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Overview & Root Cause Summary: The error Temporary failure in name resolution is a system-wide Linux network resolution failure returned by standard C library resolver functions (getaddrinfo / gethostbyname). It occurs when applications (such as apt, curl, wget, git, or Docker) attempt to resolve a domain name to an IP address, but the system receives no response from the configured DNS servers. This is typically caused by an inactive or crashed systemd-resolved daemon, a broken /etc/resolv.conf symlink, network adapter changes following a VPN disconnect, or upstream router DHCP nameserver failure.

Understanding the Root Causes

• Unreachable or Missing Nameservers in /etc/resolv.conf: The resolver configuration file contains no active nameserver entries, or lists upstream DNS IP addresses that are offline or unreachable from the current network interface.
• systemd-resolved Local Stub Resolver Failure: On Ubuntu, Debian, and modern systemd distributions, /etc/resolv.conf points to the local loopback stub resolver (127.0.0.53). If the systemd-resolved background service crashes or freezes, all system DNS queries fail instantly.
• Broken Symlink After VPN or Network Reconfiguration: Disconnecting from corporate VPN clients (e.g. OpenVPN, Cisco AnyConnect, WireGuard) often fails to restore the original /etc/resolv.conf, leaving a dead file or orphaned configuration.
• Firewall Dropping Outbound UDP Port 53: Host-level firewalls (UFW, iptables) or corporate network security gateways blocking outbound UDP/TCP port 53 traffic.

Step 1: Quick Fix (Configure Reliable Upstream Nameservers)

Restore immediate internet and package manager connectivity by adding public upstream DNS resolvers directly to your system configuration.

# 1. Temporarily populate /etc/resolv.conf with public DNS servers (Google and Cloudflare):
sudo bash -c 'cat <<EOF > /etc/resolv.conf
nameserver 8.8.8.8
nameserver 1.1.1.1
nameserver 8.8.4.4
EOF'

# 2. Test immediate name resolution:
ping -c 3 google.com

# 3. If resolution works, proceed to Step 2 to make the configuration permanent 
# (otherwise NetworkManager or systemd-resolved will overwrite /etc/resolv.conf on reboot).

Step 2: Repair systemd-resolved and the resolv.conf Symlink

On modern Linux distributions, ensure the systemd DNS daemon is active and that resolv.conf links to the correct runtime directory.

# 1. Check systemd-resolved operational status:
sudo systemctl status systemd-resolved

# 2. Start and enable systemd-resolved if inactive:
sudo systemctl enable --now systemd-resolved

# 3. Re-link /etc/resolv.conf to the standard systemd-resolved stub file:
sudo ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf

# 4. If the stub resolver fails inside containers or VMs, link to the upstream file directly:
sudo ln -sf /run/systemd/resolve/resolv.conf /etc/resolv.conf

# 5. Flush and restart the DNS cache:
sudo resolvectl flush-caches
sudo systemctl restart systemd-resolved

Step 3: Configure Permanent Static DNS via Netplan or NetworkManager

Make your DNS configuration permanent across reboots and DHCP renewals through your distribution’s primary network manager.

# --- For Netplan (Ubuntu Server / Desktop 18.04+) ---
# 1. Locate and edit your Netplan configuration file:
sudo nano /etc/netplan/01-netcfg.yaml

# 2. Under your primary network interface (e.g. eth0 or enp3s0), add nameservers:
network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      dhcp4: true
      nameservers:
        addresses: [8.8.8.8, 1.1.1.1]

# 3. Apply the Netplan configuration:
sudo netplan apply

# --- For NetworkManager (Desktop Linux / RHEL / Fedora) ---
# Set DNS via nmcli on your active connection:
nmcli connection show
nmcli connection modify "<Connection_Name>" ipv4.dns "8.8.8.8 1.1.1.1"
nmcli connection modify "<Connection_Name>" ipv4.ignore-auto-dns yes
nmcli connection up "<Connection_Name>"

Verification & Testing Steps

Confirm that DNS resolution is functioning properly and inspect active resolver endpoints.

# 1. Query the active DNS servers and link state using resolvectl:
resolvectl status
# Verify that "DNS Servers: 8.8.8.8 1.1.1.1" is displayed under your primary interface.

# 2. Test domain resolution using dig or nslookup:
dig google.com +short

# 3. Validate system package repository reachability:
sudo apt-get update
# (Or on RHEL/CentOS: sudo dnf check-update)

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