INETD(8) UNIX Programmer's Manual INETD(8) NAME inetd - internet ``super-server'' SYNOPSIS inetd [-d] [-R _r_a_t_e] [_c_o_n_f_i_g_u_r_a_t_i_o_n _f_i_l_e] DESCRIPTION The inetd program should be run at boot time by /_e_t_c/_r_c (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 pro- gram 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. -R _r_a_t_e Specifies the maximum number of times a service can be invoked in one minute; the default is 1000. Upon execution, inetd reads its configuration information from a configuration file which, by default, is /_e_t_c/_i_n_e_t_d._c_o_n_f. 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 user server program server program arguments There are two types of services that inetd can start: stan- dard and TCPMUX. A standard service has a well-known port assigned to it; it may be a service that implements an offi- cial Internet standard or is a BSD-specific service. As described in RFC 1078, TCPMUX services are nonstandard ser- vices 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. Printed 11/26/99 November 7, 1996 1 INETD(8) UNIX Programmer's Manual INETD(8) The _s_e_r_v_i_c_e-_n_a_m_e entry is the name of a valid service in the file /_e_t_c/_s_e_r_v_i_c_e_s. For ``internal'' services (discussed below), the service name must be the official name of the service (that is, the first entry in /_e_t_c/_s_e_r_v_i_c_e_s). For TCPMUX services, the value of the _s_e_r_v_i_c_e-_n_a_m_e field con- sists of the string ``tcpmux'' followed by a slash and the locally-chosen service name. The service names listed in /_e_t_c/_s_e_r_v_i_c_e_s 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 _s_o_c_k_e_t-_t_y_p_e should be one of ``stream'', ``dgram'', ``raw'', ``rdm'', or ``seqpacket'', depending on whether the socket is a stream, datagram, raw, reliably delivered mes- sage, or sequenced packet socket. TCPMUX services must use ``stream''. NOTE: ``rdm'' and ``seqpacket'' are not supported in 2.11BSD. The _p_r_o_t_o_c_o_l must be a valid protocol as given in /_e_t_c/_p_r_o_t_o_c_o_l_s. Examples might be ``tcp'' or ``udp''. TCPMUX services must use ``tcp''. The _w_a_i_t/_n_o_w_a_i_t 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 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 Printed 11/26/99 November 7, 1996 2 INETD(8) UNIX Programmer's Manual INETD(8) connection request before exiting. Such a server would nor- mally accept and process incoming connection requests until a timeout. TCPMUX services must use ``nowait''. The _u_s_e_r 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 _s_e_r_v_e_r-_p_r_o_g_r_a_m 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 inter- nally, this entry should be ``internal''. The _s_e_r_v_e_r _p_r_o_g_r_a_m _a_r_g_u_m_e_n_t_s 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 genera- tor), ``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. 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. TCPMUX RFC 1078 describes the TCPMUX protocol: ``A TCP client con- nects to a foreign host on TCP port 1. It sends the service name followed by a carriage-return line-feed . The service name is never case sensitive. The server replies with a single character indicating positive (+) or negative (-) acknowledgment, immediately followed by an optional mes- sage of explanation, terminated with a . If the reply was positive, the selected protocol begins; otherwise the connection is closed.'' The program is passed the TCP con- nection 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 _i_n_e_t_d._c_o_n_f. Printed 11/26/99 November 7, 1996 3 INETD(8) UNIX Programmer's Manual INETD(8) 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/phonebook phonebook ERROR MESSAGES The inetd server logs error messages using syslog(3). Important error messages and their explanations are: _s_e_r_v_i_c_e/_p_r_o_t_o_c_o_l server failing (looping), service ter- minated. 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 lots of hosts requesting the service within a short time period, 2) a 'broken' client program is requesting the ser- vice too frequently, 3) a malicious user is running a pro- gram to invoke the service in a 'denial of service' attack, or 4) the invoked service program has an error that causes clients to retry quickly. Use the -R option, as described above, to change the rate limit. Once the limit is reached, the service will be reenabled automatically in 10 minutes. _s_e_r_v_i_c_e/_p_r_o_t_o_c_o_l: No such user '_u_s_e_r', service ignored _s_e_r_v_i_c_e/_p_r_o_t_o_c_o_l: getpwnam: _u_s_e_r: No such user No entry for _u_s_e_r exists in the _p_a_s_s_w_d file. The first mes- sage occurs when inetd (re)reads the configuration file. The second message occurs when the service is invoked. _s_e_r_v_i_c_e: can't set uid _n_u_m_b_e_r _s_e_r_v_i_c_e: can't set gid _n_u_m_b_e_r The user or group ID for the entry's _u_s_e_r is invalid. SEE ALSO comsat(8), fingerd(8), ftpd(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. Printed 11/26/99 November 7, 1996 4