The /etc/xinetd.conf File

The /etc/xinetd.conf contains general configuration settings which effect every service under xinetd's control.
It is read once when the xinetd service is started, so in order for configuration changes to take effect, the
administrator must restart the xinetd service. Below is a sample /etc/xinetd.conf file:

defaults
{
        instances               = 60
        log_type                = SYSLOG authpriv
        log_on_success          = HOST PID
        log_on_failure          = HOST
        cps                     = 25 30
}

service telnet
{
        flags           = REUSE
        socket_type     = stream
        wait            = no
        user            = root
        server          = /usr/sbin/in.telnetd
        log_on_failure  += USERID
        disable         = yes
}

• instances — Sets the maximum number of requests xinetd can handle at once.
  
• log_type — Configures xinetd to use the authpriv log facility, which writes log entries to the /var/log/secure file. Adding a directive such as FILE /var/log/xinetdlog here would create a custom log file called xinetdlog in the /var/log/ directory.
  
• log_on_success — Configures xinetd to log if the connection is successful. By default, the remote host's IP address and the process ID of server processing the request are recorded.
  
• log_on_failure — Configures xinetd to log if there is a connection failure or if the connection is not allowed.
  
• cps — Configures xinetd to allow no more than 25 connections per second to any given service. If this limit is reached, the service is retired for 30 seconds.
  
• service — Defines the service name, usually to match a service listed in the /etc/services file.
• flags — Sets any of a number of attributes for the connection. REUSE instructs xinetd to reuse the socket for a Telnet connection.
• socket_type — Sets the network socket type to stream.
• wait — Defines whether the service is single-threaded (yes) or multi-threaded (no).
• user — Defines what user ID the process process will run under.
• server — Defines the binary executable to be launched.
• log_on_failure — Defines logging parameters for log_on_failure in addition to those already defined in xinetd.conf.
• disable — Defines whether or not the service is active.

There are a large assortment of directives available for xinetd protected services. This section highlights some of the more commonly used options.

The following logging options are available for both /etc/xinetd.conf.
Below is a list of some of the more commonly used logging options:

• ATTEMPT — Logs the fact that a failed attempt was made (log_on_failure).
• DURATION — Logs the length of time the service is used by a remote system (log_on_success).
• EXIT — Logs the exit status or termination signal of the service (log_on_success).
• HOST — Logs the remote host's IP address (log_on_failure and log_on_success).
• PID — Logs the process ID of the server receiving the request (log_on_success).
• RECORD — Records information about the remote system in the case the service cannot be started. Only particular services, such as login and finger, may use this option (log_on_failure).
• USERID — Logs the remote user using the method defined in RFC 1413 for all multi-threaded stream services (log_on_failure and log_on_success).
  

For a complete list of logging options, consult the xinetd.conf man page.

Users of xinetd services can choose to use the TCP wrappers hosts access rules, provide access control via the xinetd configuration files, or a mixture of both. Information concerning the use of TCP wrappers hosts access control files can be found in Section 15.2 TCP Wrappers Configuration Files. This section discusses using xinetd to control access to services.

Unlike TCP wrappers, changes to access control only take effect if the xinetd administrator restarts the xinetd service.

• only_from — Allows only the specified hosts to use the service.
• no_access — Blocks listed hosts from using the service.
• access_times — Specifies the time range when a particular service may be used. The time range must be stated in 24-hour format notation, HH:MM-HH:MM.
  

The only_from and no_access options can use a list of IP addresses or host names, or can specify an entire network. Like TCP wrappers, combining xinetd access control with the enhanced logging configuration can enhance security by blocking requests from banned hosts while verbosely record each connection attempt.
For example, the following can be used to block Telnet access from a particular network group and restrict the overall time range that even allowed users can log in:

service telnet
{
        disable         = no
        flags           = REUSE
        socket_type     = stream
        wait            = no
        user            = root
        server          = /usr/sbin/in.telnetd
        log_on_failure  += USERID
        no_access       = 10.0.1.0/24
        log_on_success  += PID HOST EXIT
        access_times    = 09:45-16:15
}

In this example, when client system from the 10.0.1.0/24 network, such as 10.0.1.2, tries access the Telnet service, it will receive a message stating the following message:

Connection closed by foreign host.

