Apache Tomcat 9 Configuration Reference

All implementations of Host support the following attributes:

The standard implementation of Host is org.apache.catalina.core.StandardHost. It supports the following additional attributes (in addition to the common attributes listed above):

A host is associated with the org.apache.catalina.core.ContainerBase.[engine_name].[host_name] log category. Note that the brackets are part of the name, don't omit them.

When you run a web server, one of the output files normally generated is an access log, which generates one line of information for each request processed by the server, in a standard format. Catalina includes an optional Valve implementation that can create access logs in the same standard format created by web servers, or in any number of custom formats.

You can ask Catalina to create an access log for all requests processed by an Engine, Host, or Context by nesting a Valve element like this:

<Host name="localhost" ...>
  ...
  <Valve className="org.apache.catalina.valves.AccessLogValve"
         prefix="localhost_access_log" suffix=".txt"
         pattern="common"/>
  ...
</Host>

See Access Logging Valves for more information on the configuration attributes that are supported.

If you are using the standard Host implementation with default settings then applications in the appBase or with context files in the configBase are automatically deployed when Tomcat starts (the deployOnStartup property defaults to true) and reloaded or redeployed (as appropriate) when a change is detected while Tomcat is running (the autoDeploy attribute also defaults to true).

deployOnStartup and autoDeploy trigger execution of exactly the same code so the behaviour is very similar. However, there is one key difference. When Tomcat starts it has no knowledge of which files are the same, which have been changed and which are new. It therefore treats all files as new. While Tomcat is running, it can differentiate between unchanged, modified and new files. This leads to some differences in behaviour between files being modified while Tomcat is running and files being modified while Tomcat is stopped.

When you use automatic deployment, related files (a web application may have a context.xml file, a WAR and a directory) that exist in the Host's appBase and/or configBase must conform to the expected naming convention. In short, this means files for the same web application must share the same base name.

The automatic deployment process identifies new and/or modified web applications using the following search order:

1. Web applications with a context.xml file located in the Host's configBase.
2. Web applications with a WAR file located in the Host's appBase that have not already been identified during the scan for context.xml files.
3. Web applications with a directory located in the Host's appBase that have not already been identified during the scans for context.xml and/or WAR files.

When autoDeploy is true, the automatic deployment process will monitor the deployed web applications for changes. Depending on exactly what changes, the web application will either be re-deployed or reloaded. Re-deployment involves the creation of a new web application and, if using the standard session manager, user sessions will not be retained. Reloading uses the existing web application but re-parses the web.xml and reloads any classes. If using the standard session manager, user sessions will be persisted.

Users may add to the files that the automatic deployment process monitors for reloading (i.e. any change to one of these files triggers a reload of the web application) by adding a WatchedResources element to the context.xml file. See the Context documentation for further details.

When using automatic deployment, the docBase defined by an XML Context file should be outside of the appBase directory. If this is not the case, difficulties may be experienced deploying the web application or the application may be deployed twice. The deployIgnore attribute can be used to avoid this situation.

Note that if you are defining contexts explicitly in server.xml, you should probably turn off automatic application deployment or specify deployIgnore carefully. Otherwise, the web applications will each be deployed twice, and that may cause problems for the applications.

There are many possible combinations of settings, new files, changed files and deleted files. A separate page describes the expected behaviour of the automatic deployment process in many of these scenarios.

In many server environments, Network Administrators have configured more than one network name (in the Domain Name Service (DNS) server), that resolve to the IP address of the same server. Normally, each such network name would be configured as a separate Host element in conf/server.xml, each with its own set of web applications.

However, in some circumstances, it is desirable that two or more network names should resolve to the same virtual host, running the same set of applications. A common use case for this scenario is a corporate web site, where it is desirable that users be able to utilize either www.mycompany.com or company.com to access exactly the same content and applications.

This is accomplished by utilizing one or more Alias elements nested inside your Host element. For example:

<Host name="www.mycompany.com" ...>
  ...
  <Alias>mycompany.com</Alias>
  ...
</Host>

In order for this strategy to be effective, all of the network names involved must be registered in your DNS server to resolve to the same computer that is running this instance of Catalina.

Aliases may also use the wildcard form (*.domainname) permitted for the name attribute of a Host.

