Configure and troubleshoot network interfaces, routing, and DNS on Debian.

Show interfaces and addresses:

ip addr show
ip addr show eth0

Bring interface up/down:

sudo ip link set eth0 up
sudo ip link set eth0 down

Assign IP address (temporary):

sudo ip addr add 192.168.1.100/24 dev eth0
sudo ip addr del 192.168.1.100/24 dev eth0

Show routing table:

ip route show

Add default gateway:

sudo ip route add default via 192.168.1.1

Add static route:

sudo ip route add 10.0.0.0/8 via 192.168.1.1

Show DNS servers:

cat /etc/resolv.conf

Set temporarily:

echo "nameserver 8.8.8.8" | sudo tee /etc/resolv.conf

Persistent via /etc/network/interfaces:

auto eth0
iface eth0 inet dhcp
    dns-nameservers 8.8.8.8 8.8.4.4

Or use NetworkManager (GUI preferred on desktop):

nmcli device show eth0
nmcli connection modify eth0 ipv4.dns "8.8.8.8 8.8.4.4"

Ping a host:

ping -c 4 8.8.8.8

Trace route to host:

traceroute google.com

Test DNS:

nslookup google.com
dig google.com

Check open ports:

ss -tlnp         # listening TCP ports
ss -ulnp         # listening UDP ports
netstat -tlnp    # old alternative

Show current hostname:

hostname

Set permanently:

sudo hostnamectl set-hostname newname

Or edit directly:

sudo nano /etc/hostname
sudo systemctl restart systemd-hostnamed

Edit /etc/network/interfaces:

auto eth0
iface eth0 inet static
    address 192.168.1.100
    netmask 255.255.255.0
    gateway 192.168.1.1
    dns-nameservers 8.8.8.8

Apply changes:

sudo systemctl restart networking

DHCP is simpler for most systems; use static only when necessary.