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OpenVPN somehow failed when tethering on the Android mobile hotspot from a new phone

Posted by jpluimers on 2023/04/07

A while after I got a new smartphone, I noticed that when my MacBook was connected over Wi-Fi to the mobile hotspot of my Android phone, the Tunnelblick connections over OpenVPN to my family members would not work. A telnet from the Android phone to the OpenVPN TCP port 1194 woud succeed, but not from the MacBook. Connecting from the phone using JuiceSSH to the OpenSSH endpoints at those family members would work too, so I was a bit flabbergasted.

In the end this seems to be a set of coincidences that fails in this particular setup, but I am not totally aware why.

The solution was to both re-configure the APN (Access Point Name) the smartphone uses to connect to the internet from ipv4/ipv6 to ipv4, and to reboot the phone.

For Dutch provider KPN Mobile, the APN is named internet and apparently changed default to ipv4/ipv6 without properly supporting ipv4. Note the configuration parameters are all lowercase, although they should be written IPv4 and IPv6.

Here are a few posts that got me on the right track (all via [Wayback/Archive] openvpn fails over android hotspot – Google Search):

Note that sometimes the MTU can cause similar failures:

Note too: some links to check for OpenVPN responding are below.

Various sites with (often different) APNs that KPN mobile supports:

There are quite a few APNs, some with firewall and/or proxy and/or compression, some with external IP address (which means your smartphone really needs a firewall).


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