Show all addresses

ip address show

All show commands can be used with -4 or -6 options to show only IPv4 or IPv6 addresses.

Show addresses for a single interface

ip address show ${interface name}

Examples:

ip address show eth0

Show addresses only for running interfaces

ip address show up

Show only static or dynamic IPv6 addresses

Show only statically configured addresses:

ip address show [dev ${interface}] permanent

Show only addresses learnt via autoconfiguration:

ip address show [dev ${interface}] dynamic

Add an address to an interface

ip address add ${address}/${mask} dev ${interface name}

Examples:

ip address add 192.0.2.10/27 dev eth0

ip address add 2001:db8:1::/48 dev tun10

You can add as many addresses as you want.

If you add more than one address, your machine will accept packets for
all of them. The first address you added will be used as source address for
outgoing traffic by default, it’s referred to as primary address.

All additional addresses you set will become secondary addresses.

Add an address with human-readable description

ip address add ${address}/${mask} dev ${interface name} label ${interface name}:${description}

Examples:

ip address add 192.0.2.1/24 dev eth0 label eth0:WANaddress

A label must start with the interface name followed by a colon due to some backwards compatibility issues,
otherwise you’ll get an error.
Keep the label shorter than sixteen characters, or else you’ll get this error:

RTNETLINK answers: Numerical result out of range

Notes

For IPv6 addresses this command has no effect (address will be added, but without a label).

Delete an address

ip address delete ${address}/${prefix} dev ${interface name}

Examples:

ip address delete 192.0.2.1/24 dev eth0

ip address delete 2001:db8::1/64 dev tun1

Interface name is required. Linux does allow the same address to be configured on multiple interfaces
and it has valid use cases.

Remove all addresses from an interface

ip address flush dev ${interface name}

Examples:

ip address flush dev eth1

By default this command removes both IPv4 and IPv6 addresses. If you want to remove only
IPv4 or IPv6 addresses, use “ip -4 address flush” or “ip -6 address flush”.

Notes

There is no way to swap primary and secondary addresses or explicitly set the new primary address. Try to always set the primary address first.

However, if the sysctl variable net.ipv4.conf.${interface}.promote_secondaries is set to 1, when you delete the primary address,
the first secondary address will become the new primary.

Note that net.ipv4.conf.default.promote_secondaries=1 is not the universal default setting in all Linux distributions,
so check yours before trying it. If it’s set to 0, then when you delete the primary address, all addresses will be removed
from the interface.

Secondary IPv6 addresses are always promoted to primary if the primary address is deleted so you don’t need to worry about sysctl settings.

View neighbor tables

ip neighbor show

All “show” commands support -4 and -6 options to view only IPv4 (ARP) or IPv6 (NDP) neighbors. By default
all neighbors are displayed.

View neighbors for single interface

ip neighbor show dev ${interface name}

Examples:

ip neighbor show dev eth0

Flush table for an interface

ip neighbor flush dev ${interface name}

Examples:

ip neighbor flush dev eth1

Add a neighbor table entry

ip neighbor add ${network address} lladdr ${link layer address} dev ${interface name}

Examples:

ip neighbor add 192.0.2.1 lladdr 22:ce:e0:99:63:6f dev eth0

One of the use cases for it is to add static entry for an interface with disabled ARP to
restrict interface usage only by hosts with specific MAC addresses.

Delete a neighbor table entry

ip neighbor delete ${network address} lladdr ${link layer address} dev ${interface name}

Examples:

ip neighbor delete 192.0.2.1 lladdr 22:ce:e0:99:63:6f dev eth0

Allows to delete a static entry, or get rid of an automatically learnt entry without flushing the table.

Show information about all links

ip link show

ip link list

These commands are equivalent and can be used with the same arguments.

Show information about specific link

ip link show dev ${interface name}

Examples:

ip link show dev eth0

ip link show dev tun10

The word “dev” may be omitted.

Bring a link up or down

ip link set dev ${interface name} up

ip link set dev ${interface name} down

Examples:

ip link set dev eth0 down

ip link set dev br0 up

Note: virtual links described below, like VLANs and bridges
are in down state immediately after creation. You need to bring them up to start using them.

Set human-readable link description

ip link set dev ${interface name} alias "${description}"

Examples:

ip link set dev eth0 alias "LAN interface"

Link aliases show up in “ip link show” output, like:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT qlen 1000
    link/ether 22:ce:e0:99:63:6f brd ff:ff:ff:ff:ff:ff
    alias LAN interface

Rename an interface

ip link set dev ${old interface name} name ${new interface name}

Examples:

ip link set dev eth0 name lan

Note that you can’t rename an active interface. You need to bring it down
before doing it.

