Introduce a utility class for working with forwarding chains in HTTP headers.

service/src/main/java/org/whispersystems/textsecuregcm/util/ForwardedIpUtil.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.util;
+
+import org.apache.commons.lang3.StringUtils;
+import java.util.Optional;
+
+/**
+ * Tools for working with chains of IP addresses in forwarding lists in HTTP headers.
+ *
+ * @see <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-For">X-Forwarded-For - HTTP | MDN</a>
+ */
+public class ForwardedIpUtil {
+
+  /**
+   * Returns the most recent proxy in a chain described by an {@code X-Forwarded-For} header.
+   *
+   * @param forwardedFor the value of an X-Forwarded-For header
+   *
+   * @return the IP address of the most recent proxy in the forwarding chain, or empty if none was found or
+   * {@code forwardedFor} was null
+   */
+  public static Optional<String> getMostRecentProxy(final String forwardedFor) {
+    return Optional.ofNullable(forwardedFor)
+        .filter(StringUtils::isNotBlank)
+        .map(proxies -> proxies.split(","))
+        .map(proxyArray -> proxyArray[proxyArray.length - 1])
+        .map(String::trim);
+  }
+}

service/src/test/java/org/whispersystems/textsecuregcm/util/ForwardedIpUtilTest.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.util;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.params.provider.Arguments.arguments;
+
+import java.util.Optional;
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class ForwardedIpUtilTest {
+
+    @SuppressWarnings("OptionalUsedAsFieldOrParameterType")
+    @ParameterizedTest
+    @MethodSource("argumentsForGetMostRecentProxy")
+    void getMostRecentProxy(final String forwardedFor, final Optional<String> expectedMostRecentProxy) {
+      assertEquals(expectedMostRecentProxy, ForwardedIpUtil.getMostRecentProxy(forwardedFor));
+    }
+
+    private static Stream<Arguments> argumentsForGetMostRecentProxy() {
+      return Stream.of(
+          arguments(null, Optional.empty()),
+          arguments("", Optional.empty()),
+          arguments("    ", Optional.empty()),
+          arguments("203.0.113.195", Optional.of("203.0.113.195")),
+          arguments("203.0.113.195, 70.41.3.18, 150.172.238.178", Optional.of("150.172.238.178"))
+      );
+    }
+}