From 6d29b59a708b7ff4745049370c2b09c4a4c656eb Mon Sep 17 00:00:00 2001
From: Javi Velasco <javier.velasco86@gmail.com>
Date: Mon, 17 Jan 2022 19:19:50 +0100
Subject: [PATCH 1/9] Add URL behaviour tests

---
 .../middleware/core/pages/urls/_middleware.js | 35 ++++++++++
 .../middleware/core/pages/urls/index.js       |  3 +
 .../middleware/core/pages/urls/urls-b.js      |  3 +
 .../middleware/core/test/index.test.js        | 67 +++++++++++++++++++
 .../middleware/hmr/pages/about/_middleware.js |  4 +-
 5 files changed, 110 insertions(+), 2 deletions(-)
 create mode 100644 test/integration/middleware/core/pages/urls/_middleware.js
 create mode 100644 test/integration/middleware/core/pages/urls/index.js
 create mode 100644 test/integration/middleware/core/pages/urls/urls-b.js

diff --git a/test/integration/middleware/core/pages/urls/_middleware.js b/test/integration/middleware/core/pages/urls/_middleware.js
new file mode 100644
index 0000000000000..43efc371d2e6d
--- /dev/null
+++ b/test/integration/middleware/core/pages/urls/_middleware.js
@@ -0,0 +1,35 @@
+import { NextResponse, NextRequest } from 'next/server'
+
+export function middleware(request) {
+  try {
+    if (request.nextUrl.pathname === '/urls/relative-url') {
+      return NextResponse.json({ message: String(new URL('/relative')) })
+    }
+
+    if (request.nextUrl.pathname === '/urls/relative-request') {
+      return fetch(new Request('/urls/urls-b'))
+    }
+
+    if (request.nextUrl.pathname === '/urls/relative-redirect') {
+      return Response.redirect('/urls/urls-b')
+    }
+
+    if (request.nextUrl.pathname === '/urls/relative-next-redirect') {
+      return NextResponse.redirect('/urls/urls-b')
+    }
+
+    if (request.nextUrl.pathname === '/urls/relative-next-rewrite') {
+      return NextResponse.rewrite('/urls/urls-b')
+    }
+
+    if (request.nextUrl.pathname === '/urls/relative-next-request') {
+      return fetch(new NextRequest('/urls/urls-b'))
+    }
+  } catch (error) {
+    return NextResponse.json({
+      error: {
+        message: error.message,
+      },
+    })
+  }
+}
diff --git a/test/integration/middleware/core/pages/urls/index.js b/test/integration/middleware/core/pages/urls/index.js
new file mode 100644
index 0000000000000..a1a42f2f39375
--- /dev/null
+++ b/test/integration/middleware/core/pages/urls/index.js
@@ -0,0 +1,3 @@
+export default function URLsA() {
+  return <p className="title">URLs A</p>
+}
diff --git a/test/integration/middleware/core/pages/urls/urls-b.js b/test/integration/middleware/core/pages/urls/urls-b.js
new file mode 100644
index 0000000000000..19326a7b8d8ac
--- /dev/null
+++ b/test/integration/middleware/core/pages/urls/urls-b.js
@@ -0,0 +1,3 @@
+export default function URLsB() {
+  return <p className="title">URLs B</p>
+}
diff --git a/test/integration/middleware/core/test/index.test.js b/test/integration/middleware/core/test/index.test.js
index 25f4e01634ab3..e90aa64045807 100644
--- a/test/integration/middleware/core/test/index.test.js
+++ b/test/integration/middleware/core/test/index.test.js
@@ -43,6 +43,8 @@ describe('Middleware base tests', () => {
     responseTests('/fr')
     interfaceTests()
     interfaceTests('/fr')
+    urlTests()
+    urlTests('/fr')
 
     it('should have showed warning for middleware usage', () => {
       expect(output).toContain(middlewareWarning)
@@ -78,6 +80,8 @@ describe('Middleware base tests', () => {
     responseTests('/fr')
     interfaceTests()
     interfaceTests('/fr')
+    urlTests()
+    urlTests('/fr')
 
     it('should have middleware warning during build', () => {
       expect(buildOutput).toContain(middlewareWarning)
@@ -104,6 +108,69 @@ describe('Middleware base tests', () => {
   })
 })
 
