ZyXEL Communications G-2000 Plus V2, Руководство пользователя

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ZyXEL G-2000 Plus v2 User’s Guide
142 Chapter 10 Firewalls
10.5.3 TCP Security
The ZyXEL device uses state information embedded in TCP packets. The first packet of any
new connection has its SYN flag set and its ACK flag cleared; these are "initiation" packets.
All packets that do not have this flag structure are called "subsequent" packets, since they
represent data that occurs later in the TCP stream.
If an initiation packet originates on the WAN, this means that someone is trying to make a
connection from the Internet into the LAN. Except in a few special cases (See “Upper Layer
Protocols” on page 142 shown next), these packets are dropped and logged.
If an initiation packet originates on the LAN, this means that someone is trying to make a
connection from the LAN to the Internet. Assuming that this is an acceptable part of the
security policy (as is the case with the default policy), the connection will be allowed. A cache
entry is added which includes connection information such as IP addresses, TCP ports,
sequence numbers, etc.
When the ZyXEL device receives any subsequent packet (from the Internet or from the LAN),
its connection information is extracted and checked against the cache. A packet is only
allowed to pass through if it corresponds to a valid connection (that is, if it is a response to a
connection which originated on the LAN).
10.5.4 UDP/ICMP Security
UDP and ICMP do not themselves contain any connection information (such as sequence
numbers). However, at the very minimum, they contain an IP address pair (source and
destination). UDP also contains port pairs, and ICMP has type and code information. All of
this data can be analyzed in order to build "virtual connections" in the cache.
For instance, any UDP packet that originates on the LAN will create a cache entry. Its IP
address and port pairs will be stored. For a short period of time, UDP packets from the WAN
that have matching IP and UDP information will be allowed back in through the firewall.
A similar situation exists for ICMP, except that the ZyXEL device is even more restrictive.
Specifically, only outgoing echoes will allow incoming echo replies, outgoing address mask
requests will allow incoming address mask replies, and outgoing timestamp requests will
allow incoming timestamp replies. No other ICMP packets are allowed in through the firewall,
simply because they are too dangerous and contain too little tracking information. For
instance, ICMP redirect packets are never allowed in, since they could be used to reroute
traffic through attacking machines.
10.5.5 Upper Layer Protocols
Some higher layer protocols (such as FTP and RealAudio) utilize multiple network
connections simultaneously. In general terms, they usually have a "control connection" which
is used for sending commands between endpoints, and then "data connections" which are used
for transmitting bulk information.