You may recall http://lkml.org/lkml/2006/9/29/268, wherein I described
network device enumeration and naming challenges, and several possible
fixes. Of these, Fix #1 (fix the PCI device list to be sorted
breadth-first) has been implemented in the kernel, and Fix #3 (system
board routing rules) have been implemented on Dell PowerEdge 10G and
11G servers (11G begin selling RSN).
However, these have not been completely satisfactory. In particular,
it keeps getting harder and harder to route PCI-Express lanes to
guarantee the same ordering between a depth-first and breadth-first
walk, and it turns out, that isn't sufficient anyhow.
Problem: Users expect on-motherboard NICs to be named eth0..ethN. This can be difficult to achieve.
Ethernet device names are initially assigned by the kernel, and may be
changed by udev or nameif in userspace. The initial name assigned by
the kernel is in monotonically increasing order, starting with eth0.
In this instance, the enumeration directly leads to an assigned name.
Complications:
1) Devices are discovered, and presented to the kernel for name
assignment, based on several factors:
a) the kernel hotplug mechanism emits events for udev to catch, to
b) udev may run modprobes in parallel. It guarantees that the
To get any consistent ordering now, one of two things must
happen:
i) drivers must be loaded before udev begins loading drivers
(either very early in initscripts, or in the inital ramdisk).
ii) something must "fix up" the kernel-assigned names after
udev's modprobes complete. udev does this as well.
2) udev may have rules to change the device names. This is most often
seen in the '70-persistent-net.rules' file. Here we have
additional challenges:
a) this does not exist the first time devices are discovered; the
b) it introduces state (MAC addresses) to the system, on a system
c) udev may not always be able to change a device's name. If udev
3) End users have the (reasonable?) expectation that NIC ports
4) When adding a network card to an existing system, what should the
ports on the new card be named? If it is added, they will be named
ethN+1... above the existing named cards. This means a (new)
add-in card in PCI slot 3 may have ports named eth5 and eth6, while
an add-in card in PCI slot 5 may have ports named eth2 and eth3.
This is not intuitive.