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inetd - internet "super-server"


Implementation Notes
TCP Wrappers
Ux Domain Sockets
See Also


inetd [-d] [-l] [-w] [-W] [-c maximum] [-C rate] [-a address | hostname] [-p filename] [-R rate] [-s maximum] [configuration file]


The inetd utility 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 following options are available:

-d Turn on debugging.
-l Turn on logging of successful connections.
-w Turn on TCP Wrapping for external services. See the "IMPLEMENTATION NOTES" section for more information on TCP Wrappers support.
-W Turn on TCP Wrapping for internal services which are built in to inetd.
-c maximum
Specify the default maximum number of simultaneous invocations of each service; the default is unlimited. May be overridden on a per-service basis with the "max-child" parameter.
-C rate
Specify the default maximum number of times a service can be invoked from a single IP address in one minute; the default is unlimited. May be overridden on a per-service basis with the "max-connections-per-ip-per-minute" parameter.
-R rate
Specify the maximum number of times a service can be invoked in one minute; the default is 256. A rate of 0 allows an unlimited number of invocations.
-s maximum
Specify the default maximum number of simultaneous invocations of each service from a single IP address; the default is unlimited. May be overridden on a per-service basis with the "max-child-per-ip" parameter.
-a Specify one specific IP address to bind to. Alternatively, a hostname can be specified, in which case the IPv4 or IPv6 address which corresponds to that hostname is used. Usually a hostname is specified when inetd is run inside a jail(8), in which case the hostname corresponds to the jail(8) environment.

When hostname specification is used and both IPv4 and IPv6 bindings are desired, one entry with the appropriate protocol type for each binding is required for each service in /etc/inetd.conf. For example, a TCP-based service would need two entries, one using "tcp4" for the protocol and the other using "tcp6". See the explanation of the /etc/inetd.conf protocol field below.

-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
server program
server program arguments

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

service name/version
socket type
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 Ns -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. TCPMUX requests are only accepted when the multiplexor service itself is enabled, above and beyond and specific TCPMUX-based servers; see the discussion of internal services below.

The service-name entry is the name of a valid service in the file /etc/services, or the specification of a Unix domain socket (see below). For "internal" services (discussed below), the service name should 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 or "unix". Examples are "tcp" or "udp", both of which imply IPv4 for backward compatibility. The names "tcp4" and "udp4" specify IPv4 only. The names "tcp6" and "udp6" specify IPv6 only. The names "tcp46" and "udp46" specify that the entry accepts both IPv4 and IPv6 connections via a wildcard AF_INET6 socket. If it is desired that the service is reachable via T/TCP, one should specify "tcp/ttcp", which implies IPv4 for backward compatibility. The name "tcp4/ttcp" specifies IPv4 only, while "tcp6/ttcp" specifies IPv6 only. The name "tcp46/ttcp" specify that the entry accepts both IPv6 and IPv6 connections via a wildcard AF_INET6 socket. Rpc based services are specified with the "rpc/tcp" or "rpc/udp" service type. One can use specify IPv4 and/or IPv6 with the 4, 6 or 46 suffix, for example "rpc/tcp6" or "rpc/udp46". TCPMUX services must use "tcp", "tcp4", "tcp6" or "tcp46".

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 receive 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". The comsat(8), (biff(1)) and talkd(8) utilities are both examples of the latter type of datagram server. The tftpd(8) utility 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. You can also specify the maximum number of connections per minute for a given IP address by appending a "/" followed by the number to the maximum number of outstanding child processes. Once the maximum is reached, further connections from this IP address will be dropped until the end of the minute. In addition, you can specify the maximum number of simultaneous invocations of each service from a single IP address by appending a "/" followed by the number to the maximum number of outstanding child processes. Once the maximum is reached, further connections from this IP address will be dropped.

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. Optional group part separated by ":" allows to specify group name different than default group for this user. Optional login-class part separated by "/" allows to specify login class different than default "daemon" login class.

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 service-name of the service (and any arguments to it) or the word "internal" should take the place of this entry.

