Avaya Business Secure Router 222 Configuration - Basics Manuale Utente Scaricare il pdf (Pagina 167)

Chapter 10 Firewalls 167

Nortel Business Secure Router 222 Configuration — Basics

After the Business Secure Router 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 that originated on the

LAN).

UDP/ICMP security

UDP and ICMP do not 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 creates a cache entry. Its

IP address and port pairs are stored. For a short period of time, UDP packets from

the WAN that have matching IP and UDP information are allowed back in through

the firewall.

A similar situation exists for ICMP, except that the Business Secure Router is

even more restrictive. Specifically, only outgoing echoes allow incoming echo

replies, outgoing address mask requests allow incoming address mask replies, and

outgoing timestamp requests 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 can be used to reroute traffic through

attacking machines.

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.

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Avaya Business Secure Router 222 Configuration - Basics Manuale Utente Pagina 167