nerdexam
Cisco

200-150 · Question #130

Refer to the exhibit. You are configuring an extended access list to permit a reply from Destination Server B to Source Host A for HTTP traffic. Which extended access list should you implement on the

The correct answer is B. router(config)# permit tcp host 209.165.200.230 eq www host 209.165.202.130. Extended ACLs match on source IP, optional source port, destination IP, and optional destination port - a reply from an HTTP server originates FROM port 80 on the server side.

Cisco Data Center Network Services

Question

Refer to the exhibit. You are configuring an extended access list to permit a reply from Destination Server B to Source Host A for HTTP traffic. Which extended access list should you implement on the inbound interface of Router1?

Exhibit

200-150 question #130 exhibit

Options

  • Arouter(config)# permit tcp host 209.165.200.230 host 209.165.202.130 eq www
  • Brouter(config)# permit tcp host 209.165.200.230 eq www host 209.165.202.130
  • Crouter(config)# permit tcp host 209.165.202.130 host 209.165.200.230 eq www
  • Drouter(config)# permit tcp host 209.165.202.130 eq www host 209.165.200.230

How the community answered

(22 responses)
  • A
    5% (1)
  • B
    77% (17)
  • C
    5% (1)
  • D
    14% (3)

Why each option

Extended ACLs match on source IP, optional source port, destination IP, and optional destination port - a reply from an HTTP server originates FROM port 80 on the server side.

Arouter(config)# permit tcp host 209.165.200.230 host 209.165.202.130 eq www

Placing 'eq www' after the destination matches destination port 80, which would permit a new outbound request to Server B rather than a reply originating from Server B's port 80.

Brouter(config)# permit tcp host 209.165.200.230 eq www host 209.165.202.130Correct

When Server B (209.165.200.230) replies to an HTTP request, the TCP segment originates from source port 80 (www) on Server B and is destined for Host A (209.165.202.130). The extended ACL syntax is 'permit tcp <source> [eq <port>] <destination>', so 'permit tcp host 209.165.200.230 eq www host 209.165.202.130' correctly matches Server B as the source with source port 80 sending to Host A.

Crouter(config)# permit tcp host 209.165.202.130 host 209.165.200.230 eq www

This reverses the source and destination - it permits Host A (209.165.202.130) to send traffic to Server B on destination port 80, which is the original outbound request, not the return reply.

Drouter(config)# permit tcp host 209.165.202.130 eq www host 209.165.200.230

This matches Host A as the source with source port 80 destined for Server B, which is nonsensical for a reply and has both source/destination reversed from the correct direction.

Concept tested: Cisco extended ACL syntax for TCP reply traffic

Source: https://www.cisco.com/c/en/us/support/docs/security/ios-firewall/23602-confaccesslists.html

Topics

#extended ACL#HTTP#TCP#access list syntax

Community Discussion

No community discussion yet for this question.

Full 200-150 Practice