The /etc/xinetd.d/ directory contains
the configuration files for each service managed by xinetd and the names of the files correlate to the
service. As with xinetd.conf, this
directory is read only when the xinetd
service is started. For any changes to take effect, the
administrator must restart the xinetd
service.
The format of files in the /etc/xinetd.d/ directory use the same conventions
as /etc/xinetd.conf. The primary reason
the configuration for each service is stored in a separate file is
to make customization easier and less likely to effect other
services.
To gain an understanding of how these files are structured,
consider the /etc/xinetd.d/telnet
file:
service telnet
{
flags = REUSE
socket_type = stream
wait = no
user = root
server = /usr/sbin/in.telnetd
log_on_failure += USERID
disable = yes
}
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These lines control various aspects of the telnet service:
-
service — Defines the service
name, usually one 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 runs 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 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 and the service-specific
configuration files within the /etc/xinetd.d/ directory.
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).
-
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, refer to 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 17.2 TCP Wrappers
Configuration Files.
This section discusses using xinetd to
control access to services.
 |
Note |
| |
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 within
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/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 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 a message stating the following message:
Connection closed by foreign host.
|
In addition, their login attempts are 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 a 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.
 |
Important |
| |
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. In 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 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
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.
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.
There are more resource management options available for
xinetd. Refer to the chapter titled
Server Security in the Red Hat Enterprise Linux Security Guide for more
information, as well as the xinetd.conf
man page.
