- Kerio WinRoute Firewall Administrator's Guide

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DNS
DNS (Domain Name System) A worldwide distributed database of Internet host-
names and their associated IP address. Computers use Domain Name Servers to
resolve host names to IP addresses. Names are sorted in hierarchized domains.
Firewall
Software or hardware device that protects a computer or computer network against
attacks from external sources (typically from the Internet).
In this guide, the word firewall represents the WinRoute host.
FTP
File Transfer Protocol. The FTP protocol uses two types of TCP connection: control
and data. The control connection is always established by a client. Two FTP modes
are distinguished according to a method how connection is established:
• active mode — data connection is established from the server to a client (to the
port specified by the client). This mode is suitable for cases where the firewall
is at the server’s side, however, it is not supported by some clients (e.g. by web
browsers).
• passive mode — data connection is established also by the client (to the port
required by the server). This mode is suitable for cases where the firewall is at
the client’s side. It should be supported by any FTP client.
Note: WinRoute includes special support (protocol inspector) for FTP protocol.
Therefore, both FTP modes can be used on LAN hosts.
Gateway
Network device or a computer connecting two different subnets.
Greylisting
A method of protection of SMTP servers from spam. If an email message sent by an
unknown sender is delivered to the server, the server rejects it for the first time (so
called temporary delivery error). Legitimate senders attempt resend the message
after some time. SMTP server lets the message in and considers the sender as trust-
worthy since then, not blocking their messages any longer. Most spam senders try
to send as great volume in as short time as possible and stay anonymous. There-
fore, they usually do not repeat sending the message and focus on another SMTP
server.
More information (in English) can be found for example at
Wikipedia.
IMAP
Internet Message Access Protocol (IMAP) enables clients to manage messages stored
on a mail server without downloading them to a local computer. This architecture
allows the user to access his/her mail from multiple locations (messages down-
loaded to a local host disk would not be available from other locations).