Declarative Web Application Security with Servlets and JSP
- Form-Based Authentication
- Example: Form-Based Authentication
- BASIC Authentication
- Example: BASIC Authentication
- Configuring Tomcat to Use SSL
This chapter is from the book
This chapter is from the book
This chapter is from the book
There are two major aspects to securing Web applications:
Preventing unauthorized users from accessing sensitive data. This process involves access restriction (identifying which resources need protection and who should have access to them) and authentica-tion (identifying users to determine if they are one of the authorized ones). Simple authentication involves the user entering a username and password in an HTML form or a dialog box; stronger authentica-tion involves the use of X509 certificates sent by the client to the server. This aspect applies to virtually all secure applications. Even intranets at locations with physical access controls usually require some sort of user authentication.
Preventing attackers from stealing network data while it is in transit. This process involves the use of Secure Sockets Layer (SSL) to encrypt the traffic between the browser and the server. This capa-bility is generally reserved for particularly sensitive applications or particularly sensitive pages within a larger application. After all, unless the attackers are on your local subnet, it is exceedingly difficult for them to gain access to your network traffic.
These two security aspects are mostly independent. The approaches to access restriction are the same regardless of whether or not you use SSL. With the excep-tion of client certificates (which apply only to SSL), the approaches to authentication are also identical whether or not you use SSL.
Within the Web application framework, there are two general approaches to this type of security:
Declarative security. With declarative security, the topic of this chapter, none of the individual servlets or JSP pages need any security-aware code. Instead, both of the major security aspects are handled by the server.
To prevent unauthorized access, you use the Web application deployment descriptor (web.xml) to declare that certain URLs need protection. You also designate the authentication method that the server should use to identify users. At request time, the server automatically prompts users for usernames and passwords when they try to access restricted resources, automatically checks the results against a predefined set of usernames and passwords, and automatically keeps track of which users have previously been authenticated. This process is completely transparent to the servlets and JSP pages.
To safeguard network data, you use the deployment descriptor to stipulate that certain URLs should only be accessible with SSL. If users try to use a regular HTTP connection to access one of these URLs, the server automatically redirects them to the HTTPS (SSL) equivalent.
Programmatic security. With programmatic security, the topic of the next chapter, protected servlets and JSP pages at least partially manage their own security
To prevent unauthorized access, each servlet or JSP page must either authenticate the user or verify that the user has been authenticated previously.
To safeguard network data, each servlet or JSP page has to check the network protocol used to access it. If users try to use a regular HTTP connection to access one of these URLs, the servlet or JSP page must manually redirect them to the HTTPS (SSL) equivalent.
7.1 Form-Based Authentication
The most common type of declarative security uses regular HTML forms. The developer uses the deployment descriptor to identify the protected resources and to designate a page that has a form to collect usernames and passwords. A user who attempts to access protected resources is redirected to the page containing the form. When the form is submitted, the server checks the username and password against a list of usernames, passwords and roles. If the login is successful and the user belongs to a role that is permitted access to the page, the user is granted access to the page originally requested. If the login is unsuccessful, the user is sent to a designated error page. Behind the scenes, the system uses some variation of session tracking to remember which users have already been validated.
The whole process is automatic: redirection to the login page, checking of user names and passwords, redirection back to the original resource, and tracking of already authenticated users are all performed by the container (server) in a manner that is completely transparent to the individual resources. However, there is one major caveat: the servlet specification explicitly says that form-based authentication is not guaranteed to work when the server is set to perform session tracking based on URL rewriting instead of cookies (the default session tracking mechanism).
Core Warning
Depending on your server, form-based authentication might fail when you use URL rewriting as the basis of session tracking.
This type of access restriction and authentication is completely independent of the protection of the network traffic. You can stipulate that SSL be used for all, some, or none of your application; but doing so does not change the way you restrict access or authenticate users. Nor does the use of SSL require your individual servlets or JSP pages to participate in the security process; redirection to the URL that uses SSL and encryption/decryption of the network traffic are all performed by the server in a manner that is transparent to the servlets and JSP pages.
Seven basic steps are required to set up your system to use this type of form-based security. I'll summarize the steps here, then give details on each step in the following subsections. All the steps except for the first are standardized and portable across all servers that support version 2.2 or later of the servlet API. Section 7.2 illustrates the concepts with a small application.
