46 lines
1.1 KiB
Markdown
46 lines
1.1 KiB
Markdown
+++
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title = "Issues With OpenVPN & MikroTik"
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date = 2025-10-22
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[taxonomies]
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categories = ["Networking"]
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[extra]
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author = "Emil Miler"
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I started having issues with my an existing OpenVPN tunnel after setting up a new MikroTik router in my home network.
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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.
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<!-- more -->
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After checking the OpenVPN logs, I discovered the issue was this:
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```
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read UDPv4 [EMSGSIZE Path-MTU=1460]: Message too long (fd=3,code=90)
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```
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In short, the encrypted packets being sent were too large.
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## Fixing OpenVPN
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The preferred option is to change the MTU size in the OpenVPN configuration by adding the following:
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```
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tun-mtu 1440
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mssfix 1392 fixed
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```
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## Fixing MikroTik
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Other option, albeit not preferred, is to set maximum MTU size on `ether1` (WAN) to 1460 from the default of 1500.
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```
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/interface ethernet set ether1 mtu=1460
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```
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Or in the GUI under `Interfaces > ether1 > MTU`.
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