In addition, their login attempt is logged in /var/log/secure as follows:

May 15 17:38:49 boo xinetd[16252]: START: telnet pid=16256 from=10.0.1.2
May 15 17:38:49 boo xinetd[16256]: FAIL: telnet address from=10.0.1.2
May 15 17:38:49 boo xinetd[16252]: EXIT: telnet status=0 pid=16256

When using TCP wrappers in conjunction with xinetd access controls, it is important to understand the relationship between the two access control mechanisms.

The following is the order of operations followed by xinetd when client requests a connection:

The xinetd daemon accesses the TCP wrappers hosts access rules through a libwrap.a library call. If a deny rule matches the client host, the connection is dropped. If an allow rule matches the client host, the connection is passed on to xinetd.

The xinetd daemon checks its own access control rules both for the xinetd service and the requested service. If a deny rule matches the client host the connection is dropped. Otherwise, xinetd starts an instance of the requested service and passes control of the connection to it.

Care should be taken when using TCP wrappers access controls in conjunction with xinetd access controls.
Misconfiguration can cause undesired effects.

The service configuration files for xinetd support binding the service to an IP address and redirecting incoming requests for that service to another IP address, hostname, or port.

• Binding is controlled with the bind option in the service-specific configuration files and links the service to one IP address on the system. Once configured, the bind option only allows requests for the proper IP address to access the service. This way different services can be bound to different network interfaces based on need.
  

This is particularly useful for systems with multiple network adapters or with multiple IP addresses configured. On such a system, insecure services, like Telnet, can be configured to listen only on the interface connected to a private network and not to the interface connected with the Internet.

• The redirect option accepts an IP address or hostname followed by a port number. It configures the service to redirect any requests for this service to the specified host and port number. This feature can be used to point to another port number on the same system, redirect the request to different IP address on the same machine, shift the request to a totally different system and port number, or any combination of these options. In this way, a user connecting to certain service on a system may be rerouted to another system with no disruption.
  

The xinetd daemon is able to accomplish this redirection by spawning a process that stays alive for the duration of the connection between the requesting client machine and the host actually providing the service, transferring data between the two systems.
But the advantages of the bind and redirect options are most clearly evident when they are used together. By binding a service to a particular IP address on a system and then redirecting requests for this service to a second machine that only the first machine can see, an internal system can be used to provide services for a totally different network.
Alternatively, these options can be used to limit the exposure of a particular service on a multi-homed machine to a known IP address, as well as redirect any requests for that service to another machine specially configured for that purpose.

For example, consider a system that is used as a firewall with this setting for its Telnet service:

service telnet
{
        socket_type     = stream
        wait            = no
        server          = /usr/sbin/in.telnetd
        log_on_success      += DURATION USERID
        log_on_failure      += USERID
        bind                    = 123.123.123.123
        redirect                = 10.0.1.13 21 23
}

The bind and redirect options in this file ensures that the Telnet service on the machine is bound to the external IP address (123.123.123.123), the one facing the Internet. In addition, any requests for Telnet service sent to 123.123.123.123 are redirected via a second network adapter to an internal IP address (10.0.1.13) that only the firewall and internal systems can access. The firewall then send the communication between the two systems, and the connecting system thinks it is connected to 123.123.123.123 when it is actually connected to a different machine. This feature is particularly useful for users with broadband connections and only one fixed IP address. When using Network Address Translation (NAT), the systems behind the gateway machine, which are using internal-only IP addresses, are not available from outside the gateway system. However, when certain services controlled by xinetd are configured with the bind and redirect options, the gateway machine can act as a type of proxy between outside systems and a particular internal machine configured to provide the service. In addition, the various xinetd access control and logging options are also available for additional protection, such as limiting the number of simultaneous connections for the redirected service.

The xinetd daemon can add a basic level of protection from a Denial of Service (DoS) attacks. Below is a list of directives which can aid in limiting the effectiveness of such attacks:

• per_source — Defines the maximum number of instances for a service per source IP address. It accepts only integers as an argument and can be used in both xinetd.conf and in the service-specific configuration files in the xinetd.d/ directory.
• cps — Defines the maximum of connections per second. This directive takes two integer arguments separated by white space. The first is the maximum number of connections allowed to the service per second. The second is the number of seconds xinetd must wait before re-enabling the service. It accepts only integers as an argument and can be used in both xinetd.conf and in the service-specific configuration files in the xinetd.d/ directory.
• max_load — Defines the CPU usage threshold for a service. It accepts a floating point number argument.