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.
-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
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
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.
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
.Ux
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.
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
.Ux
domain.
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.
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
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.
BUGS
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.)