Currently, the only internal service to take arguments is "auth". Without options, the service will always return "ERROR : HIDDEN-USER". The available arguments to this service that alter its behavior are:

-d fallback
Provide a fallback username. If the real "auth" service is enabled (with the -r option discussed below), return this username instead of an error when lookups fail for either socket credentials or the username. If the real "auth" service is disabled, return this username for every request. This is primarily useful when running this service on a NAT machine.
-g Instead of returning the user’s name to the ident requester, report a username made up of random alphanumeric characters, e.g. "c0c993". The -g flag overrides not only the user names, but also any fallback name, .fakeid or .noident files.
-t -Xo sec [.usec]
Specify a timeout for the service. The default timeout is 10.0 seconds.
-r Offer a real "auth" service, as per RFC 1413. All the remaining flags apply only in this case.
-i Return numeric user IDs instead of usernames.
-f If the file .fakeid exists in the home directory of the identified user, report the username found in that file instead of the real username. If the username found in .fakeid is that of an existing user, then the real username is reported. If the -i flag is also given then the username in .fakeid is checked against existing user IDs instead.
-F same as -f but without the restriction that the username in .fakeid must not match an existing user.
-n If the file .noident exists in the home directory of the identified user, return "ERROR : HIDDEN-USER". This overrides any fakeid file which might exist.
-o osname
Use osname instead of the name of the system as reported by uname(3).

The inetd utility also provides several other "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.

The TCPMUX-demultiplexing service is also implemented as an internal service. For any TCPMUX-based service to function, the following line must be included in inetd.conf:
tcpmux stream tcpnowait root internal

When given the -l option inetd will log an entry to syslog each time a connection is accepted, noting the service selected and the IP-number of the remote requester if available. Unless otherwise specified in the configuration file, and in the absence of the -W and -w options, inetd will log to the "daemon" facility.

The inetd utility 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.


TCP Wrappers

When given the -w option, inetd will wrap all services specified as "stream nowait" or "dgram" except for "internal" services. If the -W option is given, such "internal" services will be wrapped. If both options are given, wrapping for both internal and external services will be enabled. Either wrapping option will cause failed connections to be logged to the "auth" syslog facility. Adding the -l flag to the wrapping options will include successful connections in the logging to the "auth" facility.

Note that inetd only wraps requests for a "wait" service while no servers are available to service requests. Once a connection to such a service has been allowed, inetd has no control over subsequent connections to the service until no more servers are left listening for connection requests.

When wrapping is enabled, the tcpd daemon is not required, as that functionality is builtin. For more information on TCP Wrappers, see the relevant documentation (hosts_access(5)). When reading that document, keep in mind that "internal" services have no associated daemon name. Therefore, the service name as specified in inetd.conf should be used as the daemon name for "internal" services.


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.


The implementation includes a tiny hack to support IPsec policy settings for each socket. A special form of comment line, starting with "#@", is interpreted as a policy specifier. Everything after the "#@" will be used as an IPsec policy string, as described in ipsec_set_policy(3). Each policy specifier is applied to all the following lines in inetd.conf until the next policy specifier. An empty policy specifier resets the IPsec policy.

If an invalid IPsec policy specifier appears in inetd.conf, inetd will provide an error message via the syslog(3) interface and abort execution.

Ux Domain Sockets

In addition to running services on IP sockets, inetd can also manage Unix domain sockets. To do this you specify a protocol of "unix" and specify the Unix domain socket as the service-name. The service-type may be "stream" or "dgram". The specification of the socket must be an absolute path name, optionally prefixed by an owner and mode of the form :user:group:mode:. The specification:


creates a socket owned by user "news" in group "daemon" with permissions allowing only that user and group to connect. The default owner is the user that inetd is running as. The default mode only allows the socket’s owner to connect.

WARNING: while creating Unix domain socket, inetd must change the ownership and permissions on the socket. This can only be done securely if the directory in which the socket is created is writable only by root. Do NOT use inetd to create sockets in world writable directories, such as /tmp, instead use /var/run or a similar directory.

Internal services may be run on Unix domain sockets, in the usual way. In this case the name of the internal service is determined using the last component of the socket’s pathname.


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


Here are several example service entries for the various types of services:
ftpstream tcp nowait root /usr/libexec/ftpd ftpd -l
ntalk dgram udp wait root /usr/libexec/ntalkd ntalkd
telnet stream tcp6 nowait root /usr/libexec/telnetd telnetd
shell stream tcp46 nowait root /usr/libexec/rshd rshd
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
/var/run/echo stream unix nowait root internal
#@ ipsec ah/require
chargen stream tcp nowait root internal


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: No such user user, service ignored
service / protocol: getpwnam: user: No 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 utility attempted to renounce the privileged state associated with a socket but was unable to.


ipsec_set_policy(3), hosts_access(5), hosts_options(5), login.conf(5), passwd(5), rpc(5), services(5), comsat(8), fingerd(8), ftpd(8), rlogind(8), rpcbind(8), rshd(8), telnetd(8), tftpd(8)
.Rs Identification Protocol


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