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Upsun User Documentation

additional_hosts

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An additional hosts dictionary that maps hostnames to IP addresses.

Optional in single-runtime and composable images.

If you’re using a private network with specific IP addresses you need to connect to, you might want to map those addresses to hostnames to better remember and organize them. In such cases, you can add a map of those IP addresses to whatever hostnames you like. Then when your app tries to access the hostname, it’s sent to the proper IP address.

So in the following example, if your app tries to access api.example.com, it’s sent to 192.0.2.23.

.upsun/config.yaml 
applications:
  APP_NAME:
    type: 'php:8.5'
    source:
      root: "/"
    additional_hosts:
      api.example.com: "192.0.2.23"
      web.example.com: "203.0.113.42"
.upsun/config.yaml 
applications:
  APP_NAME:
    type: "composable:25.05"
    stack: 
      runtimes: [ "php@8.4" ]
    source:
      root: "/"
    additional_hosts:
      api.example.com: "192.0.2.23"
      web.example.com: "203.0.113.42"

This is equivalent to adding the mapping to the /etc/hosts file for the container.