XINETD.CONF(5) File Formats Manual XINETD.CONF(5)
NAME
xinetd.conf - Extended Internet Services Daemon configuration file
DESCRIPTION
xinetd.conf is the configuration file that determines the services provided by xinetd. Any line whose first non-white-space character is a
'#' is considered a comment line. Empty lines are ignored.
The file contains entries of the form:
service <service_name>
{
<attribute> <assign_op> <value> <value> ...
...
}
The assignment operator, assign_op, can be one of '=', '+=', '-='. The majority of attributes support only the simple assignment operator,
'='. Attributes whose value is a set of values support all assignment operators. For such attributes, '+=' means adding a value to the
set and '-=' means removing a value from the set. A list of these attributes will be given after all the attributes are described.
Each entry defines a service identified by the service_name. The following is a list of available attributes:
id This attribute is used to uniquely identify a service. This is useful because there exist services that can use different
protocols and need to be described with different entries in the configuration file. By default, the service id is the
same as the service name.
type Any combination of the following values may be used:
RPC if this is an RPC service
INTERNAL if this is a service provided by xinetd.
TCPMUX/TCPMUXPLUS
if this is a service that will be started according to the RFC 1078 protocol on the TCPMUX well-known port.
See the section describing TCPMUX services below.
UNLISTED if this is a service not listed in a standard system file (like /etc/rpc for RPC services, or /etc/services
for non-RPC services).
flags Any combination of the following flags may be used:
INTERCEPT Intercept packets or accepted connections in order to verify that they are coming from acceptable locations
(internal or multi-threaded services cannot be intercepted).
NORETRY Avoid retry attempts in case of fork failure.
IDONLY Accept connections only when the remote end identifies the remote user (i.e. the remote host must run an iden-
tification server). This flag applies only to connection-based services. This flag is ineffective if the
USERID log option is not used.
NAMEINARGS This will cause the first argument in "server_args" to be argv[0] when executing the server, as specified in
"server". This allows you to use tcpd by putting tcpd in "server" and the name of the server in "server_args"
like in normal inetd.
NODELAY If the service is a tcp service and the NODELAY flag is set, then the TCP_NODELAY flag will be set on the
socket. If the service is not a tcp service, this option has no effect.
KEEPALIVE If the service is a tcp service and the KEEPALIVE flag is set, then the SO_KEEPALIVE socket flag will be set
on the socket. If the service is not a tcp service, this option has no effect.
NOLIBWRAP This disables internal calling of the tcpwrap library to determine access to the service. This may be needed
in order to use libwrap functionality not available to long-running processes such as xinetd; in this case,
the tcpd program can be called explicitly (see also the NAMEINARGS flag).
SENSOR This replaces the service with a sensor that detects accesses to the specified port. NOTE: It will NOT detect
stealth scans. This flag should be used only on services that you know you don't need. When an access is made
to this service's port, the IP Address is added to a global no_access list. This causes all subsequent
accesses from the originating IP address to be denied access until the deny_time setting expires. The amount
of time spent on this list is configurable as the deny_time attribute. The SENSOR flag will also cause xinetd
to consider the server attribute to be INTERNAL no matter what is typed on the same line. Another important
thing to remember is that if the socket_type is set to stream, then the wait attribute should be set to no.
IPv4 Sets the service to be an IPv4 service (AF_INET).
IPv6 Sets the service to be an IPv6 service (AF_INET6), if IPv6 is available on the system.
REUSE The REUSE flag is deprecated. All services now implicitly use the REUSE flag.
disable This is boolean "yes" or "no". This will result in the service being disabled and not starting. See the DISABLE flag
description.
socket_type Possible values for this attribute include:
stream stream-based service
dgram datagram-based service
raw service that requires direct access to IP
seqpacket service that requires reliable sequential datagram transmission
protocol determines the protocol that is employed by the service. The protocol must exist in /etc/protocols. If this attribute is
not defined, the default protocol employed by the service will be used.
wait This attribute determines if the service is single-threaded or multi-threaded and whether or not xinetd accepts the con-
nection or the server program accepts the connection. If its value is yes, the service is single-threaded; this means that
xinetd will start the server and then it will stop handling requests for the service until the server dies and that the
server software will accept the connection. If the attribute value is no, the service is multi-threaded and xinetd will
keep handling new service requests and xinetd will accept the connection. It should be noted that udp/dgram services nor-
mally expect the value to be yes since udp is not connection oriented, while tcp/stream servers normally expect the value
to be no.