Change link layer address (usually MAC address)

ip link set dev ${interface name} address ${address}

Link layer address is a pretty broad concept. The most known
example is MAC address for ethernet devices. To change MAC
address you would need something like:

ip link set dev eth0 address 22:ce:e0:99:63:6f

Change link MTU

ip link set dev ${interface name} mtu ${MTU value}

Examples:

ip link set dev tun0 mtu 1480

MTU stands for “Maximum Transmission Unit”, the maximum size
of a frame an interface can transmit at once.

Apart from reducing fragmentation in tunnels like
in example above, this is also used to increase performance of
gigabit ethernet links that support so called “jumbo frames”
(frames up to 9000 bytes large). If all your equipment
supports gigabit ethernet, you may want to do something like

ip link set dev eth0 mtu 9000

Note that you may need to configure it on your L2 switches too,
some of them have it disabled by default.

Delete a link

ip link delete dev ${interface name}

Obviously, only virtual links like VLANs or bridges can be deleted.

Enable or disable multicast on an interface

ip link set ${interface name} multicast on

ip link set ${interface name} multicast off

Unless you really understand what you are doing, better not to touch this.

Enable or disable ARP on an interface

ip link set ${interface name} arp on

ip link set ${interface name} arp off

One may want to disable ARP to enforce a security policy and allow only specific MACs
to communicate with the interface. In this case neighbor table entries for whitelisted MACs
should be created manually (see neighbor table management
section), or nothing will be able to communicate with that interface.

In most cases it’s better to configure MAC policy on an access layer switch though.
Do not change this flag unless you are sure what you are going to do and why.

Create a VLAN interface

ip link add name ${VLAN interface name} link ${parent interface name} type vlan id ${tag}

Examples:

ip link add name eth0.110 link eth0 type vlan id 110

The only type of VLAN supported in Linux is IEEE 802.1q VLAN, legacy implementations like ISL
are not supported.

Once you create a VLAN interface, all frames tagged with ${tag} you specified in id option
received by ${parent interface} will be processed by that VLAN interface.

eth0.100 name format is traditional, but not required, you can name the interface as you want,
just like with other interface types.

VLANs can be created over bridge, bonding and other interfaces capable of processing ethernet frames too.

Create a QinQ interface (VLAN stacking)

ip link add name ${service interface} link ${physical interface} type vlan proto 802.1ad id ${service tag}

ip link add name ${client interface} link ${service interface} type vlan proto 802.1q id ${client tag}

Example:

ip link add name eth0.100 link eth0 type vlan proto 802.1ad id 100 # Create service tag interface

ip link add name eth0.100.200 link eth0.100 type vlan proto 802.1q id 200 # Create client tag interface

VLAN stacking (aka 802.1ad QinQ) is a way to transmit VLAN tagged traffic over another VLAN. The common use case
for it is like this: suppose you are a service provider and you have a customer who wants to use your network
infrastructure to connect parts of their network to each other. They use multiple VLANs in their network, so
an ordinary rented VLAN is not an option. With QinQ you can add a second tag to the customer traffic when it enters your network
and remove that tag when it exits, so there are no conflicts and you don’t need to waste VLAN numbers.

The service tag is the VLAN tag the provider uses to carry client traffic through their network. The client tag
is the tag set by the customer.

Note that link MTU for the client VLAN interface is not adjusted automatically, you need to take care
of it yourself and either decrease the client interface MTU by at least 4 bytes, or increase the parent MTU accordingly.

Standards-compliant QinQ is available since Linux 3.10.

Create pseudo-ethernet (aka macvlan) interface

ip link add name ${macvlan interface name} link ${parent interface} type macvlan

Examples:

ip link add name peth0 link eth0 type macvlan

You can think of macvlan interfaces as additional virtual MAC addresses on the parent interface.
They look like normal ethernet interfaces from user point of view, and handle all traffic for
MAC address they are assigned with received by their parent interface.

This is commonly used for testing, or for using several instances of a service identified by
MAC when only one physical interface is available.

They also can be used just for IP address separation instead of assigning multiple addresses
to the same physical interface, especially if some service can’t operate on a secondary address properly.

Create a dummy interface

ip link add name ${dummy interface name} type dummy

Examples:

ip link add name dummy0 type dummy

Dummy interfaces work pretty much like loopback interfaces, just there can be as many
of them as you want.

The first purpose of them is for communication of programs inside the host.

The second purpose exploits the fact they are always up (unless administratively taken down).
This is often used to assign service addresses to them on routers with more than one physical
interface. As long as the traffic to the address assigned to a loopback or dummy interface
is routed to the machine that owns it, you can access it through any of its interfaces.

Create a bridge interface

ip link add name ${bridge name} type bridge

Examples:

ip link add name br0 type bridge

Bridge interfaces are virtual ethernet switches. They can be used to relay traffic
transparently between ethernet interfaces, and, increasingly common, as ethernet switches
for virtual machines running inside hypervisors.

You can assign an IP address to a bridge and it will be visible from all bridge ports.

