When troubleshooting network connectivity or configuring DNS settings in Linux, many administrators instinctively check /etc/resolv.conf first. However, modern Linux distributions using NetworkManager or systemd-resolved may not store DHCP-assigned DNS servers in this traditional location.

Here are several reliable approaches to identify DNS servers provided by DHCP:

Method 1: Using nmcli (NetworkManager)

For systems using NetworkManager:

nmcli dev show | grep DNS

Example output:

IP4.DNS[1]:                             192.168.1.1
IP4.DNS[2]:                             8.8.8.8

Method 2: Using resolvectl (systemd-resolved)

On systems with systemd-resolved:

resolvectl status

This provides comprehensive DNS information including DHCP-assigned servers.

Method 3: Querying DHCP Lease Files

Directly examining DHCP lease information:

cat /var/lib/NetworkManager/dhclient-*.lease | grep dhcp-server-identifier

Or for systemd-networkd:

journalctl -u systemd-networkd | grep "DNS server"

For scripting or programmatic access:

Python Example

import subprocess
def get_dhcp_dns():
    result = subprocess.run(['nmcli', 'dev', 'show'], 
                          capture_output=True, text=True)
    return [line.split()[-1] for line in result.stdout.splitlines() 
           if 'DNS' in line]

print(get_dhcp_dns())

Bash One-Liner

nmcli -f IP4.DNS dev show $(nmcli -g DEVICE dev) | awk '{print $2}'

The results may show multiple DNS servers - typically the first is the primary, while others are fallbacks. Servers marked with "DHCP" or appearing in lease files are DHCP-assigned, while static configurations may appear in network scripts.

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When working with DHCP-configured network interfaces in Linux, DNS servers are typically stored in /etc/resolv.conf. However, modern Linux distributions using systemd-resolved, NetworkManager, or other network management tools may not populate this file directly. Here's how to reliably discover your DHCP-assigned DNS servers through various methods.

For systems using NetworkManager, this is the most straightforward approach:

nmcli dev show | grep DNS

Example output showing both IPv4 and IPv6 DNS servers:

IP4.DNS[1]: 192.168.1.1
IP6.DNS[1]: 2606:4700:4700::1111

On systems using systemd-resolved (common in Ubuntu and others):

systemd-resolve --status | grep "DNS Servers" -A2

Alternatively, query the DNS configuration directly:

resolvectl dns

For systems not using NetworkManager, check the DHCP lease files:

cat /var/lib/dhcp/dhclient.leases | grep domain-name-servers

Or for a specific interface (e.g., eth0):

cat /var/lib/dhcp/dhclient.eth0.leases | grep domain-name-servers

You can also find DNS information through the iproute2 utilities:

ip route show dev eth0 | grep via

While this shows the gateway rather than DNS, it's often the same address in home networks.

For programmatic access, use NetworkManager's D-Bus API:

import dbus

bus = dbus.SystemBus()
proxy = bus.get_object('org.freedesktop.NetworkManager', '/org/freedesktop/NetworkManager')
manager = dbus.Interface(proxy, 'org.freedesktop.NetworkManager')

active_connections = manager.GetActiveConnections()
for ac_path in active_connections:
    ac_proxy = bus.get_object('org.freedesktop.NetworkManager', ac_path)
    ac = dbus.Interface(ac_proxy, 'org.freedesktop.NetworkManager.Connection.Active')
    ip4_config = ac.Ip4Config
    if ip4_config:
        ip4_proxy = bus.get_object('org.freedesktop.NetworkManager', ip4_config)
        ip4 = dbus.Interface(ip4_proxy, 'org.freedesktop.NetworkManager.IP4Config')
        print("DNS servers:", ip4.Get('org.freedesktop.NetworkManager.IP4Config', 'Nameservers'))

For systems using dhclient directly, check the lease acquisition process:

dhclient -v eth0

The verbose output will show the received DNS server information during the DHCP negotiation.