user determines the uid for the server process. The user attribute can either be numeric or a name. If a name is given (recom-
mended), the user name must exist in /etc/passwd. This attribute is ineffective if the effective user ID of xinetd is
not super-user.
group determines the gid for the server process. The group attribute can either be numeric or a name. If a name is given (recom-
mended), the group name must exist in /etc/group. If a group is not specified, the group of user will be used (from
/etc/passwd). This attribute is ineffective if the effective user ID of xinetd is not super-user and if the groups
attribute is not set to 'yes'.
instances determines the number of servers that can be simultaneously active for a service (the default is no limit). The value of
this attribute can be either a number or UNLIMITED which means that there is no limit.
nice determines the server priority. Its value is a (possibly negative) number; check nice(3) for more information.
server determines the program to execute for this service.
server_args determines the arguments passed to the server. In contrast to inetd, the server name should not be included in
server_args.
libwrap overrides the service name passed to libwrap (which defaults to the server name, the first server_args component with
NAMEINARGS, the id for internal services and the service name for redirected services). This attribute is only valid if
xinetd has been configured with the libwrap option.
only_from determines the remote hosts to which the particular service is available. Its value is a list of IP addresses which can
be specified in any combination of the following ways:
a) a numeric address in the form of %d.%d.%d.%d. If the rightmost components are 0, they are treated as wildcards (for
example, 128.138.12.0 matches all hosts on the 128.138.12 subnet). 0.0.0.0 matches all Internet addresses. IPv6
hosts may be specified in the form of abcd:ef01::2345:6789. The rightmost rule for IPv4 addresses does not apply to
IPv6 addresses.
b) a factorized address in the form of %d.%d.%d.{%d,%d,...}. There is no need for all 4 components (i.e.
%d.%d.{%d,%d,...%d} is also ok). However, the factorized part must be at the end of the address. This form does not
work for IPv6 hosts.
c) a network name (from /etc/networks). This form does not work for IPv6 hosts.
d) a host name. When a connection is made to xinetd, a reverse lookup is performed, and the canonical name returned is
compared to the specified host name. You may also use domain names in the form of .domain.com. If the reverse
lookup of the client's IP is within .domain.com, a match occurs.
e) an ip address/netmask range in the form of 1.2.3.4/32. IPv6 address/netmask ranges in the form of 1234::/46 are also
valid.
Specifying this attribute without a value makes the service available to nobody.
no_access determines the remote hosts to which the particular service is unavailable. Its value can be specified in the same way as
the value of the only_from attribute. These two attributes determine the location access control enforced by xinetd. If
none of the two is specified for a service, the service is available to anyone. If both are specified for a service, the
one that is the better match for the address of the remote host determines if the service is available to that host (for
example, if the only_from list contains 128.138.209.0 and the no_access list contains 128.138.209.10 then the host with
the address 128.138.209.10 can not access the service).
access_times determines the time intervals when the service is available. An interval has the form hour:min-hour:min (connections will
be accepted at the bounds of an interval). Hours can range from 0 to 23 and minutes from 0 to 59.
log_type determines where the service log output is sent. There are two formats:
SYSLOG syslog_facility [syslog_level]
The log output is sent to syslog at the specified facility. Possible facility names include: daemon, auth, auth-
priv, user, mail, lpr, news, uucp, ftp local0-7. Possible level names include: emerg, alert, crit, err, warning,
notice, info, debug. If a level is not present, the messages will be recorded at the info level.
FILE file [soft_limit [hard_limit]]
The log output is appended to file which will be created if it does not exist. Two limits on the size of the log
file can be optionally specified. The first limit is a soft one; xinetd will log a message the first time this
limit is exceeded (if xinetd logs to syslog, the message will be sent at the alert priority level). The second
limit is a hard limit; xinetd will stop logging for the affected service (if the log file is a common log file,
then more than one service may be affected) and will log a message about this (if xinetd logs to syslog, the mes-
sage will be sent at the alert priority level). If a hard limit is not specified, it defaults to the soft limit
increased by 1% but the extra size must be within the parameters LOG_EXTRA_MIN and LOG_EXTRA_MAX which default to
5K and 20K respectively (these constants are defined in xconfig.h).
log_on_success determines what information is logged when a server is started and when that server exits (the service id is always
included in the log entry). Any combination of the following values may be specified:
PID logs the server process id (if the service is implemented by xinetd without forking another process the logged
process id will be 0)
HOST logs the remote host address
USERID logs the user id of the remote user using the RFC 1413 identification protocol. This option is available only
for multi-threaded stream services.
