listens for connections on certain internet sockets.
When a connection is found on one of its sockets, it decides what service the
socket corresponds to, and invokes a program to service the request. After the
program is finished, it continues to listen on the socket (except in some
cases which will be described below). Essentially, inetd
allows running one daemon to invoke several others, reducing load on the
The options are as follows:
- Turns on debugging.
- Prevents inetd from laundering the
environment. Without this option a selection of potentially harmful
environent variables, including PATH, will be removed
and not inherited by services.
- Makes the program not daemonize itself.
- Turns on libwrap connection logging and access control.
Internal services cannot be wrapped. When enabled,
/usr/sbin/tcpd is silently not executed even if present
in /etc/inetd.conf and instead libwrap is called
directly by inetd.
- Specify the length of the listen(2)
connections queue; the default is 128.
- Specify the maximum number of times a service can be
invoked in one minute; the default is 256. If a service exceeds this
limit, inetd will log the problem and stop servicing
requests for the specific service for ten minutes. See also the
wait/nowait configuration fields below.
Upon execution, inetd
reads its configuration information from
a configuration file which, by default, is /etc/inetd.conf
There must be an entry for each field of the configuration file, with entries
for each field separated by a tab or a space. Comments are denoted by a
“#” at the beginning of a line. The fields of the configuration
file are as follows:
user[.group] or user[:group]
server program arguments
To specify a Sun-RPC based service, the entry would contain these fields.
user[.group] or user[:group]
server program arguments
For internet services, the first field of the line may also have a host address
specifier prefixed to it, separated from the service name by a colon. If this
is done, the string before the colon in the first field indicates what local
should use when listening for that service.
Multiple local addresses can be specified on the same line, separated by
commas. Numeric IP addresses in dotted-quad notation can be used as well as
symbolic hostnames. Symbolic hostnames are looked up using
(). If a hostname has multiple address
mappings, inetd creates a socket to listen on each address.
The single character “*” indicates
, meaning “all local addresses”.
To avoid repeating an address that occurs frequently, a line with a host
address specifier and colon, but no further fields, causes the host address
specifier to be remembered and used for all further lines with no explicit
host specifier (until another such line or the end of the file). A line
is implicitly provided at the top of the file; thus, traditional configuration
files (which have no host address specifiers) will be interpreted in the
traditional manner, with all services listened for on all local addresses. If
the protocol is “unix”, this value is ignored.
The service name
entry is the name of a valid service in the
or a port number. For
“internal” services (discussed below), the service name
be the official name of the service (that is, the first
entry in /etc/services
). When used to specify a Sun-RPC
based service, this field is a valid RPC service name in the file
. The part on the right of the “/” is
the RPC version number. This can simply be a single numeric argument or a
range of versions. A range is bounded by the low version to the high version -
“rusers/1-3”. For UNIX
-domain sockets this
field specifies the path name of the socket.
The socket type
should be one of “stream”,
“dgram”, “raw”, “rdm”, or
“seqpacket”, depending on whether the socket is a stream,
datagram, raw, reliably delivered message, or sequenced packet socket.
must be a valid protocol as given in
“unix”. Examples might be
“tcp” or “udp”. RPC based services are specified with
the “rpc/tcp” or “rpc/udp” service type.
“tcp” and “udp” will be recognized as “TCP or
UDP over default IP version”. This is currently IPv4, but in the future
it will be IPv6. If you need to specify IPv4 or IPv6 explicitly, use something
like “tcp4” or “udp6”. A protocol
“unix” is used to specify a socket in the
In addition to the protocol, the configuration file may specify the send and
receive socket buffer sizes for the listening socket. This is especially
useful for TCP as the window scale factor, which is based on the receive
socket buffer size, is advertised when the connection handshake occurs, thus
the socket buffer size for the server must be set on the listen socket. By
increasing the socket buffer sizes, better TCP performance may be realized in
some situations. The socket buffer sizes are specified by appending their
values to the protocol specification as follows:
A literal value may be specified, or modified using ‘k’ to indicate
kilobytes or ‘m’ to indicate megabytes.
