«An SMTP Filtering Proxy»
- Authentication. In the context of mail, SMTP AUTH (RFC 2554,
4954), is an extension that allows for
assorted different mechanisms to be used to accept authentication creditials, ie.
account name and password details. Most common are LOGIN and
PLAIN (RFC 2595, 4616),
which are insecure clear text mechanisms support by most MUA.
Some others mechanisms include CRAM-MD5 (RFC 2195),
DIGEST-MD5 (RFC 2831),
KERBEROS_V5 (RFC 4752).
- Domain Name Service is a distributed database that acts a phone book to the Internet.
Typically used to map domain names and machine names into IP addresses, though many other related
pieces of information can be found through the DNS.
See RFC 1034, 1035.
- DNS A or AAAA record
- A domain name record used to map a domain name into an IPv4 or IPv6 address.
This is the opposite to a PTR record discussed below.
See RFC 1035, 3596.
- DNS MX record
- A mail exchange record that specifies where mail destined for a domain name should be sent.
See RFC 1035 and
- DNS PTR record
- A domain name pointer record used to map an IP address into a domain name. This is the
opposite to an A or AAAA record.
See RFC 1035.
- DNS TXT record
- A domain name text record used to store arbitrary text or binary data.
Used by many DNS blacklists for comments and for SPF tests.
See RFC 1035.
- DSN, NDR
Delivery Status Notification is more commonly referred to as a "bounce message"
or Non-Delivery Report. It provides a summary as to why a messsage could not be delivered.
Enhanced Message-ID as Email Watermark.
- Full Qualified Domain Name is typically a host name of a machine connected to the Internet
that is three or more labels in length, such as
smtp.snertsoft.com. A host name may
also be a domain name, such as
such usage is frowned upon. While not strictly a FQDN, an IP-domain literal, which is an IP address
between square brackets (
]), for example "
[192.0.2.9]" and can
often be used in place of host name.
Hyper-Text Transfer Protocol is used for requesting web resources such
as documents and images. See RFC 2616.
- Internet Engineering Task Force is an open international community
concerned with the evolution of the Internet architecture and the smooth operation; they review
Internet Drafts and published RFC documents, which typically pertain to interoperability over the Internet.
- Internet Protocol version 6, the successor to the original Internet Protocol version 4. See
- Message Disposition Notification is the term for the various flavours of "return receipts"
defined by RFC 3798,
- Mail Submission Agent is a special SMTP server that listens on port 587 and is the entry point for new mail
into the mail system. It can typically validate and/or authenticate the origin of the mail
as coming from a known user.
- Mail Transfer Agent is an SMTP client/server that listens normally on port 25 and handles the routing and
delivery of mail between remote locations. It makes up the backbone of the Internet mail system.
When smtpf is installed, it is configured to listen on port 25. Any MTA that was
previous configured to listen on port 25 of the same machine, has to be configured to listen
on an unused port, such as port 26; ideally only on the localhost interface.
- Mail User Agent is the end user's mail program that is used to compose, send, and read mail.
- Multipurpose Internet Mail Extensions used to specify how mail attachments can encoded and
transfered using mail messages. Also used for HTTP. There are many RFC documents pertaining to MIME. The initial
set to start with are
- Request For Comments: originally intended as published technical documents related
to Internet operations that was intended to solicit feedback. Now RFC are more
formal documents providing information, approved standards, protocols, experimental options, etc.
New RFC documents start life as an Internet Draft to be discussed and are later voted on for
approval, further review, or dropped by the IETF.
- Proxy or Gateway
An intermediary server or application that accepts requests from clients, screening and/or caching
them, before forward the requests to other proxy servers, origin servers, or services.
Gateway is often used as a synonym for "proxy server".
- Simple Mail Transfer Protocol. See RFC
There are several other RFC documents for SMTP extensions not mentioned here.
Briefly, an SMTP session follows these states: connection, HELO/EHLO, AUTH,
MAIL, RCPT, DATA, content, dot, QUIT. Each successful MAIL command during the
SMTP session starts a new message transaction, which ends when either the final
dot is sent or RSET is given. For each message, there can be more than
one RCPT given.
Of the information obtained from each state, only the IP
address of the SMTP client and each valid RCPT address specified
can be relied upon. Even then, the connecting IP might be questionable,
because it's possibly in a dynamic IP address pool, the reverse DNS
of the IP is often poorly configured or non-existent, or the
WhoIs information about IP and domain assignment might be restricted,
due to privacy concerns (RFC 3912).
As for the other states, the HELO, MAIL, and message content can be
misrepresented or faked, and thus cannot be immediately trusted.
Most spam filtering techniques fall into two classes: those that
act on the SMTP client's IP address and envelope information
(pre-DATA) and those that act on the message content (post-DATA).
This distinction is important, because once the DATA command is accepted
by the receiving server, it is generally committed to reading the
entire message until the SMTP client indicates it has finished. This,
of course, consumes bandwidth and system resources, so several filtering
techniques attempt to make a decision based on policy or behaviour
before accepting DATA in order to avoid/reduce more expensive forms
of filtering after.
- Sender Policy Framework is an experimental protocol. See RFC
- TLD, gTLD, ccTLD
- Global Top Level Domain such as
In most cases Country Code Top Level Domain that have a secondary level classification,
.gouv.fr can be lump
together under TLD.
- Time To Live is the life span for some piece of locally stored information before it expires
and is retested or refetched. This value is typically expressed in seconds.
- URI, URL, URN
Uniform Resource Identifiers, Uniform Resource Locators, and Uniform Resource Names are
used to specify how and where an object or resource can be found. See RFC
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