kuznet@alisa:~ $ ip link ls eth0
3: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc cbq qlen 100
link/ether 00:a0:cc:66:18:78 brd ff:ff:ff:ff:ff:ff
kuznet@alisa:~ $ ip link ls sit0
5: sit0@NONE: <NOARP,UP> mtu 1480 qdisc noqueue
link/sit 0.0.0.0 brd 0.0.0.0
kuznet@alisa:~ $ ip link ls dummy
2: dummy: <BROADCAST,NOARP> mtu 1500 qdisc noop
link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
kuznet@alisa:~ $
The number before each colon is an interface index or ifindex.
This number uniquely identifies the interface. This is followed by the interface name
(eth0, sit0 etc.). The interface name is also
unique at every given moment. However, the interface may disappear from the
list (f.e. when the corresponding driver module is unloaded) and another
one with the same name may be created later. Besides that,
the administrator may change the name of any device with
ip link set name
to make it more intelligible.
The interface name may have another name or NONE appended
after the @ sign. This means that this device is bound to some other
device,
i.e. packets send through it are encapsulated and sent via the ``master''
device. If the name is NONE, the master is unknown.
Then we see the interface mtu (``maximal transfer unit''). This determines the maximal size of data which can be sent as a single packet over this interface.
qdisc (``queuing discipline'') shows the queuing algorithm used
on the interface. Particularly, noqueue means that this interface
does not queue anything and noop means that the interface is in blackhole
mode i.e. all packets sent to it are immediately discarded.
qlen is the default transmit queue length of the device measured
in packets.
The interface flags are summarized in the angle brackets.
UP -- the device is turned on. It is ready to accept
packets for transmission and it may inject into the kernel packets received
from other nodes on the network.
LOOPBACK -- the interface does not communicate with other
hosts. All packets sent through it will be returned
and nothing but bounced packets can be received.
BROADCAST -- the device has the facility to send packets
to all hosts sharing the same link. A typical example is an Ethernet link.
POINTOPOINT -- the link has only two ends with one node
attached to each end. All packets sent to this link will reach the peer
and all packets received by us came from this single peer.
If neither LOOPBACK nor BROADCAST nor POINTOPOINT
are set, the interface is assumed to be NMBA (Non-Broadcast Multi-Access).
This is the most generic type of device and the most complicated one, because
the host attached to a NBMA link has no means to send to anyone
without additionally configured information.
MULTICAST -- is an advisory flag indicating that the interface
is aware of multicasting i.e. sending packets to some subset of neighbouring
nodes. Broadcasting is a particular case of multicasting, where the multicast
group consists of all nodes on the link. It is important to emphasize
that software must not interpret the absence of this flag as the inability
to use multicasting on this interface. Any POINTOPOINT and
BROADCAST link is multicasting by definition, because we have
direct access to all the neighbours and, hence, to any part of them.
Certainly, the use of high bandwidth multicast transfers is not recommended
on broadcast-only links because of high expense, but it is not strictly
prohibited.
PROMISC -- the device listens to and feeds to the kernel all
traffic on the link even if it is not destined for us, not broadcasted
and not destined for a multicast group of which we are member. Usually
this mode exists only on broadcast links and is used by bridges and for network
monitoring.
ALLMULTI -- the device receives all multicast packets
wandering on the link. This mode is used by multicast routers.
NOARP -- this flag is different from the other ones. It has
no invariant value and its interpretation depends on the network protocols
involved. As a rule, it indicates that the device needs no address
resolution and that the software or hardware knows how to deliver packets
without any help from the protocol stacks.
DYNAMIC -- is an advisory flag indicating that the interface is
dynamically created and destroyed.
SLAVE -- this interface is bonded to some other interfaces
to share link capacities.
1cm NB.
There are other flags but they are either obsolete (NOTRAILERS)
or not implemented (DEBUG) or specific to some devices
(MASTER, AUTOMEDIA and PORTSEL). We do not discuss
them here.
The second line contains information on the link layer addresses
associated with the device. The first word (ether, sit)
defines the interface hardware type. This type determines the format and semantics
of the addresses and is logically part of the address.
The default format of the station address and the broadcast address
(or the peer address for pointopoint links) is a
sequence of hexadecimal bytes separated by colons, but some link
types may have their natural address format, f.e. addresses
of tunnels over IP are printed as dotted-quad IP addresses.
1cm NB. NBMA links have no well-defined broadcast or peer address, however this field may contain useful information, f.e. about the address of broadcast relay or about the address of the ARP server.
1cm NB.
Multicast addresses are not shown by this command, see
ip maddr ls in Sec.9 (p.![[*]](crossref.png)
of this
document).