Set up usernames, passwords, and roles. In this step, you designate a list of users and associate each with a password and one or more abstract roles (e.g., normal user or administrator). This is a completely server-specific process. In general, you'll have to read your server's documentation, but I'll summarize the process for Tomcat, JRun, and ServletExec.
Tell the server that you are using form-based authentication. Designate the locations of the login and login-failure page. This process uses the web.xml login-configelement with an auth-methodsubelement of FORMand a form-login-config subelement that gives the locations of the two pages.
Create a login page. This page must have a form with an ACTIONof j_security_check, a METHODof POST, a textfield named j_username, and a password field named j_password.
Create a page to report failed login attempts. This page can simply say something like "username and password not found" and perhaps give a link back to the login page.
Specify which URLs should be password protected. For this step, you use the security-constraintelement of web.xml. This element, in turn, uses web-resource-collectionand auth-constraintsubelements. The first of these (web-resource-collection) designates the URL patterns to which access should be restricted, and the second (auth-constraint) specifies the abstract roles that should have access to the resources at the given URLs.
Specify which URLs should be available only with SSL. If your server supports SSL, you can stipulate that certain resources are available only through encrypted HTTPS (SSL) connections. You use the user-data-constraintsubelement of security-constraint for this purpose.
Turn off the invoker servlet. If your application restricts access to servlets, the access restrictions are placed on the custom URLs that you associate with the servlets. But, most servers have a default servlet URL: http://host/webAppPrefix/servlet/ServletName. To prevent users from bypassing the security settings, disable default servlet URLs of this form. To disable these URLs, use the servlet-mappingele-ment with a url-patternsubelement that designates a pattern of /servlet/*.
Details follow.
Setting Up Usernames, Passwords, and Roles
When a user attempts to access a protected resource in an application that is using form-based authentication, the system uses an HTML form to ask for a username and password, verifies that the password matches the user, determines what abstract roles (regular user, administrator, executive, etc.) that user belongs to, and sees whether any of those roles has permission to access the resource. If so, the server redirects the user to the originally requested page. If not, the server redirects the user to an error page.
The good news regarding this process is that the server (container) does a lot of the work for you. The bad news is that the task of associating users with passwords and logical roles is server specific. So, although you would not have to change the web.xml file or any of the actual servlet and JSP code to move a secure Web application from system to system, you would still have to make custom changes on each system to set up the users and passwords.
In general, you will have to read your server's documentation to determine how to assign passwords and role membership to users. However, I'll summarize the process for Tomcat, JRun, and ServletExec.
Setting Passwords with Tomcat
Tomcat permits advanced developers to configure custom username and password management schemes (e.g., by accessing a database, looking in the Unix /etc/passwd file, checking the Windows NT/2000 User Account settings, or making a Kerberos call). For details, see http://jakarta.apache.org/tomcat/tomcat-4.0-doc/realm-howto.html. However, this configuration is a lot of work, so Tomcat also provides a default mechanism. With this mechanism, Tomcat stores usernames, passwords, and roles in install_dir/ conf/tomcat-users.xml. This file should contain an XML header followed by a tomcat-users element containing any number of user elements. Each user element should have three attributes: name (the username), password (the plain text password), and roles (a comma-separated list of logical role names). Listing 7.1 presents a simple example that defines four users (valjean, bishop, javert, thenardier), each of whom belongs to two logical roles.
Listing 7.1 install_dir/conf/tomcat-users.xml (Sample)
<?xml version="1.0" encoding="ISO-8859-1"?>
<tomcat-users>
<user name="valjean" password="forgiven"
roles="lowStatus,nobleSpirited" />
<user name="bishop" password="mercy"
roles="lowStatus,nobleSpirited" />
<user name="javert" password="strict"
roles="highStatus,meanSpirited" />
<user name="thenardier" password="grab"
roles="lowStatus,meanSpirited" />
</tomcat-users>
Note that the default Tomcat strategy of storing unencrypted passwords is a poor one. First, an intruder that gains access to the server's file system can obtain all the passwords. Second, even system administrators who are authorized to access server resources should not be able to obtain user's passwords. In fact, since many users reuse passwords on multiple systems, passwords should never be stored in clear text. Instead, they should be encrypted with an algorithm that cannot easily be reversed. Then, when a user supplies a password, it is encrypted and the encrypted version is compared with the stored encrypted password. Nevertheless, the default Tomcat approach makes it easy to set up and test secure Web applications. Just keep in mind that for real applications you'll want to replace the simple file-based password scheme with something more robust (e.g., a database or a system call to Kerberos or the Windows NT/2000 User Account system).
