# Host rules

{% hint style="success" %}
New in version 17: Read host rules from the OS' hosts file. See below for more details.
{% endhint %}

## What are the host rules?

Host rules assist your operating system to map a hostname to an IP address. Historically it was used in first computers before the first DNS (Domain Naming System) was created to tell the system what is the IP address of the remote machine while having its name only. Today, it is primarily used to alter host - IP address mapping to something different than DNS tells or to create a mapping that DNS is unaware of.

## Why would you need it in ARC?

While developing a web server or an API often the server is run locally or in the local network. In most cases, the IP address, that is well known to you, is enough to run a request. However, sometimes you need to test a configuration that depends on the value of the `host` header. This is quite common for servers that are hosting several virtual hosts.

Technically you can manually set the URL to an IP address and then put `host` header in the headers editor to test for virtual hosts. Another option is to alter the system's `hosts` file to create a mapping but this usually requires administrative privileges. In ARC, however, recommended way of doing this is to define custom host rules that map parts of your URL to some other value but keep the original URL to generate a host header when connecting to a server.

When you define a rule and the rule matches the URL of the currently executed request, the URL that is used to make a connection is replaced by the mapping value. This way, say, instead of connecting to `host.com` you would be connecting to `127.0.0` (localhost). However, when the HTTP message is constructed, the original URL value is used to set a value for the `host` header which in this example will be `host: host.com`

## Defining rules

From the menu Request select Hosts. A host rules mapping screen appears. Click on the add button (right bottom corner of the screen) to add a new entry.

![Empty rule in hosts rules mapping](https://2300023309-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LgJa7u7Cw8HjSR0oUlx%2F-LrWMpqz1d247BzlevvZ%2F-LrWRDvf0tvl5bMzEETP%2Fimage.png?alt=media\&token=d869152e-fe6a-40d5-8887-b39c6e82518a)

In the `from` field enter the part of the URL that should be replaced with another value. It doesn't have to be a hostname only. It can be a full URL or a part of it.

![Defining a rule in hosts mapping editor](https://2300023309-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LgJa7u7Cw8HjSR0oUlx%2F-LrWMpqz1d247BzlevvZ%2F-LrWRjICncS4Jz8zYps1%2Fimage.png?alt=media\&token=8079fa8e-8a1a-459e-89e8-761a808fa407)

In this example, the rules mapping maps `https://hosts.com` to `http://127.0.0.1`. Notice that we are not only changing the host but also the scheme.

Now you can test whether the mapping meets your expectations by running a request URL through the rules tester.

![Rules tester with result](https://2300023309-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LgJa7u7Cw8HjSR0oUlx%2F-LrWMpqz1d247BzlevvZ%2F-LrWSakrrYeb6BML791e%2Fimage.png?alt=media\&token=6f891b7b-8f4e-4745-b9a3-b51b225ccab2)

We are testing `https://host.com/index.html` URL against defined rules. The result shows that the expected result is `http://127.0.0.1/index.html`.

Now, we can run a request that uses `host.com` in the URL and the connection is made to the localhost.

![Request made to mapped location](https://2300023309-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LgJa7u7Cw8HjSR0oUlx%2F-LrWMpqz1d247BzlevvZ%2F-LrWTX6bcu1UtcBOz4zW%2Fimage.png?alt=media\&token=b3bf49f4-318d-4881-a0f4-9d98af357fbd)

Even though the request URL contains a non-existing URL the request returned data from my locally run server.

{% hint style="info" %}
All rules are evaluated in order from the first one to the last one. Already altered URL can be altered again by another rule.
{% endhint %}

## Examples

### Host-only mapping

From: `212.77.100.101` To: `127.0.0.1`

This will result in translating the following URL `https://212.77.100.101/index.html` into `https://127.0.0.1/index.html`.

### URI mapping

From: `http://domain.com/api` To: `http://127.0.0.1:8081`

This will result in translating the following URL `http://domain.com/api/endpoint/?query=something` into `https://127.0.0.1:8081/endpoint/?query=something`.

### With asterisk

From: `http://212.77.100.101/*/` To: `http://localhost/path/to/endpoint/`

This will result in translating the following URL `http://212.77.100.101/api/whathever/here/test?query=something` into `http://localhost/path/to/endpoint/test?query=something`.

## Reading OS' hosts file

Since version 17 you can configure the application to read the operating system's hosts file. These entries are translated into the same data as described above. You won't see the OS' hosts file entries in the host rules editor. They are read each time you send the request.

To enable the setting go to the application settings (ctrl/cmd + ,) and scroll to the Request section. Toggle the Read OS' hosts file option.

![Enabled option to read OS' hosts file](https://2300023309-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-LgJa7u7Cw8HjSR0oUlx%2Fuploads%2FcCxsigUCwGAFzXEpAngd%2Fimage.png?alt=media\&token=244fbda7-4ee2-42e5-9f24-f36cc3a51a1c)