+function urlTests(locale = '') {
+  it('rewrites by default to a target location', async () => {
+    const res = await fetchViaHTTP(context.appPort, `${locale}/urls`)
+    const html = await res.text()
+    const $ = cheerio.load(html)
+    expect($('.title').text()).toBe('URLs A')
+  })
+
+  it('throws when using URL with a relative URL', async () => {
+    const res = await fetchViaHTTP(
+      context.appPort,
+      `${locale}/urls/relative-url`
+    )
+    const json = await res.json()
+    expect(json.error.message).toEqual('Invalid URL')
+  })
+
+  it('throws when using Request with a relative URL', async () => {
+    const res = await fetchViaHTTP(
+      context.appPort,
+      `${locale}/urls/relative-request`
+    )
+    const json = await res.json()
+    expect(json.error.message).toEqual('Invalid URL')
+  })
+
+  it('throws when using Response.redirect with a relative URL', async () => {
+    const res = await fetchViaHTTP(
+      context.appPort,
+      `${locale}/urls/relative-redirect`
+    )
+    const json = await res.json()
+    expect(json.error.message).toEqual('Invalid URL')
+  })
+
+  it('throws when using NextRequest with a relative URL', async () => {
+    const res = await fetchViaHTTP(
+      context.appPort,
+      `${locale}/urls/relative-next-request`
+    )
+    const json = await res.json()
+    expect(json.error.message).toEqual('Invalid URL')
+  })
+
+  it('throws when using NextResponse.redirect with a relative URL', async () => {
+    const res = await fetchViaHTTP(
+      context.appPort,
+      `${locale}/urls/relative-next-redirect`
+    )
+    const json = await res.json()
+    expect(json.error.message).toEqual('Invalid URL')
+  })
+
+  it('throws when using NextResponse.rewrite with a relative URL', async () => {
+    const res = await fetchViaHTTP(
+      context.appPort,
+      `${locale}/urls/relative-next-rewrite`
+    )
+    const json = await res.json()
+    expect(json.error.message).toEqual('Invalid URL')
+  })
+}
+
 function rewriteTests(locale = '') {
   it('should rewrite to fallback: true page successfully', async () => {
     const randomSlug = `another-${Date.now()}`
diff --git a/test/integration/middleware/hmr/pages/about/_middleware.js b/test/integration/middleware/hmr/pages/about/_middleware.js
index 64dba899b58b9..66902bd191d7f 100644
--- a/test/integration/middleware/hmr/pages/about/_middleware.js
+++ b/test/integration/middleware/hmr/pages/about/_middleware.js
@@ -1,5 +1,5 @@
 import { NextResponse } from 'next/server'
 
-export function middleware() {
-  return NextResponse.rewrite('/about/a')
+export function middleware(request) {
+  return NextResponse.rewrite(new URL('/about/a', request.url))
 }

From 67350857acf3f5a7d2e969af0ca80b6657bf28e9 Mon Sep 17 00:00:00 2001
From: Javi Velasco <javier.velasco86@gmail.com>
Date: Mon, 17 Jan 2022 19:20:11 +0100
Subject: [PATCH 2/9] Make the runtime work only with absolute URLs

---
 packages/next/server/web/spec-compliant/response.ts | 2 +-
 packages/next/server/web/spec-extension/response.ts | 8 +++-----
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/packages/next/server/web/spec-compliant/response.ts b/packages/next/server/web/spec-compliant/response.ts
index c7d5c746e9316..3991c8ecef80c 100644
--- a/packages/next/server/web/spec-compliant/response.ts
+++ b/packages/next/server/web/spec-compliant/response.ts
@@ -45,7 +45,7 @@ class BaseResponse extends Body implements Response {
       )
     }
 
-    return new Response(url, {
+    return new Response(String(new URL(url)), {
       headers: { Location: url },
       status,
     })
diff --git a/packages/next/server/web/spec-extension/response.ts b/packages/next/server/web/spec-extension/response.ts
index 76b40a8490407..027bfaf7ac530 100644
--- a/packages/next/server/web/spec-extension/response.ts
+++ b/packages/next/server/web/spec-extension/response.ts
@@ -79,7 +79,8 @@ export class NextResponse extends Response {
         'Failed to execute "redirect" on "response": Invalid status code'
       )
     }
-    const destination = typeof url === 'string' ? url : url.toString()
+
+    const destination = String(new URL(String(url)))
     return new NextResponse(destination, {
       headers: { Location: destination },
       status,
@@ -89,10 +90,7 @@ export class NextResponse extends Response {
   static rewrite(destination: string | NextURL) {
     return new NextResponse(null, {
       headers: {
-        'x-middleware-rewrite':
-          typeof destination === 'string'
-            ? destination
-            : destination.toString(),
+        'x-middleware-rewrite': String(new URL(String(destination))),
       },
     })
   }

