Linux Networking Cheatsheet

1. Interface & Routing (iproute2)

• ip l : List all network interfaces (L2/MAC).
• ip l set eth0 up : Bring interface up.
• ip a : List all IP addresses.
• ip a add 192.168.1.10/24 dev eth0 : Assign temporary IPv4.
• ip r : Display IPv4 routing table.
• ip r add default via 192.168.1.1 : Set default gateway.
• ip n : Show ARP table (IPv4) / NDP table (IPv6).

2. Port Checks (netstat & ss)

Run as root to see PIDs.

• netstat -tuln : Show TCP/UDP Listening ports (Numbers only).
• netstat -tupan : Show active connections + Program/PID.
• netstat -rn : Display kernel routing table.
• ss -tuln : Modern, faster alternative to netstat.

3. Packet Capture (tcpdump)

• tcpdump -i eth0 : Capture all on eth0.
• tcpdump -i eth0 -n : Disable DNS resolution (faster).
• tcpdump -i eth0 port 80 : Filter by HTTP traffic.
• tcpdump -i eth0 host 10.0.0.5 : Filter by specific IP.
• tcpdump -i eth0 -vvv -X : Verbose, show HEX and ASCII.
• tcpdump -i eth0 -w cap.pcap : Write to file for Wireshark.

4. IPv6 Focus

• ip -6 a : Show only IPv6 addresses.
• ip -6 r : Show IPv6 routing table.
• ip -6 addr add 2001:db8::1/64 dev eth0 : Assign static IPv6.
• ip -6 r add 2001:db8::1/64 via 2001:db8::1 dev eth0 : Assign static IPv6 Route.
• ip -6 neigh : View IPv6 neighbor cache (NDP).
• ping6 2001:db8::1 : Ping IPv6 address.
• sysctl -w net.ipv6.conf.all.forwarding=1 : Enable IPv6 routing.

5. Quick L2 Bridge

Connects interfaces at Layer 2.

# Create bridge & attach interfaces
ip link add name br0 type bridge
ip link set dev eth0 master br0
ip link set dev eth1 master br0

# Bring everything up
ip link set dev br0 up
ip link set dev eth0 up
ip link set dev eth1 up

6. Quick NAT (IPv4 & IPv6)

Assuming eth0 is WAN and eth1 is LAN.

# Enable forwarding
sysctl -w net.ipv4.ip_forward=1
sysctl -w net.ipv6.conf.all.forwarding=1

# Set up Masquerading (NAT)
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
ip6tables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

7. Quick DHCP (dnsmasq)

Fast temporary DHCP/DNS server on eth1.

# Listen on eth1, hand out .50 to .150
dnsmasq -d \
  --interface=eth1 \
  --dhcp-range=192.168.1.50,192.168.1.150,12h

8. WiFi AP (hostapd)

1. Create /etc/hostapd/minimal.conf:

interface=wlan0
driver=nl80211
ssid=MyLinuxAP
hw_mode=g
channel=6
wpa=2
wpa_passphrase=SuperSecretPassword
wpa_key_mgmt=WPA-PSK
rsn_pairwise=CCMP

2. Start the Access Point:

hostapd -d /etc/hostapd/minimal.conf