Setting Passwords with JRun
JRun, like Tomcat, permits developers to customize the username and password management scheme. For details, see Chapter 39 (Web Application Authentication) of http://www.allaire.com/documents/jr31/devapp.pdf. Also like Tomcat, JRun provides a file-based default mechanism. Unlike Tomcat, however, JRun encrypts the passwords before storing them in the file. This approach makes the default JRun strategy usable even in real-world applications.
With the default mechanism, JRun stores usernames, encrypted passwords, and roles in install_dir/lib/users.properties. This file contains entries of three types: user.username entries that associate a password with a user; group.groupname entries that group users together; and role.rolename entries that place users and/ or groups into logical roles. Encrypted passwords can be obtained from an existing Unix-based password or .htaccess file or by using the PropertyFileAuthentica-tion class supplied with JRun. To use this class, temporarily set your CLASSPATH (not the server's CLASSPATH) to include install_dir/lib/jrun.jar and install_dir/lib/ ext/servlet.jar, change directory to install_dir/lib, and add a user at a time with the -add flag, as below. For real applications you would probably set up the server to automate this process.
java allaire.jrun.security.PropertyFileAuthentication valjean grace
After adding the users, edit the file to assign the roles. Listing 7.2 shows an example that sets up the same users, passwords, and roles as in the previous Tomcat example (Listing 7.1).
Listing 7.2 install_dir/lib/users.properties (Sample)
user.valjean=vaPoR2yIzbfdI user.bishop=bic5wknlJ8QFE user.javert=jaLULvqM82wfk user.thenardier=thvwKJbcM0s7o role.lowStatus=valjean,thenardier role.highStatus=bishop,javert role.nobleSpirited=valjean,bishop role.meanSpirited=javert,thenardier
Setting Passwords with ServletExec
The process of setting up usernames, passwords, and roles is particularly simple with ServletExec. Simply open the administrator home page and select Users within the Web Applications heading (Figure 7–1). From there, you can interactively enter usernames, passwords, and roles (Figure 7–2). Voila!
){kind=link}
){kind=link}
With the free desktop debugger version, ServletExec stores the usernames and passwords in plain text in install_dir/ServletExec Data/users.properties. The passwords are encrypted in the deployment version.
Figure 7–1 ServletExec user editing interface.
Figure 7–2 Adding a user, password, and role in ServletExec.
Telling the Server You Are Using Form-Based Authentication; Designating Locations of Login and Login-Failure Pages
You use the login-config element in the deployment descriptor (web.xml) to control the authentication method. Recall from Chapters 4 and 5 that this file goes in the WEB-INF directory of your Web application. Although a few servers support nonstandard web.xml files (e.g., Tomcat has one in install_dir/conf that provides defaults for multiple Web applications), those files are entirely server specific. I am addressing only the standard version that goes in the Web application's WEB-INF directory.
To use form-based authentication, supply a value of FORM for the auth-method subelement and use the form-login-config subelement to give the locations of the login (form-login-page) and login-failure (form-error-page) pages. In the next sections I'll explain exactly what these two files should contain. But for now, note that nothing mandates that they use dynamic content. Thus, these pages can consist of either JSP or ordinary HTML.
For example, Listing 7.3 shows part of a web.xml file that stipulates that the container use form-based authentication. Unauthenticated users who attempt to access protected resources will be redirected to http://host/webAppPrefix/login.jsp. If they log in successfully, they will be returned to whatever resource they first attempted to access. If their login attempt fails, they will be redirected to http://host/webApp-Prefix/login-error.html.