If this command fails, check if “bridge” module is loaded.

Add an interface to bridge

ip link set dev ${interface name} master ${bridge name}

Examples:

ip link set dev eth0 master br0

Interface you added to a bridge becomes a virtual switch port. It operates only on datalink
layer and ceases all network layer operation.

Remove interface from bridge

ip link set dev ${interface name} nomaster

Examples:

ip link set dev eth0 nomaster

Create a bonding interface

ip link add name ${name} type bond

Examples:

ip link add name bond1 type bond

Note: This is not enough to configure bonding (link aggregation)
in any meaningful way. You need to set up bonding parameters according to your situation.
This is far beyond the cheat sheet scope, so consult the documentation.

Interfaces are added to the bond group the same way to bridge group, just note that you can’t
add it until you take it down.

Create an intermediate functional block interface

ip link add ${interface name} type ifb

Example:

ip link add ifb10 type ifb

Intermediate functional block devices are used for traffic redirection and mirroring
in conjunction with tc. This is also far beyond the scope of this document, consult tc
documentation.

Create a pair of virtual ethernet devices

Virtual ethernet (veth) devices always come in pairs and work as a bidirectional pipe,
whatever comes into one of them, comes out of another. They are used in conjunction with
system partitioning features such as network namespaces and containers (OpenVZ and LXC)
for connecting one partition to another.

ip link add name ${first device name} type veth peer name ${second device name}

Examples:

ip link add name veth-host type veth peer name veth-guest

Note: virtual ethernet devices are created in UP state, no need to
bring them up manually after creation.

Add an interface to a group

ip link set dev ${interface name} group ${group number}

Examples:

ip link set dev eth0 group 42

ip link set dev eth1 group 42

Remove an interface from a group

This can be done by assigning it to the default group.

ip link set dev ${interface name} group 0

ip link set dev ${interface} group default

Examples:

ip link set dev tun10 group 0

Assign a symbolic name to a group

Group names are stored in /etc/iproute2/group file. Symbolic name “default”
for group 0 comes exactly from there. You can add your own, one per line,
following the same “${number} ${name}” format. You can have up to 255 named groups.

Once you configured a group name, number and name can be used interchangeably
in ip commands.

Example:

echo "10    customer-vlans" >> /etc/iproute2/group

After that you can use that name in all operations, like in

ip link set dev eth0.100 group customer-vlans

Perform an operation on a group

ip link set group ${group number} ${operation and arguments}

Examples:

ip link set group 42 down

ip link set group uplinks mtu 1200

View information about links from specific group

Use usual information viewing command with “group ${group}” modifier.

Examples:

ip link list group 42

ip address show group customers

View tun/tap devices

ip tuntap show

Note: this command can be abbreviated to “ip tuntap”.

This command is the only way to find out if some device is in tun or tap mode.

Add an tun/tap device useable by the root user

ip tuntap add dev ${interface name} mode ${mode}

Examples:

ip tuntap add dev tun0 mode tun

ip tuntap add dev tap9 mode tap

Add an tun/tap device usable by an ordinary user

ip tuntap add dev ${interface name} mode ${mode} user ${user} group ${group}

Example:

ip tuntap add dev tun1 mode tun user me group mygroup

ip tuntap add dev tun2 mode tun user 1000 group 1001

Add an tun/tap device using an alternate packet format

Add meta information to each packet received over the file
descriptor. Very few programs expect this information, and
including it when it isn’t expected will break things.

ip tuntap add dev ${interface name} mode ${mode} pi

Example:

ip tuntap add dev tun1 mode tun pi

Add an tun/tap ignoring flow control

Normally packets sent to a tun/tap device travel in the same way
as packets sent to any other device: they are put on a queue handled by the
traffic control engine (which is configured by the tc command).
This can be bypassed, thus disabling the traffic control engine
for this tun/tap device.

ip tuntap add dev ${interface name} mode ${mode} one_queue

Example:

ip tuntap add dev tun1 mode tun one_queue

Delete tun/tap device

ip tuntap del dev ${interface name} mode ${mode}

Examples:

ip tuntap delete dev tun0 mode tun

ip tuntap delete dev tap1 mode tap

Note: you must specify the mode. The mode is not displayed in “ip link show”, so
if you don’t know if it’s TUN or TAP, consult the output of “ip tuntap show”.

Create an IPIP tunnel

ip tunnel add ${interface name} mode ipip local ${local endpoint address} remote ${remote endpoint address}

Examples:

ip tunnel add tun0 mode ipip local 192.0.2.1 remote 198.51.100.3

ip link set dev tun0 up

ip address add 10.0.0.1/30 dev tun0

Create a SIT tunnel

sudo ip tunnel add ${interface name} mode sit local ${local endpoint address} remote ${remote endpoint address}

Examples:

ip tunnel add tun9 mode sit local 192.0.2.1 remote 198.51.100.3

ip link set dev tun9 up

ip address add 2001:db8:1::1/64 dev tun9

This type of tunnels is commonly used to provide an IPv4-connected network with
IPv6 connectivity. There are so called “tunnel brokers” that provide it to everyone interested,
e.g. Hurricane Electric tunnelbroker.net.

