Linux: nmcli, networkctl, resolvectl, ss, ethtool and the config files

systemctl is-active NetworkManager systemd-networkd
nmcli general status 2>/dev/null
networkctl list 2>/dev/null
ls /etc/netplan/ /etc/network/interfaces /etc/sysconfig/network-scripts/ 2>/dev/null
resolvectl status 2>/dev/null | head -5

nmcli                                # overview: every device with addresses, routes, DNS
nmcli general status
nmcli device status                  # devices and their state
nmcli device show enp3s0             # full detail for one device
nmcli connection show                # saved profiles
nmcli connection show "Wired connection 1"   # every property of a profile
nmcli -g ip4.address device show enp3s0        # one field, script-friendly
nmcli -t -f DEVICE,STATE device      # terse output

sudo nmcli connection modify "Wired connection 1" \
  ipv4.method manual \
  ipv4.addresses 192.168.1.50/24 \
  ipv4.gateway 192.168.1.1 \
  ipv4.dns "1.1.1.1 1.0.0.1" \
  ipv4.dns-search home.arpa
sudo nmcli connection up "Wired connection 1"    # apply

sudo nmcli connection modify "Wired connection 1" ipv4.method auto ipv4.addresses "" ipv4.gateway "" ipv4.dns ""
sudo nmcli connection up "Wired connection 1"

sudo nmcli connection down "Wired connection 1" && sudo nmcli connection up "Wired connection 1"
sudo nmcli device reapply enp3s0
sudo nmcli device disconnect enp3s0 && sudo nmcli device connect enp3s0
sudo nmcli networking off && sudo nmcli networking on
sudo systemctl restart NetworkManager

nmcli device wifi list
nmcli device wifi list --rescan yes
sudo nmcli device wifi connect "MySSID" password "secret"
sudo nmcli device wifi connect "MySSID" password "secret" hidden yes
nmcli connection show --active
sudo nmcli connection delete "MySSID"
nmcli radio wifi off && nmcli radio wifi on
sudo nmcli device wifi hotspot ifname wlp2s0 ssid MyHotspot password "12345678"
nmcli -s connection show "MySSID" | grep psk      # show the saved password

# clone or randomise MAC
sudo nmcli con mod "Wired connection 1" ethernet.cloned-mac-address 02:11:22:33:44:55
sudo nmcli con mod "MySSID" wifi.cloned-mac-address random
# ignore DHCP-supplied DNS and use your own
sudo nmcli con mod "Wired connection 1" ipv4.ignore-auto-dns yes ipv4.dns 1.1.1.1
# disable IPv6 on a profile
sudo nmcli con mod "Wired connection 1" ipv6.method disabled
# MTU
sudo nmcli con mod "Wired connection 1" ethernet.mtu 1400
# route metric (lower = preferred)
sudo nmcli con mod "Wired connection 1" ipv4.route-metric 50
# add a static route
sudo nmcli con mod "Wired connection 1" +ipv4.routes "10.0.0.0/8 192.168.1.254"
# don't use this connection as the default route
sudo nmcli con mod "Wired connection 1" ipv4.never-default yes
# tell NetworkManager to leave an interface alone
sudo nmcli device set enp3s0 managed no
# add a new profile from scratch
sudo nmcli con add type ethernet ifname enp3s0 con-name lan ipv4.method manual ipv4.addresses 192.168.1.50/24 ipv4.gateway 192.168.1.1
# interactive editor
nmcli connection edit "Wired connection 1"

networkctl list
networkctl status                 # overview with addresses, gateway, DNS
networkctl status enp3s0          # one interface, including DHCP lease and driver
networkctl lldp                   # LLDP neighbours (switch port names)
sudo networkctl reconfigure enp3s0
sudo networkctl renew enp3s0      # renew DHCP
sudo networkctl reload            # re-read config files
sudo networkctl up enp3s0 / down enp3s0   # systemd 248+

sudo netplan get                 # show the merged config
sudo netplan try                 # apply with automatic rollback after 120 s unless confirmed
sudo netplan apply
sudo netplan --debug apply
netplan status                   # Ubuntu 22.10+: live state per interface

sudo ifdown enp3s0 && sudo ifup enp3s0
sudo ifup -a
sudo systemctl restart networking
ifquery --state

sudo ifdown eth0 && sudo ifup eth0
sudo systemctl restart network      # RHEL 7
sudo nmcli connection reload        # RHEL 8, which reads ifcfg files through NetworkManager