Listing 7.3 web.xml (Excerpt designating form-based authentication)
<?xml version="1.0" encoding="ISO-8859-1"?> <!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN" "http://java.sun.com/j2ee/dtds/web-app_2_2.dtd"> <web-app> <!-- ... --> <security-constraint>...</security-constraint> <login-config> <auth-method>FORM</auth-method> <form-login-config> <form-login-page>/login.jsp</form-login-page> <form-error-page>/login-error.html</form-error-page> </form-login-config> </login-config> <!-- ... --> </web-app>
Creating the Login Page
OK, so the login-config element tells the server to use form-based authentication and to redirect unauthenticated users to a designated page. Fine. But what should you put in that page? The answer is surprisingly simple: all the login page requires is a form with an ACTION of j_security_check, a textfield named j_username, and a password field named j_password. And, since using GET defeats the whole point of password fields (protecting the password from prying eyes looking over the user's shoulder), all forms that have password fields should use a METHOD of POST. Note that j_security_check is a "magic" name; you don't preface it with a slash even if your login page is in a subdirectory of the main Web application directory. Listing 7.4 gives an example.
Listing 7.4 login.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD><TITLE>...</TITLE></HEAD> <BODY> ... <FORM ACTION="j_security_check" METHOD="POST"> <TABLE> <TR><TD>User name: <INPUT TYPE="TEXT" NAME="j_username"> <TR><TD>Password: <INPUT TYPE="PASSWORD" NAME="j_password"> <TR><TH><INPUT TYPE="SUBMIT" VALUE="Log In"> </TABLE> </FORM> ... </BODY></HTML>
OK, that was the page for logging in. What about a page for logging out? The session should time out eventually, but what if users want to log out immediately without closing the browser? Well, the servlet specification says that invalidating the HttpSession should log out users and cause them to be reauthenticated the next time they try to access a protected resource. So, in principle you should be able to create a logout page by making servlet or JSP page that looks up the session and calls invalidate on it. In practice, however, not all servers support this process. Fortunately, changing users is simple: you just visit the login page a second time. This is in contrast to BASIC authentication (Section 7.3), where neither logging out nor changing your username is supported without the user quitting and restarting the browser.
Creating the Page to Report Failed Login Attempts
The main login page must contain a form with a special-purpose ACTION (j_security_check), a textfield with a special name (j_username), and a password field with yet another reserved name (j_password). So, what is required to be in the login-failure page? Nothing! This page is arbitrary; it can contain a link to an unrestricted section of the Web application, a link to the login page, or a simple "login failed" message.
Specifying URLs That Should Be Password Protected
The login-config element tells the server which authentication method to use. Good, but how do you designate the specific URLs to which access should be restricted? Designating restricted URLs and describing the protection they should have is the purpose of the security-constraint element. The security-constraint element should come immediately before login-config in web.xml and contains four possible subelements: display-name (an optional element giving a name for IDEs to use), web-resource-collection (a required element that specifies the URLs that should be protected), auth-constraint (an optional element that designates the abstract roles that should have access to the URLs), and user-data-constraint (an optional element that specifies whether SSL is required). Note that multiple web-resource-collection entries are permitted within security-constraint.
For a quick example of the use of security-constraint, Listing 7.5 instructs the server to require passwords for all URLs of the form http://host/webAppPrefix/ sensitive/blah. Users who supply passwords and belong to the administrator or executive logical roles should be granted access; all others should be denied access. The rest of this subsection provides details on the web-resource-collection, auth-constraint, and display-name elements. The role of user-data-constraint is explained in the next subsection (Specifying URLs That Should Be Available Only with SSL).
Listing 7.5 web.xml (Excerpt specifying protected URLs)
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE web-app PUBLIC
"-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN"
"http://java.sun.com/j2ee/dtds/web-app_2_2.dtd">
<web-app>
<!-- ... -->
<security-constraint>
<web-resource-collection>
<web-resource-name>Sensitive</web-resource-name>
<url-pattern>/sensitive/*</url-pattern>
</web-resource-collection>
<auth-constraint>
<role-name>administrator</role-name>
<role-name>executive</role-name>
</auth-constraint>
</security-constraint>
<login-config>...</login-config>
<!-- ... -->
</web-app>
isplay-name
This rarely used optional subelement of security-constraint gives a name to the security constraint entry. This name might be used by an IDE or other graphical tool.
web-resource-collection
This subelement of security-constraint identifies the resources that should be protected. Each security-constraint element must contain one or more web-resource-collection entries; all other security-constraint subelements are optional. The web-resource-collection element consists of a web-resource-name element that gives an arbitrary identifying name, a url-pattern element that identifies the URLs that should be protected, an optional http-method element that designates the HTTP commands to which the protection applies (GET, POST, etc.; the default is all methods), and an optional description element providing documentation. For example, the following web-resource-collection entries (within a security-constraint element) specify that password protection should be applied to all documents in the proprietary directory (and subdirectories thereof) and to the delete-account.jsp page in the admin directory.