Create an IPIP6 tunnel

ip -6 tunnel add ${interface name} mode ipip6 local ${local endpoint address} remote ${remote endpoint address}

Examples:

ip -6 tunnel add tun8 mode ipip6 local 2001:db8:1::1 remote 2001:db8:1::2

This type of tunnels will be widely used when transit operators phase IPv4 out (i.e. not any soon).

Create an IP6IP6 tunnel

ip -6 tunnel add ${interface name} mode ip6ip6 local ${local endpoint address} remote ${remote endpoint address}

Examples:

ip -6 tunnel add tun3 mode ip6ip6 local 2001:db8:1::1 remote 2001:db8:1::2

ip link set dev tun3 up

ip address add 2001:db8:2:2::1/64 dev tun3

Just like IPIP6 these ones aren’t going to be generally useful any soon.

Create a gretap (ethernet over GRE) device

ip link add ${interface name} type gretap local ${local endpoint address} remote ${remote endpoint address}

Examples:

ip link add gretap0 type gretap local 192.0.2.1 remote 203.0.113.3

This type of tunnels encapsulates ethernet frames into IPv4 packets.

Recent kernel and iproute2 versions also support gretap over IPv6, you need to replace the mode with “ip6gretap”
to create an IPv6-based link.

This probably should have been in “Links management” section, but as it involves encapsulation, it’s here.
Tunnel interface created this way looks like an L2 link, and it can be added to a bridge group. This is used
to connect L2 segments via a routed network.

Create a GRE tunnel

ip tunnel add ${interface name} mode gre local ${local endpoint address} remote ${remote endpoint address}

Examples:

ip tunnel add tun6 mode gre local 192.0.2.1 remote 203.0.113.3

ip link set dev tun6 up

ip address add 192.168.0.1/30 dev tun6

ip address add 2001:db8:1::1/64 dev tun6

GRE can encapsulate both IPv4 and IPv6 at the same time. However, by default it uses IPv4 for transport,
for GRE over IPv6 there is a separate tunnel mode, “ip6gre”.

Create multiple GRE tunnels to the same endpoint

ip tunnel add ${interface name} mode gre local ${local endpoint address} remote ${remote endpoint address} key ${key value}

Examples:

ip tunnel add tun4 mode gre local 192.0.2.1 remote 203.0.113.6 key 123

ip tunnel add tun5 mode gre local 192.0.2.1 remote 203.0.113.6 key 124

Keyed tunnels can be used at the same time to unkeyed too. Key may be in dotted decimal IPv4-like format.

Note that key does not add any security to the tunnel. It’s just an identifier used to distinguish one tunnel from another.

Create a point-to-multipoint GRE tunnel

ip tunnel add ${interface name} mode gre local ${local endpoint address} key ${key value}

Examples:

ip tunnel add tun8 mode gre local 192.0.2.1 key 1234

ip link set dev tun8 up

ip address add 10.0.0.1/27 dev tun8

Note the absence of ${remote endpoint address}. This is the same to what is called “mode gre multipoint” in Cisco IOS.

In the absence of remote endpoint address the key is the only way to identify the tunnel traffic, so ${key value} is required.

This type of tunnels allows you to communicate with multiple endpoints by using the same tunnel interface. It’s commonly used in
complex VPN setups with multiple endpoints communicating to one another (in Cisco terminology, “dynamic multipoint VPN”).

As there is no explicit remote endpoint address, obviously it is not enough to just create a tunnel. Your system needs to know
where the other endpoints are.

In real life NHRP (Next Hop Resolution Protocol) is used for it. For testing you can add peers manually (given remote endpoint
uses 203.0.113.6 address on its physical interface and 10.0.0.2 on the tunnel):

ip neighbor add 10.0.0.2 lladdr 203.0.113.6 dev tun8

You will have to do it on the remote endpoint too, like:

ip neighbor add 10.0.0.1 lladdr 192.0.2.1 dev tun8

Note that link-layer address and neighbor address are both IP addresses, so they are on the same OSI layer.
This one of the cases where link-layer address concept gets interesting.

Create a GRE tunnel over IPv6

Recent kernel and iproute2 versions support GRE over IPv6. Point-to-point with no key:

ip -6 tunnel add name ${interface name} mode ip6gre local ${local endpoint} remote ${remote endpoint}

It should support all options and features supported by the IPv4 GRE described above.

Delete a tunnel

ip tunnel del ${interface name}

Examples:

ip tunnel del gre1

Note that in older iproute2 versions this command did not support the full “delete” word, only “del”.
Recent versions allow both full and abbreviated forms (tested in iproute2-ss131122).

