1.1 KiB
+++ title = "Issues With OpenVPN & MikroTik" date = 2025-10-22
[taxonomies] categories = ["Networking"]
[extra] author = "Emil Miler" +++
I started having issues with my an existing OpenVPN tunnel after setting up a new MikroTik router in my home network.
The symptoms were strange: I could resolve addresses through DNS, ping all hosts on the network, and access HTTP sites without issues. However, when I tried to SSH into any machine or access HTTPS services, the connection would time out.
After checking the OpenVPN logs, I discovered the issue was this:
read UDPv4 [EMSGSIZE Path-MTU=1460]: Message too long (fd=3,code=90)
In short, the encrypted packets being sent were too large.
Fixing OpenVPN
The preferred option is to change the MTU size in the OpenVPN configuration by adding the following:
tun-mtu 1440
mssfix 1392 fixed
Fixing MikroTik
Other option, albeit not preferred, is to set maximum MTU size on ether1 (WAN) to 1460 from the default of 1500.
/interface ethernet set ether1 mtu=1460
Or in the GUI under Interfaces > ether1 > MTU.
