Openvpn “WARNING: Bad encapsulated packet length from peer”

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I run a VPN from my Linode VM for various reasons, the most important of which is so that I and other family members can submit email over SMTP without having to worry about braindead networks that block outgoing port 587 for makey-uppey “security” reasons. Since my brother and I both have jobs that entail connecting to random corporate wireless networks, this is critical.

The problem was that I was running openvpn over the standard port 1194, which is also blocked by many networks – including my own employer’s. Openvpn uses a mock-HTTP protocol that will work over HTTP proxies, so I configured squid on the server’s port 8080 to forward packets to localhost:1194 and told the laptop openvpn client to use myserver:8080 as a proxy.

This worked well for my employer’s network, but did not agree with the guest wireless network of one of my clients, which had absolutely no problem with port 1194, but uses its own transparent proxy that doesn’t play nice with daisychained proxies. I kept having to comment and uncomment the proxy directive in my laptop’s openvpn.conf and restart, depending on location.

So I decided to do it the proper way, by connecting directly to openvpn on port 8080. My employer’s network would allow this through directly, and the client’s network should route through its transparent proxy without complaint. I don’t want to turn off port 1194 though, as this would rudely nobble all my brother’s devices, so I configured the server’s iptables to masquerade 8080->1194. I could then remove the proxy config from the laptop, change its connecting port to 8080 and restart the vpn client.

Problem solved! Except then I started getting the following error in my server logs:

Apr 28 13:02:43 xxx ovpn-server[13110]: xxx.xxx.xxx.xxx:57458 WARNING: Bad encapsulated packet length from peer (17231), which must be > 0 and <= 1560 -- please ensure that --tun-mtu or --link-mtu is equal on both peers -- this condition could also indicate a possible active attack on the TCP link -- [Attempting restart...]

It turned out this was being generated by another client which had also been configured to use the proxy, but which had slipped my mind. The error stems from the client connecting to an openvpn port directly but sending requests formatted for a web proxy. Not sure why it shows up as an MTU error, but changing the other client config to match the laptop solved it.

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