Modify a tunnel

ip tunnel change ${interface name} ${options}

Examples:

ip tunnel change tun0 remote 203.0.113.89

ip tunnel change tun10 key 23456

Note: Apparently you can’t add a key to previously unkeyed tunnel.
Not sure if it’s a bug or a feature. Also, you can’t change tunnel mode on the fly,
for obvious reasons.

View tunnel information

ip tunnel show

ip tunnel show ${interface name}

Examples:

$ip tun show tun99 
tun99: gre/ip  remote 10.46.1.20  local 10.91.19.110  ttl inherit

Create an L2TPv3 tunnel over UDP

ip l2tp add tunnel \
tunnel_id ${local tunnel numeric identifier} \
peer_tunnel_id ${remote tunnel numeric identifier} \
udp_sport ${source port} \
udp_dport ${destination port} \
encap udp \
local ${local endpoint address} \
remote ${remote endpoint address}

Examples:

ip l2tp add tunnel \
tunnel_id 1 \
peer_tunnel_id 1 \
udp_sport 5000 \
udp_dport 5000 \ 
encap udp \
local 192.0.2.1 \ 
remote 203.0.113.2

Note: Tunnel identifiers and other settings on both endpoints must match.

Create an L2TPv3 tunnel over IP

ip l2tp add tunnel \
tunnel_id ${local tunnel numeric identifier} \
peer_tunnel_id {remote tunnel numeric identifier } \
encap ip \
local 192.0.2.1 \
remote 203.0.113.2

L2TPv3 encapsulated directly into IP offers less overhead,
bug generally is unable to pass through NAT.

Create an L2TPv3 session

ip l2tp add session tunnel_id ${local tunnel identifier} \
session_id ${local session numeric identifier} \
peer_session_id ${remote session numeric identifier}

Examples:

ip l2tp add session tunnel_id 1 \ 
session_id 10 \
peer_session_id 10

Notes: tunnel_id value must match a value of previously created tunnel.
Session identifiers on both endpoints must match.

Once you create a tunnel and a session, l2tpethX interface will appear, in down state. Change the state
to up and bridge it with another interface or assign an address.

Delete an L2TPv3 session

ip l2tp del session tunnel_id ${tunnel identifier} \
session_id ${session identifier}

Examples

ip l2tp del session tunnel_id 1 session_id 1

Delete an L2TPv3 tunnel

ip l2tp del tunnel tunnel_id ${tunnel identifier}

Examples

ip l2tp del tunnel tunnel_id 1

Note: You need to delete all sessions associated
with a tunnel before deleting it.

View L2TPv3 tunnel information

ip l2tp show tunnel

ip l2tp show tunnel tunnel_id ${tunnel identifier}

Examples:

ip l2tp show tunnel tunnel_id 12

View L2TPv3 session information

ip l2tp show session

ip l2tp show session session_id ${session identifier} \
tunnel_id ${tunnel identifier}

Examples:

ip l2tp show session session_id 1 tunnel_id 12

Create unicast VXLAN link

ip link add name ${interface name} type vxlan \

id <0-16777215> \

dev ${source interface}\

remote ${remote endpoint address} \

local ${local endpoint address} \

dstport ${VXLAN destination port}

Example:

ip link add name vxlan0 type vxlan \

id 42 dev eth0 remote 203.0.113.6 local 192.0.2.1 dstport 4789

Note: id options means VXLAN Network Identifier (VNI).

Create multicast VXLAN link

ip link add name ${interface name} type vxlan \

id <0-16777215> \

dev ${source interface} \

group ${multicast address} \

dstport ${VXLAN destination port}

Example:

ip link add name vxlan0 type vxlan \

id 42 dev eth0 group 239.0.0.1 dstport 4789

After that you need to bring the link up and either bridge it with
another interface or assign an address.

Connected routes

Some routes appear in the system without explicit configuration (against your will).

Once you assign an address to an interface, the system calculates its network address
and creates a route to it (this is why the subnet mask is required). These routes are
called connected routes.

For example, if you assign 203.0.113.25/24 to eth0, a connected route to 203.0.113.0/24 network
will be created and the system will know that hosts from that network can be reached
directly.

When an interface goes down, connected routes associated with it are removed.
This is used for inaccessible gateway detection so routes through gateways that
went inaccessible are removed. Same mechanism prevents you from creating
routes through inaccessible gateways.

View all routes

ip route

ip route show

You can use -4 and -6 options to view only IPv4 or IPv6 routes.
If no options given, IPv4 routes are displayed. To view IPv6 routes, use:

ip -6 route

View routes to a network and all its subnets

ip route show to root ${address}/${mask}

For example, if you use 192.168.0.0/24 subnet in your network
and it’s broken into 192.168.0.0/25 and 192.168.0.128/25, you can see
all those routes with:

ip route show to root 192.168.0.0/24

Note: the word “to” in this and other show commands is optional.

