R – Is it possible to prevent DoSing on Google App Engine

There are no built-in tools to prevent DoS. If you are writing Google Apps using java then you can use the service.FloodFilter filter. The following piece of code will execute before any of your Servlets do.

package service;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;


/**
 * 
 * This filter can protect web server from simple DoS attacks
 * via request flooding.
 * 
 * It can limit a number of simultaneously processing requests
 * from one ip and requests to one page.
 *
 * To use filter add this lines to your web.xml file in a <web-app> section.
 * 
    <filter>
        <filter-name>FloodFilter</filter-name>
        <filter-class>service.FloodFilter</filter-class>
        <init-param>
            <param-name>maxPageRequests</param-name>
            <param-value>50</param-value>
        </init-param>
        <init-param>
            <param-name>maxClientRequests</param-name>
            <param-value>5</param-value>
        </init-param>
        <init-param>
            <param-name>busyPage</param-name>
            <param-value>/busy.html</param-value>
        </init-param>
    </filter>

    <filter-mapping>
        <filter-name>JSP flood filter</filter-name>
        <url-pattern>*.jsp</url-pattern>
    </filter-mapping>
 *  
 *  PARAMETERS
 *  
 *  maxPageRequests:    limits simultaneous requests to every page
 *  maxClientRequests:  limits simultaneous requests from one client (ip)
 *  busyPage:           busy page to send to client if the limit is exceeded
 *                      this page MUST NOT be intercepted by this filter
 *  
 */
public class FloodFilter implements Filter
{
    private Map <String, Integer> pageRequests;
    private Map <String, Integer> clientRequests;

    private ServletContext context;
    private int maxPageRequests = 50;
    private int maxClientRequests = 10;
    private String busyPage = "/busy.html";


    public void doFilter( ServletRequest request, ServletResponse response, FilterChain chain ) throws IOException, ServletException
    {
        String page = null;
        String ip = null;

        try {
            if ( request instanceof HttpServletRequest ) {
                // obtaining client ip and page URI without parameters & jsessionid
                HttpServletRequest req = (HttpServletRequest) request;
                page = req.getRequestURI();

                if ( page.indexOf( ';' ) >= 0 )
                    page = page.substring( 0, page.indexOf( ';' ) );

                ip = req.getRemoteAddr();

                // trying & registering request
                if ( !tryRequest( page, ip ) ) {
                    // too many requests in process (from one client or for this page) 
                    context.log( "Flood denied from "+ip+" on page "+page );
                    page = null;
                    // forwarding to busy page
                    context.getRequestDispatcher( busyPage ).forward( request, response );
                    return;
                }
            }

            // requesting next filter or servlet
            chain.doFilter( request, response );
        } finally {
            if ( page != null )
                // unregistering the request
                releaseRequest( page, ip );
        }
    }


    private synchronized boolean tryRequest( String page, String ip )
    {
        // checking page requests
        Integer pNum = pageRequests.get( page );

        if ( pNum == null )
            pNum = 1;
        else {
            if ( pNum > maxPageRequests )
                return false;

            pNum = pNum + 1;
        }

        // checking client requests
        Integer cNum = clientRequests.get( ip );

        if ( cNum == null )
            cNum = 1;
        else {
            if ( cNum > maxClientRequests )
                return false;

            cNum = cNum + 1;
        }

        pageRequests.put( page, pNum );
        clientRequests.put( ip, cNum );

        return true;
    }


    private synchronized void releaseRequest( String page, String ip )
    {
        // removing page request
        Integer pNum = pageRequests.get( page );

        if ( pNum == null ) return;

        if ( pNum <= 1 )
            pageRequests.remove( page );
        else
            pageRequests.put( page, pNum-1 );

        // removing client request
        Integer cNum = clientRequests.get( ip );

        if ( cNum == null ) return;

        if ( cNum <= 1 )
            clientRequests.remove( ip );
        else
            clientRequests.put( ip, cNum-1 );
    }


    public synchronized void init( FilterConfig config ) throws ServletException
    {
        // configuring filter
        this.context = config.getServletContext();
        pageRequests = new HashMap <String,Integer> ();
        clientRequests = new HashMap <String,Integer> ();
        String s = config.getInitParameter( "maxPageRequests" );

        if ( s != null ) 
            maxPageRequests = Integer.parseInt( s );

        s = config.getInitParameter( "maxClientRequests" );

        if ( s != null ) 
            maxClientRequests = Integer.parseInt( s );

        s = config.getInitParameter( "busyPage" );

        if ( s != null ) 
            busyPage = s;
    }


    public synchronized void destroy()
    {
        pageRequests.clear();
        clientRequests.clear();
    }
}

• From: http://forums.sun.com/thread.jspa?threadID=5125779&tstart=13545

If you are using python, then you may have to roll your own filter.