From b80aa39c1cbdf66d191bdb1a1eb9ff3bd6ecba4c Mon Sep 17 00:00:00 2001
From: Javi Velasco <javier.velasco86@gmail.com>
Date: Mon, 17 Jan 2022 19:20:20 +0100
Subject: [PATCH 3/9] Fix remaining tests

---
 .../middleware/core/pages/interface/_middleware.js  |  3 ++-
 .../middleware/core/pages/redirects/_middleware.js  |  2 +-
 .../middleware/core/pages/rewrites/_middleware.js   | 13 ++++++++-----
 3 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/test/integration/middleware/core/pages/interface/_middleware.js b/test/integration/middleware/core/pages/interface/_middleware.js
index 81b57cba1fb4d..1aee8917ed5c9 100644
--- a/test/integration/middleware/core/pages/interface/_middleware.js
+++ b/test/integration/middleware/core/pages/interface/_middleware.js
@@ -81,7 +81,8 @@ export async function middleware(request) {
   }
 
   if (url.pathname.endsWith('/dynamic-replace')) {
-    return NextResponse.rewrite('/_interface/dynamic-path')
+    url.pathname = '/_interface/dynamic-path'
+    return NextResponse.rewrite(url)
   }
 
   return new Response(null, {
diff --git a/test/integration/middleware/core/pages/redirects/_middleware.js b/test/integration/middleware/core/pages/redirects/_middleware.js
index e97cb99011b04..0a8dafc5c8b26 100644
--- a/test/integration/middleware/core/pages/redirects/_middleware.js
+++ b/test/integration/middleware/core/pages/redirects/_middleware.js
@@ -56,6 +56,6 @@ export async function middleware(request) {
 
   if (url.pathname === '/redirects/infinite-loop-1') {
     url.pathname = '/redirects/infinite-loop'
-    return Response.redirect(url.pathname)
+    return Response.redirect(url)
   }
 }
diff --git a/test/integration/middleware/core/pages/rewrites/_middleware.js b/test/integration/middleware/core/pages/rewrites/_middleware.js
index 13d4453938251..20113e2978d16 100644
--- a/test/integration/middleware/core/pages/rewrites/_middleware.js
+++ b/test/integration/middleware/core/pages/rewrites/_middleware.js
@@ -5,24 +5,27 @@ export async function middleware(request) {
 
   if (url.pathname.startsWith('/rewrites/to-blog')) {
     const slug = url.pathname.split('/').pop()
-    console.log('rewriting to slug', slug)
-    return NextResponse.rewrite(`/rewrites/fallback-true-blog/${slug}`)
+    url.pathname = `/rewrites/fallback-true-blog/${slug}`
+    return NextResponse.rewrite(url)
   }
 
   if (url.pathname === '/rewrites/rewrite-to-ab-test') {
     let bucket = request.cookies.bucket
     if (!bucket) {
       bucket = Math.random() >= 0.5 ? 'a' : 'b'
-      const response = NextResponse.rewrite(`/rewrites/${bucket}`)
+      url.pathname = `/rewrites/${bucket}`
+      const response = NextResponse.rewrite(url)
       response.cookie('bucket', bucket, { maxAge: 10000 })
       return response
     }
 
-    return NextResponse.rewrite(`/rewrites/${bucket}`)
+    url.pathname = `/rewrites/${bucket}`
+    return NextResponse.rewrite(url)
   }
 
   if (url.pathname === '/rewrites/rewrite-me-to-about') {
-    return NextResponse.rewrite('/rewrites/about')
+    url.pathname = '/rewrites/about'
+    return NextResponse.rewrite(url)
   }
 
   if (url.pathname === '/rewrites/rewrite-me-to-vercel') {

From 6dea23980fd93f28d865c35374de712406399934 Mon Sep 17 00:00:00 2001
From: Javi Velasco <javier.velasco86@gmail.com>
Date: Wed, 19 Jan 2022 11:42:07 +0100
Subject: [PATCH 4/9] Make tests pass for all node versions