View routes to a network and all supernets

ip route show to match ${address}/${mask}

If you want to view routes to 192.168.0.0/24 and all larger subnets,
use:

ip route show to match 192.168.0.0/24

As routers prefer more specific routes to less specific,
this is often useful for debugging in situations when traffic to specific
subnet is sent the wrong way because a route to it is missing
but routes to larger subnets exist.

View routes to exact subnet

ip route show to exact ${address}/${mask}

If you want to see the routes to 192.168.0.0/24, but not to,
say 192.168.0.0/25 and 192.168.0.0/16, you can use:

ip route show to exact 192.168.0.0/24

View only the route actually used by the kernel

ip route get ${address}/${mask}

Example:

ip route get 192.168.0.0/24

Note that in complex routing scenarios like multipath routing,
the result may be “correct but not complete”, as it always shows
one route that will be used first. In most situations it’s not
a problem, but never forget to look at the corresponding “show” command
output too.

View route cache (pre 3.6 kernels only)

ip route show cached

Until the version 3.6, Linux used
route caching. In older kernels, this command displays the contents
of the route cache. It can be used with modifiers
described above. In newer kernels it does nothing.

Add a route via gateway

ip route add ${address}/${mask} via ${next hop}

Examples:

ip route add 192.0.2.128/25 via 192.0.2.1

ip route add 2001:db8:1::/48 via 2001:db8:1::1

Add a route via interface

ip route add ${address}/${mask} dev ${interface name}

Example:

ip route add 192.0.2.0/25 dev ppp0

Interface routes are commonly used with point-to-point interfaces
like PPP tunnels where next hop address is not required.

Change or replace a route

You may use “change” command to change parameters of existing routes.
“Replace” command can be used to modify existing route or add new one if
it doesn’t exist. Examples:

ip route change 192.168.2.0/24 via 10.0.0.1

ip route replace 192.0.2.1/27 dev tun0

Delete a route

ip route delete ${rest of the route statement}

Examples:

ip route delete 10.0.1.0/25 via 10.0.0.1

ip route delete default dev ppp0

Default route

There is a shortcut to add default route.

ip route add default via ${address}/${mask}

ip route add default dev ${interface name}

These are equivalent to:

ip route add 0.0.0.0/0 via ${address}/${mask}

ip route add 0.0.0.0/0 dev ${interface name}

With IPv6 routes it also works and is equivalent to ::/0

ip -6 route add default via 2001:db8::1

Blackhole routes

ip route add blackhole ${address}/${mask}

Examples:

ip route add blackhole 192.0.2.1/32

Traffic to destinations that match a blackhole route is silently discarded.

Blackhole routes have dual purpose. First one is straightforward, to discard traffic
sent to unwanted destinations, e.g. known malicious hosts.

The second one is less obvious and uses the “longest match rule” as per RFC1812.
In some cases you may want the router to think it has a route to a larger subnet,
while you are not using it as a whole,
e.g. when advertising the whole subnet via dynamic routing protocols. Large subnets
are commonly broken into smaller parts, so if your subnet is 192.0.2.0/24, and you
have assigned 192.0.2.1/25 and 192.0.2.129/25 to your interfaces, your system creates connected
routes to the /25’s, but not the whole /24, and routing daemons may not want to
advertise /24 because you have no route to that exact subnet. The solution is to setup
a blackhole route to 192.0.2.0/24. Because routes to smaller subnets are preferred
over larger subnets, it will not affect actual routing, but will convince routing
daemons there’s a route to the supernet.

Other special routes

ip route add unreachable ${address}/${mask}

ip route add prohibit ${address}/${mask}

ip route add throw ${address}/${mask}

These routes make the system discard packets and reply with an ICMP error message to the sender.

unreachable

Sends ICMP “host unreachable”.

prohibit

Sends ICMP “administratively prohibited”.

throw

Sends “net unreachable”.

Unlike blackhole routes, these can’t be recommended for stopping unwanted traffic (e.g. DDoS)
because they generate a reply packet for every discarded packet and thus create even greater traffic flow.
They can be good for implementing internal access policies, but consider firewall for this purpose first.

“Throw” routes may be used for implementing policy-based routing, in non-default tables they
stop current table lookup, but don’t send ICMP error messages.

Routes with different metric

ip route add ${address}/${mask} via ${gateway} metric ${number}

Examples:

ip route add 192.168.2.0/24 via 10.0.1.1 metric 5

ip route add 192.168.2.0 dev ppp0 metric 10

If there are several routes to the same network with different metric value,
the one with the lowest metric will be preferred.

Important part of this concept is that when an interface goes down, routes
that would be rendered useless by this event disappear from the routing table (see the Connected Routes section), and
the system will fall back to higher metric routes.

