New feature: Default destination

README.md

@@ -30,6 +30,20 @@ https://udl.fdo.cr/?r=https://reservation.com/restaurants/silvestre
 
 This will bypass the limitation of Safari (embedded webview) that doesn't allow your Universal Links to trigger. You should now have a working "open in app" UX.
 
+### Default destination support
+If the `DEFAULT_DESTINATION` environment variable is set and no `r` query parameter is provided:
+
+- A request to the root path:
+
+    - `https://udl.fdo.cr/` will redirect to `DEFAULT_DESTINATION`
+
+- A request with a path:
+
+    - `https://udl.fdo.cr/restaurants/silvestre` will redirect to `DEFAULT_DESTINATION + "/restaurants/silvestre"`
+
+The original `?r=` format continues to work as before. If `DEFAULT_DESTINATION` is not set, behavior remains unchanged.
+
+
 `https://udl.fdo.cr` is a **public (free to use) UDL Server** instance for anyone to try out and use on your own. It has usage limits (throttling), which should be more than enough for most low-medium traffic websites.
 
 If this service adds value to you or your company please consider sponsoring me right here on GitHub. I offer different sponsor tiers too where I will host a private instance without usage limits for ensured reliability. [Read more about this on my profile](https://github.com/sponsors/fdocr) to support the OSS work I do on my free time.

spec/server_spec.cr

@@ -1,5 +1,25 @@
 require "./spec_helper"
 
+def with_default_destination(value)
+  original = ENV["DEFAULT_DESTINATION"]?
+  ENV["DEFAULT_DESTINATION"] = value
+  yield
+ensure
+  if original
+    ENV["DEFAULT_DESTINATION"] = original
+  else
+    ENV.delete("DEFAULT_DESTINATION")
+  end
+end
+
+def without_default_destination
+  original = ENV["DEFAULT_DESTINATION"]?
+  ENV.delete("DEFAULT_DESTINATION")
+  yield
+ensure
+  ENV["DEFAULT_DESTINATION"] = original if original
+end
+
 describe "UDL Server" do
   context "success" do
     it "redirects to the Target URI parameter if valid" do
@@ -16,6 +36,24 @@ describe "UDL Server" do
       response.headers["Location"].should eq(target_url)
     end
 
+    it "redirects to DEFAULT_DESTINATION when root path has no r parameter and DEFAULT_DESTINATION is set" do
+      with_default_destination("https://example.com") do
+        get "/"
+
+        response.status_code.should eq(302)
+        response.headers["Location"].should eq("https://example.com")
+      end
+    end
+
+    it "redirects to DEFAULT_DESTINATION + path when path has no r parameter and DEFAULT_DESTINATION is set" do
+      with_default_destination("https://example.com") do
+        get "/about"
+
+        response.status_code.should eq(302)
+        response.headers["Location"].should eq("https://example.com/about")
+      end
+    end
+
     it "populates apple-app-site-association file" do
       get "/.well-known/apple-app-site-association"
 
@@ -26,20 +64,24 @@ describe "UDL Server" do
   end
 
   context "failure" do
-    it "renders fallback page if target redirect not provided" do
-      get "/"
+    it "renders fallback page if target redirect not provided and DEFAULT_DESTINATION is not set" do
+      without_default_destination do
+        get "/"
 
-      response.status_code.should eq(200)
-      response.body.should contain("Something went wrong")
-      response.body.should contain("Check out the <a href=\"https://github.com/fdocr/udl-server#Troubleshooting\">README</a> for more details")
+        response.status_code.should eq(200)
+        response.body.should contain("Something went wrong")
+        response.body.should contain("Check out the <a href=\"https://github.com/fdocr/udl-server#Troubleshooting\">README</a> for more details")
+      end
     end
 
-    it "renders fallback page if requesting any other path" do
-      get "/about-us"
+    it "renders fallback page if requesting any other path and DEFAULT_DESTINATION is not set" do
+      without_default_destination do
+        get "/about-us"
 
-      response.status_code.should eq(200)
-      response.body.should contain("Something went wrong")
-      response.body.should contain("Check out the <a href=\"https://github.com/fdocr/udl-server#Troubleshooting\">README</a> for more details")
+        response.status_code.should eq(200)
+        response.body.should contain("Something went wrong")
+        response.body.should contain("Check out the <a href=\"https://github.com/fdocr/udl-server#Troubleshooting\">README</a> for more details")
+      end
     end
 
     it "renders fallback page if r parameter is an invalid URL" do

src/server.cr

@@ -13,7 +13,19 @@ error_context = "Use the root path instead, i.e. `/?r=TARGET_URL_HERE`"
 
 get "/" do |env|
   begin
-    target_uri = URI.parse(env.params.query["r"])
+    redirect_param = env.params.query["r"]?
+
+    # If no r parameter, redirect to DEFAULT_DESTINATION (only if set)
+    unless redirect_param
+      if default_destination = ENV["DEFAULT_DESTINATION"]?
+        env.redirect default_destination
+        next
+      else
+        raise "Missing redirect parameter"
+      end
+    end
+
+    target_uri = URI.parse(redirect_param)
 
     # Check that it's a valid URL
     valid_uri = /https?/ =~ target_uri.scheme && target_uri.host
@@ -48,8 +60,15 @@ get "/.well-known/apple-app-site-association" do |env|
 end
 
 get "/*" do |env|
-  udl_error = "Invalid path `#{env.request.path}` - #{error_context}"
-  render "src/views/fallback.ecr"
+  # If DEFAULT_DESTINATION is set, redirect to it + path; otherwise show fallback.
+  if default_target = ENV["DEFAULT_DESTINATION"]?
+    path = env.request.path
+    final_url = default_target.rstrip("/") + "/" + path.lstrip("/")
+    env.redirect final_url
+  else
+    udl_error = "Invalid path `#{env.request.path}` - #{error_context}"
+    render "src/views/fallback.ecr"
+  end
 end
 
 error 404 do