Access Control Options

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. Refer to "TCP Wrappers Configuration Files" for more information about TCP Wrappers hosts access control 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.

Also, unlike TCP Wrappers, access control through xinetd only affects services controlled by xinetd.

The xinetd hosts access control differs from the method used by TCP Wrappers. While TCP Wrappers places all of the access configuration within two files, /etc/hosts.allow and /etc/hosts.deny, xinetd's access control is found in each service's configuration file in the /etc/xinetd.d/ directory.

The following hosts access options are supported by xinetd:

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 increase security by blocking requests from banned hosts while verbosely recording each connection attempt.

For example, the following /etc/xinetd.d/telnet file 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/kerberos/sbin/telnetd
  log_on_failure  += USERID
  no_access       = 172.16.45.0/24
  log_on_success  += PID HOST EXIT
  access_times    = 09:45-16:15
}

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

Connection closed by foreign host.

In addition, their login attempts are logged in /var/log/messages as follows:

Sep  7 14:58:33 localhost xinetd[5285]: FAIL: telnet address from=172.16.45.107
Sep  7 14:58:33 localhost xinetd[5283]: START: telnet pid=5285 from=172.16.45.107
Sep  7 14:58:33 localhost xinetd[5283]: EXIT: telnet status=0 pid=5285 duration=0(sec)

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 sequence of events followed by xinetd when a client requests a connection:

The xinetd daemon accesses the TCP Wrappers hosts access rules using a libwrap.a library call. If a deny rule matches the client, the connection is dropped. If an allow rule matches the client, the connection is passed 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, the connection is dropped. Otherwise, xinetd starts an instance of the requested service and passes control of the connection to that service.
Care should be taken when using TCP Wrappers access controls in conjunction with xinetd access controls. Misconfiguration can cause undesirable effects.