Netopia R310 사용자 설명서
Security 12-23
This header information is what the packet filter uses to make filtering decisions. It is impor tant to note that a
packet filter does not look into the IP datastream (the User Data from above) to make filtering decisions.
packet filter does not look into the IP datastream (the User Data from above) to make filtering decisions.
Basic Protocol Types
TCP: Transmission Control Protocol. TCP provides reliable packet deliver y and has a retransmission
mechanism (so packets are not lost). RFC 793 is the specification for TCP.
mechanism (so packets are not lost). RFC 793 is the specification for TCP.
UDP: User Datagram Protocol. Unlike TCP, UDP does not guarantee reliable, sequenced packet deliver y. If data
does not reach its destination, UDP does not re transmit the data. RFC 768 is the specification for UDP.
does not reach its destination, UDP does not re transmit the data. RFC 768 is the specification for UDP.
And there are many more por ts defined in the Assigned Addresses RFC.
Example TCP/UDP Ports
Firewall design rules
There are two basic rules to firewall design:
■
“What is not explicitly allowed is denied...”
and
■
“What is not explicitly denied is allowed...”
The first rule is far more secure, and is the best approach to firewall design. It is far easier (and more secure)
to allow in or out only cer tain ser vices and deny anything else. If the other rule is used, you would have to figure
out ever ything that you want to disallow, now and future.
to allow in or out only cer tain ser vices and deny anything else. If the other rule is used, you would have to figure
out ever ything that you want to disallow, now and future.
Source Por t
2541
Destination Por t
80
Protocol
TCP
ACK Bit
Yes
DATA
User Data
TCP Port
Service
UDP Port
Service
20/21
FTP
161
SNMP
23
Telnet
69
TFTP
25
SMTP
80
WWW
144
News