Linux Networking

Network interface configuration, socket inspection, NetworkManager control, and DNS diagnostics.

IP Address Management

Show all interfaces with addresses

ip addr show

Show only IPv4 addresses — cut the noise

ip -4 addr show

Show a specific interface

ip addr show dev eth0

Add an IP address temporarily — lost on reboot

sudo ip addr add 10.50.1.100/24 dev eth0

Remove an IP address

sudo ip addr del 10.50.1.100/24 dev eth0

Link Layer

Show link status for all interfaces — UP/DOWN, MTU, MAC

ip link show

Bring an interface up or down

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

Set MTU — jumbo frames for storage networks

sudo ip link set eth0 mtu 9000

Routing

Show routing table

ip route show

Show the route a packet would take to a destination

ip route get 8.8.8.8

Add a static route

sudo ip route add 192.168.100.0/24 via 10.50.1.1 dev eth0

Add a default gateway

sudo ip route add default via 10.50.1.1

Delete a route

sudo ip route del 192.168.100.0/24

Socket Statistics — ss

ss replaced netstat. Faster, more information, same muscle memory.

Show all listening TCP ports with process names

sudo ss -tlnp

Flags: -t TCP, -l listening, -n numeric (no DNS), -p process.

Show all established connections

ss -tn state established

Show listening UDP sockets

sudo ss -ulnp

Filter by port — what’s listening on 443?

sudo ss -tlnp sport = :443

Show socket memory usage and timer info

ss -tmi

NetworkManager — nmcli

Show all connections

nmcli connection show

Show active connections only

nmcli connection show --active

Show device status

nmcli device status

Create a static IP connection

sudo nmcli connection add type ethernet con-name "static-eth0" ifname eth0 \
  ipv4.addresses 10.50.1.100/24 \
  ipv4.gateway 10.50.1.1 \
  ipv4.dns "10.50.1.50" \
  ipv4.method manual

Modify an existing connection — change DNS

sudo nmcli connection modify "static-eth0" ipv4.dns "10.50.1.50 8.8.8.8"

Activate a connection profile

sudo nmcli connection up "static-eth0"

Deactivate a connection

sudo nmcli connection down "static-eth0"

Switch from DHCP to static

sudo nmcli connection modify "eth0" ipv4.method manual ipv4.addresses 10.50.1.100/24 ipv4.gateway 10.50.1.1
sudo nmcli connection up "eth0"

Reload connection files from disk

sudo nmcli connection reload

Hostname

Show current hostname

hostnamectl

Set hostname persistently

sudo hostnamectl set-hostname server01.example.com

DNS Tools

Query a DNS record — dig is the standard tool

dig example.com A

Short answer only

dig +short example.com

Query a specific DNS server

dig @10.50.1.50 example.com

Reverse DNS lookup

dig -x 10.50.1.50

nslookup — simpler but less powerful

nslookup example.com 10.50.1.50

/etc/hosts — local name resolution, checked before DNS

10.50.1.50    dc01.inside.domusdigitalis.dev    dc01
10.50.1.20    ise-01.inside.domusdigitalis.dev  ise-01

/etc/resolv.conf — DNS client configuration

search inside.domusdigitalis.dev
nameserver 10.50.1.50
nameserver 8.8.8.8

On systems with NetworkManager, edit DNS via nmcli, not by editing resolv.conf directly.

Firewall — firewalld

Check firewall status

sudo firewall-cmd --state

List all rules in the default zone

sudo firewall-cmd --list-all

Add a service permanently

sudo firewall-cmd --permanent --add-service=https
sudo firewall-cmd --reload

Add a port permanently

sudo firewall-cmd --permanent --add-port=8443/tcp
sudo firewall-cmd --reload

Remove a service

sudo firewall-cmd --permanent --remove-service=http
sudo firewall-cmd --reload

List available services

sudo firewall-cmd --get-services

Add a rich rule — source-based access control

sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="10.50.1.0/24" service name="ssh" accept'
sudo firewall-cmd --reload

Diagnostics

Ping — basic connectivity test

ping -c 4 10.50.1.1

Traceroute — show path to destination

tracepath 10.50.1.1

curl — test HTTP endpoints, follow redirects, show headers

curl -ILs https://example.com | head -20

tcpdump — capture packets on an interface (requires root)

sudo tcpdump -i eth0 -nn port 53 -c 20

Flags: -nn no name/port resolution, -c 20 capture 20 packets then stop.

tcpdump write to file for Wireshark analysis

sudo tcpdump -i eth0 -w /tmp/capture.pcap -c 1000