【Typical Case】How to Troubleshoot Domain Name Resolution Failures of NBR6205-E?

Keywords:

Domain Name System (DNS), Domain Name Resolution
1. Fault Description

The NBR device is connected to an NBS device at downlink. It obtains an IP address of 192.168.10.3 through DHCP and uses 8.8.8.8 as the DNS address.
While the NBS device can ping 8.8.8.8, it is unable to resolve domain names such as ryrcmq.ruijienetworks.com.
2. Device Model and Firmware

Device Type	Device Model	Firmware Version
Reyee NBR router	NBR6215-E	NBR_RGOS 11.9(6)B17P3
Reyee switch	NBS5200-24SFP/8GT4XS	Reyee OS 2.280.0.1709

3. Cause Analysis

• The basic settings of the device are incorrect, resulting in Internet access failures.
• A policy such as authentication or ACL is configured on the NBR device, preventing LAN devices from accessing the Internet.
  

• Communication between the device and the DNS server is abnormal.
  

4. Solution
4.1 Check Device Basic Settings

Check if the device is online, the DHCP settings are correct, and the NAT translation table entries are correct.
It is found that:
1. The NBR device can access the Internet and is connected to Ruijie Cloud.
2. The NBS device can obtain an IP address.
3. The NAT table contains 192.168.10.3, indicatingthe NAT function is normal.

4.2 Check Policy Configurations on the NBR Device

There are no special policies (such as ACLs, authentication, or website blocking) configured on the NBR device that would prevent LAN devices from accessing the Internet.
4.3 Check the Communication between the NBR Device and the DNS Server

• Check the DNS configuration
  

DNS server configured on the NBR device:
ip name-server 8.8.8.8
ip name-server 192.168.18.1
DNS server settings delivered to LAN devices:
ip dhcppool 10
lease 0 8 0
network 192.168.10.0 255.255.255.0192.168.10.2 192.168.10.254
dns-server8.8.8.8
default-router 192.168.10.1

• Perform packet capture
  
  

Only DNS Query packets are obtained on the NBR device, and no response packets from 8.8.8.8 are captured. This suggests that the NBR device may be unable to communicate with the DNS server.
However, the DNS server 192.168.18.1 sends DNS response packets.

• Check the flow table on the NBR device.
  

Run the showip f f command to display the flow table. It is found that packets are sentand received between the NBR device and the DNS server 192.168.18.1.
In contrary, the NBR device only sends packets to the DNS server 8.8.8.8 but doesn't receive any packets from the DNS server 8.8.8.8.
This confirms that communication between the NBR device and 8.8.8.8 is abnormal. Next,verify the communication between the NBR device and 8.8.8.8 on the NBS device.

• Run a DNS lookup test on the NBS device.
  

When the DNS server address is set to 8.8.8.8, the domain name cannot be resolved.
The domain name is resolved properly when the DNS server address is set to192.168.18.1.

• Change the DNS server address in the DHCP poolon the NBR device to 192.168.18.1. After the NBS device obtains a new IPaddress, it can access the Internet normally.
  

Note: Take the following action as aworkaround.
1. Enable the DNS serverfunction of the NBR device.
conf t
ip dns server enable
ip dns server nameserver8.8.8.8 end
wr
2. Configure the LANdevice's DNS address as the LAN port address of the NBR device.
Example:
conft
ipdns server enable
ipdns server host ryrcmq.ruijienetworks.com 34.120.84.165
ipdns server nameserver
223.5.5.5
end
wr