LFCS Networking Practice Question
A server has multiple IP aliases on eth0. Remote hosts cannot reach the secondary IP addresses. What should the administrator check?
⚠ Common exam trap
A common mix-up: candidates assume secondary IPs must be in the same subnet as the primary (Option D) or that routing entries are needed (Option A), when the actual cause is the kernel's default ARP behavior, which is controlled by sysctl settings.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ARP flux settings are configured correctly.
When multiple IP aliases are configured on a single Ethernet interface, the kernel may respond to ARP requests inconsistently, a behavior known as ARP flux. This causes remote hosts to receive conflicting MAC addresses for the secondary IPs, preventing connectivity. Correctly configuring ARP flux settings (e.g., using `arp_ignore=1` and `arp_announce=2` via sysctl) ensures the kernel responds only from the appropriate IP and advertises the correct MAC.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The server's routing table includes routes for the secondary IPs' subnets.
Why it's wrong here
Routing is typically fine if the primary IP routes packets.
- ✓
The ARP flux settings are configured correctly.
Why this is correct
Incorrect arp_ignore/arp_announce can cause secondary IPs to be unreachable.
- ✗
The interface is set to NOARP.
Why it's wrong here
NOARP disables ARP completely, which would break all communication.
- ✗
The secondary IPs are in the same subnet as the primary.
Why it's wrong here
They can be in different subnets; this is not the cause.
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
Go deeper
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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