately (yet another reason not to try to change
_vconfig_'s VLAN name format). Fedora is perfectly willing to bring up
VLANs that have no assigned IP address, and this is how I have mine set
up. I name my VLAN ifcfg files things like '_ifcfg-vlan6_', but I believe
this name format is not required.

There's an alternate format for the VLAN ID and base device information:

>  DEVICE=vlan6
>  PHYSDEV=eth0
>  VLAN=yes
>  ONBOOT=yes

(For VLAN ID 6 on eth0 again.)

As far as I can see, you still get a device called '_eth0.6_' out of
this, not one called '_vlan6_'.

Fedora's _tcpdump_ understands VLANs and so can be used to dump the
traffic on _eth0_ so you can see what VLANs are actually reaching your
machine. However, just to confuse you, it will not print the VLAN
ID information unless you ask it for link-level headers with _-e_.
(Although it will happily receive and dump the packets, which can be
really confusing; you need to remember to ask for '_not vlan and ..._'
if you want to see just the untagged base traffic on your link.)

Because VLAN devices are regular Ethernet devices, you can use
_tcpdump_ on them to see just traffic for that particular VLAN.
This traffic is naturally already detagged.

(This is the kind of entry I write so that I have all of this
information in one place the next time I need it.)