This feature is commonly used to implement backup connections to important destinations.

Multipath routing

ip route add ${addresss}/${mask} nexthop via ${gateway 1} weight ${number} nexthop via ${gateway 2} weight ${number}

Multipath routes make the system balance packets across several links according to the weight (higher weight is preferred, so
gateway/interface with weight of 2 will get roughly two times more traffic than another one with weight of 1). You can have as many gateways
as you want and mix gateway and interface routes, like:

ip route add default nexthop via 192.168.1.1 weight 1 nexthop dev ppp0 weight 10

Warning: the downside of this type of balancing is that packets are not guaranteed to be sent back
through the same link they came in. This is called “asymmetric routing”. For routers that simply forward packets and don’t
do any local traffic processing such as NAT, this is usually normal, and in some cases even unavoidable.

If your system does anything but forwarding packets between interfaces, this may cause problems with incoming connections
and some measures should be taken to prevent it.

Create a policy route

ip route add ${route options} table ${table id or name}

Examples:

ip route add 192.0.2.0/27 via 203.0.113.1 table 10

ip route add 0.0.0.0/0 via 192.168.0.1 table ISP2

ip route add 2001:db8::/48 dev eth1 table 100

Notes: You can use any route options described in “Route management” section
in policy routes too, the only difference is the “table ${table id/name}” part at the end.

Numeric table identifiers and names can be used interchangeably. To create your own symbolic
names, edit /etc/iproute2/rt_tables config file.

“delete”, “change”, “replace”, or any other route actions work with any table too.

“ip route … table main” or “ip route … table 254” would have exact same effect to
commands without a table part.

View policy routes

ip route show table ${table id or name}

Examples:

ip route show table 100

ip route show table test

Note: in this case you need the “show” word, the shorthand like “ip route table 120”
do not work because the command would be ambiguous.

General rule syntax

ip rule add ${options} <lookup ${table id or name}|blackhole|prohibit|unreachable>

Traffic that matches the ${options} (described below) will be routed
according to the table with specified name/id instead of the “main”/254 table if “lookup” action is used.

“blackhole”, “prohibit”, and “unreachable” actions that work the same way to route types with same names.
In most of examples we will use “lookup” action as the most common.

For IPv6 rules, use “ip -6”, the rest of the syntax is the same.

“table ${table id or name}” can be used as alias to “lookup ${table id or name}”.

Create a rule to match a source network

ip rule add from ${source network} ${action}

Examples:

ip rule add from 192.0.2.0/24 lookup 10

ip -6 rule add from 2001:db8::/32 prohibit

Notes: “all” can be used as shorthand to 0.0.0.0/0 or ::/0

Create a rule to match a destination network

ip rule add to ${destination network} ${action}

Examples:

ip rule add to 192.0.2.0/24 blackhole

ip -6 rule add to 2001:db8::/32 lookup 100

Create a rule to match a ToS field value

ip rule add tos ${ToS value} ${action}

Examples:

ip rule add tos 0x10 lookup 110

Create a rule to match a firewall mark value

ip rule add fwmark ${mark} ${action}

Examples:

ip rule add fwmark 0x11 lookup 100

Note: See iptables documentation to find out how to set the mark.

Create a rule to match inbound interface

ip rule add iif ${interface name} ${action}

Examples:

ip rule add iif eth0 lookup 10

ip rule add iif lo lookup 20

Rule with “iif lo” (loopback) will match locally generated traffic.

Create a rule to match outbound interface

ip rule add oif ${interface name} ${action}

Examples:

ip rule add oif eth0 lookup 10

Note: this works only for locally generated traffic.

Set rule priority

ip rule add ${options} ${action} priority ${value}

Examples:

ip rule add from 192.0.2.0/25 lookup 10 priority 10

ip rule add from 192.0.2.0/24 lookup 20 priority 20

Note: As rules are traversed from the lowest to the highest
priority and processing stops at first match, you need to put more specific rules
before less specific. The above example demonstrates rules for 192.0.2.0/24 and its
subnet 192.0.2.0/25. If the priorities were reversed and the rule for /25 was placed
after the rule for /24, it would never be reached.

Show all rules

ip rule show

ip -6 rule show

Delete a rule

ip rule del ${options} ${action}

Examples:

ip rule del 192.0.2.0/24 lookup 10

Notes: You can copy/paste from the output of “ip rule show”/”ip -6 rule show”.

Delete all rules

ip rule flush

ip -6 rule flush

Notes: this operation is highly disruptive. Even if you have not
configured any rules, “from all lookup main” rules are initialized by default. On an unconfigured
machine you can see this:

$ ip rule show
0:	from all lookup local 
32766:	from all lookup main 
32767:	from all lookup default 

$ ip -6 rule show
0:	from all lookup local 
32766:	from all lookup main

The “from all lookup local” rule is special and cannot be deleted. The “from all lookup main” is not,
there may be valid reasons not to have it, e.g. if you want to route only traffic you created explicit
rules for. As a side effect, if you do “ip rule flush”, this rule will be deleted, which will make the
system stop routing any traffic until you restore your rules.

