Related articles * [VLAN](/index.php/VLAN "VLAN") ## Contents * [1 Link status](#Link_status) * [1.1 RTNETLINK answers: Cannot assign requested address](#RTNETLINK_answers:_Cannot_assign_requested_address) * [2 IP address](#IP_address) * [3 Ping & Tracepath/Traceroute](#Ping_.26_Tracepath.2FTraceroute) ## Link status In the overview of `ip a`, the link status will already be displayed. But it can also be displayed by running: ``` $ ip link show dev eth0 ``` This will provide an output along the lines of: ``` 2: eth0: mtu 1500 qdisc pfifo_fast state DOWN qlen 1000 link/ether 70:5a:b6:8a:a0:87 brd ff:ff:ff:ff:ff:ff ``` Bringing up an interface can be done by issuing: ``` # ip link set dev eth0 up ``` #### RTNETLINK answers: Cannot assign requested address If you get this error when trying to set an interface up, its most probably because you've got an invalid MAC address. To set a working MAC, see [MAC address spoofing](/index.php/MAC_address_spoofing "MAC address spoofing"). ## IP address In the overview provided by `ip a`, the ip address will already be displayed. But it can also be displayed by running: ``` $ ip addr show dev eth0 ``` This will provide an output along the lines of: ``` 2: eth0: mtu 1500 qdisc pfifo_fast state UP qlen 1000 link/ether 70:5a:b6:8a:a0:87 brd ff:ff:ff:ff:ff:ff inet 192.168.1.143/24 brd 192.168.1.255 scope global eth0 inet6 fe80::725a:b6ff:fe8a:a087/64 scope link valid_lft forever preferred_lft forever ``` Adding a temporary ip address: ``` # ip addr add 192.168.1.143/24 dev eth0 ``` Removing an ip address: ``` # ip addr del 192.168.1.143/24 dev eth0 ``` ## Ping & Tracepath/Traceroute The ping command can help test connectivity towards a specific host. The first step would be verifying connectivity towards the default gateway (replace the ip address with your own default gateway): ``` $ ping -c4 192.168.1.1 ``` When erasing the "-c4" parameter, the ping will continue endlessly. It can be aborted by hitting "Control-C". ``` PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data. 64 bytes from 192.168.1.1: icmp_req=1 ttl=64 time=0.193 ms 64 bytes from 192.168.1.1: icmp_req=2 ttl=64 time=0.190 ms 64 bytes from 192.168.1.1: icmp_req=3 ttl=64 time=0.192 ms 64 bytes from 192.168.1.1: icmp_req=4 ttl=64 time=0.189 ms --- 192.168.1.1 ping statistics --- 4 packets transmitted, 4 received, 0% packet loss, time 2999ms rtt min/avg/max/mdev = 0.165/0.184/0.193/0.014 ms ``` The output above indicated the default gateway is reachable. When instead a "`Destination Host Unreachable`" message is displayed, doublecheck the ip address, netmask and default gateway config. This message can also be displayed when ICMP traffic is not permitted towards the default gateway (blocked by a firewall, router,...). The next step is verifying connectivity towards the configured dns server(s). When no reply is received, `tracepath` or `traceroute` can be used to verify the routing towards said server and get an idea of where the issue lies. ``` $ traceroute 8.8.4.4 ``` Traceroute also used ICMP to determine the path and hence there can be "no reply" answers as well when ICMP traffic is blocked.