LFCS Networking Practice Question
A Linux server has a single interface ens3 with address 203.0.113.10/24. The administrator runs `ss -tulnp` and sees that a service is listening on 127.0.0.1:8080 only. Remote clients on the Internet report they cannot reach the service on port 8080 even though the firewall allows the port. Which change is the most appropriate to allow remote access while preserving the service's intended exposure?
⚠ Common exam trap
The trap here is assuming a firewall or routing problem when the listening socket itself is bound only to the loopback address.
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
✓
Restart the service after changing its configuration to bind to 0.0.0.0 or the specific external address instead of 127.0.0.1.
A listener bound to 127.0.0.1 accepts only connections that originate on the local machine. Firewall rules and routing cannot change this. The correct remedy is to reconfigure the service to bind to 0.0.0.0 or the external address, then restart it so the new socket is created.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Add a route for 127.0.0.1 via ens3 so that external packets can reach the loopback address.
Why it's wrong here
Loopback traffic is handled internally by the kernel and is not routed over a physical interface. Adding a route for 127.0.0.1 via ens3 is invalid and would not make the bound socket accept external connections; it also risks breaking local loopback semantics.
- ✓
Restart the service after changing its configuration to bind to 0.0.0.0 or the specific external address instead of 127.0.0.1.
Why this is correct
The service is bound to the loopback address, so it only accepts connections from the local host regardless of firewall rules. Rebinding to 0.0.0.0 or 203.0.113.10 makes the socket reachable on the external interface, which is the direct fix for the observed behavior.
- ✗
Enable IP forwarding with `sysctl -w net.ipv4.ip_forward=1` so that packets can be forwarded to the loopback listener.
Why it's wrong here
IP forwarding only matters when the host routes traffic between interfaces. It does not change which address a listening process is bound to, so the service would remain unreachable from remote clients even with forwarding enabled.
- ✗
Configure a DNAT rule redirecting external port 8080 to 127.0.0.1:8080 on the same host.
Why it's wrong here
A DNAT rule on the same host would still deliver packets to a socket bound to loopback, which does not accept connections arriving on the external interface. The root cause is the bind address, not the destination port mapping.
Quick reference
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Linux Foundation exam blueprint
This LFCS practice question is part of Courseiva's free Linux Foundation certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LFCS exam.