EXIT logs the fact that a server exited along with the exit status or the termination signal (the process id is
also logged if the PID option is used)
DURATION logs the duration of a service session
TRAFFIC logs the total bytes in and out for a redirected service.
log_on_failure determines what information is logged when a server cannot be started (either because of a lack of resources or because of
access control restrictions). The service id is always included in the log entry along with the reason for failure. Any
combination of the following values may be specified:
HOST logs the remote host address.
USERID logs the user id of the remote user using the RFC 1413 identification protocol. This option is available only
for multi-threaded stream services.
ATTEMPT logs the fact that a failed attempt was made (this option is implied by all others).
rpc_version determines the RPC version for a RPC service. The version can be a single number or a range in the form number-number.
rpc_number determines the number for an UNLISTED RPC service (this attribute is ignored if the service is not unlisted).
env The value of this attribute is a list of strings of the form 'name=value'. These strings will be added to the environment
before starting a server (therefore the server's environment will include xinetd's environment plus the specified
strings).
passenv The value of this attribute is a list of environment variables from xinetd's environment that will be passed to the
server. An empty list implies passing no variables to the server except for those explicitly defined using the env
attribute. (notice that you can use this attribute in conjunction with the env attribute to specify exactly what environ-
ment will be passed to the server).
port determines the service port. If this attribute is specified for a service listed in /etc/services, it must be equal to the
port number listed in that file.
redirect Allows a tcp service to be redirected to another host. When xinetd receives a tcp connection on this port it spawns a
process that establishes a connection to the host and port number specified, and forwards all data between the two hosts.
This option is useful when your internal machines are not visible to the outside world. Syntax is: redirect = (ip
address) (port). You can also use a hostname instead of the IP address in this field. The hostname lookup is performed
only once, when xinetd is started, and the first IP address returned is the one that is used until xinetd is restarted.
The "server" attribute is not required when this option is specified. If the "server" attribute is specified, this
attribute takes priority.
bind Allows a service to be bound to a specific interface on the machine. This means you can have a telnet server listening on
a local, secured interface, and not on the external interface. Or one port on one interface can do something, while the
same port on a different interface can do something completely different. Syntax: bind = (ip address of interface).
interface Synonym for bind.
banner Takes the name of a file to be splatted at the remote host when a connection to that service is established. This banner
is printed regardless of access control. It should *always* be printed when a connection has been made. xinetd outputs
the file as-is, so you must ensure the file is correctly formatted for the service's protocol. In paticular, if the pro-
tocol requires CR-LF pairs for line termination, you must supply them.
banner_success Takes the name of a file to be splatted at the remote host when a connection to that service is granted. This banner is
printed as soon as access is granted for the service. xinetd outputs the file as-is, so you must ensure the file is cor-
rectly formatted for the service's protocol. In paticular, if the protocol requires CR-LF pairs for line termination, you
must supply them.
banner_fail Takes the name of a file to be splatted at the remote host when a connection to that service is denied. This banner is
printed immediately upon denial of access. This is useful for informing your users that they are doing something bad and
they shouldn't be doing it anymore. xinetd outputs the file as-is, so you must ensure the file is correctly formatted for
the service's protocol. In paticular, if the protocol requires CR-LF pairs for line termination, you must supply them.
per_source Takes an integer or "UNLIMITED" as an argument. This specifies the maximum instances of this service per source IP
address. This can also be specified in the defaults section.
cps Limits the rate of incoming connections. Takes two arguments. The first argument is the number of connections per second
to handle. If the rate of incoming connections is higher than this, the service will be temporarily disabled. The second
argument is the number of seconds to wait before re-enabling the service after it has been disabled. The default for this
setting is 50 incoming connections and the interval is 10 seconds.
max_load Takes a floating point value as the load at which the service will stop accepting connections. For example: 2 or 2.5.
