Support Floodgate Global Linking

src/main/java/moe/yushi/authlibinjector/AuthlibInjector.java

@@ -44,6 +44,7 @@
 import java.util.stream.Stream;
 import moe.yushi.authlibinjector.httpd.AntiFeaturesFilter;
 import moe.yushi.authlibinjector.httpd.DefaultURLRedirector;
+import moe.yushi.authlibinjector.httpd.FloodgateGlobalLinkingFilter;
 import moe.yushi.authlibinjector.httpd.LegacySkinAPIFilter;
 import moe.yushi.authlibinjector.httpd.ProfileKeyFilter;
 import moe.yushi.authlibinjector.httpd.PublickeysFilter;
@@ -259,6 +260,11 @@ private static List<URLFilter> createFilters(APIMetadata config) {
 			filters.add(new ProfileKeyFilter());
 		}
 
+		boolean floodgateGlobalLinkingDefault = Boolean.TRUE.equals(config.getMeta().get("feature.floodgate_global_linking"));
+		if (floodgateGlobalLinkingDefault) {
+			filters.add(new FloodgateGlobalLinkingFilter(config.getApiRoot()));
+		}
+
 		filters.add(new PublickeysFilter());
 
 		return filters;

src/main/java/moe/yushi/authlibinjector/httpd/FloodgateGlobalLinkingFilter.java

@@ -0,0 +1,65 @@
+/*
+ * Copyright (C) 2026  Haowei Wen <yushijinhun@gmail.com> and contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package moe.yushi.authlibinjector.httpd;
+
+import static moe.yushi.authlibinjector.util.IOUtils.CONTENT_TYPE_TEXT;
+import static moe.yushi.authlibinjector.util.Logging.log;
+import static moe.yushi.authlibinjector.util.Logging.Level.WARNING;
+
+import java.io.IOException;
+import java.util.Optional;
+import java.util.regex.Pattern;
+
+import moe.yushi.authlibinjector.internal.fi.iki.elonen.IHTTPSession;
+import moe.yushi.authlibinjector.internal.fi.iki.elonen.Response;
+import moe.yushi.authlibinjector.internal.fi.iki.elonen.Status;
+
+/**
+ * Proxies Floodgate Global Linking query requests to the custom authentication server.
+ */
+public class FloodgateGlobalLinkingFilter implements URLFilter {
+
+	private static final String GEYSER_GLOBAL_API_DOMAIN = "api.geysermc.org";
+	private static final Pattern LINK_QUERY_PATH = Pattern.compile("^/v2/link/(bedrock|java)/[^/]+$");
+
+	private String apiRoot;
+
+	public FloodgateGlobalLinkingFilter(String apiRoot) {
+		this.apiRoot = apiRoot.endsWith("/") ? apiRoot : apiRoot + "/";
+	}
+
+	@Override
+	public boolean canHandle(String domain) {
+		return domain.equals(GEYSER_GLOBAL_API_DOMAIN);
+	}
+
+	@Override
+	public Optional<Response> handle(String domain, String path, IHTTPSession session) throws IOException {
+		if (!domain.equals(GEYSER_GLOBAL_API_DOMAIN) || !session.getMethod().equals("GET") || !LINK_QUERY_PATH.matcher(path).find()) {
+			return Optional.empty();
+		}
+
+		String target = apiRoot + "geyser" + path;
+
+		try {
+			return Optional.of(URLProcessor.reverseProxy(session, target));
+		} catch (IOException e) {
+			log(WARNING, "Failed to proxy Floodgate Global Linking request to [" + target + "]", e);
+			return Optional.of(Response.newFixedLength(Status.BAD_GATEWAY, CONTENT_TYPE_TEXT, "Bad Gateway"));
+		}
+	}
+}

src/main/java/moe/yushi/authlibinjector/httpd/URLProcessor.java

@@ -171,7 +171,7 @@ public Response serve(IHTTPSession session) {
 	private static final Set<String> ignoredHeaders = new HashSet<>(Arrays.asList("host", "expect", "connection", "keep-alive", "transfer-encoding"));
 
 	@SuppressWarnings("resource")
-	private Response reverseProxy(IHTTPSession session, String upstream) throws IOException {
+	static Response reverseProxy(IHTTPSession session, String upstream) throws IOException {
 		String method = session.getMethod();
 
 		String url = session.getQueryParameterString() == null ? upstream : upstream + "?" + session.getQueryParameterString();