Inspecting Linux Network Configuration

Linux provides multiple ways to query network configuration. While the underlying concepts remain consistent, modern tools have evolved from the older net-tools utilities.

IP Address, MAC Address, and Netmask

The modern replacement for ifconfig is ip addr:

$ ip addr show

This displays all network interfaces with their IPv4 and IPv6 addresses, MAC addresses, and subnet masks:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:26:22:a1:25:0f brd ff:ff:ff:ff:ff:ff
    inet 143.89.135.175/24 brd 143.89.135.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::226:22ff:fea1:250f/64 scope link
       valid_lft forever preferred_lft forever

To get information about a specific interface:

$ ip addr show eth0
$ ip link show eth0

The CIDR notation (e.g., /24) replaces the old netmask format. If you need the netmask in dotted-decimal notation, use ipcalc:

$ ipcalc 143.89.135.175/24

For quick reference, hostname -I shows all IP addresses on the system.

DNS Configuration

Modern systems using systemd typically manage DNS through systemd-resolved. Query the current DNS configuration with:

$ resolvectl status

This shows active DNS servers, search domains, and DNSSEC status.

Legacy /etc/resolv.conf may still exist but is often a symlink to systemd’s managed configuration:

$ cat /etc/resolv.conf

If you need to configure DNS manually (for systems not using systemd or for overrides), edit /etc/resolv.conf:

nameserver 8.8.8.8
nameserver 8.8.4.4

For persistent DNS configuration on systems with NetworkManager:

$ nmcli dev show

Or edit the NetworkManager connection profile directly:

$ sudo nmcli connection modify "Connection Name" ipv4.dns "8.8.8.8 8.8.4.4"
$ sudo nmcli connection up "Connection Name"

The Hosts File

The /etc/hosts file provides local hostname-to-IP mapping, bypassing DNS lookups. The loopback entries should remain unchanged:

127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4
::1         localhost localhost.localdomain localhost6 localhost6.localdomain6
10.0.1.8    mysite.mydomain mysite

The system consults this file before DNS queries when configured properly in Name Service Switch.

Name Service Switch Order

The /etc/nsswitch.conf file controls the order of hostname resolution:

hosts: files mdns4_minimal [NOTFOUND=return] dns

This line means: check /etc/hosts first, then mDNS, then DNS. Modify the order based on your needs.

Gateway and Routing

View the routing table with:

$ ip route show

Output example:

default via 143.89.135.254 dev eth0 proto static
143.89.135.0/24 dev eth0 proto kernel scope link src 143.89.135.175

The default via line shows your gateway. To view routes for a specific interface:

$ ip route show dev eth0

Add a static route:

$ sudo ip route add 192.168.1.0/24 via 10.0.0.1 dev eth0

For persistence across reboots, use your distribution’s network configuration tool (NetworkManager, netplan, systemd-networkd) rather than manual route commands.

Hostname Configuration

View the current hostname:

$ hostname

Display the fully qualified domain name:

$ hostname -f

Change the hostname temporarily (reverts on reboot):

$ sudo hostname newhostname

For permanent changes, use hostnamectl on systemd systems:

$ sudo hostnamectl set-hostname newhostname

This updates /etc/hostname and applies the change immediately.

Alternatively, edit /etc/hostname directly and reboot, or edit both /etc/hostname and /etc/hosts to ensure consistency.

Complete Network Diagnostics

For comprehensive network information in one command, use:

$ ip -a

This shows all addresses, routes, and interface statistics.

Test DNS resolution:

$ nslookup example.com
$ dig example.com
$ getent hosts example.com

These tools verify both your DNS configuration and network connectivity.