Red Hat System Admin - Glossary

Action flag

A flag that modifies the behavior of a procmail recipe's action.

Action line

A line in a procmail recipe that specifies the recipe's action.

Agent

A program in the email system that inspects an email and decides how to move it further toward the ultimate destination.

Automount point

A directory monitored by automount.

Backdoor

A hole placed in you r security by a cracker. It allows the intruder to gain easy access to your system by bypassing normal security.

Bit bucket

UNIX jargon for /dev/null. It is a device that data can be written into, but never retrieved from. In effect, it's an information trashcan.

Blacklist

A list of known email abusers, such as spammers.

Bounce

To resend an email to another recipient, but retain the original email envelope. This makes an email you bounce to someone look as though he or she received it from the original sender.

Checksum

A value computed from a quantity of data and transmitted alongside the data. The receiver can verify the data's integrity by computing the checksum again and comparing the newly computed value to the transmitted one.

Client-server model

A computer design model, where servers offer up one or more services for client use.

Condition

The regular expressions in a procmail recipe.

Condition flags

Flags that modify a procmail recipe's condition.

Cracker

An individual who breaks into systems or breaks copy protection of software products.

Daemon

A daemon is a program that waits for a request from another program. The daemon then performs the desired action, such as creating an http session, or opening and maintaining a communications socket. Some common daemons include httpd, telnetd, and ftpd.

Delivering recipes

Recipes that save the email to a folder.

Delivery headers

Routing and control information found at the beginning of every email. Common headers include the destination (the "To:" header), the sender (the "From:" header), and the subject (the "Subj:" header).

Domain Name System (DNS)

The Internet system for translating IP addresses into names and vice-versa.

Epoch

In UNIX parlance, the epoch is the beginning of UNIX time. All events in the system (for example creating a file) are measured relative to the epoch. Most UNIX systems, Linux included, use January 1, 1970 12:00AM as the epoch, and all other times are measured as seconds after this date.

Extended Simple Mail Transfer Protocol (ESMTP)

An extension of SMTP, ESMTP is a ubiquitous protocol for email transfer.

File access semantics

When a file is opened, closed, read from, or written to, certain guidelines, or semantics must be in place. These semantics are "right-of-way" indicators, describing what happens when programs make conflicting requests. For example, a read/write semantic would define who has the right-of-way when one program reads from a file that is being written to by another program.

File lock

A flag the operating system sets to prevent multiple programs from updating the same file simultaneously.

File Transfer Protocol (FTP)

FTP is one way to move a file from computer to computer.

Fully-Qualified Domain Names (FQDN)

A domain name that has all required DNS information: the host, network, and top-level domain.

Gateway

An entrance into another network, responsible for routing packets into its own network or onto a peer network.

Heterogeneous environments

A computing environment in which differing operating systems interact.

Host-based access control

Host-based access control allows you to specify the hosts and networks that may access your system's services.

Inbox

A user's primary email mailbox, into which all unfiltered mail flows.

Internet Assigned Numbers Authority (IANA)

The IANA is responsible for assigned standard port numbers to well-known Internet services, such as HTTP, FTP, and DNS.

Internet Message Access Protocol (IMAP)

A standard protocol for accessing mail from your local MDA.

Internet Protocol (IP)

The fundamental unit for sending data across the Internet.

Mail Abuse Prevention System (MAPS)

A nonprofit organization dedicated to defending the Internet's email system from abuse.

Mail Delivery Agents (MDA)

The email agents responsible for local network email delivery.

Mail Exchanger (MX)

A special host designated to handle all the network's email.

Mail Transfer Agents (MTA)

The email agents responsible for Internet-wide email delivery.

Mail User Agents (MUA)

Programs responsible for displaying and composing email.

MD5 check

Uses a message digest algorithm to determine file integrity.

Metacharacter

A character that has a special meaning to a regular expression.

MX records

Records in the DNS that describe which host handles mail for a given domain.

NetBIOS

A network protocol that allows hosts to have separate names--tied directly with their capabilities--that may be queried in real-time.

Network File System (NFS)

A network filesystem that allows remote filesystems to attach to the local filesystem. Once attached, remote files are accessible the same way as local files.

Non-delivering recipes

Recipes that do not save email to a folder, but perform some other action (such as sending the email to an external program) on the email.

Packet

A unit of information, usually composed of two parts: a header with controlling information and a body with actual information.

POP3

An acronym for Post Office Protocol, version 3, POP3 is an older protocol that enabled MUAs to send and receive mail from MDAs.

Process accounting

Maintains an account of every process ever executed.

Process auditing

Examines the process accounting logs and retrieves useful information.

Protocol

A grammar for communicating an idea between two end points.

Real-time Blackhole List (RBL)

A system designed to block sites known for spamming.

Recipes

A concise set of procmail instructions that filter or otherwise manipulate an email.

Regular expressions

A string expression that uses special metacharacters (wildcards, for example) to match zero or more strings. Regular expressions are the foundation for many Linux programs, including the UNIX command line file-matching routines (e.g., "ls myfile.*"), grep (e.g., "grep bar foo.txt"), and procmail.

Rejection banners

Rejection banners are TCP wrappers messages displayed when a connection is denied.

Remote procedure calls (RPC)

A method for invoking a procedure (a specific section within a program) on a remote computer.

RPM

An acronym for Red Hat Package Manager, RPMS is a standard source and binary distribution format.

Samba

A collection of programs that implements SMB on Linux.

Server Message Blocks (SMB)

The core of file and printer sharing under Microsoft Windows operating systems.

Simple Mail Transfer Protocol (SMTP)

The Internet's fundamental mail transfer protocol.

Sniffer

A program that looks at all traffic on the network, trying to gain access to other systems.

Social engineering

The use of social techniques, such as masquerading as a system's administrator, in order to gain access to confidential user information like a password.

SPAM

A common name for unsolicited email, named after a Monty Python sketch.

Spammer

Anyone who sends SPAM.

Spoofing

Faking a hostname to bypass one or more security mechanisms.

Standalone server

A server that opens, maintains, and closes its own network connections.

Superserver

A server process responsible for listening on a group of ports and spawning other servers when connections arrive for them.

System accounting

Refers to the creation and maintenance of logs, which keep track of processes executed, user activity, and network connections.

Tcpd

A program that provides host-based security for many Linux Internet applications.

Transient servers

A server that performs its service for a single client, starting fresh every time a client request arrives. Transient servers do not listen for client requests, but rather are spawned by another program.

Transmission Control Protocol (TCP)

A connected, reliable method for sending data across a network.

Unexport

The opposite of exporting. A jargon term meaning to make a previously exported NFS filesystem unavailable.

User auditing

Examines the system logs to determine information about user access.

User Datagram Protocol (UDP)

A connectionless, unreliable method for delivering packets across the Internet.

Vacuously

A statement applies vacuously when it is obviously true in all contexts.

Variable definitions

Assigning a value to a variable.

Well-known services (WKS)

WKSs are services that are prevalent across the Internet. Examples include HTTP for World Wide Web traffic, FTP for File Transfer Protocol, and DNS for the Domain Name System.

Whitespace

As the name implies, any space between text that is empty. Whitespace includes spaces, tabs, new lines, and form feeds, though programs may restrict what they consider to be whitespace to just a subset of these.

Windows domain

A more organized form of Windows workgroup.

Windows Internet Name Server (WINS)

A Microsoft Windows NT component, a WINS server can dynamically update hostnames to IP addresses without requiring manual intervention from the system administrator.

Workgroup

In Windows terminology, a group of computers acting as part of a cohesive working unit that can share resources allocated to that workgroup.