TPLINK ER6120 Router - DNS look up issues (Randomly)

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TPLINK ER6120 Router - DNS look up issues (Randomly)

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TPLINK ER6120 Router - DNS look up issues (Randomly)
TPLINK ER6120 Router - DNS look up issues (Randomly)
2023-01-27 01:43:38 - last edited 2023-01-27 01:45:53
Model: TL-ER6120  
Hardware Version: V2
Firmware Version: 2.0.3 Build 20180929 Rel.56286

I have some random issues occurring whilst connected to an iiNet NBN FTTN service.

 

1x WAN port connected to a VDSL modem - is assigned a DHCP Dynamic address from the ISP.

 

All LAN devices were randomly still connected, but only the DNS lookups on client devices were failing.

So could ping external IPv4 addresses, but not via the DNS name. The DNS Name resolution failed.

This occurs at random time throughout the day for 30 decs to minutes at a time.

 

iiNet (The ISP) have performed some tests on their end and confirmed its not an issue with their service.

 

The ER6120 is setup as a DHCP server with DHCP reservations for local client.

The DNS (Optional PDNS and SDNS were blank).

 

DHCP Range : 192.168.1.5 to 1.79

Leasetime was 120.

Default gateway 1.254.

 

LAN5 is connected to a 24 Port Gigabit TPLINK Switch for the LAN across the office.

All clients connected via the TPLINK 24 Port switch were seeing similar symptoms.

 

The office is split between to business functions.  "Printing" and "Building"

 

** 

I am trying to isolate which piece of equipment or config function / firmware causing the issue.

 

The System Tools - System log on the ER6120 is showing the following...

 

I suspect it may be related to some of the following events?

 

11 2023-01-27 08:31:51 DHCP Client NOTICE WAN1: DHCP client renewing IP succeeded. (IP-Address=203.59.xx.xxx, Mask=255.255.224.0, Gateway=203.59.xx.xxx)
12 2023-01-27 08:31:51 DHCP Client NOTICE WAN1: DHCP client lease expired. Began renewing the lease.
13 2023-01-27 08:29:18 Firewall WARNING Detected Ping of Death attack. Dropped 5 packets.
14 2023-01-27 08:29:12 Firewall WARNING Detected Ping of Death attack. Dropped 6 packets.
15 2023-01-27 08:29:06 Firewall WARNING Detected Ping of Death attack. Dropped 6 packets.
16 2023-01-27 08:29:01 Firewall WARNING Detected Ping of Death attack. Dropped 5 packets.
17 2023-01-27 08:28:55 Firewall WARNING Detected Ping of Death attack. Dropped 6 packets.
18 2023-01-27 08:28:49 Firewall WARNING Detected Ping of Death attack. Dropped 6 packets.

 

How do I stop the WAN1 renewing lease? (This happens very frequently in the logs).

The "Detected Ping of Death attack" - I suspect this is normal and the firewall is doing it job. (This is also frequent).

 

**

 

In an attempt to isolate the DNS lookup issue, I have now done the following...

 

I have set the PDNS and SDNS setting in the DHCP Server setup on the ER6120. PDNS is : 192.168.1.254 and the SDNS is 8.8.8.8 (Google DNS)

This is now propogating across to the LAN Client devices and PC's appear to be more stable.

Lease time is extended from 120 to 2880.

 

I have also installed a 2nd switch.

"Printing" LAN devices all connected to the LAN5 port via the existing 24 Port TP LINK Gigabit switch. (They also have IPv4 IP Phones)

"Building" LAN Devices all connected to the LAN4 port via a new 8 Port TP LINK Gigabit Switch.

 

Connection speed to the internet via speedtest.net is providing : 65 Mbps Down / 29 Mbps Up. (So OK speeds for the size of the office)

 

Some clients on "Building" side are still experiencing issues with internet browsing and have reverted back to mobile phone hot spots.

 

**

Also the ER6120 is setup to provide some external client L2TP VPN access. This appears to work ok with external clients connecting remotely.

 

Suggestions / Any recommendations or further fault finding steps?

 

Firmware update on the ER6120?

 

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Re:TPLINK ER6120 Router - DNS look up issues (Randomly)
2023-01-27 12:05:50

  @dcs730 

Try 8.8.8.8 as the primary DNS;

 

Check the WAN IP of your ER6120, if it is a private IP, try to modify your modem as pure modem.

Double NAT may cause DNS lookup issue. I can't really explain why but I saw this issue happens on many double NAT network.

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