---
 test/integration/middleware/core/test/index.test.js | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/test/integration/middleware/core/test/index.test.js b/test/integration/middleware/core/test/index.test.js
index e90aa64045807..ca6a8f6e277f6 100644
--- a/test/integration/middleware/core/test/index.test.js
+++ b/test/integration/middleware/core/test/index.test.js
@@ -122,7 +122,7 @@ function urlTests(locale = '') {
       `${locale}/urls/relative-url`
     )
     const json = await res.json()
-    expect(json.error.message).toEqual('Invalid URL')
+    expect(json.error.message).toContain('Invalid URL')
   })
 
   it('throws when using Request with a relative URL', async () => {
@@ -131,7 +131,7 @@ function urlTests(locale = '') {
       `${locale}/urls/relative-request`
     )
     const json = await res.json()
-    expect(json.error.message).toEqual('Invalid URL')
+    expect(json.error.message).toContain('Invalid URL')
   })
 
   it('throws when using Response.redirect with a relative URL', async () => {
@@ -140,7 +140,7 @@ function urlTests(locale = '') {
       `${locale}/urls/relative-redirect`
     )
     const json = await res.json()
-    expect(json.error.message).toEqual('Invalid URL')
+    expect(json.error.message).toContain('Invalid URL')
   })
 
   it('throws when using NextRequest with a relative URL', async () => {
@@ -149,7 +149,7 @@ function urlTests(locale = '') {
       `${locale}/urls/relative-next-request`
     )
     const json = await res.json()
-    expect(json.error.message).toEqual('Invalid URL')
+    expect(json.error.message).toContain('Invalid URL')
   })
 
   it('throws when using NextResponse.redirect with a relative URL', async () => {
@@ -158,7 +158,7 @@ function urlTests(locale = '') {
       `${locale}/urls/relative-next-redirect`
     )
     const json = await res.json()
-    expect(json.error.message).toEqual('Invalid URL')
+    expect(json.error.message).toContain('Invalid URL')
   })
 
   it('throws when using NextResponse.rewrite with a relative URL', async () => {
@@ -167,7 +167,7 @@ function urlTests(locale = '') {
       `${locale}/urls/relative-next-rewrite`
     )
     const json = await res.json()
-    expect(json.error.message).toEqual('Invalid URL')
+    expect(json.error.message).toContain('Invalid URL')
   })
 }
 

From e7066d95b05988c0f792a8e3111a31ea087d2fce Mon Sep 17 00:00:00 2001
From: Javi Velasco <javier.velasco86@gmail.com>
Date: Wed, 19 Jan 2022 13:14:22 +0100
Subject: [PATCH 5/9] Update URL tests to show a warning

---
 .../middleware/core/test/index.test.js        | 62 ++++++++-----------
 1 file changed, 26 insertions(+), 36 deletions(-)

diff --git a/test/integration/middleware/core/test/index.test.js b/test/integration/middleware/core/test/index.test.js
index ca6a8f6e277f6..32e2b99c2f4a2 100644
--- a/test/integration/middleware/core/test/index.test.js
+++ b/test/integration/middleware/core/test/index.test.js
@@ -19,18 +19,20 @@ const context = {}
 context.appDir = join(__dirname, '../')
 
 const middlewareWarning = 'using beta Middleware (not covered by semver)'
+const urlsWarning = 'using relative URLs for Middleware will be deprecated soon'
 
 describe('Middleware base tests', () => {
   describe('dev mode', () => {
-    let output = ''
+    const log = { output: '' }
+
     beforeAll(async () => {
       context.appPort = await findPort()
       context.app = await launchApp(context.appDir, context.appPort, {
         onStdout(msg) {
-          output += msg
+          log.output += msg
         },
         onStderr(msg) {
-          output += msg
+          log.output += msg
         },
       })
     })
@@ -43,16 +45,16 @@ describe('Middleware base tests', () => {
     responseTests('/fr')
     interfaceTests()
     interfaceTests('/fr')
-    urlTests()
-    urlTests('/fr')
+    urlTests(log)
+    urlTests(log, '/fr')
 
