INETD(8)

NAME
inetd - internet “super-server”

SYNOPSIS
inetd [-d] [-l] [-R rate] [-a address] [-p filename] [configuration file]

DESCRIPTION
The inetd program 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. The server program is invoked with the service socket as its standard input, output and error descriptors. After the program is finished, inetd 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 inetd:
-d  Turns on debugging.
-l  Turns on logging.
-R rate
    Specifies the maximum number of times a service can be invoked in one minute; the default is 256.
-a  Specify a specific IP address to bind to.
-p  Specify an alternate file in which to store the process ID.

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. The fields of the configuration file are as follows:

    service name
    socket type
    protocol
    {wait|nowait}[/max-child]
    user
    server program
    server program arguments

To specify an ONC RPC-based service, the entry would contain these fields:

    service name/version
    socket type
    rpc/protocol
    user
    server program
    server program arguments

There are two types of services that inetd can start: standard and TCPMUX. A standard service has a well-known port assigned to it; it may be a service that implements an official Internet standard or is a BSD-specific service. As described in RFC 1078, TCPMUX services are nonstandard services that do not have a well-known port assigned to them. They are invoked from inetd when a program connects to the “tcpmux” well-known port and specifies the service name. This feature is useful for adding locally-developed servers.

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 an ONC 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 TCPMUX services, the value of the service-name field consists of the string “tcpmux” followed by a slash and the locally-chosen service name. The service names listed in /etc/services and the name “help” are reserved. Try to choose unique names for your TCPMUX services by prefixing them with your organization's name and suffixing them with a version number.

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. TCPMUX services must use “stream”.

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. TCPMUX services must use “tcp”.

The wait/nowait entry specifies whether the server that is invoked by inetd will take over the socket associated with the service access point, and thus whether inetd should wait for the server to exit before listening for new service requests. Datagram servers must use “wait”, as they are always invoked with the original datagram socket bound to the specified service address. These servers must read at least one datagram from the socket before exiting. If a datagram server connects to its peer, freeing the socket so inetd can received further messages on the socket, it is said to be a “multi-threaded” server; it should read one datagram from the socket and create a new socket connected to the peer. It should fork, and the parent should then exit to allow inetd to check for new service requests to spawn new servers. Datagram servers which process all incoming datagrams on a socket and eventually time out are said to be “single-threaded”. Comsat(8), (biff(1)) and talkd(8) are both examples of the latter type of datagram server. Tftpd(8) is an example of a multi-threaded datagram server.

Servers using stream sockets generally are multi-threaded and use the “nowait” entry. Connection requests for these services are accepted by inetd, and the server is given only the newly-accepted socket connected to a client of the service. Most stream-based services operate in this manner. Stream-based servers that use “wait” are started with the listening service socket, and must accept at least one connection request before exiting. Such a server would normally accept and process incoming connection requests until a timeout. TCPMUX services must use “nowait”.

The maximum number of outstanding child processes (or ``threads'') for a ``nowait'' service may be explicitly specified by appending a ``/'' followed by the number to the ``nowait'' keyword. Normally (or if a value of zero is specified) there is no maximum. Otherwise, once the maximum is reached, further connection attempts will be queued up until an existing child process exits. This also works in the case of ``wait'' mode, although a value other than one (the default) might not make sense in some cases.

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.

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.

The inetd program 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 available in both TCP and UDP versions; the UDP versions will refuse service if the request specifies a reply port corresponding to any internal service. (This is done as a defense against looping attacks; the remote IP address is logged.) For details of these services, consult the appropriate RFC document.

When given the -l option Inetd will log an entry to syslog each time an accept(2) is made, which notes the service selected and the IP-number of the remote requestor.

The inetd program rereads its configuration file when it receives a hangup signal, SIGHUP. Services may be added, deleted or modified when the configuration file is reread. Except when started in debugging mode, inetd records its process ID in the file /var/run/inetd.pid to assist in reconfiguration.

TCPMUX

RFC 1078 describes the TCPMUX protocol: ``A TCP client connects to a foreign host on TCP port 1. It sends the service name followed by a carriage-return line-feed <CRLF>. The service name is never case sensitive. The server replies with a single character indicating positive (+) or negative (-) acknowledgment, immediately followed by an optional message of explanation, terminated with a <CRLF>. If the reply was positive, the selected protocol begins; otherwise the connection is closed.'' The program is passed the TCP connection as file descriptors 0 and 1.

If the TCPMUX service name begins with a ``+'', inetd returns the positive reply for the program. This allows you to invoke programs that use stdin/stdout without putting any special server code in them.

The special service name “help” causes inetd to list TCPMUX services in inetd.conf.

FILES
/etc/inetd.conf
    configuration file.
/etc/rpc
    translation of service names to RPC program numbers.
/etc/services
    translation of service names to port numbers.
/var/run/inetd.pid
    the pid of the currently running inetd.

EXAMPLES

Here are several example service entries for the various types of services:

    ftp          stream  tcp   nowait root  /usr/libexec/ftpd        ftpd -l
    ntalk        dgram   udp   wait   root  /usr/libexec/ntalkd      ntalkd
    tcpmux/+date stream  tcp   nowait guest /bin/date                date
    tcpmux/phonebook stream tcp nowait guest /usr/local/bin/phonebook phonebook
    rstatd/1-3   dgram   rpc/udp wait root  /usr/libexec/rpc.rstatd  rpc.rstatd

ERROR MESSAGES
The inetd server logs error messages using syslog(3). Important error messages and their explanations are:

service/protocol server failing (looping), service terminated. The number of requests for the specified service in the past minute exceeded the limit. The limit exists to prevent a broken program or a malicious user from swamping the system. This message may occur for several reasons:

1.  There are many hosts requesting the service within a short time period.

2.  A broken client program is requesting the service too frequently.

3.  A malicious user is running a program to invoke the service in a denial-of-service attack.

4.  The invoked service program has an error that causes clients to retry quickly.

Use the -R rate option, as described above, to change the rate limit. Once the limit is reached, the service will be reenabled automatically in 10 minutes.

service/protocol: such user user, service ignored

service/protocol: getpwnam: user: such user No entry for user exists in the passwd(5) database. The first message occurs when inetd (re)reads the configuration file. The second message occurs when the service is invoked.

service: can't set uid uid

service: can't set gid gid The user or group ID for the entry's user field is invalid.

setsockopt(SO_PRIVSTATE): Operation not supported
The inetd program attempted to renounce the privileged state associated with a socket but was unable to.

SEE ALSO
passwd(5), rpc(5), services(5), comsat(8), fingerd(8), ftpd(8), portmap(8), rexecd(8), rlogind(8), rshd(8), telnetd(8), tftpd(8)

HISTORY
The inetd command appeared in 4.3BSD. TCPMUX is based on code and documentation by Mark Lottor. Support for ONC RPC based services is modeled after that provided by SunOS 4.1.