Networking Fundamentals

Linux has powerful built-in networking capabilities. Whether you are configuring a server, debugging connectivity issues, or securing a firewall, understanding Linux networking tools is essential.

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Network Interfaces

A network interface is the point of connection between your system and a network. Common interface names:

Name Pattern	Type
eth0, ens33	Ethernet (wired)
wlan0, wlp2s0	Wireless
lo	Loopback (localhost, 127.0.0.1)
docker0, br-xxx	Bridge (Docker, containers)
veth	Virtual Ethernet (containers)

ip — Network Configuration

The ip command is the modern replacement for the older ifconfig:

ip addr show                     # show all interfaces and IP addresses
ip addr show eth0                # show specific interface
ip link show                     # show link-layer information
ip route show                    # show routing table
ip neigh show                    # show ARP table (neighbours)

Configuring IP Addresses

# Temporary (lost on reboot)
sudo ip addr add 192.168.1.100/24 dev eth0
sudo ip link set eth0 up

# Permanent — depends on distribution
# Ubuntu/Debian: /etc/netplan/*.yaml
# Fedora/RHEL: /etc/NetworkManager/system-connections/

Example Netplan configuration (Ubuntu):

# /etc/netplan/01-config.yaml
network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 192.168.1.100/24
      gateway4: 192.168.1.1
      nameservers:
        addresses:
          - 8.8.8.8
          - 8.8.4.4

sudo netplan apply               # apply netplan configuration

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Checking Connectivity

ping — Test Network Reachability

ping google.com                  # ping continuously (Ctrl+C to stop)
ping -c 4 google.com             # send 4 pings then stop
ping -c 4 192.168.1.1            # ping an IP address

traceroute / tracepath — Trace Network Path

traceroute google.com            # show the route packets take
tracepath google.com             # similar, no root required

mtr — Combined Ping and Traceroute

mtr google.com                   # real-time traceroute with statistics

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Viewing Connections and Ports

ss — Socket Statistics

ss is the modern replacement for netstat:

ss -tuln                         # show TCP/UDP listening ports
ss -tulnp                        # include process names
ss -s                            # show socket summary statistics
ss -t state established          # show established TCP connections
ss -t dst 10.0.0.5              # connections to a specific destination

Understanding ss Output

State    Recv-Q  Send-Q  Local Address:Port  Peer Address:Port  Process
LISTEN   0       128     0.0.0.0:22          0.0.0.0:*          users:(("sshd",pid=1234))
LISTEN   0       511     0.0.0.0:80          0.0.0.0:*          users:(("nginx",pid=5678))
ESTAB    0       0       10.0.0.5:22         10.0.0.10:54321    users:(("sshd",pid=2345))

Common Port Numbers

Port	Service
22	SSH
80	HTTP
443	HTTPS
53	DNS
25 / 587	SMTP (email)
3306	MySQL
5432	PostgreSQL
6379	Redis
8080	HTTP (alternative)
27017	MongoDB

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DNS

Resolving Hostnames

dig google.com                   # detailed DNS query
dig google.com +short            # show just the IP address
dig -x 142.250.187.206          # reverse DNS lookup
nslookup google.com              # simpler DNS lookup
host google.com                  # another DNS lookup tool

DNS Configuration