NAME¶
inetd
—
internet “super-server”
SYNOPSIS¶
inetd |
[ -d ]
[-E ]
[-i ]
[-l ]
[-q
length ]
[-R
rate ]
[configuration_file ] |
DESCRIPTION¶
inetd
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 system.
The options are as follows:
-d
- Turns on debugging.
-E
- Prevents
inetd
from laundering the
environment. Without this option a selection of potentially harmful
environment variables, including PATH,
will be removed and not inherited by services.
-i
- Makes the program not daemonize itself.
-l
- 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.
-q
length
- Specify the length of the listen(2)
connections queue; the default is 128.
-R
rate
- 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:
service name
socket type
protocol[,sndbuf=size][,rcvbuf=size]
wait/nowait[.max]
user[.group] or user[:group]
server program
server program arguments
To specify a Sun-RPC based service, the entry would contain these fields.
service name/version
socket type
rpc/protocol[,sndbuf=size][,rcvbuf=size]
wait/nowait[.max]
user[.group] or user[:group]
server program
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
address
inetd
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
gethostbyname
(). If a hostname has multiple
address mappings, inetd creates a socket to listen on each address.
The single character “*” indicates
INADDR_ANY
, 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 file
/etc/services or a port
number. For “internal” services (discussed below), the service
name
must 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
/etc/rpc. 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.
The
protocol must be a valid protocol as given in
/etc/protocols or “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 of “unix” is used to
specify a socket in the
UNIX-domain.
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:
tcp,rcvbuf=16384
tcp,sndbuf=64k
tcp,rcvbuf=64k,sndbuf=1m
A literal value may be specified, or modified using ‘k’ to
indicate kilobytes or ‘m’ to indicate megabytes.
The
wait/nowait 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.
comsat(8)
(
biff(1)) and
talkd(8) are both examples of the latter type of
datagram server. 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
option above.
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.
inetd
will continue to listen on the master
socket for connections, so the server should not close it when it exits.
The
user 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[0], which is the name of the
program. If the service is provided internally, the word
“internal” should take the place of this entry.
inetd
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 Information Center.
inetd
rereads its configuration file when it
receives a hangup signal,
SIGHUP
. 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
identifier.
libwrap¶
Support for TCP wrappers is included with
inetd
to provide 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
uses
/etc/hosts.allow and
/etc/hosts.deny for access control facility
configurations, as described in
hosts_access(5).
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 “tcp6”.
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 “tcp6”.
- 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.
SEE ALSO¶
fingerd(8),
ftpd(8),
identd(8),
talkd(8)
HISTORY¶
The
inetd
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.
BUGS¶
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
inetd
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.)