The service will stop accepting connections at this load. This is the one minute load average. This is an OS dependent
feature, and currently only Linux, Solaris, and FreeBSD are supported for this. This feature is only avaliable if xinetd
was configured with the -with-loadavg option.
groups Takes either "yes" or "no". If the groups attribute is set to "yes", then the server is executed with access to the
groups that the server's effective UID has access to. Alternatively, if the group attribute is set, the server is exe-
cuted with access to the groups specified. If the groups attribute is set to "no", then the server runs with no supple-
mentary groups. This attribute must be set to "yes" for many BSD systems. This attribute can be set in the defaults sec-
tion as well.
mdns Takes either "yes" or "no". On systems that support mdns registration of services (currently only Mac OS X), this will
enable or disable registration of the service. This defaults to "yes".
umask Sets the inherited umask for the service. Expects an octal value. This option may be set in the "defaults" section to
set a umask for all services. xinetd sets its own umask to the previous umask OR'd with 022. This is the umask that will
be inherited by all child processes if the umask option is not used.
enabled Takes a list of service ID's to enable. This will enable only the services listed as arguments to this attribute; the
rest will be disabled. If you have 2 ftp services, you will need to list both of their ID's and not just ftp. (ftp is the
service name, not the ID. It might accidentally be the ID, but you better check.) Note that the service "disable"
attribute and "DISABLE" flag can prevent a service from being enabled despite being listed in this attribute.
include Takes a filename in the form of "include /etc/xinetd/service". The file is then parsed as a new configuration file. It
is not the same thing as pasting the file into xinetd.conf where the include directive is given. The included file must
be in the same form as xinetd.conf. This may not be specified from within a service. It must be specified outside a ser-
vice declaration.
includedir Takes a directory name in the form of "includedir /etc/xinetd.d". Every file inside that directory, excluding files with
names containing a dot ('.') or ending with a tilde ('~'), will be parsed as xinetd configuration files. The files will
be parsed in alphabetical order according to the C locale. This allows you to specify services one per file within a
directory. The includedir directive may not be specified from within a service declaration.
rlimit_as Sets the Address Space resource limit for the service. One parameter is required, which is either a positive integer rep-
resenting the number of bytes to set the limit to (K or M may be used to specify kilobytes/megabytes) or "UNLIMITED". Due
to the way Linux's libc malloc is implemented, it is more useful to set this limit than rlimit_data, rlimit_rss and
rlimit_stack. This resource limit is only implemented on Linux systems.
rlimit_cpu Sets the maximum number of CPU seconds that the service may use. One parameter is required, which is either a positive
integer representing the number of CPU seconds limit to, or "UNLIMITED".
rlimit_data Sets the maximum data size resource limit for the service. One parameter is required, which is either a positive integer
representing the number of bytes or "UNLIMITED".
rlimit_rss Sets the maximum resident set size limit for the service. Setting this value low will make the process a likely candidate
for swapping out to disk when memory is low. One parameter is required, which is either a positive integer representing
the number of bytes or "UNLIMITED".
rlimit_stack Set the maximum stack size limit for the service. One parameter is required, which is either a positive integer repre-
senting the number of bytes or "UNLIMITED".
deny_time Sets the time span that access to all services on all IP addresses are denied to someone that sets off the SENSOR. The
unit of time is in minutes. Valid options are: FOREVER, NEVER, and a numeric value. FOREVER causes the IP address not to
be purged until xinetd is restarted. NEVER has the effect of just logging the offending IP address. A typical time value
would be 60 minutes. This should stop most DOS attacks while allowing IP addresses that come from a pool to be recycled
for legitimate purposes. This option must be used in conjunction with the SENSOR flag.
You don't need to specify all of the above attributes for each service. The necessary attributes for a service are:
socket_type
user (non-internal services only)
server (non-internal services only)
wait
protocol (RPC and unlisted services only)
rpc_version (RPC services only)
rpc_number (unlisted RPC services only)
port (unlisted non-RPC services only)
The following attributes support all assignment operators:
only_from
no_access
log_on_success
log_on_failure
passenv
env (does not support the '-=' operator)
These attributes can also appear more than once in a service entry. The remaining attributes support only the '=' operator and can appear
at most once in a service entry.