If you have implemented a Java object that needs to know when this Host is started or stopped, you can declare it by nesting a Listener element inside this element. The class name you specify must implement the org.apache.catalina.LifecycleListener interface, and it will be notified about the occurrence of the corresponding lifecycle events. Configuration of such a listener looks like this:

<Host name="localhost" ...>
  ...
  <Listener className="com.mycompany.mypackage.MyListener" ... >
  ...
</Host>

Note that a Listener can have any number of additional properties that may be configured from this element. Attribute names are matched to corresponding JavaBean property names using the standard property method naming patterns.

You can ask Catalina to check the IP address, or host name, on every incoming request directed to the surrounding Engine, Host, or Context element. The remote address or name will be checked against configured "accept" and/or "deny" filters, which are defined using java.util.regex Regular Expression syntax. Requests that come from locations that are not accepted will be rejected with an HTTP "Forbidden" error. Example filter declarations:

<Host name="localhost" ...>
  ...
  <Valve className="org.apache.catalina.valves.RemoteHostValve"
         allow=".*\.mycompany\.com|www\.yourcompany\.com"/>
  <Valve className="org.apache.catalina.valves.RemoteAddrValve"
         deny="192\.168\.1\.\d+"/>
  ...
</Host>

See Remote Address Filter and Remote Host Filter for more information about the configuration options that are supported.

In many environments, but particularly in portal environments, it is desirable to have a user challenged to authenticate themselves only once over a set of web applications deployed on a particular virtual host. This can be accomplished by nesting an element like this inside the Host element for this virtual host:

<Host name="localhost" ...>
  ...
  <Valve className="org.apache.catalina.authenticator.SingleSignOn"/>
  ...
</Host>

The Single Sign On facility operates according to the following rules:

• All web applications configured for this virtual host must share the same Realm. In practice, that means you can nest the Realm element inside this Host element (or the surrounding Engine element), but not inside a Context element for one of the involved web applications.
• As long as the user accesses only unprotected resources in any of the web applications on this virtual host, they will not be challenged to authenticate themselves.
• As soon as the user accesses a protected resource in any web application associated with this virtual host, the user will be challenged to authenticate himself or herself, using the login method defined for the web application currently being accessed.
• Once authenticated, the roles associated with this user will be utilized for access control decisions across all of the associated web applications, without challenging the user to authenticate themselves to each application individually.
• As soon as the user logs out of one web application (for example, by invalidating the corresponding session if form based login is used), the user's sessions in all web applications will be invalidated. Any subsequent attempt to access a protected resource in any application will require the user to authenticate himself or herself again.
• The Single Sign On feature utilizes HTTP cookies to transmit a token that associates each request with the saved user identity, so it can only be utilized in client environments that support cookies.

Many web servers can automatically map a request URI starting with a tilde character ("~") and a username to a directory (commonly named public_html) in that user's home directory on the server. You can accomplish the same thing in Catalina by using a special Listener element like this (on a Unix system that uses the /etc/passwd file to identify valid users):

<Host name="localhost" ...>
  ...
  <Listener className="org.apache.catalina.startup.UserConfig"
            directoryName="public_html"
            userClass="org.apache.catalina.startup.PasswdUserDatabase"/>
  ...
</Host>

On a server where /etc/passwd is not in use, you can request Catalina to consider all directories found in a specified base directory (such as c:\Homes in this example) to be considered "user home" directories for the purposes of this directive:

<Host name="localhost" ...>
  ...
  <Listener className="org.apache.catalina.startup.UserConfig"
            directoryName="public_html"
            homeBase="c:\Homes"
            userClass="org.apache.catalina.startup.HomesUserDatabase"/>
  ...
</Host>

If a user home directory has been set up for a user named craigmcc, then its contents will be visible from a client browser by making a request to a URL like:

http://www.mycompany.com:8080/~craigmcc

Successful use of this feature requires recognition of the following considerations:

• Each user web application will be deployed with characteristics established by the global and host level default context settings.
• It is legal to include more than one instance of this Listener element. This would only be useful, however, in circumstances where you wanted to configure more than one "homeBase" directory.
• The operating system username under which Catalina is executed MUST have read access to each user's web application directory, and all of its contents.

You can override the default values found in conf/context.xml and conf/web.xml files from $CATALINA_BASE for each virtual host. Tomcat will look for files named context.xml.default and web.xml.default in the directory specified by xmlBase and merge the files into those found in the default ones.