SMTP is the standard protocol for transferring mail between hosts over TCP. A TC
ID: 3558524 • Letter: S
Question
SMTP is the standard protocol for transferring mail between hosts over TCP. A TCP connection is set up between a user agent and a server program. The server listens on TCP port 25 for incoming connection requests. The user end of the connection is on a TCP port number above 1023. Suppose you wish to build a packet filter rule set allowing inbound and outbound SMTP traffic. You generate the following rule set:
Source Address Source Port Dest Address Dest Port Action
1 Any Any 192.168.1.0 >1023 Allow
2 192.168.1.1 Any Any Any Deny
3 Any Any 192.168.1.1 Any Deny
4 192.168.1.0 Any Any Any Allow
5 Any Any 192.168.1.2 SMTP Allow
6 Any Any 192.168.1.3 HTTP Allow
7 Any Any Any Any Deny
a. Describe the effect of each rule.
Explanation / Answer
1 Any Any 192.168.1.0 >1023 Allow
From Source Any to Destination 192.168.1.0 on port 1023 Allow
This rule will allow inbound traffic onport 1023 from external world to destination 192.168.1.0
2 192.168.1.1 Any Any Any Deny
From 192.168.1.1 to Any on Any Port Deny
This rule will block outbound traffic from 192.168.1.1 to any destination and on any port. So, 192.168.1.1 will not be able to communicate with both internal(outside 192.168.1.0/24 subnet) or external world.
3 Any Any 192.168.1.1 Any Deny
From Any to 192.168.1.1 on Any port Deny
This rule will block inbound traffic to 192.168.1.1 from Any (external or internal other network/subnet) on any port.
4 192.168.1.0 Any Any Any Allow
From 192.168.1.0 to Any on Any port Allow
This rule will allow oubound traffic from 192.168.1.0 to any destination on any port.
5 Any Any 192.168.1.2 SMTP Allow
From Any to 192.168.1.2 on port 25 Allow
This rule will allow inbound traffic of port 25 from any to 192.168.1.2. Hence, 192.168.1.2 can be used as mail server.
6 Any Any 192.168.1.3 HTTP Allow
From Any to 192.168.1.3 on port 80 Allow
This rule will allow inbound traffic on port 80 from Any to 192.168.1.3. Hence, 192.168.1.3 can be used as webserver.
7 Any Any Any Any Deny
From Any to Any on port Any Deny(Clean Up Rule)
This rule will block any kind of traffic passing through the firewall. this rule is used as cleanup to block any unwanted traffic for which a specific allow rule is not configured.
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