The configuration file may also contain a single defaults entry that has the form
defaults
{
<attribute> = <value> <value> ...
...
}
This entry provides default attribute values for service entries that don't specify those attributes. Possible default attributes:
log_type (cumulative effect)
bind
per_source
umask
log_on_success (cumulative effect)
log_on_failure (cumulative effect)
only_from (cumulative effect)
no_access (cumulative effect)
passenv (cumulative effect)
instances
disabled (cumulative effect)
enabled (cumulative effect)
banner
banner_success
banner_fail
per_source
groups
cps
max_load
Attributes with a cumulative effect can be specified multiple times
with the values specified each time accumulating (i.e. '=' does the same thing as '+='). With the exception of disabled they all
have the same meaning as if they were specified in a service entry. disabled determines services that are disabled even if they
have entries in the configuration file. This allows for quick reconfiguration by specifying disabled services with the disabled
attribute instead of commenting them out. The value of this attribute is a list of space separated service ids. enabled has the
same properties as disabled. The difference being that enabled is a list of which services are to be enabled. If enabled is speci-
fied, only the services specified are available. If enabled is not specified, all services are assumed to be enabled, except those
listed in disabled.
INTERNAL SERVICES
xinetd provides the following services internally (both stream and datagram based): echo, time, daytime, chargen, and discard. These ser-
vices are under the same access restrictions as all other services except for the ones that don't require xinetd to fork another process
for them. Those ones (time, daytime, and the datagram-based echo, chargen, and discard) have no limitation in the number of instances.
TCPMUX Services
xinetd supports TCPMUX services that conform to RFC 1078. These services may not have a well-known port associated with them, and can be
accessed via the TCPMUX well-known port.
For each service that is to be accessed via TCPMUX, a service entry in /etc/xinetd.conf or in a configuration file in an includedir direc-
tory must exist.
The service_name field (as defined above for each service in any xinetd configuration file) must be identical to the string that is passed
(according to RFC 1078 protocol) to xinetd when the remote service requestor first makes the connection on the TCPMUX well-known port.
Private protocols should use a service name that has a high probability of being unique. One way is to prepend the service name with some
form of organization ID.
The type field can be either TCPMUX or TCPMUXPLUS. If the type is TCPMUXPLUS, xinetd will handle the initial protocol handshake (as defined
in RFC 1078) with the calling process before initiating the service. If the type is TCPMUX, the server that is started is responsible for
performing the handshake.
The type field should also include UNLISTED if the service is not listed in a standard system file (like /etc/rpc for RPC services, or
/etc/services for non-RPC services).
The socket_type for these services must be stream, and the protocol must be tcp.
Following is a sample TCPMUX service configuration:
service myorg_server
{
disable = no
type = TCPMUX
socket_type = stream
protocol = tcp
wait = no
user = root
server = /usr/etc/my_server_exec
}
Besides a service entry for each service that can be accessed via the TCPMUX well-known port, a service entry for TCPMUX itself must also
be included in the xinetd configuration. Consider the following sample:
service tcpmux
{
type = INTERNAL
id = tcpmux
socket_type = stream
protocol = tcp
user = root
wait = no
}
NOTES
1. The following service attributes cannot be changed on reconfiguration: socket_type, wait, protocol, type.
2. When the attributes only_from and no_access are not specified for a service (either directly or via defaults) the address check is con-
sidered successful (i.e. access will not be denied).
3. The address check is based on the IP address of the remote host and not on its domain address. We do this so that we can avoid remote
name lookups which may take a long time (since xinetd is single-threaded, a name lookup will prevent the daemon from accepting any
other requests until the lookup is resolved). The down side of this scheme is that if the IP address of a remote host changes, then
access to that host may be denied until xinetd is reconfigured. Whether access is actually denied or not will depend on whether the
new host IP address is among those allowed access. For example, if the IP address of a host changes from 1.2.3.4 to 1.2.3.5 and
only_from is specified as 1.2.3.0 then access will not be denied.
4. If the USERID log option is specified and the remote host either does not run an identification server or the server sends back a bad
reply, access will not be denied unless the IDONLY service flag is used.
