Virtual VIR'TUAL, a. [See Virtue.]
1. Potential; having the power of acting or of invisible efficacy without the material or sensible part.
Every kind that lives, fomented by his virtual power, and warm'd.
Neither an actual nor virtual intention of the mind, but only that which may be gathered from the outward acts.
2. Being in essence or effect, not in fact; as the virtual presence of a man in his agent or substitute.

virtual adj 1: being actually such in almost every respect; "a practical failure"; "the once elegant temple lay in virtual ruin" [syn: virtual(a), practical(a)] 2: existing in essence or effect though not in actual fact; "a virtual dependence on charity"; "a virtual revolution"; "virtual reality"

virtual adjective Etymology: Middle English, efficacious, potential, from Medieval Latin virtualis, from Latin virtus strength, virtue Date: 15th century 1. being such in essence or effect though not formally recognized or admitted <a virtual dictator> 2. of, relating to, or using virtual memory 3. of, relating to, or being a hypothetical particle whose existence is inferred from indirect evidence <virtual photons> — compare real 3 4. being on or simulated on a computer or computer network <print or virtual books> <a virtual keyboard>: as a. occurring or existing primarily online <a virtual library> <virtual shopping> b. of, relating to, or existing within a virtual reality <a virtual world> <a virtual tour>

virtual
adj.
1 that is such for practical purposes though not in name or according to strict definition (is the virtual manager of the business; take this as a virtual promise).
2 Optics relating to the points at which rays would meet if produced backwards (virtual focus; virtual image).
3 Mech. relating to an infinitesimal displacement of a point in a system.
4 Computing not physically existing as such but made by software to appear to do so (virtual memory).
Derivatives:
virtuality n. virtually adv.
Etymology: ME f. med.L virtualis f. L virtus after LL virtuosus

virtual 1. You can use virtual to indicate that something is so nearly true that for most purposes it can be regarded as true. Argentina came to a virtual standstill while the game was being played... ...conditions of virtual slavery. ADJ: ADJ n 2. Virtual objects and activities are generated by a computer to simulate real objects and activities. (COMPUTING) This is a virtual shopping centre offering visitors entry to a clutch of well-known e-tailers without going to their different websites. ADJ: ADJ n • virtuality People speculate about virtuality systems, but we're already working on it. N-UNCOUNT

virtual ˈvə:tjuəl adj. 1 that is such for practical purposes though not in name or according to strict definition (is the virtual manager of the business; take this as a virtual promise). 2 Optics relating to the points at which rays would meet if produced backwards (virtual focus; virtual image). 3 Mech. relating to an infinitesimal displacement of a point in a system. 4 Computing not physically existing as such but made by software to appear to do so (virtual memory). øøvirtuality n. virtually adv. [ME f. med.L virtualis f. L virtus after LL virtuosus]

Virtual \Vir"tu*al\ (?; 135), a. [Cf. F. virtuel. See Virtue.] 1. Having the power of acting or of invisible efficacy without the agency of the material or sensible part; potential; energizing. Heat and cold have a virtual transition, without communication of substance. --Bacon. Every kind that lives, Fomented by his virtual power, and warmed. --Milton. 2. Being in essence or effect, not in fact; as, the virtual presence of a man in his agent or substitute. A thing has a virtual existence when it has all the conditions necessary to its actual existence. --Fleming. To mask by slight differences in the manners a virtual identity in the substance. --De Quincey. Principle of virtual velocities (Mech.), the law that when several forces are in equilibrium, the algebraic sum of their virtual moments is equal to zero. Virtual focus (Opt.), the point from which rays, having been rendered divergent by reflection of refraction, appear to issue; the point at which converging rays would meet if not reflected or refracted before they reach it. Virtual image. (Optics) See under Image. Virtual moment (of a force) (Mech.), the product of the intensity of the force multiplied by the virtual velocity of its point of application; -- sometimes called virtual work}. Virtual velocity (Mech.), a minute hypothetical displacement, assumed in analysis to facilitate the investigation of statical problems. With respect to any given force of a number of forces holding a material system in equilibrium, it is the projection, upon the direction of the force, of a line joining its point of application with a new position of that point indefinitely near to the first, to which the point is conceived to have been moved, without disturbing the equilibrium of the system, or the connections of its parts with each other. Strictly speaking, it is not a velocity but a length. Virtual work. (Mech.) See Virtual moment, above.

virtual adj. [via the technical term `virtual memory', prob. from the term `virtual image' in optics] 1. Common alternative to logical; often used to refer to the artificial objects (like addressable virtual memory larger than physical memory) simulated by a computer system as a convenient way to manage access to shared resources. 2. Simulated; performing the functions of something that isn't really there. An imaginative child's doll may be a virtual playmate. Oppose real.

virtual a. 1. Substantial, constructive, practical, essential, equivalent, implied, implicit, indirect. 2. Potential, in essence or effect.

virtual ˈvə:tjuəl adj. effective, essential; practical, understood, accepted: In rush hours, the motorway traffic comes to a virtual standstill.

