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''Amavis'' is an open source content filter for electronic mail, implementing mail message transfer, decoding, some processing and checking, and interfacing with external content filters to provide protection against spam, viruses and other malware. It can be considered an interface between a mailer (MTA, Mail Transfer Agent) and one or more content filters. ''Amavis'' can be used to: * detect viruses, spam, banned content types or syntax errors in mail messages * block, tag, redirect (using sub-addressing), or forward mail depending on its content, origin or size * quarantine (and release), or archive mail messages to files, to mailboxes, or to an SQL database * sanitize passed messages using an external sanitizer * generate DKIM signatures * verify DKIM signatures and provide DKIM-based whitelisting. Notable features: * provides SNMP statistics and status monitoring using an extensive MIB with more than 300 variables * provides structured event log in JSON format * IPv6 protocol is supported in interfacing, and IPv6 address forms in mail header section * properly honors per-recipient settings even in multi-recipient messages, while scanning a message only once. * supports Internationalized Email (RFC 6530, SMTPUTF8, EAI, IDN) A common mail filtering installation with ''Amavis'' consists of a Postfix as an MTA, SpamAssassin as a spam classifier, and ClamAV as an anti-virus protection, all running under a Unix-like operating system. Many other virus scanners (about 30) and some other spam scanners (CRM114, DSPAM, Bogofilter) are supported too, as well as some other MTAs. == Interfacing topology == Three topologies for interfacing with an MTA are supported. The ''amavisd'' process can be sandwiched between two instances of an MTA, yielding a classical after-queue〔 mail filtering setup, or ''amavisd'' can be used as an SMTP proxy filter in a before-queue〔 filtering setup, or the ''amavisd'' process can be consulted to provide mail classification but not to forward a mail message by itself, in which case the consulting client remains in charge of mail forwarding. This last approach is used in a Milter setup (with some limitations), or with a historical client program ''amavisd-submit''. Since version 2.7.0 a before-queue setup is preferred, as it allows for a mail message transfer to be rejected during an SMTP session〔 with a sending client. In an after-queue setup filtering takes place after a mail message has already been received and enqueued by an MTA, in which case a mail filter can no longer reject a message, but can only deliver it (possibly tagged), or discard it, or generate a non-delivery notification, which can cause unwanted backscatter in case of bouncing a message with a fake sender address. A disadvantage of a before-queue setup〔 is that it requires resources (CPU, memory) proportional to a current (peak) mail transfer rate, unlike an after-queue setup, where some delay is acceptable and resource usage corresponds to average mail transfer rate. With introduction of an option ''smtpd_proxy_options=speed_adjust'' in Postfix 2.7.0 the resource requirements for a before-queue content filter have been much reduced.〔 In some countries〔 the legislation does not permits mail filtering to discard a mail message once it has been accepted by an MTA, so this rules out an after-queue filtering setup with discarding or quarantining of messages, but leaves a possibility of delivering (possibly tagged) messages, or rejecting them in a before-queue setup (SMTP proxy or milter). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「''Amavis'' is an open source content filter for electronic mail,implementing mail message transfer, decoding, some processing and checking,and interfacing with external content filters to provide protection againstspam, viruses and other malware.It can be considered an interface between a mailer (MTA, Mail Transfer Agent)and one or more content filters.''Amavis'' can be used to:* detect viruses, spam, banned content types or syntax errors in mail messages* block, tag, redirect (using sub-addressing), or forward mail depending on its content, origin or size* quarantine (and release), or archive mail messages to files, to mailboxes, or to an SQL database* sanitize passed messages using an external sanitizer* generate DKIM signatures* verify DKIM signatures and provide DKIM-based whitelisting.Notable features:* provides SNMP statistics and status monitoring using an extensive MIB with more than 300 variables* provides structured event log in JSON format* IPv6 protocol is supported in interfacing, and IPv6 address forms in mail header section* properly honors per-recipient settings even in multi-recipient messages, while scanning a message only once.* supports Internationalized Email (RFC 6530, SMTPUTF8, EAI, IDN)A common mail filtering installation with ''Amavis'' consists of aPostfix as an MTA, SpamAssassin as a spamclassifier, and ClamAV as an anti-virus protection, all running undera Unix-like operating system. Many other virus scanners (about 30) andsome other spam scanners (CRM114, DSPAM, Bogofilter)are supported too, as well as some other MTAs.== Interfacing topology ==Three topologies for interfacing with an MTA are supported. The ''amavisd''process can be sandwiched between two instances of an MTA, yielding a classicalafter-queue mail filtering setup, or ''amavisd'' canbe used as an SMTP proxy filter in a before-queuefiltering setup, or the ''amavisd'' process can be consulted to provide mailclassification but not to forward a mail message by itself, in which case theconsulting client remains in charge of mail forwarding. This last approach isused in a Milter setup (with some limitations), or with a historical clientprogram ''amavisd-submit''.Since version 2.7.0 a before-queue setup is preferred, as it allows for a mailmessage transfer to be rejected during an SMTP sessionwith a sending client. In an after-queue setup filtering takes place aftera mail message has already been received and enqueued by an MTA, in whichcase a mail filter can no longer reject a message, but can only deliver it(possibly tagged), or discard it, or generate a non-delivery notification,which can cause unwanted backscatter in case ofbouncing a message with a fake sender address.A disadvantage of a before-queue setup is thatit requires resources (CPU, memory) proportional to a current (peak) mailtransfer rate, unlike an after-queue setup, where some delay is acceptableand resource usage corresponds to average mail transfer rate.With introduction of an option ''smtpd_proxy_options=speed_adjust''in Postfix 2.7.0 the resource requirements for a before-queue contentfilter have been much reduced.In some countries the legislationdoes not permits mail filtering to discard a mail message once it hasbeen accepted by an MTA, so this rules out an after-queue filtering setupwith discarding or quarantining of messages, but leaves a possibility ofdelivering (possibly tagged) messages, or rejecting them in a before-queuesetup (SMTP proxy or milter).」の詳細全文を読む スポンサード リンク
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