The export command makes the shell variable named VARIABLE visible to subshells. This means that any child process that is spawned from the current shell will inherit the value of VARIABLE. The export command does not need a dollar sign ($) before the variable name, as that would expand the variable to its value. The set command only affects the current shell and does not export the variable to subshells. The env command can be used to run a command in a modified environment, but it does not export the variable to subshells either. Reference:
[LPI Linux Essentials - Topic 105: Shells, Scripting and Data Management]
[LPI Linux Administrator - Exam 102 Objectives - Topic 105: Shells and Shell Scripting]

The logrotate command is a tool for rotating, compressing, and mailing system logs. It is designed to ease the administration of systems that generate large numbers of log files. It allows automatic rotation, compression, removal, and mailing of log files. Each log file may be handled daily, weekly, monthly, or when it grows too large. Normally, logrotate is run as a daily cron job1. The logrotate command reads the configuration files specified on the command line or in the /etc/logrotate.conf and /etc/logrotate.d directories. These configuration files can set global options and specify log files to rotate and how to handle them. For example, the compress option enables compression of old log files, the mail option sends the log files to a specified email address before being rotated, and the rotate option sets the number of log files to keep12. The logrotate command is part of the LPI's multi-level Linux professional certification program, and it is covered in the topic 106.1 System logging of the exam 102objectives3. References: 1: logrotate(8) - Linux man page 2: logrotate command in Linux with examples - Linux command line tutorial 3: Exam 102 Objectives

The tcpd program is a wrapper for network services that use the TCP protocol. It intercepts incoming connection requests and checks them against the rules specified in the /etc/hosts.allow and /etc/hosts.deny files. If the connection is allowed, tcpd executes the actual service program and passes the connection to it. If the connection is denied, tcpd logs the attempt and sends anerror message to the client. The tcpd program can be used to enhance the security and control of network access to various services, such as SSH, FTP, Telnet, etc.
The other programs listed are not directly related to the hosts.allow file, although they may be affected by it if they are wrapped by tcpd. The inetd and xinetd programs are super-servers that listen for incoming connections and launch the appropriate service program. The fingerd program is a service that provides information about users on a remote system. The mountd program is a service that handles NFS mount requests from clients. References:
* tcpd(8) - Linux man page
* Control server access using hosts.allow and hosts.deny files
* hosts.allow format and example on Linux

The configuration file that contains the default options for SSH clients is /etc/ssh/ssh_config. This file is read by the ssh program when it connects to a remote SSH server. It can contain global options that apply to all hosts, or host-specific options that only apply to certain hosts or patterns. The /etc/ssh/ssh_config file is the system-wide default SSH client configuration file. It can be overridden by a user-specific configuration file ~/.ssh/config, which is located in the user's home directory. The /etc/ssh/ssh_config file is part of the topic 110.1: Perform security administration tasks, which is one of the objectives of the LPI Linux Administrator - 102 exam12. Reference: 1: LPI Linux Administrator - 102 (LPIC-1) 2: Exam 102 Objectives