<security-constraint> <web-resource-collection> <web-resource-name>Proprietary</web-resource-name> <url-pattern>/proprietary/*</url-pattern> </web-resource-collection> <web-resource-collection> <web-resource-name>Account Deletion</web-resource-name> <url-pattern>/admin/delete-account.jsp</url-pattern> </web-resource-collection> <!-- ... --> </security-constraint>
It is important to note that the url-pattern applies only to clients that access the resources directly. In particular, it does not apply to pages that are accessed through the MVC architecture with a RequestDispatcher (Section 3.8) or by the similar means of jsp:forward or jsp:include (Section 3.5). This asymmetry is good if used properly. For example, with the MVC architecture a servlet looks up data, places it in beans, and forwards the request to a JSP page that extracts the data from the beans and displays it. You want to ensure that the JSP page is never accessed directly but instead is accessed only through the servlet that sets up the beans the page will use. The url-pattern and auth-constraint (see next subsection) elements can provide this guarantee by declaring that no user is permitted direct access to the JSP page. But, this asymmetric behavior can catch developers off guard and allow them to accidentally provide unrestricted access to resources that should be protected.
Core Warning
These protections apply only to direct client access. The security model does not apply to pages accessed by means of a RequestDispatcher, jsp:forward, or jsp:include.
auth-constraint
Whereas the web-resource-collection element designates the URLs that should be protected, the auth-constraint element designates the users that should have access to protected resources. It should contain one or more role-name elements identifying the class of users that have access and, optionally, a description element describing the role. For instance, the following part of the security-constraint element in web.xml states that only users who are designated as either Administrators or Big Kahunas (or both) should have access to the designated resource.
<security-constraint> <web-resource-collection>...</web-resource-collection> <auth-constraint> <role-name>administrator</role-name> <role-name>kahuna</role-name> </auth-constraint> </security-constraint> >
If you want all authenticated users to have access to a resource, use * as the role-name. Technically, the auth-constraint element is optional. Omitting it means that no roles have access. Although at first glance it appears pointless to deny access to all users, remember that these security restrictions apply only to direct client access. So, for example, suppose you had a JSP snippet that is intended to be inserted into another file with jsp:include (Section 3.5). Or, suppose you have a JSP page that is the forwarding destination of a servlet that is using a RequestDispatcher as part of the MVC architecture (Section 3.8). In both cases, users should be prohibited from directly accessing the JSP page. A security-constraint element with no auth-constraint would enforce this restriction nicely.
Specifying URLs That Should Be Available Only with SSL
Suppose your servlet or JSP page collects credit card numbers. User authentication keeps out unauthorized users but does nothing to protect the network traffic. So, for instance, an attacker that runs a packet sniffer on the end user's local area network could see that user's credit card number. This scenario is exactly what SSL protects against—it encrypts the traffic between the browser and the server.
Use of SSL does not change the basic way that form-based authentication works. Regardless of whether you are using SSL, you use the login-config element to indicate that you are using form-based authentication and to identify the login and login-failure pages. With or without SSL, you designate the protected resources with the url-pattern subelement of web-resource-collection. None of your servlets or JSP pages need to be modified or moved to different locations when you enable or disable SSL. That's the beauty of declarative security.
The user-data-constraint subelement of security-constraint can mandate that certain resources be accessed only with SSL. So, for example, attempts to access https://host/webAppPrefix/specialURL are handled normally, whereas attempts to access http://host/webAppPrefix/specialURL are redirected to the https URL. This behavior does not mean that you cannot supply an explicit https URL for a hypertext link or the ACTION of a form; it just means that you aren't required to. You can stick with the simpler and more easily maintained relative URLs and still be assured that certain URLs will only be accessed with SSL.
