Network Security Engineer Interview Questions UK: 8 Real Questions and What Interviewers Want

Applying for a Network Security Engineer job in the UK? Here is what you are likely to be asked. These questions come from our Network Security Engineer question bank of 16 questions, written for UK employers at mid-level level.

Eight Network Security Engineer interview questions, and what the interviewer is thinking

1. Talk me through how you would build a site-to-site IPsec VPN between a head office firewall and a branch, and how you would troubleshoot it if the tunnel won't come up.

Why they ask: To test whether you understand VPN mechanics rather than just clicking through a vendor wizard.

2. How would you implement 802.1X port-based authentication across a wired and wireless estate, and what problems would you expect during rollout?

Why they ask: To check hands-on knowledge of network access control, a staple of mid-level network security work.

3. What is VLAN hopping, how can it happen, and what would you configure on a switch to prevent it?

Why they ask: To test practical Layer 2 security knowledge beyond simply saying 'we use VLANs'.

4. An application team asks you to open 'any to any' on a firewall rule so their new system can talk to a database server. What do you do?

Why they ask: To see whether you can uphold least privilege while still helping the business deliver.

5. Users report that an application connection is failing and you suspect the firewall. How do you prove whether it is the firewall, and what tools do you use?

Why they ask: To assess your methodical troubleshooting approach with real tooling.

6. How would you protect an internet-facing service from a volumetric or application-layer DDoS attack?

Why they ask: To check you understand layered mitigation and the limits of on-premises devices.

7. Tell me about a time you led or delivered a firewall, VPN or network security migration. How did you manage the risk?

Why they ask: To gauge your experience delivering change on live infrastructure.

8. What does a typical week look like for a network security engineer, and which parts do you enjoy most?

Why they ask: To check your expectations of the role are realistic.

A sample answer to question 1

Talk me through how you would build a site-to-site IPsec VPN between a head office firewall and a branch, and how you would troubleshoot it if the tunnel won't come up.

I'd start by agreeing the parameters with the other end: peer IPs, IKEv2, AES-256-GCM, SHA-384 and a strong Diffie-Hellman group such as 19 or 20, plus the local and remote subnets. Phase one, the IKE SA, authenticates the peers, ideally with certificates rather than a pre-shared key. Phase two, the child SA, negotiates the ESP parameters and the traffic selectors. I'd configure the tunnel interface or policy-based proxy IDs, add routing, and write firewall policy so only required traffic crosses it. If it won't establish, I check reachability on UDP 500 and 4500 first, then compare proposals on both sides, because mismatched encryption, lifetimes or traffic selectors are the usual culprits. Debug logs and the IKE gateway status show which phase fails. NAT-T and an ISP blocking ESP also catch people out. Once it's up, I verify with a packet capture and set up dead peer detection and monitoring so a drop raises an alert.

Common mistake to avoid: Quoting only vendor GUI steps; ignoring NAT traversal and routing

Want the rest?

Full model answers, key points and common mistakes for all 16 questions are on the Network Security Engineer interview questions page. The first five are free, no sign-up needed.