Setup two Undertow instances: Application Proxy - has ProxyHandler behind ProxyPeerAddressHandler import io.undertow.Handlers; import io.undertow.Undertow; import io.undertow.server.handlers.proxy.LoadBalancingProxyClient; import io.undertow.servlet.Servlets; import io.undertow.servlet.api.DeploymentInfo; import io.undertow.servlet.api.DeploymentManager; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.IOException; import java.net.URI; public class App { public static void main( String [] args) throws ServletException { // Application DeploymentInfo servletBuilder = Servlets.deployment() .setClassLoader(App. class. getClassLoader()) .setContextPath( "/" ) .setDeploymentName( "test.war" ) .addServlets(Servlets.servlet( "x-forwarded- for -printer" , RequestPrinter.class).addMapping( "/*" )); DeploymentManager manager = Servlets.defaultContainer().addDeployment(servletBuilder); manager.deploy(); Undertow applicationServer = Undertow.builder() .addHttpListener(8080, "localhost" ) .setHandler(manager.start()) .build(); applicationServer.start(); // Load Balancer LoadBalancingProxyClient proxyClient = new LoadBalancingProxyClient(); proxyClient.addHost(URI.create( "http: //localhost:8080" )); Undertow loadBalancer = Undertow.builder() .addHttpListener(8180, "localhost" ) // ProxyHandler behind ProxyPeerAddressHandler .setHandler(Handlers.proxyPeerAddress(Handlers.proxyHandler(proxyClient))) .build(); loadBalancer.start(); } public static class RequestPrinter extends HttpServlet { @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { resp.getOutputStream().println( "X-Forwarded-For: " + req.getHeader( "X-Forwarded-For" )); } } } Pretend that we are a load balancer and the real client ip is 1.2.3.4: curl -H 'X-Forwarded-For: 1.2.3.4' http://localhost:8180/ Expected result: X-Forwarded-For: 1.2.3.4 Actual result: X-Forwarded-For: 127.0.0.1