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.
Содержание BSR252
Страница 28: ...28 Tables NN47923 500 ...
Страница 44: ...44 Chapter 1 Getting to know your Business Secure Router NN47923 500 ...
Страница 52: ...52 Chapter 2 Introducing the WebGUI NN47923 500 ...
Страница 70: ...70 Chapter 3 Wizard setup NN47923 500 ...
Страница 91: ...Chapter 5 System screens 91 Nortel Business Secure Router 252 Configuration Basics Figure 20 Time and Date ...
Страница 96: ...96 Chapter 5 System screens NN47923 500 ...
Страница 100: ...100 Chapter 6 LAN screens NN47923 500 Configuring IP Click LAN to open the IP screen Figure 22 LAN IP ...
Страница 111: ...Chapter 7 WAN screens 111 Nortel Business Secure Router 252 Configuration Basics Figure 26 WAN WAN ISP ...
Страница 114: ...114 Chapter 7 WAN screens NN47923 500 Figure 27 WAN IP ...
Страница 120: ...120 Chapter 7 WAN screens NN47923 500 Figure 31 Dial Backup Setup ...
Страница 128: ...128 Chapter 7 WAN screens NN47923 500 ...
Страница 152: ...152 Chapter 9 Static Route screens NN47923 500 ...
Страница 194: ...194 Chapter 11 Firewall screens NN47923 500 ...
Страница 210: ...210 Chapter 13 VPN NN47923 500 Figure 67 Summary IP Policies ...
Страница 222: ...222 Chapter 13 VPN NN47923 500 Figure 71 VPN Branch Office rule setup ...
Страница 231: ...Chapter 13 VPN 231 Nortel Business Secure Router 252 Configuration Basics Figure 72 VPN Branch Office IP Policy ...
Страница 249: ...Chapter 13 VPN 249 Nortel Business Secure Router 252 Configuration Basics Figure 78 VPN Client Termination ...
Страница 256: ...256 Chapter 13 VPN NN47923 500 Figure 81 VPN Client Termination advanced ...
Страница 260: ...260 Chapter 13 VPN NN47923 500 ...
Страница 264: ...264 Chapter 14 Certificates NN47923 500 Figure 83 My Certificates ...
Страница 270: ...270 Chapter 14 Certificates NN47923 500 Figure 85 My Certificate create ...
Страница 274: ...274 Chapter 14 Certificates NN47923 500 Figure 86 My Certificate details ...
Страница 282: ...282 Chapter 14 Certificates NN47923 500 Figure 89 Trusted CA details ...
Страница 298: ...298 Chapter 14 Certificates NN47923 500 ...
Страница 316: ...316 Chapter 16 IEEE 802 1x NN47923 500 ...
Страница 320: ...320 Chapter 17 Authentication server NN47923 500 Figure 106 Local User database edit ...
Страница 328: ...328 Chapter 17 Authentication server NN47923 500 ...
Страница 376: ...376 Chapter 20 Logs Screens NN47923 500 Figure 150 Log settings ...
Страница 386: ...386 Chapter 20 Logs Screens NN47923 500 ...
Страница 392: ...392 Chapter 21 Call scheduling screens NN47923 500 Figure 157 Applying Schedule Sets to a remote node ...
Страница 393: ...Chapter 21 Call scheduling screens 393 Nortel Business Secure Router 252 Configuration Basics ...
Страница 394: ...394 Chapter 21 Call scheduling screens NN47923 500 ...
Страница 409: ...Chapter 22 Maintenance 409 Nortel Business Secure Router 252 Configuration Basics Figure 170 Restart screen ...
Страница 410: ...410 Chapter 22 Maintenance NN47923 500 ...
Страница 430: ...430 Appendix A Troubleshooting NN47923 500 ...