# ISC dhclient (Debian/Ubuntu until 24.04, RHEL until 8)
sudo dhclient -r enp3s0      # release
sudo dhclient enp3s0         # obtain
sudo dhclient -v enp3s0      # verbose: see DISCOVER/OFFER/REQUEST/ACK
cat /var/lib/dhcp/dhclient.leases

# dhcpcd (Raspberry Pi OS Bullseye, Arch, Alpine)
sudo dhcpcd -k enp3s0        # release
sudo dhcpcd enp3s0
sudo dhcpcd -n enp3s0        # rebind/renew
dhcpcd -U enp3s0             # dump lease variables

# systemd-networkd's built-in client
sudo networkctl renew enp3s0

# NetworkManager's built-in client (default since NM 1.20)
sudo nmcli device reapply enp3s0

# BusyBox udhcpc
sudo udhcpc -i eth0

resolvectl status                     # servers per interface, DNSSEC, DNS-over-TLS state
resolvectl dns                        # just the servers
resolvectl domain                     # search domains
resolvectl query example.com
resolvectl query -t MX example.com
resolvectl statistics                 # cache hits and misses
sudo resolvectl flush-caches
sudo resolvectl dns enp3s0 1.1.1.1 1.0.0.1      # set servers for this boot
sudo resolvectl domain enp3s0 home.arpa
sudo resolvectl revert enp3s0
ls -l /etc/resolv.conf
# older name for the same tool
systemd-resolve --status
sudo systemd-resolve --flush-caches

ss -tulpn                  # TCP+UDP listening sockets with process (sudo for all PIDs)
ss -tan                    # all TCP, numeric
ss -tan state established
ss -tp dst 192.168.1.10
ss -s                      # summary counts
ss -tln sport = :22
ss -x                      # Unix domain sockets
ss -i                      # TCP internals: cwnd, rtt, retransmits
ss -K dst 203.0.113.5      # kill connections (needs CONFIG_INET_DIAG_DESTROY)

sudo ethtool enp3s0             # speed, duplex, auto-negotiation, link detected
ethtool -i enp3s0               # driver and firmware version
ethtool -S enp3s0               # detailed NIC statistics
ethtool -k enp3s0               # offload features (tso, gro, checksum)
sudo ethtool -K enp3s0 tso off gro off    # turn offloads off (for capture accuracy or driver bugs)
sudo ethtool -s enp3s0 speed 100 duplex full autoneg off
sudo ethtool -s enp3s0 autoneg on
sudo ethtool -p enp3s0 10       # blink the port LED for 10 seconds to find the cable
ethtool -g enp3s0               # ring buffer sizes
sudo ethtool -G enp3s0 rx 4096
sudo ethtool -s enp3s0 wol g    # Wake-on-LAN magic packet
ethtool --show-eee enp3s0

iw dev                          # wireless interfaces
iw dev wlp2s0 link              # SSID, BSSID, signal, bitrate
iw dev wlp2s0 info
sudo iw dev wlp2s0 scan | grep -E 'SSID|signal|freq'
iw dev wlp2s0 station dump      # per-station stats (AP mode or connected)
iw reg get                      # regulatory domain
sudo iw reg set GB
iw list                         # hardware capabilities: bands, channels, modes
rfkill list                     # is the radio blocked?
sudo rfkill unblock wifi
iwconfig                        # legacy wireless-tools; may not be installed
iwgetid -r                      # just the SSID (wireless-tools)

hostname                 # short name
hostname -f              # FQDN
hostname -I              # all addresses on one line (capital i); useful in scripts
hostname -i              # address per /etc/hosts; often 127.0.1.1 and not what you want
hostnamectl              # static, transient and pretty names, plus OS info
sudo hostnamectl set-hostname server1
cat /etc/hostname /etc/hosts

cat /proc/net/dev                        # counters per interface
cat /sys/class/net/enp3s0/address        # MAC
cat /sys/class/net/enp3s0/operstate      # up / down
cat /sys/class/net/enp3s0/speed          # Mb/s
cat /sys/class/net/enp3s0/mtu
cat /sys/class/net/enp3s0/carrier        # 1 = link
ls /sys/class/net/                       # interface names
cat /proc/net/route                      # routes in hex
cat /proc/net/arp                        # ARP cache
cat /proc/net/if_inet6                   # IPv6 addresses
cat /proc/sys/net/ipv4/ip_forward        # forwarding on?
cat /proc/net/wireless