INETD(8)

NAME
inetd, inetd.conf - internet “super-server”

SYNOPSIS
inetd [-d] [-l] [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 available for:

-d  Turns on debugging.

-l  Turns on libwrap connection logging.

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. There must be an entry for each field (except for one special case, described below). The fields of the configuration file are as follows:

    [addr:]service-name
    socket-type
    protocol[,sndbuf=size][,rcvbuf=size]
    wait/nowait[.max]
    user[.group]
    server-program
    server program arguments

To specify an 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]
    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 indiciates what local address inetd should use when listening for that service, or the single character “*” to indicate 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.

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”.

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.

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. Socket buffer sizes may be specified for all services and protocols except for tcpmux services.

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 server instances that may be spawned from inetd within an interval of 60 seconds. When omitted, “max” defaults to 40.

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. identd(8) is usually the only stream server marked as wait.

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). 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.

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.)

SEE ALSO
comsat(8), fingerd(8), ftpd(8), rexecd(8), rlogind(8), rshd(8), telnetd(8), tftpd(8), hosts_access(5), hosts_options(5)

HISTORY
The inetd command appeared in 4.3BSD. Support for Sun-RPC based services is modeled after that provided by SunOS 4.1. Support for specifying the socket buffer sizes was added in NetBSD 1.4.

SECURITY CONSIDERATIONS
Enabling the “echo”, “discard”, and “chargen” built-in trivial services is not recommended because remote users may abuse these to cause a denial of network service to or from the local host.