     it('should have showed warning for middleware usage', () => {
-      expect(output).toContain(middlewareWarning)
+      expect(log.output).toContain(middlewareWarning)
     })
   })
   describe('production mode', () => {
+    let serverOutput = { output: '' }
     let buildOutput
-    let serverOutput
 
     beforeAll(async () => {
       const res = await nextBuild(context.appDir, undefined, {
@@ -64,10 +66,10 @@ describe('Middleware base tests', () => {
       context.appPort = await findPort()
       context.app = await nextStart(context.appDir, context.appPort, {
         onStdout(msg) {
-          serverOutput += msg
+          serverOutput.output += msg
         },
         onStderr(msg) {
-          serverOutput += msg
+          serverOutput.output += msg
         },
       })
     })
@@ -80,15 +82,15 @@ describe('Middleware base tests', () => {
     responseTests('/fr')
     interfaceTests()
     interfaceTests('/fr')
-    urlTests()
-    urlTests('/fr')
+    urlTests(serverOutput)
+    urlTests(serverOutput, '/fr')
 
     it('should have middleware warning during build', () => {
       expect(buildOutput).toContain(middlewareWarning)
     })
 
     it('should have middleware warning during start', () => {
-      expect(serverOutput).toContain(middlewareWarning)
+      expect(serverOutput.output).toContain(middlewareWarning)
     })
 
     it('should have correct files in manifest', async () => {
@@ -108,7 +110,7 @@ describe('Middleware base tests', () => {
   })
 })
 
-function urlTests(locale = '') {
+function urlTests(log, locale = '') {
   it('rewrites by default to a target location', async () => {
     const res = await fetchViaHTTP(context.appPort, `${locale}/urls`)
     const html = await res.text()
@@ -134,15 +136,6 @@ function urlTests(locale = '') {
     expect(json.error.message).toContain('Invalid URL')
   })
 
-  it('throws when using Response.redirect with a relative URL', async () => {
-    const res = await fetchViaHTTP(
-      context.appPort,
-      `${locale}/urls/relative-redirect`
-    )
-    const json = await res.json()
-    expect(json.error.message).toContain('Invalid URL')
-  })
-
   it('throws when using NextRequest with a relative URL', async () => {
     const res = await fetchViaHTTP(
       context.appPort,
@@ -152,22 +145,19 @@ function urlTests(locale = '') {
     expect(json.error.message).toContain('Invalid URL')
   })
 
-  it('throws when using NextResponse.redirect with a relative URL', async () => {
-    const res = await fetchViaHTTP(
-      context.appPort,
-      `${locale}/urls/relative-next-redirect`
-    )
-    const json = await res.json()
-    expect(json.error.message).toContain('Invalid URL')
+  it('warns when using Response.redirect with a relative URL', async () => {
+    await fetchViaHTTP(context.appPort, `${locale}/urls/relative-redirect`)
+    expect(log.output).toContain(urlsWarning)
   })
 
-  it('throws when using NextResponse.rewrite with a relative URL', async () => {
-    const res = await fetchViaHTTP(
-      context.appPort,
-      `${locale}/urls/relative-next-rewrite`
-    )
-    const json = await res.json()
-    expect(json.error.message).toContain('Invalid URL')
+  it('warns when using NextResponse.redirect with a relative URL', async () => {
+    await fetchViaHTTP(context.appPort, `${locale}/urls/relative-next-redirect`)
+    expect(log.output).toContain(urlsWarning)
+  })
+
+  it('warns when using NextResponse.rewrite with a relative URL', async () => {
+    await fetchViaHTTP(context.appPort, `${locale}/urls/relative-next-rewrite`)
+    expect(log.output).toContain(urlsWarning)
   })
 }
 

From 45d50e773af2af2acfce2ea7f7563dce8c19f50a Mon Sep 17 00:00:00 2001
From: Javi Velasco <javier.velasco86@gmail.com>
Date: Wed, 19 Jan 2022 13:37:43 +0100
Subject: [PATCH 6/9] Add clone method for NextURL

