The correct answer is to apply a firewall filter to the loopback interface that polices SSH traffic. This works because the loopback interface (lo0) is the termination point for all control-plane traffic on a Junos device, including SSH sessions used for login attempts. By configuring a firewall filter with a policer that limits the rate of SSH packets from a specific source IP, excessive failed logins will cause the policer to drop subsequent packets, automatically blocking the attacker without manual intervention. On the JNCIA-Junos exam, this tests your understanding of control plane policing and loopback firewall filters as a security mechanism—a common trap is confusing this with data-plane filters applied to transit interfaces. Remember that the loopback interface is the device’s own IP, so any filter applied there protects the control plane directly. A useful memory tip: “Loopback locks out logins” — the loopback filter is your first line of defense against brute-force SSH attacks.
JNCIA-JUNOS Junos OS Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of junos os fundamentals. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
Refer to the exhibit.
root@router> show log messages | match "login"
Jun 15 10:30:22 router login: LOGIN FAILED FOR user1 FROM 10.0.0.2
Jun 15 10:30:25 router login: LOGIN FAILED FOR user1 FROM 10.0.0.2
Jun 15 10:30:28 router login: LOGIN FAILED FOR user1 FROM 10.0.0.2
Refer to the exhibit. A security analyst sees repeated login failures from 10.0.0.2 for user1. Which Junos feature can be used to automatically block further login attempts from that IP?
Refer to the exhibit.
root@router> show log messages | match "login"
Jun 15 10:30:22 router login: LOGIN FAILED FOR user1 FROM 10.0.0.2
Jun 15 10:30:25 router login: LOGIN FAILED FOR user1 FROM 10.0.0.2
Jun 15 10:30:28 router login: LOGIN FAILED FOR user1 FROM 10.0.0.2
A
Configure SSH to accept only public key authentication for user1.
Why wrong: This prevents password brute force but does not block the IP.
B
Disable the user1 account.
Why wrong: This just prevents that user, but the IP can attack other users.
C
Set the 'session-limit' for user1 to prevent multiple login attempts.
Why wrong: Session-limit limits concurrent sessions, not failed attempts.
D
Apply a firewall filter to the loopback interface that polices SSH traffic.
A firewall filter with a policer can rate-limit SSH attempts from a source IP, effectively blocking after excessive failures.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Apply a firewall filter to the loopback interface that polices SSH traffic.
Option D is correct because applying a firewall filter to the loopback interface (lo0) that polices SSH traffic can automatically block further login attempts from a specific IP address, such as 10.0.0.2. The loopback interface is the termination point for all control-plane traffic on a Junos device, including SSH sessions. By configuring a firewall filter with a policer that limits the rate of SSH packets from a source IP, excessive login failures can trigger the policer to drop subsequent packets, effectively blocking the attacker without manual intervention.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Configure SSH to accept only public key authentication for user1.
Why it's wrong here
This prevents password brute force but does not block the IP.
✗
Disable the user1 account.
Why it's wrong here
This just prevents that user, but the IP can attack other users.
✗
Set the 'session-limit' for user1 to prevent multiple login attempts.
Why it's wrong here
Session-limit limits concurrent sessions, not failed attempts.
✓
Apply a firewall filter to the loopback interface that polices SSH traffic.
Why this is correct
A firewall filter with a policer can rate-limit SSH attempts from a source IP, effectively blocking after excessive failures.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse control-plane policing (applied to lo0) with data-plane firewall filters applied to interfaces like ge-0/0/0, or mistakenly think that session limits or disabling accounts are automated responses to brute-force attacks.
Detailed technical explanation
How to think about this question
Under the hood, Junos firewall filters applied to the loopback interface operate at the control plane, inspecting packets before they reach the routing engine. A policer configured with a bandwidth limit and burst size can enforce a rate limit on SSH packets (TCP port 22) from a specific source; once the policer exceeds its threshold, packets are dropped, effectively rate-limiting login attempts. In real-world scenarios, this is often combined with a 'term' that matches on source IP and protocol, and a 'then policer' action, providing a lightweight DoS mitigation without requiring external tools like IDS/IPS.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Junos OS Fundamentals — This question tests Junos OS Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Apply a firewall filter to the loopback interface that polices SSH traffic. — Option D is correct because applying a firewall filter to the loopback interface (lo0) that polices SSH traffic can automatically block further login attempts from a specific IP address, such as 10.0.0.2. The loopback interface is the termination point for all control-plane traffic on a Junos device, including SSH sessions. By configuring a firewall filter with a policer that limits the rate of SSH packets from a source IP, excessive login failures can trigger the policer to drop subsequent packets, effectively blocking the attacker without manual intervention.
What should I do if I get this JNCIA-JUNOS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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