The user-data-constraint element, if used, must contain a transport-guarantee subelement (with legal values NONE, INTEGRAL, or CONFIDENTIAL) and can optionally contain a description element. A value of NONE for transport-guarantee puts no restrictions on the communication protocol used. Since NONE is the default, there is little point in using user-data-constraint or transport-guarantee if you specify NONE. A value of INTEGRAL means that the communication must be of a variety that prevents data from being changed in transit without detection. A value of CONFIDENTIAL means that the data must be transmitted in a way that prevents anyone who intercepts it from reading it. Although in principle (and perhaps in future HTTP versions) there may be a distinction between INTEGRAL and CONFIDENTIAL, in current practice they both simply mandate the use of SSL.
For example, the following instructs the server to permit only https connections to the associated resource:
<security-constraint>
<!-- ... -->
<user-data-constraint>
<transport-guarantee>CONFIDENTIAL</transport-guarantee>
</user-data-constraint>
</security-constraint>
In addition to simply requiring SSL, the servlet API provides a way to stipulate that users must authenticate themselves with client certificates. You supply a value of CLIENT-CERT for the auth-method subelement of login-config (see "Specifying URLs That Should Be Password Protected" earlier in this section). However, only servers that have full J2EE support are required to support this capability.
Now, although the method of prohibiting non-SSL access is standardized, servers that are compliant with the servlet 2.3 and JSP 1.2 specifications are not required to support SSL. So, Web applications that use a transport-guarantee of CONFIDENTIAL (or, equivalently, INTEGRAL) are not necessarily portable. For example, JRun and ServletExec are usually used as plugins in Web servers like iPlanet/ Netscape or IIS. In this scenario, the network traffic between the client and the Web server is encrypted with SSL, but the local traffic from the Web server to the servlet/ JSP container is not encrypted. Consequently, a CONFIDENTIAL transport-guarantee will fail. Tomcat, however, can be set up to use SSL directly. Details on this process are given in Section 7.5. Some server plugins maintain SSL even on the local connection between the main Web server and the servlet/JSP engine; for example, the BEA WebLogic plugin for IIS, Apache, and Netscape Enterprise Server does so. Furthermore, integrated application servers like the standalone version of WebLogic have no "separate" servlet and JSP engine, so SSL works exactly as described here. Nevertheless, it is important to realize that these features, although useful, are not mandated by the servlet and JSP specifications.
Core Warning
Web applications that rely on SSL are not necessarily portable.
Turning Off the Invoker Servlet
When you restrict access to certain resources, you do so by specifying the URL patterns to which the restrictions apply. This pattern, in turn, matches a pattern that you set with the servlet-mapping web.xml element (see Section 5.3, "Assigning Names and Custom URLs"). However, most servers use an "invoker servlet" that provides a default URL for servlets: http://host/webAppPrefix/servlet/ServletName. You need to make sure that users don't access protected servlets with this URL, thus bypassing the access restrictions that were set by the url-pattern subelement of web-resource-collection. For example, suppose that you use security-constraint, web-resource-collection, and url-pattern to say that the URL /admin/DeclareChapter11 should be protected. You also use the auth-constraint and role-name elements to say that only users in the director role can access this URL. Next, you use the servlet and servlet-mapping elements to say that the servlet BankruptcyServlet.class in the disaster package should correspond to /admin/ DeclareChapter11. Now, the security restrictions are in force when clients use the URL http://host/webAppPrefix/admin/DeclareChapter11. No restrictions apply to http://host/webAppPrefix/servlet/disaster.BankruptcyServlet. Oops.
Section 5.4 (Disabling the Invoker Servlet) discusses server-specific approaches to turning off the invoker. The most portable approach, however, is to simply remap the /servlet pattern in your Web application so that all requests that include the pattern are sent to the same servlet. To remap the pattern, you first create a simple servlet that prints an error message or redirects users to the top-level page. Then, you use the servlet and servlet-mapping elements (Section 5.3) to send requests that include the /servlet pattern to that servlet. Listing 7.6 gives a brief example.
Listing 7.6
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE web-app PUBLIC
"-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN"
"http://java.sun.com/j2ee/dtds/web-app_2_2.dtd">
<web-app>
<!-- ... -->
<servlet>
<servlet-name>Error</servlet-name>
<servlet-class>somePackage.ErrorServlet</servlet-class>
</servlet>
<!-- ... -->
<servlet-mapping>
<servlet-name>Error</servlet-name>
<url-pattern>/servlet/*</url-pattern>
</servlet-mapping>
<!-- ... -->
</web-app>