A Citrix Administrator needs to block all DNS requests from subnet 10.107.149.0/24. Which expressions can the administrator use to match the required traffic?

The expression must satisfy two conditions: the traffic must originate from the 10.107.149.0/24 subnet, AND the traffic must be a DNS request. DNS uses both UDP and TCP on destination port 53. The expression CLIENT.IP.SRC INSUBNET(10.107.149.0/24) correctly identifies traffic originating from the specified subnet. The expression (client.UDP.DSTPORT.EQ(53) || client.TCP.DSTPORT.EQ(53)) correctly identifies DNS traffic by checking for either UDP or TCP destination port 53. The parentheses are crucial to group the OR condition, ensuring that the logic correctly evaluates to (Source Subnet) AND (UDP Port 53 OR TCP Port 53).

A. The function CLIENT.IP.SRC(10.107.149.0) is syntactically incorrect for matching a subnet; it would attempt to match a single host IP. The correct function is INSUBNET.
B. This expression lacks parentheses around the OR condition. Due to operator precedence, it would be evaluated as (CLIENT.IP.SRC INSUBNET(...) && client.UDP.DSTPORT.EQ(53)) || client.TCP.DSTPORT.EQ(53), incorrectly matching all TCP port 53 traffic, regardless of source.
C. This option has two errors: it uses the incorrect function CLIENT.IP.SRC(...) for subnet matching and lacks the necessary parentheses for correct logical evaluation of the port checks.

1. Citrix ADC 13.1 Documentation

"Policy Expression Reference": This document details the components of advanced policy expressions.

Section: "Textual operators": Describes the INSUBNET operator

stating it "Evaluates to TRUE if the IP address is in the specified subnet." This confirms the correct syntax for matching the subnet.

Section: "Qualifiers for TCP

UDP

and IP packets": Lists CLIENT.IP.SRC

CLIENT.TCP.DSTPORT

and CLIENT.UDP.DSTPORT as valid qualifiers for accessing packet information.

Section: "Logical and relational operators": Explains the use of && (AND) and || (OR) and the standard operator precedence

which necessitates the use of parentheses () to group the OR condition in this scenario.

2. Citrix ADC 13.1 Documentation

"Advanced policy expressions: The basics": This guide provides foundational knowledge on constructing expressions.

Section: "Compound Expressions": This section explains how to combine simple expressions using logical operators like && and ||. It implicitly supports the structure used in the correct answer

where multiple conditions are joined to create a specific rule.