29.2. The inetd «Super-Server»

Contributed by Chern Lee.
Updated for FreeBSD 6.1-RELEASE by The FreeBSD Documentation Project.

29.2.1. Overview

inetd(8) is sometimes referred to as the «Internet Super-Server» because it manages connections for several services. When a connection is received by inetd, it determines which program the connection is destined for, spawns the particular process and delegates the socket to it (the program is invoked with the service socket as its standard input, output and error descriptors). Running inetd for servers that are not heavily used can reduce the overall system load, when compared to running each daemon individually in stand-alone mode.

Primarily, inetd is used to spawn other daemons, but several trivial protocols are handled directly, such as chargen, auth, and daytime.

This section will cover the basics in configuring inetd through its command-line options and its configuration file, /etc/inetd.conf.

29.2.2. Settings

inetd is initialized through the rc(8) system. The inetd_enable option is set to NO by default, but may be turned on by sysinstall during installation, depending on the configuration chosen by the user. Placing:

inetd_enable="YES"

or

inetd_enable="NO"

into /etc/rc.conf will enable or disable inetd starting at boot time. The command:

/etc/rc.d/inetd rcvar

can be run to display the current effective setting.

Additionally, different command-line options can be passed to inetd via the inetd_flags option.

29.2.3. Command-Line Options

Like most server daemons, inetd has a number of options that it can be passed in order to modify its behaviour. The full list of options reads:

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

Options can be passed to inetd using the inetd_flags option in /etc/rc.conf. By default, inetd_flags is set to -wW -C 60, which turns on TCP wrapping for inetd's services, and prevents any single IP address from requesting any service more than 60 times in any given minute.

Novice users may be pleased to note that these parameters usually do not need to be modified, although we mention the rate-limiting options below as they be useful should you find that you are receiving an excessive amount of connections. A full list of options can be found in the inetd(8) manual.

-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 maximum number of times a service can be invoked from a single IP address at any one time; the default is unlimited. May be overridden on a per-service basis with the max-child-per-ip parameter.

29.2.4. inetd.conf

Configuration of inetd is done via the file /etc/inetd.conf.

When a modification is made to /etc/inetd.conf, inetd can be forced to re-read its configuration file by running the command:

Παράδειγμα 29.1. Reloading the inetd configuration file
# /etc/rc.d/inetd reload

Each line of the configuration file specifies an individual daemon. Comments in the file are preceded by a «#». The format of each entry in /etc/inetd.conf is as follows:

service-name
socket-type
protocol
{wait|nowait}[/max-child[/max-connections-per-ip-per-minute[/max-child-per-ip]]]
user[:group][/login-class]
server-program
server-program-arguments

An example entry for the ftpd(8) daemon using IPv4 might read:

ftp     stream  tcp     nowait  root    /usr/libexec/ftpd       ftpd -l
service-name

This is the service name of the particular daemon. It must correspond to a service listed in /etc/services. This determines which port inetd must listen to. If a new service is being created, it must be placed in /etc/services first.

socket-type

Either stream, dgram, raw, or seqpacket. stream must be used for connection-based, TCP daemons, while dgram is used for daemons utilizing the UDP transport protocol.

protocol

One of the following:

ProtocolExplanation
tcp, tcp4TCP IPv4
udp, udp4UDP IPv4
tcp6TCP IPv6
udp6UDP IPv6
tcp46Both TCP IPv4 and v6
udp46Both UDP IPv4 and v6
{wait|nowait}[/max-child[/max-connections-per-ip-per-minute[/max-child-per-ip]]]

wait|nowait indicates whether the daemon invoked from inetd is able to handle its own socket or not. dgram socket types must use the wait option, while stream socket daemons, which are usually multi-threaded, should use nowait. wait usually hands off multiple sockets to a single daemon, while nowait spawns a child daemon for each new socket.

The maximum number of child daemons inetd may spawn can be set using the max-child option. If a limit of ten instances of a particular daemon is needed, a /10 would be placed after nowait. Specifying /0 allows an unlimited number of children

In addition to max-child, two other options which limit the maximum connections from a single place to a particular daemon can be enabled. max-connections-per-ip-per-minute limits the number of connections from any particular IP address per minutes, e.g. a value of ten would limit any particular IP address connecting to a particular service to ten attempts per minute. max-child-per-ip limits the number of children that can be started on behalf on any single IP address at any moment. These options are useful to prevent intentional or unintentional excessive resource consumption and Denial of Service (DoS) attacks to a machine.

In this field, either of wait or nowait is mandatory. max-child, max-connections-per-ip-per-minute and max-child-per-ip are optional.

A stream-type multi-threaded daemon without any max-child, max-connections-per-ip-per-minute or max-child-per-ip limits would simply be: nowait.

The same daemon with a maximum limit of ten daemons would read: nowait/10.

The same setup with a limit of twenty connections per IP address per minute and a maximum total limit of ten child daemons would read: nowait/10/20.

These options are utilized by the default settings of the fingerd(8) daemon, as seen here:

finger stream  tcp     nowait/3/10 nobody /usr/libexec/fingerd fingerd -s

Finally, an example of this field with a maximum of 100 children in total, with a maximum of 5 for any one IP address would read: nowait/100/0/5.

user

This is the username that the particular daemon should run as. Most commonly, daemons run as the root user. For security purposes, it is common to find some servers running as the daemon user, or the least privileged nobody user.

server-program

The full path of the daemon to be executed when a connection is received. If the daemon is a service provided by inetd internally, then internal should be used.

server-program-arguments

This works in conjunction with server-program by specifying the arguments, starting with argv[0], passed to the daemon on invocation. If mydaemon -d is the command line, mydaemon -d would be the value of server-program-arguments. Again, if the daemon is an internal service, use internal here.

29.2.5. Security

Depending on the choices made at install time, many of inetd's services may be enabled by default. If there is no apparent need for a particular daemon, consider disabling it. Place a «#» in front of the daemon in question in /etc/inetd.conf, and then reload the inetd configuration. Some daemons, such as fingerd, may not be desired at all because they provide information that may be useful to an attacker.

Some daemons are not security-conscious and have long, or non-existent, timeouts for connection attempts. This allows an attacker to slowly send connections to a particular daemon, thus saturating available resources. It may be a good idea to place max-connections-per-ip-per-minute, max-child or max-child-per-ip limitations on certain daemons if you find that you have too many connections.

By default, TCP wrapping is turned on. Consult the hosts_access(5) manual page for more information on placing TCP restrictions on various inetd invoked daemons.

29.2.6. Miscellaneous

daytime, time, echo, discard, chargen, and auth are all internally provided services of inetd.

The auth service provides identity network services, and is configurable to a certain degree, whilst the others are simply on or off.

Consult the inetd(8) manual page for more in-depth information.

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