feat: add internalRewriteLocationHeader option

README.md

@@ -163,6 +163,14 @@ configuration passed to the route.
 
 Object with [reply options](https://github.com/fastify/fastify-reply-from#replyfromsource-opts) for `@fastify/reply-from`.
 
+### `internalRewriteLocationHeader`
+By default, `@fastify/http-proxy` will rewrite the `location` header when a request redirects to a relative path.
+In other words, the [prefix](https://github.com/fastify/fastify-http-proxy#prefix) will be added to the relative path.
+
+If you want to preserve the original path, this option will disable this internal operation. Default: `true`.
+
+Note that the [rewriteHeaders](https://github.com/fastify/fastify-reply-from#rewriteheadersheaders-request) option of [`@fastify/reply-from`](http://npm.im/fastify-reply-from) will retrieve headers modified (reminder: only `location` is updated among all headers) in parameter but with this option, the headers are unchanged.
+
 ### `httpMethods`
 An array that contains the types of the methods. Default: `['DELETE', 'GET', 'HEAD', 'PATCH', 'POST', 'PUT', 'OPTIONS']`.
 

index.js

@@ -234,6 +234,7 @@ async function fastifyHttpProxy (fastify, opts) {
   fromOpts.base = opts.upstream
   fromOpts.prefix = undefined
 
+  const internalRewriteLocationHeader = opts.internalRewriteLocationHeader ?? true
   const oldRewriteHeaders = (opts.replyOptions || {}).rewriteHeaders
   const replyOpts = Object.assign({}, opts.replyOptions, {
     rewriteHeaders
@@ -249,7 +250,7 @@ async function fastifyHttpProxy (fastify, opts) {
 
   function rewriteHeaders (headers, req) {
     const location = headers.location
-    if (location && !isExternalUrl(location)) {
+    if (location && !isExternalUrl(location) && internalRewriteLocationHeader) {
       headers.location = location.replace(rewritePrefix, fastify.prefix)
     }
     if (oldRewriteHeaders) {

test/test.js

@@ -29,6 +29,11 @@ async function run () {
     throw new Error('kaboom')
   })
 
+  origin.post('/redirect-to-relative-url', async (request, reply) => {
+    reply.header('location', '/relative-url')
+    return { status: 'ok' }
+  })
+
   origin.get('/api2/a', async (request, reply) => {
     return 'this is /api2/a'
   })
@@ -567,6 +572,32 @@ async function run () {
     t.equal(location, '/api/something')
   })
 
+  test('location headers is preserved when internalRewriteLocationHeader option is false', async t => {
+    const proxyServer = Fastify()
+
+    proxyServer.register(proxy, {
+      upstream: `http://localhost:${origin.server.address().port}`,
+      prefix: '/my-prefix',
+      internalRewriteLocationHeader: false
+    })
+
+    await proxyServer.listen({ port: 0 })
+
+    t.teardown(() => {
+      proxyServer.close()
+    })
+
+    const {
+      headers: { location }
+    } = await got(
+      `http://localhost:${proxyServer.server.address().port}/my-prefix/redirect-to-relative-url`,
+      {
+        method: 'POST'
+      }
+    )
+    t.equal(location, '/relative-url')
+  })
+
   test('passes onResponse option to reply.from() calls', async t => {
     const proxyServer = Fastify()