Description

 This KB explains the steps to identify and rectify the DNS entry resolution faced by the customer when the config was pushed from MIST APs to core switches.

Symptoms

The customer was unable to resolve DNS entries in both their core switches:

Model: qfx5120-48y-8c
Junos: 20.4R3-S7.2 flex

core1> ping inet google.com
ping: cannot resolve google.com: Host name lookup failure

The issue started when the config was pushed from mist AI to the switch. Earlier DNS was working fine.

Solution

- As investigated, we saw the ICMP requests are going out using the loopback as the source to the destination DNS server.
- No ICMP replies were received.
- Identified that Mist is pushing a loopback IP that was not configured earlier via CLI and the switch is using that loopback IP to reach the DNS server.
- The loopback IP is not advertised to the outer network, there is no reverse patch for ICMP replies and that is why DNS is not working when config is pushed via mist AI. 
- It is a routing issue. The customer advertised the loopback to resolve the loopback reachability.
- The DNS issue was fixed after that.

Modification History

2023-09-12: <initial publication>