31 Moby Thesaurus words for "virtual": accepted, basic, between the lines, constructive, covert, cryptic, delitescent, dormant, effective, esoteric, essential, fundamental, hibernating, hidden, latent, lurking, muffled, mystic, obfuscated, obscured, occult, possible, potential, practical, sleeping, submerged, under the surface, underlying, understood, unmanifested, veiled

virtual VIRTUAL(8) VIRTUAL(8) NAME virtual - Postfix virtual domain mail delivery agent SYNOPSIS virtual [generic Postfix daemon options] DESCRIPTION The virtual(8) delivery agent is designed for virtual mail hosting ser- vices. Originally based on the Postfix local(8) delivery agent, this agent looks up recipients with map lookups of their full recipient address, instead of using hard-coded unix password file lookups of the address local part only. This delivery agent only delivers mail. Other features such as mail forwarding, out-of-office notifications, etc., must be configured via virtual_alias maps or via similar lookup mechanisms. MAILBOX LOCATION The mailbox location is controlled by the virtual_mailbox_base and vir- tual_mailbox_maps configuration parameters (see below). The vir- tual_mailbox_maps table is indexed by the recipient address as described under TABLE SEARCH ORDER below. The mailbox pathname is constructed as follows: $virtual_mailbox_base/$virtual_mailbox_maps(recipient) where recipient is the full recipient address. UNIX MAILBOX FORMAT When the mailbox location does not end in /, the message is delivered in UNIX mailbox format. This format stores multiple messages in one textfile. The virtual(8) delivery agent prepends a "From sender time_stamp" enve- lope header to each message, prepends a Delivered-To: message header with the envelope recipient address, prepends an X-Original-To: header with the recipient address as given to Postfix, prepends a Return-Path: message header with the envelope sender address, prepends a > character to lines beginning with "From ", and appends an empty line. The mailbox is locked for exclusive access while delivery is in progress. In case of problems, an attempt is made to truncate the mail- box to its original length. QMAIL MAILDIR FORMAT When the mailbox location ends in /, the message is delivered in qmail maildir format. This format stores one message per file. The virtual(8) delivery agent prepends a Delivered-To: message header with the final envelope recipient address, prepends an X-Original-To: header with the recipient address as given to Postfix, and prepends a Return-Path: message header with the envelope sender address. By definition, maildir format does not require application-level file locking during mail delivery or retrieval. MAILBOX OWNERSHIP Mailbox ownership is controlled by the virtual_uid_maps and vir- tual_gid_maps lookup tables, which are indexed with the full recipient address. Each table provides a string with the numerical user and group ID, respectively. The virtual_minimum_uid parameter imposes a lower bound on numerical user ID values that may be specified in any virtual_uid_maps. TABLE SEARCH ORDER Normally, a lookup table is specified as a text file that serves as input to the postmap(1) command. The result, an indexed file in dbm or db format, is used for fast searching by the mail system. The search order is as follows. The search stops upon the first suc- cessful lookup. o When the recipient has an optional address extension the user+extension@domain.tld address is looked up first. With Postfix versions before 2.1, the optional address extension is always ignored. o The user@domain.tld address, without address extension, is looked up next. o Finally, the recipient @domain is looked up. When the table is provided via other means such as NIS, LDAP or SQL, the same lookups are done as for ordinary indexed files. Alternatively, a table can be provided as a regular-expression map where patterns are given as regular expressions. In that case, only the full recipient address is given to the regular-expression map. SECURITY The virtual(8) delivery agent is not security sensitive, provided that the lookup tables with recipient user/group ID information are ade- quately protected. This program is not designed to run chrooted. The virtual(8) delivery agent disallows regular expression substitution of $1 etc. in regular expression lookup tables, because that would open a security hole. The virtual(8) delivery agent will silently ignore requests to use the proxymap(8) server. Instead it will open the table directly. Before Postfix version 2.2, the virtual delivery agent will terminate with a fatal error. STANDARDS RFC 822 (ARPA Internet Text Messages) DIAGNOSTICS Mail bounces when the recipient has no mailbox or when the recipient is over disk quota. In all other cases, mail for an existing recipient is deferred and a warning is logged. Problems and transactions are logged to syslogd(8). Corrupted message files are marked so that the queue manager can move them to the corrupt queue afterwards. Depending on the setting of the notify_classes parameter, the postmas- ter