---
 packages/next/server/web/next-url.ts   |  4 ++++
 test/unit/web-runtime/next-url.test.ts | 18 ++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/packages/next/server/web/next-url.ts b/packages/next/server/web/next-url.ts
index 8ee57ff66a6cb..532b93e64ab38 100644
--- a/packages/next/server/web/next-url.ts
+++ b/packages/next/server/web/next-url.ts
@@ -244,6 +244,10 @@ export class NextURL {
   toJSON() {
     return this.href
   }
+
+  clone() {
+    return new NextURL(String(this), this[Internal].options)
+  }
 }
 
 const REGEX_LOCALHOST_HOSTNAME =
diff --git a/test/unit/web-runtime/next-url.test.ts b/test/unit/web-runtime/next-url.test.ts
index e3914a938e920..ed13e63eb217f 100644
--- a/test/unit/web-runtime/next-url.test.ts
+++ b/test/unit/web-runtime/next-url.test.ts
@@ -170,3 +170,21 @@ it('allows to change the port', () => {
   url.port = ''
   expect(url.href).toEqual('https://localhost/foo')
 })
+
+it('allows to clone a new copy', () => {
+  const url = new NextURL('/root/es/bar', {
+    base: 'http://127.0.0.1',
+    basePath: '/root',
+    i18n: {
+      defaultLocale: 'en',
+      locales: ['en', 'es', 'fr'],
+    },
+  })
+
+  const clone = url.clone()
+  clone.pathname = '/test'
+  clone.basePath = '/root-test'
+
+  expect(url.toString()).toEqual('http://localhost/root/es/bar')
+  expect(clone.toString()).toEqual('http://localhost/root-test/es/test')
+})

From 3ef00507abd63993de9f32b829d693c4502bea34 Mon Sep 17 00:00:00 2001
From: Javi Velasco <javier.velasco86@gmail.com>
Date: Wed, 19 Jan 2022 13:45:18 +0100
Subject: [PATCH 7/9] Add a util to warn on relative URLs

---
 errors/manifest.json               |  4 ++++
 errors/middleware-relative-urls.md | 38 ++++++++++++++++++++++++++++++
 packages/next/server/web/utils.ts  | 17 +++++++++++++
 3 files changed, 59 insertions(+)
 create mode 100644 errors/middleware-relative-urls.md

diff --git a/errors/manifest.json b/errors/manifest.json
index 4aaeead555a5a..69d0763077574 100644
--- a/errors/manifest.json
+++ b/errors/manifest.json
@@ -527,6 +527,10 @@
         {
           "title": "invalid-styled-jsx-children",
           "path": "/errors/invalid-styled-jsx-children.md"
+        },
+        {
+          "title": "middleware-relative-urls",
+          "path": "/errors/middleware-relative-urls.md"
         }
       ]
     }
diff --git a/errors/middleware-relative-urls.md b/errors/middleware-relative-urls.md
new file mode 100644
index 0000000000000..db832e3faea98
--- /dev/null
+++ b/errors/middleware-relative-urls.md
@@ -0,0 +1,38 @@
+# Middleware Relative URLs
+
+#### Why This Error Occurred
+
+You are using a Middleware function that uses `Response.redirect(url)`, `NextResponse.redirect(url)` or `NextResponse.rewrite(url)` where `url` is a relative or an invalid URL. Currently this will work, but building a request with `new Request(url)` or running `fetch(url)` when `url` is a relative URL will **not** work. For this reason and to bring consistency to Next.js Middleware, this behavior will be deprecated soon in favor of always using absolute URLs.
+
+#### Possible Ways to Fix It
+
+To fix this warning you must always pass absolute URL for redirecting and rewriting. There are several ways to get the absolute URL but the recommended way is to clone `NextURL` and mutate it:
+
+```typescript
+import type { NextRequest } from 'next/server'
+import { NextResponse } from 'next/server'
+
+export function middleware(request: NextRequest) {
+  const url = request.nextUrl.clone()
+  url.pathname = '/dest'
+  return NextResponse.rewrite(url)
+}
+```
+
+Another way to fix this error could be to use the original URL as the base but this will not consider configuration like `basePath` or `locale`:
+
+```typescript
+import type { NextRequest } from 'next/server'
+import { NextResponse } from 'next/server'
+
+export function middleware(request: NextRequest) {
+  return NextResponse.rewrite(new URL('/dest', request.url))
+}
+```
+
+Or you can simply pass a string containing a valid absolute URL.
+
+### Useful Links
+
+- [URL Documentation](https://developer.mozilla.org/en-US/docs/Web/API/URL)
+- [Response Documentation](https://developer.mozilla.org/en-US/docs/Web/API/Response)
diff --git a/packages/next/server/web/utils.ts b/packages/next/server/web/utils.ts
index 2c5dae0e83dcd..06e257457f09d 100644
--- a/packages/next/server/web/utils.ts
+++ b/packages/next/server/web/utils.ts
@@ -147,3 +147,20 @@ export function splitCookiesString(cookiesString: string) {
 
