Host Header Override with HTTPProxy

This guide shows how to override the Host header when forwarding requests to an upstream backend using a Datum HTTPProxy. This is useful when:

The upstream expects a specific Host header
You are proxying to a shared or virtual-hosted service
The backend’s DNS name does not match the public hostname

Overview

At a high level, this setup:

Accepts public traffic at a configured hostname
Forwards all requests to an upstream backend
Rewrites the Host header before the request reaches the upstream

Prerequisites

datumctl installed and authenticated
A valid Project

Configuration Steps

Step 1: Set Variables

Windows (PowerShell)

$PROJECT    = "your-project-id"
$NAMESPACE  = "default"
$PROXY_NAME = "host-header-demo"
$HOSTNAME   = "your-app.example.com"

macOS / Linux

PROJECT="your-project-id"
NAMESPACE="default"
PROXY_NAME="host-header-demo"
HOSTNAME="your-app.example.com"

Step 2: Apply Host Header Configuration

The RequestHeaderModifier filter overwrites the Host header before the request is forwarded upstream. The type field is required.

Windows (PowerShell)

@"
apiVersion: networking.datumapis.com/v1alpha
kind: HTTPProxy
metadata:
  name: $PROXY_NAME
spec:
  hostnames:
  - $HOSTNAME
  rules:
  - name: host-header-override
    matches:
    - path:
        type: PathPrefix
        value: /
    filters:
    - type: RequestHeaderModifier
      requestHeaderModifier:
        set:
        - name: Host
          value: example.internal
    backends:
    - endpoint: https://httpbin.org
"@ | datumctl apply --project $PROJECT --namespace $NAMESPACE -f -

macOS / Linux

cat <<EOF | datumctl apply --project $PROJECT --namespace $NAMESPACE -f -
apiVersion: networking.datumapis.com/v1alpha
kind: HTTPProxy
metadata:
  name: $PROXY_NAME
spec:
  hostnames:
  - $HOSTNAME
  rules:
  - name: host-header-override
    matches:
    - path:
        type: PathPrefix
        value: /
    filters:
    - type: RequestHeaderModifier
      requestHeaderModifier:
        set:
        - name: Host
          value: example.internal
    backends:
    - endpoint: https://httpbin.org
EOF

Verification

httpbin.org/headers echoes the request headers received by the upstream, making it easy to confirm the rewrite.

curl https://$HOSTNAME/headers

Look for Host in the response:

{
  "headers": {
    "Host": "example.internal",
    ...
  }
}

If Host shows example.internal, the rewrite is working correctly.

Cleanup

Windows (PowerShell)

datumctl delete httpproxy $PROXY_NAME `
  --project $PROJECT --namespace $NAMESPACE --ignore-not-found

macOS / Linux

datumctl delete httpproxy $PROXY_NAME \
  --project $PROJECT --namespace $NAMESPACE --ignore-not-found

Troubleshooting

Symptom	Root Cause	Resolution
API rejects the resource	`type` field missing from filter	Add `type: RequestHeaderModifier` to the filter
`Host` header not rewritten	Filter applied at wrong level	Confirm `filters` is under `rules[].filters`, not `backends[].filters`
Upstream returns 404 or 421	Upstream rejects the rewritten `Host` value	Verify the upstream accepts the hostname you configured
`Host` shows original value	`set` vs `add` confusion	Use `set` to overwrite; `add` appends to an existing value

Best Practices

Use set to overwrite the Host header — add appends to any existing value instead of replacing it
Keep the rewritten hostname consistent with what the upstream backend expects in its virtual host configuration
Test with httpbin.org/headers before pointing at a real upstream — it removes backend variables from the equation

Summary

Host header rewriting uses a RequestHeaderModifier filter on the rule’s filters list
The type: RequestHeaderModifier field is required — omitting it will cause the API to reject the resource
set overwrites the header; add appends
The rewrite happens at the proxy — the client sees no change to the URL or response

Documentation

TLDR

Platform

AI Edge

Connectors

Galactic VPC

Domains & DNS

Agents

Tools

Host Header Override with HTTPProxy

Overview

Prerequisites

Configuration Steps

Step 1: Set Variables

Windows (PowerShell)

macOS / Linux

Step 2: Apply Host Header Configuration

Windows (PowerShell)

macOS / Linux

Verification

Cleanup

Windows (PowerShell)

macOS / Linux

Troubleshooting

Best Practices

Summary

​Overview

​Prerequisites

​Configuration Steps

​Step 1: Set Variables

​Windows (PowerShell)

​macOS / Linux

​Step 2: Apply Host Header Configuration

​Windows (PowerShell)

​macOS / Linux

​Verification

​Cleanup

​Windows (PowerShell)

​macOS / Linux

​Troubleshooting

​Best Practices

​Summary

Overview

Prerequisites

Configuration Steps

Step 1: Set Variables

Windows (PowerShell)

macOS / Linux

Step 2: Apply Host Header Configuration

Windows (PowerShell)

macOS / Linux

Verification

Cleanup

Windows (PowerShell)

macOS / Linux

Troubleshooting

Best Practices

Summary