entry is used to tell inetd
if it should wait for the server program to return, or continue processing
connections on the socket. If a datagram server connects to its peer, freeing
the socket so inetd
can receive further messages on the
socket, it is said to be a “multi-threaded” server, and should use
the “nowait” entry. For datagram servers which process all
incoming datagrams on a socket and eventually time out, the server is said to
be “single-threaded” and should use a “wait” entry.
are both examples of the latter type of datagram
is an exception; it is a datagram server
that establishes pseudo-connections. It must be listed as “wait”
in order to avoid a race; the server reads the first packet, creates a new
socket, and then forks and exits to allow inetd
to check for
new service requests to spawn new servers. The optional “max”
suffix (separated from “wait” or “nowait” by a dot)
specifies the maximum number of times a service can be invoked in one minute;
the default is 256. If a service exceeds this limit, inetd
will log the problem and stop servicing requests for the specific service for
ten minutes. See also the -R
Stream servers are usually marked as “nowait” but if a single server
process is to handle multiple connections, it may be marked as
“wait”. The master socket will then be passed as fd 0 to the
server, which will then need to accept the incoming connection. The server
should eventually time out and exit when no more connections are active.
will continue to listen on the master socket for
connections, so the server should not close it when it exits.
entry should contain the user name of the user as
whom the server should run. This allows for servers to be given less
permission than root. An optional group name can be specified by appending a
dot to the user name followed by the group name. This allows for servers to
run with a different (primary) group ID than specified in the password file.
If a group is specified and user is not root, the supplementary groups
associated with that user will still be set.
The server program
entry should contain the pathname of the
program which is to be executed by inetd
when a request is
found on its socket. If inetd
provides this service
internally, this entry should be “internal”.
The server program arguments
should be just as arguments
normally are, starting with argv, which is the name of the program. If the
service is provided internally, the word “internal” should take
the place of this entry.
provides several “trivial” services
internally by use of routines within itself. These services are
“echo”, “discard”, “chargen” (character
generator), “daytime” (human readable time), and
“time” (machine readable time, in the form of the number of
seconds since midnight, January 1, 1900). All of these services are TCP based.
For details of these services, consult the appropriate RFC from the Network
rereads its configuration file when it receives a hangup
. Services may be added, deleted or
modified when the configuration file is reread. inetd
creates a file /var/run/inetd.pid
that contains its process
Support for TCP wrappers is included with inetd
built-in tcpd-like access control functionality. An external tcpd program is
not needed. You do not need to change the /etc/inetd.conf
server-program entry to enable this capability. inetd
access control facility configurations, as described in
IPv6 TCP/UDP behavior¶
If you wish to run a server for IPv4 and IPv6 traffic, you'll need to run two
separate processes for the same server program, specified as two separate
lines in inetd.conf
, for “tcp4” and
Under various combinations of IPv4/v6 daemon settings, inetd
will behave as follows:
- If you have only one server
on “tcp4”, IPv4 traffic will be routed to the server. IPv6
traffic will not be accepted.
- If you have two servers on
“tcp4” and “tcp6”, IPv4 traffic will be routed to
the server on “tcp4”, and IPv6 traffic will go to server on
- If you have only one server
on “tcp6”, only IPv6 traffic will be routed to the server.
The special “tcp46” parameter can be used for obsolete servers
which require to receive IPv4 connections mapped in an IPv6 socket. Its
usage is discouraged.
command appeared in 4.3BSD
Support for Sun-RPC based services is modelled after that provided by SunOS
4.1. IPv6 support was added by the KAME project in 1999.
Marco d'Itri ported this code from OpenBSD in summer 2002 and added socket
buffers tuning and libwrap support from the NetBSD source tree.
On Linux systems, the daemon cannot reload its configuration and needs to be
restarted when the host address for a service is changed between
“*” and a specific address.
Server programs used with “dgram” “udp”
“nowait” must read from the network socket, or
will spawn processes until the maximum is reached.
Host address specifiers, while they make conceptual sense for RPC services, do
not work entirely correctly. This is largely because the portmapper interface
does not provide a way to register different ports for the same service on
different local addresses. Provided you never have more than one entry for a
given RPC service, everything should work correctly. (Note that default host
address specifiers do apply to RPC lines with no explicit specifier.)