Create a namespace

ip netns add ${namespace name}

Examples:

ip netns add foo

List existing namespaces

ip netns list

Delete a namespace

ip netns delete ${namespace name}

Examples:

ip netns delete foo

Run a process inside a namespace

ip netns exec ${namespace name} ${command}

Examples:

ip netns exec foo /bin/sh

Note: assigning a process to a non-default namespace requires
root privileges.

You can run any processes inside a namespace, in particular you can run “ip”
itself, commands like in this “ip netns exec foo ip link list” in this section
are not a special syntax but simply executing another copy of “ip” in a namespace.
You can run an interactive shell inside a namespace as well.

List all processes assigned to a namespace

ip netns pids ${namespace name}

The output will be a list of PIDs.

Identify process’ primary namespace

ip netns identify ${pid}

Examples:

ip netns identify 9000

Assign network interface to a namespace

ip link set dev ${interface name} netns ${namespace name}

ip link set dev ${interface name} netns ${pid}

Examples:

ip link set dev eth0.100 netns foo

Note: once you assign an interface to a namespace, it disappears
from the default namespace and you will have to perform all operations with it via
“ip netns exec ${netspace name}”, as in “ip netns exec ${netspace name} ip link set dev dummy0 down”.

Moreover, when you move an interface to another namespace, it loses all existing configuration
such as IP addresses configured on it and goes to DOWN state. You need to bring it back up and
reconfigure.

If you specify a PID instead of a namespace name, the interface gets assigned to the primary
namespace of the process with that PID. This way you can reassign an interface back to default
namespace with e.g. “ip netns exec ${namespace name} ip link set dev ${intf} netns 1”
(since init or another process with PID 1 is pretty much guaranteed to be in default namespace).

Connect one namespace to another

This can be done by creating two veth links and assigning them two different namespaces.
Suppose you want to connect namespace “foo” to the default namespace.

Create a pair of veth devices:

ip link add name veth1 type veth peer name veth2

Move veth2 to namespace foo:

ip link set dev veth2 netns foo

Bring veth2 and add an address in “foo” namespace:

ip netns exec foo ip link set dev veth2 up

ip netns exec foo ip address add 10.1.1.1/24 dev veth2

Add an address to veth1, which stays in the default namespace:

ip address add 10.1.1.2/24 dev veth1

Now you can ping 10.1.1.1 which if in foo namespace, and setup routes to
subnets configured in other interfaces of that namespace.

If you want switching instead of routing, you can bridge those veth
interfaces with other interfaces in corresponding namespaces. Same technique
can be used to connect namespaces to physical networks.

Monitor network namespace subsystem events

ip netns monitor

Displays events such as creation and deletion of namespaces when they occur.

View multicast groups

ip maddress show

ip maddress show ${interface name}

Example:

$ip maddress show dev lo
1:	lo
	inet  224.0.0.1
	inet6 ff02::1
	inet6 ff01::1

Add a link-layer multicast address

You cannot join an IP multicast group manually, but you can add a multicast MAC
address (even though it’s rarely needed).

ip maddress add ${MAC address} dev ${interface name}

Example:

ip maddress add 01:00:5e:00:00:ab dev eth0

View multicast routes

Multicast routes cannot be added manually, so this command can only show
multicast routes installed by a routing daemon. It supports the same modifiers to
unicast route viewing commands (iif, table, from etc.).

ip mroute show

Monitor all events

You may either call the command without parameters or explicitly specify “all”.

ip monitor

ip monitor all

Monitor specific events

ip monitor ${event type}

Event type can be:

link

Link state: interfaces going up and down, virtual interfaces getting created or
destroyed etc.

address

Link address changes.

route

Routing table changes.

mroute

Multicast routing changes.

neigh

Changes in neighbor (ARP and NDP) tables.

When there are distinct IPv4 and IPv6 subsystems, the usual “-4” and “-6” options
allow you to display events only for specified protocol. As in:

ip -4 monitor route

ip -6 monitor neigh

ip -4 monitor address

Read a log file produced by rtmon

iproute2 includes a program called “rtmon” that serves essentially the same purpose,
but writes events to a binary log file instead of displaying them. “ip monitor” command
allows you to read files created by the program”.

ip monitor ${event type} file ${path to the log file}

rtmon syntax is similar to that of “ip monitor”, except event type is limited to
link, address, route, and all; and address family is specified in “-family” option.

rtmon [-family <inet|inet6>] [<route|link|address|all>] file ${log file path}

View sysctl configuration for all interfaces

ip netconf show

View sysctl configuration for specific interface

ip netconf show dev ${interface}

Examples:

ip netconf show dev eth0