This is by default in EPiServer 5 and 6. 

Put this piece of code in Global.asax.cs

protected void Application_Start(Object sender, EventArgs e)
{
EPiServer.Web.InitializationModule.FirstBeginRequest += new EventHandler(InitializationModule_FirstBeginRequest);
}

void InitializationModule_FirstBeginRequest(object sender, EventArgs e)
{
EPiServer.Global.UrlRewriteProvider.ConvertedToExternal += new EventHandler<EPiServer.Web.UrlRewriteEventArgs>(UrlRewriteProvider_ConvertedToInternal);
}

void UrlRewriteProvider_ConvertedToInternal(object sender, EPiServer.Web.UrlRewriteEventArgs e)
{

//Redirect to real page if permalink if it exists
if (e.IsModified)
{
PageReference pageReference = (PageReference)e.Internal;
PageData page = DataFactory.Instance.GetPage(pageReference);
if (page["PageExternalURL"] != null && ((string)page["PageExternalURL"]) == HttpContext.Current.Request.Url.LocalPath.Replace("/", string.Empty) &&
GetExternalUrl(page.LinkURL) != HttpContext.Current.Request.Url.LocalPath)
{
HttpContext.Current.Response.Status = "301 Moved Permanently";
HttpContext.Current.Response.AddHeader("Location", Helper.Instance.GetExternalUrl(page.LinkURL) + HttpContext.Current.Request.Url.Query);
}
}

}

 public string GetExternalUrl(string internalUrl)
        {
            UrlBuilder ub = new EPiServer.UrlBuilder(internalUrl);
            Global.UrlRewriteProvider.ConvertToExternal(ub, null, System.Text.Encoding.Default);

            return ub.ToString();
        }

Hello Erik,

Thanks a lot for your time and reply and clarifying my doubt.

In the code snippet, I could only see the method for UrlRewriteProvider_ConvertedToInternal but not the UrlRewriteProvider_ConvertedToExternal. Can you please let me know if the method is correct.

Based on your hint, I tried to write on my own, but couldn't succeed. I get into infinite loop.

Can you please help? 

Can you please help?

I have edited the code to fix the typo with Internal/External. I also added a method GetExternalUrl, which I forgot to type here.