Network Debugging
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: <BROADCAST,MULTICAST,UP,LOWER_UP> 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 have an invalid MAC address. To set a working MAC, see 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: <BROADCAST,MULTICAST,UP,LOWER_UP> 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.
Tcpdump
tcpdump, and its underlying library libpcap, are multi-platform user space interfaces to the packets on the network. It should be emphasized they do see, they can capture, any inbound packets that reach the local NIC. No matter if the local software firewall is blocking those packets, or not. On the other hand, they can only see outbound packets the firewall passes through: [1]
A short, unintimidating introduction to tcpdump, with examples, is at: [2]