is notified of bounces and of other trouble. BUGS This delivery agent supports address extensions in email addresses and in lookup table keys, but does not propagate address extension informa- tion to the result of table lookup. Postfix should have lookup tables that can return multiple result attributes. In order to avoid the inconvenience of maintaining three tables, use an LDAP or MYSQL database. CONFIGURATION PARAMETERS Changes to main.cf are picked up automatically, as virtual(8) processes run for only a limited amount of time. Use the command "postfix reload" to speed up a change. The text below provides only a parameter summary. See postconf(5) for more details including examples. MAILBOX DELIVERY CONTROLS virtual_mailbox_base (empty) A prefix that the virtual(8) delivery agent prepends to all pathname results from $virtual_mailbox_maps table lookups. virtual_mailbox_maps (empty) Optional lookup tables with all valid addresses in the domains that match $virtual_mailbox_domains. virtual_minimum_uid (100) The minimum user ID value that the virtual(8) delivery agent accepts as a result from $virtual_uid_maps table lookup. virtual_uid_maps (empty) Lookup tables with the per-recipient user ID that the virtual(8) delivery agent uses while writing to the recipient's mailbox. virtual_gid_maps (empty) Lookup tables with the per-recipient group ID for virtual(8) mailbox delivery. Available in Postfix version 2.0 and later: virtual_mailbox_domains ($virtual_mailbox_maps) Postfix is final destination for the specified list of domains; mail is delivered via the $virtual_transport mail delivery transport. virtual_transport (virtual) The default mail delivery transport for domains that match the $virtual_mailbox_domains parameter value. LOCKING CONTROLS virtual_mailbox_lock (see 'postconf -d' output) How to lock a UNIX-style virtual(8) mailbox before attempting delivery. deliver_lock_attempts (20) The maximal number of attempts to acquire an exclusive lock on a mailbox file or bounce(8) logfile. deliver_lock_delay (1s) The time between attempts to acquire an exclusive lock on a mailbox file or bounce(8) logfile. stale_lock_time (500s) The time after which a stale exclusive mailbox lockfile is removed. RESOURCE AND RATE CONTROLS virtual_destination_concurrency_limit ($default_destination_concur- rency_limit) The maximal number of parallel deliveries to the same destina- tion via the virtual message delivery transport. virtual_destination_recipient_limit ($default_destination_recipi- ent_limit) The maximal number of recipients per delivery via the virtual message delivery transport. virtual_mailbox_limit (51200000) The maximal size in bytes of an individual mailbox or maildir file, or zero (no limit). MISCELLANEOUS CONTROLS config_directory (see 'postconf -d' output) The default location of the Postfix main.cf and master.cf con- figuration files. daemon_timeout (18000s) How much time a Postfix daemon process may take to handle a request before it is terminated by a built-in watchdog timer. ipc_timeout (3600s) The time limit for sending or receiving information over an internal communication channel. max_idle (100s) The maximum amount of time that an idle Postfix daemon process waits for the next service request before exiting. max_use (100) The maximal number of connection requests before a Postfix dae- mon process terminates. process_id (read-only) The process ID of a Postfix command or daemon process. process_name (read-only) The process name of a Postfix command or daemon process. queue_directory (see 'postconf -d' output) The location of the Postfix top-level queue directory. syslog_facility (mail) The syslog facility of Postfix logging. syslog_name (postfix) The mail system name that is prepended to the process name in syslog records, so that "smtpd" becomes, for example, "post- fix/smtpd". SEE ALSO qmgr(8), queue manager bounce(8), delivery status reports postconf(5), configuration parameters syslogd(8), system logging README_FILES Use "postconf readme_directory" or "postconf html_directory" to locate this information. VIRTUAL_README, domain hosting howto LICENSE The Secure Mailer license must be distributed with this software. HISTORY This delivery agent was originally based on the Postfix local delivery agent. Modifications mainly consisted of removing code that either was not applicable or that was not safe in this context: aliases, ~user/.forward files, delivery to "|command" or to /file/name. The Delivered-To: message header appears in the qmail system by Daniel Bernstein. The maildir structure appears in the qmail system by Daniel Bernstein. AUTHOR(S) Wietse Venema IBM T.J. Watson Research P.O. Box 704 Yorktown Heights, NY 10598, USA Andrew McNamara andrewm@connect.com.au connect.com.au Pty. Ltd. Level 3, 213 Miller St North Sydney 2060, NSW, Australia VIRTUAL(8)

virtual VIRTUAL(5) VIRTUAL(5) NAME virtual - Postfix virtual alias table format SYNOPSIS postmap /etc/postfix/virtual postmap -q "string" /etc/postfix/virtual postmap -q - /etc/postfix/virtual

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