https://blog.thisisjonathan.com/index.xml Recent content on Jonathan Lloyd | Blog Hugo -- gohugo.io en &copy; Jonathan Lloyd 2018 Sun, 22 Apr 2018 19:18:45 +0100 https://blog.thisisjonathan.com/post/why-you-cant-see-localhost-on-your-local-ip/ Sun, 22 Apr 2018 19:18:45 +0100 https://blog.thisisjonathan.com/post/why-you-cant-see-localhost-on-your-local-ip/ <p><em>Note: this post is based on experience with Linux.</em></p> <p>We&rsquo;ve all been there - you&rsquo;re working on a server on your local machine and need to show it to someone else on your local network. You grab your machine&rsquo;s IP using <code>netstat</code>, <code>ifconfig</code>, or similar and send it over. And&hellip; they can&rsquo;t connect.</p> <p>If there isn&rsquo;t an issue with your firewall settings, and you Google the right thing, you may find a StackOverflow post telling you that your server has to bind on <code>0.0.0.0</code> rather than <code>127.0.0.1</code>. But what does this mean and how does it work?</p> <h1 id="the-loopback-interface">The Loopback Interface</h1> <p>Network Interfaces are objects within the kernel that represent an entry point for the host on a particular network for sending/receiving packets. They can be used to represent physical Network Interface Cards but they can also be virtualised for other purposes.</p> <p>The purpose of the loopback interface, <code>lo</code>, is to give the host the ability to efficiently send packets to itself. The kernel implements this by returning packets straight back to the interface without going any deeper into the kernel&rsquo;s networking stack.</p> <p>When your server process binds to <code>127.0.0.1</code> it is bound to the <code>lo</code> Network Interface. This is why you cannot connect to it from your local network - the <code>lo</code> interface can only be reached from your local machine.</p> <h1 id="0-0-0-0-special-case">0.0.0.0 Special Case</h1> <p>If you can&rsquo;t connect to the server process from your local network when it is bound to <code>127.0.0.1</code>, what should you do instead? One option is to bind to your machine&rsquo;s address on your local network (<code>192.168.1.12</code> or whatever). However, it can be quite tedious to work out which interface is connected to your local network and the IP that has been assigned to that interface.</p> <p>Another option is to bind to an address that can be routed to from <em>all</em> Network Interfaces. This is what happens when you bind to <code>0.0.0.0</code>.</p> <p><code>0.0.0.0</code> is a non-routable address that, by convention, will receive packets from every Network Interface on the host.</p> <p>So, the next time you want to host something on your local network, remember to bind to <code>0.0.0.0</code>.</p>