inetd, inetd.conf — internet super-server
inetd [-d] [-E] [-i] [-l] [-q length] [-R rate] [configuration_file]
inetd 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:
Turns on debugging.
Prevents inetd from laundering the environment. Without this option a selection of potentially harmful environment variables, including PATH, will be removed and not inherited by services.
Makes the program not daemonize itself.
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.
Specify the length of the listen(2) connections queue; the default is 128.
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:
user[.group] or user[:group]
server program arguments
To specify a Sun-RPC based service, the entry would contain these fields.
user[.group] or user[:group]
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 getaddrinfo(). 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” or “dgram”, depending on whether the socket is a stream or datagram 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 both IPv4 and 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:
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. 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, 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.
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).
The default is to run two servers: one for IPv4 and one for IPv6 traffic. If you have different requirements then you may specify one or 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 the server on “tcp6”, which is identical to the default behaviour when only “tcp” is specified.
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.
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.
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.)
BSD February 10, 2020 BSD