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Can You Use Roundrobin IPs in /etc/hosts? Explained

Posted on by William Zheng

Ever wondered how you can distribute network traffic evenly among multiple servers without setting up a complex DNS solution? Many turn to the /etc/hosts file, hoping for a simple round-robin setup.

This question is surprisingly common, especially when quick load balancing or fault tolerance is needed on local networks or test environments. Knowing the limitations and possibilities can save time and frustration.

In this article, you’ll find clear answers, practical steps, key insights, and tips for handling round-robin host entries.

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Can You Use Round-Robin IPs in /etc/hosts?

When managing applications or services on Linux or Windows, you may wonder if it’s possible to mimic DNS round-robin behavior using the /etc/hosts file by assigning multiple IP addresses to a single hostname. This is a common topic for those looking to implement simple load balancing or redundancy at the local resolver level.

Let’s uncover how hosts file resolution works, why standard round-robin balancing doesn’t occur here, and what your real options are for distributing traffic across multiple servers.

Understanding How /etc/hosts Works

The /etc/hosts file is a simple, local method for mapping hostnames to IP addresses. Each line ties an IP (or multiple IPs) to a hostname.

Example:

192.168.1.101  myservice
192.168.1.102  myservice

You can define the same hostname several times with different IPs or put several IPs on one line. But does this provide round-robin name resolution and basic load balancing? The answer is…it’s not that simple.

The Expectation: What Is Round-Robin DNS?

Round-robin DNS is a straightforward load balancing method where a DNS server responds to queries for a domain name with a list of IP addresses. Each time a client queries, the order of the IP addresses changes, distributing traffic more evenly across servers.

The intended effect:
Users connect to the same hostname, but requests are distributed across several IPs in sequence.

The Reality: /etc/hosts Does NOT Provide Round-Robin Behavior

Here are the key things to know:

  • The /etc/hosts file is parsed sequentially from top to bottom.
  • The name resolver (gethostbyname, getaddrinfo, etc.) will often return the first matching entry for a hostname.
  • Many applications and system functions only use the first IP they find for any given hostname.
  • Round-robin selection—a rotating response of IPs—is not performed by the OS when reading /etc/hosts.

Result:
Even if you list multiple IPs for the same hostname, connections will likely always go to the first IP listed. Standard /etc/hosts does not cycle or randomize the IP addresses per connection.

Testing and Observations

  1. Multiple Entries, Same Hostname:
    192.168.1.101 myservice
    192.168.1.102 myservice
    Most systems and tools will return 192.168.1.101 (the first entry) for myservice.

  2. Multiple IPs on One Line:
    192.168.1.101 192.168.1.102 myservice
    Only the first IP may be used for most client lookups—rarely both.

  3. Rationale:
    The /etc/hosts mechanism was designed for fixed, small networks, not for load balancing.

Why Applications Behave This Way

Applications rely on OS resolver libraries, and these typically:

  • Use the first IP found in /etc/hosts when resolving hostnames.
  • Ignore any additional addresses for the same hostname on subsequent lines or after the first on a single line.
  • Do not “rotate” or randomize like a DNS server configured for round-robin.

There are some exceptions:
– Some programming languages and custom scripts may retrieve a list of all associated IPs and try each in order (for example, Python’s socket.getaddrinfo()).
– Web browsers and common tools generally use the first.

Common Misconceptions

Let’s clear up a few myths:

  • Myth: Adding multiple IPs for a hostname in /etc/hosts creates load balancing.
  • Fact: Only the first IP is generally used by most applications.
  • Myth: The OS will round-robin through all IPs for a hostname if you list them on separate lines.
  • Fact: The resolver grabs the first match and stops looking.

What Are the Alternatives for Round-Robin or Load Balancing?

If you need real round-robin or load balancing, consider these approaches:

1. Proper Round-Robin DNS

  • Use a DNS server.
  • For your hostname (“myservice.example.com”), create multiple A records with different IPs.
  • DNS servers can be configured to rotate the order of returned IPs per query.
  • Most client OSes and applications do honor round-robin answers from DNS.

2. Load Balancer or Reverse Proxy

  • Use software or hardware solutions like HAProxy, NGINX, or dedicated balancers.
  • Distribute incoming requests to one of several backend IPs based on flexibility and algorithms.
  • Much more reliable and feature-rich for high-traffic or production environments.

3. Custom Client Behavior

  • If writing your own software, resolve all IPs for a hostname and cycle through them.
  • This only works if you control the client code.

4. OS-Specific or Network Workarounds

  • Some advanced configurations for enterprise OSs may offer more flexible host file resolution, but these are rare and often undocumented.

Practical Tips and Best Practices

  • Avoid using /etc/hosts for load balancing. The hosts file is best for static, local, or legacy mappings—not for distributing application load.
  • Keep /etc/hosts minimal. Unnecessary or sprawling entries make troubleshooting more difficult.
  • For redundancy, use a real DNS solution. If one server fails and all clients use one IP from /etc/hosts, you’ll have manual recovery headaches.
  • Test your configuration. If trying multiple IPs, use commands like getent hosts or application-specific methods to see which IP is actually used.
  • Don’t rely on undocumented behavior. Different OSes and resolver libraries may behave in subtly different ways.

Summary

While it’s technically possible to add multiple IP addresses for a single hostname in the /etc/hosts file, this does not result in round-robin or load-balanced resolution. The standard resolver stops at the first matching entry, and most applications use the first IP found. For true round-robin or load balancing, you need to use real DNS servers configured with multiple A records or use a load balancer.

The /etc/hosts file remains a simple and effective tool for static, single-IP name resolution. For distributed or balanced services, however, growth means looking beyond this file to DNS and more scalable load balancing solutions.

Frequently Asked Questions (FAQs)

1. Can I assign multiple IP addresses to one hostname in /etc/hosts?
Yes, you can list multiple lines with the same hostname or multiple IPs on a single line. However, most systems and applications will only use the first matching IP address listed in the file.

2. Will adding multiple IPs in /etc/hosts provide load balancing or failover?
No. The /etc/hosts file does not support round-robin or any load balancing. The name resolver will typically only use the first IP address it finds for a given hostname.

3. How can I achieve true round-robin or load-balanced hostname resolution?
Use a DNS server with multiple A records for your hostname. DNS servers can rotate the order of IPs when responding to queries, which distributes the load more evenly. Alternatively, use dedicated load balancing hardware or software.

4. Are there risks in putting many entries in /etc/hosts?
Yes. Large or complex /etc/hosts files can slow down name resolution, complicate troubleshooting, and may lead to configuration errors. This approach does not scale well and is discouraged for environments needing redundancy or high availability.

5. Is there any scenario where /etc/hosts should be used for managing multiple servers?
It can be used temporarily in small test setups, legacy systems, or situations where DNS is unavailable. For anything requiring scalability, resilience, or load distribution, rely on DNS servers and dedicated network appliances.

By understanding the limitations and purpose of the /etc/hosts file, you can better architect your systems for reliability, scalability, and maintainability. For anything beyond simple local hostname resolution, leverage the power of DNS and modern load balancing solutions.

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