HomeIncident briefing
Back to the library
INCIDENT #079 · SECURITY

One DNS transport path is missing

Hard25 minutesPro lab

The incident

The network must carry both UDP/53 and TCP/53 to 10.159.30.10. TCP works but UDP is denied. Validate packet forwarding only; this lab does not run a DNS resolver.

Your objective

Restore the documented application transport flows and return reachability. Keep SSH blocked, preserve the working telemetry branch, and do not remove security boundaries or change interface addresses.

The network

Network topology4 devices 3 links

One DNS transport path is missing — network topologyR1: Client edge. R2: Transit core. R3: Application edge. R4: Telemetry edge. R1 connects to R2. R2 connects to R3. R2 connects to R4. Connections show physical links, not service health.R1 ↔ R2 · 10.159.12.0/30R2 ↔ R3 · 10.159.23.0/30R2 ↔ R4 · 10.159.24.0/30R1Client edgeR1 · Client edge · Starting deviceR2Transit coreR2 · Transit coreR3ApplicationedgeR3 · Application edgeR4Telemetry edgeR4 · Telemetry edge
Physical links · Service health not shown
Link addressing (3)
  • R1 R210.159.12.0/30
  • R2 R310.159.23.0/30
  • R2 R410.159.24.0/30
4 devices · Cisco IOS syntax · Simulated network

What you’ll practice

acl ordermulti service validationtcp vs udpacl policy

Interview exercises are inspired by common industry competencies. They are not proprietary interview questions.