   return cookiesStrings
 }
+
+/**
+ * We will be soon deprecating the usage of relative URLs in Middleware introducing
+ * URL validation. This helper puts the future code in place and prints a warning
+ * for cases where it will break. Meanwhile we preserve the previous behavior.
+ */
+export function validateURL(url: string | URL): string {
+  try {
+    return String(new URL(String(url)))
+  } catch (error: any) {
+    console.log(
+      `warn  -`,
+      'using relative URLs for Middleware will be deprecated soon - https://nextjs.org/docs/messages/middleware-relative-urls'
+    )
+    return String(url)
+  }
+}

From 743f3241d09c6855b2a0458b4969223448ae7430 Mon Sep 17 00:00:00 2001
From: Javi Velasco <javier.velasco86@gmail.com>
Date: Wed, 19 Jan 2022 13:45:36 +0100
Subject: [PATCH 8/9] Roll back to warn instead of failing

---
 packages/next/server/web/spec-compliant/response.ts | 3 ++-
 packages/next/server/web/spec-extension/response.ts | 6 +++---
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/packages/next/server/web/spec-compliant/response.ts b/packages/next/server/web/spec-compliant/response.ts
index 3991c8ecef80c..3ea1a55650f6b 100644
--- a/packages/next/server/web/spec-compliant/response.ts
+++ b/packages/next/server/web/spec-compliant/response.ts
@@ -1,5 +1,6 @@
 import { Body, BodyInit, cloneBody, extractContentType } from './body'
 import { NextURL } from '../next-url'
+import { validateURL } from '../utils'
 
 const INTERNALS = Symbol('internal response')
 const REDIRECTS = new Set([301, 302, 303, 307, 308])
@@ -45,7 +46,7 @@ class BaseResponse extends Body implements Response {
       )
     }
 
-    return new Response(String(new URL(url)), {
+    return new Response(validateURL(url), {
       headers: { Location: url },
       status,
     })
diff --git a/packages/next/server/web/spec-extension/response.ts b/packages/next/server/web/spec-extension/response.ts
index 027bfaf7ac530..240096c3bd30d 100644
--- a/packages/next/server/web/spec-extension/response.ts
+++ b/packages/next/server/web/spec-extension/response.ts
@@ -1,6 +1,6 @@
 import type { I18NConfig } from '../../config-shared'
 import { NextURL } from '../next-url'
-import { toNodeHeaders } from '../utils'
+import { toNodeHeaders, validateURL } from '../utils'
 import cookie from 'next/dist/compiled/cookie'
 import { CookieSerializeOptions } from '../types'
 
@@ -80,7 +80,7 @@ export class NextResponse extends Response {
       )
     }
 
-    const destination = String(new URL(String(url)))
+    const destination = validateURL(url)
     return new NextResponse(destination, {
       headers: { Location: destination },
       status,
@@ -90,7 +90,7 @@ export class NextResponse extends Response {
   static rewrite(destination: string | NextURL) {
     return new NextResponse(null, {
       headers: {
-        'x-middleware-rewrite': String(new URL(String(destination))),
+        'x-middleware-rewrite': validateURL(destination),
       },
     })
   }

From 1b86acab00549284667a42e359637e36b67178b1 Mon Sep 17 00:00:00 2001
From: Javi Velasco <javier.velasco86@gmail.com>
Date: Wed, 19 Jan 2022 13:57:23 +0100
Subject: [PATCH 9/9] Fix lint warning

---
 errors/middleware-relative-urls.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/errors/middleware-relative-urls.md b/errors/middleware-relative-urls.md
index db832e3faea98..dabdf33060855 100644
--- a/errors/middleware-relative-urls.md
+++ b/errors/middleware-relative-urls.md
@@ -30,7 +30,7 @@ export function middleware(request: NextRequest) {
 }
 ```
 
-Or you can simply pass a string containing a valid absolute URL.
+You can also pass directly a string containing a valid absolute URL.
 
 ### Useful Links