Skip to main content

🗓️ 04102025 0030
📎

Network Interface & Configuration Tools

Tools for viewing and configuring network interfaces, IP addresses, and routing.

ip - Network Configuration (Modern)​

Commands​

ip addr show                 # Show IP addresses
ip route show               # Show routing table
ip link show                # Show network interfaces
ip addr add 192.168.1.100/24 dev eth0  # Add IP address
ip route get 8.8.8.8        # Show route to specific IP
ip neigh                    # Show ARP table (MAC addresses)
ip -s link                  # Show interface statistics

Use for: Checking IPs, configuring network, viewing routing table.

What it does: Modern, powerful tool for network configuration on Linux.

Interpreting Output​

ip addr show​

$ ip addr show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
    inet 127.0.0.1/8 scope host lo
    inet6 ::1/128 scope host

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP
    inet 192.168.1.100/24 brd 192.168.1.255 scope global eth0
    inet6 fe80::a00:27ff:fe4e:66a1/64 scope link

Key parts:

  • lo: Loopback interface (localhost, always 127.0.0.1)
  • eth0: Ethernet interface (or en0, wlan0, etc.)
  • State flags:
    • UP - Interface is active
    • LOWER_UP - Cable plugged in/connected
    • BROADCAST - Supports broadcasting
  • inet 192.168.1.100/24: IPv4 address with subnet mask (/24 = 255.255.255.0)
  • brd 192.168.1.255: Broadcast address
  • scope global: Globally routable (vs. scope host = localhost only)
  • mtu 1500: Maximum transmission unit (packet size)

ip route show​

$ ip route show
default via 192.168.1.1 dev eth0 proto static metric 100
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100

Key parts:

  • default via 192.168.1.1: Default gateway (router) - where to send packets for unknown destinations
  • dev eth0: Which interface to use
  • 192.168.1.0/24: Local network (packets to this range go directly, not through gateway)
  • src 192.168.1.100: Source IP to use

Use cases:

Find your IP address:

$ ip addr show | grep "inet " | grep -v 127.0.0.1
    inet 192.168.1.100/24 brd 192.168.1.255 scope global eth0

Check if interface is up:

$ ip link show eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> ...

→ UP means active

See route to specific destination:

$ ip route get 8.8.8.8
8.8.8.8 via 192.168.1.1 dev eth0 src 192.168.1.100

→ To reach 8.8.8.8, go through gateway 192.168.1.1

ifconfig - Interface Configuration (Legacy)​

Commands​

ifconfig                    # Show all interfaces
ifconfig eth0               # Show specific interface
ifconfig eth0 up            # Bring interface up
ifconfig eth0 down          # Bring interface down

Use for: Quick IP check on older Linux systems or macOS.

Note: Deprecated on modern Linux, replaced by ip command. Still default on macOS.

Interpreting Output​

$ ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.100  netmask 255.255.255.0  broadcast 192.168.1.255
        inet6 fe80::a00:27ff:fe4e:66a1  prefixlen 64  scopeid 0x20<link>
        ether 08:00:27:4e:66:a1  txqueuelen 1000  (Ethernet)
        RX packets 12345  bytes 8901234 (8.9 MB)
        TX packets 6789  bytes 456789 (456.7 KB)

Key parts:

  • UP: Interface is active
  • RUNNING: Cable connected
  • inet 192.168.1.100: IPv4 address
  • netmask 255.255.255.0: Subnet mask (/24)
  • ether 08:00:27:4e:66:a1: MAC address
  • RX packets: Received packets
  • TX packets: Transmitted packets

/etc/resolv.conf - DNS Configuration​

What It Is​

Contains the DNS servers your system uses for name resolution (used by networking_dns_tools).

Viewing​

cat /etc/resolv.conf

Example Content​

nameserver 8.8.8.8
nameserver 1.1.1.1
search local.domain
domain example.com

Key parts:

  • nameserver: DNS servers to query (in order)
    • 8.8.8.8 - Google DNS
    • 1.1.1.1 - Cloudflare DNS
    • Usually 1-3 servers listed
  • search: Domains to append for non-FQDN lookups
    • If you type ping server, it tries server.local.domain
  • domain: Default domain name

Troubleshooting:

No nameservers listed?

$ cat /etc/resolv.conf
# (empty or no nameserver lines)

→ DNS won't work! Add a nameserver:

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

Check which DNS server is actually used:

$ dig example.com | grep SERVER
;; SERVER: 8.8.8.8#53(8.8.8.8)

Common DNS servers:

  • 8.8.8.8, 8.8.4.4 - Google DNS
  • 1.1.1.1, 1.0.0.1 - Cloudflare DNS
  • 208.67.222.222, 208.67.220.220 - OpenDNS

Note: Some systems (systemd-resolved, NetworkManager) auto-generate this file. Your changes may be overwritten!

/etc/hosts - Local DNS Override​

See networking_etc_hosts for detailed information.

Quick summary: This file is checked before DNS, so entries here override DNS resolution.

$ cat /etc/hosts
127.0.0.1       localhost
192.168.1.100   myserver.local

Network Configuration Flow​

1. Application needs to connect to "example.com"
   ↓
2. Check /etc/hosts first ([[networking_etc_hosts]])
   ↓ (not found)
3. Query DNS servers from /etc/resolv.conf
   ↓ (returns 93.184.216.34)
4. Check routing table (ip route show)
   ↓ (route via gateway 192.168.1.1)
5. Send packets out interface eth0
   ↓ (using source IP 192.168.1.100)
6. Connection established!

References