In classic Unix/Linux systems, we typically see hostname resolution configured in /etc/hosts like this:

127.0.0.1   localhost myhostname

However, modern Debian-based distributions (including Ubuntu/Xubuntu) implement a different approach:

127.0.0.1   localhost
127.0.1.1   myhostname

The separation serves several technical purposes:

1. Name Resolution Order: DNS lookups prioritize 127.0.0.1 for 'localhost' while using 127.0.1.1 for the actual hostname
2. Systemd Compatibility: Maintains compatibility with systemd's hostnamed service
3. Network Configuration: Helps distinguish between purely local services and those bound to the hostname

While both configurations ultimately resolve to loopback addresses, the separation has real consequences:

# Test with curl to see the difference
curl http://localhost      # Uses 127.0.0.1
curl http://$(hostname)    # Uses 127.0.1.1

This becomes important when:

• Configuring web servers with virtual hosts
• Setting up container networking
• Developing applications that bind to specific interfaces

You might want to modify this setup in these scenarios:

# For systems without permanent network connectivity
127.0.0.1   localhost
127.0.0.1   myhostname

# For multi-homed systems
127.0.0.1   localhost
192.168.1.100   myhostname

If you experience resolution problems, check:

1. The output of hostname -f matches your /etc/hosts entry
2. No conflicting entries exist in /etc/hosts
3. The hostname resolves correctly with getent hosts $(hostname)

Remember that systemd-based systems may also check /etc/hostname and use systemd-resolved for additional resolution.

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In most Unix-like systems, the default /etc/hosts file typically contains:

127.0.0.1   localhost
::1         localhost ip6-localhost ip6-loopback

This maps both IPv4 (127.0.0.1) and IPv6 (::1) loopback addresses to 'localhost'. Many administrators manually add their hostname to this line:

127.0.0.1   localhost myhostname

Debian-based systems (including Ubuntu/Xubuntu) use a different convention:

127.0.0.1   localhost
127.0.1.1   myhostname

This separation serves several technical purposes:

The primary reasons for this design choice are:

1. Network Interface Binding: Some services bind specifically to 127.0.0.1. Using a separate address prevents potential conflicts.
2. DNS Fallback: When the system has no other network configuration, this ensures hostname resolution still works.
3. Historical Compatibility: Early Debian systems used this approach for systems without permanent IP addresses.

In most cases, both configurations will work similarly. However, consider these scenarios:

# Test hostname resolution
ping $(hostname)  # Should return 127.0.1.1 in Debian systems

For applications that specifically check IP addresses:

# Python example checking localhost
import socket
print(socket.gethostbyname('localhost'))  # Returns 127.0.0.1
print(socket.gethostbyname(socket.gethostname()))  # Returns 127.0.1.1

You might want to change this if:

• Running services that validate reverse DNS lookups
• Working with containers that expect different resolution
• Debugging network-related issues

To merge the entries (traditional style):

# Edit /etc/hosts
127.0.0.1   localhost myhostname
::1         localhost ip6-localhost ip6-loopback

Modern Linux systems using systemd may override these settings. Check:

systemd-resolve --status
# Or for older systems:
cat /etc/nsswitch.conf

Remember to restart networking after changes:

sudo systemctl restart systemd-networkd