When your Ubuntu system suddenly stops honoring entries in /etc/hosts, you'll typically observe:

$ ping test
PING localhost (127.0.0.1) 56(84) bytes of data. # works as expected

$ host test
test.mydomain.com has address xx.xxx.161.201 # bypasses hosts file

Ubuntu uses multiple layers for name resolution. Let's examine the critical components:

$ cat /etc/nsswitch.conf | grep hosts
hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4

The proper order should be:

1. files (checks /etc/hosts first)
2. mdns4_minimal (multicast DNS with fallback)
3. dns (standard DNS resolution)

NetworkManager can override traditional UNIX configurations. Check for these indicators:

$ systemctl status NetworkManager
$ journalctl -u NetworkManager --since "1 hour ago"

Watch for these common offenders:

• DHCP pushing custom DNS servers
• DNS caching services like systemd-resolved
• Conflicting resolv.conf management

Option 1: Hardcode DNS priorities

# Edit /etc/NetworkManager/NetworkManager.conf
[main]
dns=none
systemd-resolved=false

# Then restart
sudo systemctl restart NetworkManager

Option 2: Full manual control

sudo chattr +i /etc/resolv.conf # Make file immutable
sudo nano /etc/resolv.conf # Set your preferred nameservers

Use these diagnostic commands:

$ getent hosts test # Should show 127.0.0.1
$ systemd-resolve --statistics # Check caching behavior
$ dig +short test # Bypasses local resolution

For developers needing consistent local resolution during testing, consider adding this to your ~/.bashrc:

export RES_OPTIONS="rotate timeout:1 attempts:1"

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Recently, I encountered a frustrating issue on my Ubuntu desktop where the system seemed to completely ignore my /etc/hosts entries. While ping worked correctly, other tools like host and browser requests were resolving names through DNS instead.

Let's examine the relevant configuration files:

# /etc/hosts contents
127.0.0.1   localhost test
127.0.1.1   desktop

# /etc/nsswitch.conf (critical section)
hosts:          files mdns4_minimal [NOTFOUND=return] dns mdns4

The configuration appears correct at first glance - the files source is listed first in nsswitch.conf, which should prioritize /etc/hosts.

The issue stems from how modern Linux systems handle name resolution. Even with correct configurations, there are several layers of caching that might interfere:

1. nscd (Name Service Cache Daemon)
2. systemd-resolved
3. Application-level DNS caching

Here's how to properly troubleshoot and fix this issue:

# First, check if nscd is running
sudo systemctl status nscd

# If running, flush its cache
sudo nscd -i hosts

# Check systemd-resolved status
sudo systemctl status systemd-resolved

# Flush systemd-resolved cache
sudo systemd-resolve --flush-caches

# Verify resolution order
sudo getent hosts test

For systems using systemd-resolved (common in modern Ubuntu), create or modify:

# /etc/systemd/resolved.conf
[Resolve]
DNSStubListener=no

Then restart the service:

sudo systemctl restart systemd-resolved

If you prefer not to use systemd-resolved for DNS:

sudo ln -sf /run/systemd/resolve/resolv.conf /etc/resolv.conf

After making changes, verify with:

host test
dig test
getent hosts test

All three commands should now return the IP specified in your /etc/hosts file.

Remember to:

• Check for any custom network manager configurations
• Verify no applications have their own DNS caching
• Test after reboot to ensure changes persist