27.2. The inetd Super-Server

Contributed by Chern Lee.

27.2.1. Overview

inetd(8) is 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 one instance of inetd reduces the overall system load as 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.

27.2.2. Settings

inetd is initialized through the /etc/rc.conf system. The inetd_enable option is set to NO by default, but is often times turned on by sysinstall with the medium security profile. Placing:

inetd_enable="YES"

or

inetd_enable="NO"

into /etc/rc.conf can enable or disable inetd starting at boot time.

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

27.2.3. Command-Line Options

inetd synopsis:

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

-d

Turn on debugging.

-l

Turn on logging of successful connections.

-w

Turn on TCP Wrapping for external services (on by default).

-W

Turn on TCP Wrapping for internal services which are built into inetd (on by default).

-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.

-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.

-p

Specify an alternate file in which to store the process ID.

These options can be passed to inetd using the inetd_flags option in /etc/rc.conf. By default, inetd_flags is set to -wW, which turns on TCP wrapping for inetd's internal and external services. For novice users, these parameters usually do not need to be modified or even entered in /etc/rc.conf.

注意:

An external service is a daemon outside of inetd, which is invoked when a connection is received for it. On the other hand, an internal service is one that inetd has the facility of offering within itself.

27.2.4. inetd.conf

Configuration of inetd is controlled through the /etc/inetd.conf file.

When a modification is made to /etc/inetd.conf, inetd can be forced to re-read its configuration file by sending a HangUP signal to the inetd process as shown:

範例 27.1. Sending inetd a HangUP Signal
# kill -HUP `cat /var/run/inetd.pid`

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

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

An example entry for the ftpd daemon using IPv4:

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]]

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.

In addition to max-child, another option limiting the maximum connections from a single place to a particular daemon can be enabled. max-connections-per-ip-per-minute does just this. A value of ten here would limit any particular IP address connecting to a particular service to ten attempts per minute. This is useful to prevent intentional or unintentional resource consumption and Denial of Service (DoS) attacks to a machine.

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

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

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

Additionally, 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 all utilized by the default settings of the fingerd daemon, as seen here:

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

27.2.5. Security

Depending on the security profile chosen at install, many of inetd's daemons may be enabled by default. If there is no apparent need for a particular daemon, disable it! Place a # in front of the daemon in question in /etc/inetd.conf, and then send a hangup signal to inetd. Some daemons, such as fingerd, may not be desired at all because they provide an attacker with too much information.

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 and max-child limitations on certain daemons.

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.

27.2.6. Miscellaneous

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

The auth service provides identity (ident, identd) network services, and is configurable to a certain degree.

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

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