Chapter 10 Firewalls
165
Nortel Business Secure Router 252 Configuration — Basics
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.
Consider the FTP protocol. A user on the LAN opens a control connection to a
server on the Internet and requests a file. At this point, the remote server opens a
data connection from the Internet. For FTP to work properly, this connection must
be allowed to pass through even though a connection from the Internet is normally
rejected.
In order to achieve the above scenario, the Business Secure Router inspects the
application level FTP data. Specifically, it searches for outgoing PORT
commands, and when it sees these; it adds a cache entry for the anticipated data
connection. This can be done safely, since the PORT command contains address
and port information, which can be used to uniquely identify the connection.
Summary of Contents for BSR252
Page 28: ...28 Tables NN47923 500 ...
Page 44: ...44 Chapter 1 Getting to know your Business Secure Router NN47923 500 ...
Page 52: ...52 Chapter 2 Introducing the WebGUI NN47923 500 ...
Page 70: ...70 Chapter 3 Wizard setup NN47923 500 ...
Page 96: ...96 Chapter 5 System screens NN47923 500 ...
Page 114: ...114 Chapter 7 WAN screens NN47923 500 Figure 27 WAN IP ...
Page 120: ...120 Chapter 7 WAN screens NN47923 500 Figure 31 Dial Backup Setup ...
Page 128: ...128 Chapter 7 WAN screens NN47923 500 ...
Page 152: ...152 Chapter 9 Static Route screens NN47923 500 ...
Page 194: ...194 Chapter 11 Firewall screens NN47923 500 ...
Page 210: ...210 Chapter 13 VPN NN47923 500 Figure 67 Summary IP Policies ...
Page 222: ...222 Chapter 13 VPN NN47923 500 Figure 71 VPN Branch Office rule setup ...
Page 256: ...256 Chapter 13 VPN NN47923 500 Figure 81 VPN Client Termination advanced ...
Page 260: ...260 Chapter 13 VPN NN47923 500 ...
Page 264: ...264 Chapter 14 Certificates NN47923 500 Figure 83 My Certificates ...
Page 270: ...270 Chapter 14 Certificates NN47923 500 Figure 85 My Certificate create ...
Page 274: ...274 Chapter 14 Certificates NN47923 500 Figure 86 My Certificate details ...
Page 282: ...282 Chapter 14 Certificates NN47923 500 Figure 89 Trusted CA details ...
Page 298: ...298 Chapter 14 Certificates NN47923 500 ...
Page 316: ...316 Chapter 16 IEEE 802 1x NN47923 500 ...
Page 320: ...320 Chapter 17 Authentication server NN47923 500 Figure 106 Local User database edit ...
Page 328: ...328 Chapter 17 Authentication server NN47923 500 ...
Page 376: ...376 Chapter 20 Logs Screens NN47923 500 Figure 150 Log settings ...
Page 386: ...386 Chapter 20 Logs Screens NN47923 500 ...
Page 393: ...Chapter 21 Call scheduling screens 393 Nortel Business Secure Router 252 Configuration Basics ...
Page 394: ...394 Chapter 21 Call scheduling screens NN47923 500 ...
Page 410: ...410 Chapter 22 Maintenance NN47923 500 ...