5. Interception works by forking a process which acts as a filter between the remote host(s) and the local server. This obviously has a
performance impact so it is up to you to make the compromise between security and performance for each service. The following tables
show the overhead of interception. The first table shows the time overhead-per-datagram for a UDP-based service using various datagram
sizes. For TCP-based services we measured the bandwidth reduction because of interception while sending a certain amount of data from
client to server (the time overhead should the same as for UDP-based services but it is "paid" only by the first packet of a continuous
data transmission). The amount of data is given in the table as system_callsxdata_sent_per_call, i.e. each send(2) system call trans-
ferred so many bytes of data. The bandwidth reduction is given in terms of bytes per second and as a percentage of the bandwidth when
interception is not performed. All measurements were done on a SparcStation IPC running SunOS 4.1.
Datagram size (bytes) Latency (msec)
--------------------- --------------
64 1.19
256 1.51
1024 1.51
4096 3.58
Bytes sent Bandwidth reduction
---------- -------------------
10000x64 941 (1.2%)
10000x256 4,231 (1.8%)
10000x1024 319,300 (39.5%)
10000x4096 824,461 (62.1%)
EXAMPLE
#
# Sample configuration file for xinetd
#
defaults
{
log_type = FILE /var/log/servicelog
log_on_success = PID
log_on_failure = HOST
only_from = 128.138.193.0 128.138.204.0
only_from = 128.138.252.1
instances = 10
disabled = rstatd
}
#
# Note 1: the protocol attribute is not required
# Note 2: the instances attribute overrides the default
#
service login
{
socket_type = stream
protocol = tcp
wait = no
user = root
server = /usr/etc/in.rlogind
instances = UNLIMITED
}
#
# Note 1: the instances attribute overrides the default
# Note 2: the log_on_success flags are augmented
#
service shell
{
socket_type = stream
wait = no
user = root
instances = UNLIMITED
server = /usr/etc/in.rshd
log_on_success += HOST
}
service ftp
{
socket_type = stream
wait = no
nice = 10
user = root
server = /usr/etc/in.ftpd
server_args = -l
instances = 4
log_on_success += DURATION HOST USERID
access_times = 2:00-9:00 12:00-24:00
}
# Limit telnet sessions to 8 Mbytes of memory and a total
# 20 CPU seconds for child processes.
service telnet
{
socket_type = stream
wait = no
nice = 10
user = root
server = /usr/etc/in.telnetd
rlimit_as = 8M
rlimit_cpu = 20
}
#
# This entry and the next one specify internal services. Since
# this is the same service using a different socket type, the
# id attribute is used to uniquely identify each entry
#
service echo
{
id = echo-stream
type = INTERNAL
socket_type = stream
user = root
wait = no
}
service echo
{
id = echo-dgram
type = INTERNAL
socket_type = dgram
user = root
wait = no
}
#
# Sample RPC service
#
service rstatd
{
type = RPC
socket_type = dgram
protocol = udp
server = /usr/etc/rpc.rstatd
wait = yes
user = root
rpc_version = 2-4
env = LD_LIBRARY_PATH=/etc/securelib
}
#
# Sample unlisted service
#
service unlisted
{
type = UNLISTED
socket_type = stream
protocol = tcp
wait = no
server = /home/user/some_server
port = 20020
}
SEE ALSO
xinetd(1L),
xinetd.log(5)
Postel J., Echo Protocol, RFC 862, May 1983
Postel J., Discard Protocol, RFC 863, May 1983
Postel J., Character Generator Protocol, RFC 864, May 1983
Postel J., Daytime Protocol, RFC 867, May 1983
Postel J., Harrenstien K., Time Protocol, RFC 868, May 1983
M. Lottor, TCP Port Service Multiplexer (TCPMUX), RFC 1078 Nov 1988
StJohns M., Identification Protocol, RFC 1413, February 1993
BUGS
If the INTERCEPT flag is not used, access control on the address of the remote host is not performed when wait is yes and socket_type is
stream.
If the INTERCEPT flag is not used, access control on the address of the remote host for services where wait is yes and socket_type is dgram
is performed only on the first packet. The server may then accept packets from hosts not in the access control list. This can happen with
RPC services.
There is no way to put a SPACE in an environment variable.
When wait is yes and socket_type is stream, the socket passed to the server can only accept connections.
The INTERCEPT flag is not supported for internal services or multi-threaded services.
14 June 2001 XINETD.CONF(5)