ip

Useful basic commands

1. View all network interfaces

$ ip link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
...

# eg. show specific network interface
$ ip link show test0
4: test0: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/ether 32:25:da:4e:93:fa brd ff:ff:ff:ff:ff:ff
...

1. Add a new network interface

$ ip link add [iface_name] type [iface_type]

# eg. add a dummy interface with name test0
$ ip link add test0 type dummy

1. Configure IP address on the interface

$ ip addr add [ip_addr] dev [iface_name]

# eg.
$ ip addr add 192.168.1.222 dev test0

1. Set the network interface up/down

$ ip link set [iface_name] <up/down>

# eg.
$ ip link set test0 up

Example 1: Create a dummy interface

Create a dummy interface test0 with IP address value of 192.168.1.222

$ ip link add test0 type dummy
$ ip link set test0 down
$ ip addr add 192.168.1.222 dev test0
$ ip link test0 up
$ ip link show test0
...

Example 2: Reset configurations on interface

# release DHCP
$ dhclient -r <iface>
# flush interface
$ ip addr flush dev <iface>
# set interface DOWN
$ ip link set <iface> down

# retrieve DHCP
$ dhclient <iface>
# set interface UP
$ ip link set <iface> up  

Example 3: Replace default route

Suppose we have an interface eth0 with a lower metric (preferred) over another interface eth1

• Now, all traffic not destined for 192.168.0.0 (eth1 specific address) will exit via eth0, with the default gateway of 192.168.1.1

• We can run a few commands to make eth1 the preferred route, to force all traffic to exit via the gateway address 192.168.0.1 instead

Note that all traffic destined for 192.168.1.0 will still exit via the eth0 interface

# view routing table
$ route -n 
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 eth0
0.0.0.0         192.168.0.1     0.0.0.0         UG    100    0        0 eth1
192.168.0.0     0.0.0.0         255.255.255.0   U     100    0        0 eth1
192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 eth0

# update default route
$ ip route replace default via 192.168.0.1 dev eth1 metric 0

# check route
$ ip route get 1.1.1.1
1.1.1.1 via 192.168.0.1 dev eth1 src 192.168.0.5 uid 1000 
    cache 

$ ip route get 8.8.8.8
8.8.8.8 via 192.168.0.1 dev eth1 src 192.168.0.5 uid 1000 
    cache 

$ ip route get 192.168.1.1
192.168.1.1 dev eth0 src 192.168.1.xxx uid 1000 
    cache 
    
$ ip route get 192.168.1.2
192.168.1.2 dev eth0 src 192.168.1.xxx uid 1000