NAME
inetd - internet “super-server”
SYNOPSIS
inetd [-d] [-E] [-i] [-l] [-R rate] [configuration file]
DESCRIPTION
inetd should be run at boot time by /etc/rc (see rc(8)). It then 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 environent variables,
including PATH, will be removed and not inherited by services.
-d 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.
-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. tftpd(8) 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 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), rshd(8), talkd(8), tftpd(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.)