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Bad compression stub decompression header byte

Viscosity 1.7 added support for OpenVPN 2.4. In this release, OpenVPN 2.4 also became the default version your connections use. OpenVPN 2.4 adds many security and performance improvements. OpenVPN 2.3 is still available as an option to use if your server will not support OpenVPN 2.4, but Viscosity will eventually stop supporting older versions of OpenVPN so we do recommend getting your connections setup for OpenVPN 2.4 correctly.

Because OpenVPN 2.4 is a major release, there are some setting incompatibilities you may have from your OpenVPN 2.3 configurations. The most common incompatibility is compression settings and the deprecation of the comp-lzo command.

OpenVPN 2.3 was far more lenient about compression configuration mismatches between client and server. You may have wanted your connection to use compression, but if there is a configuration mismatch, OpenVPN 2.3 would generally not show any errors or warnings, instead compression would simply not be used.

OpenVPN 2.4 is much more strict. The first thing to check is your OpenVPN Log, pleasee see Viewing the OpenVPN Log if you are unsure where this is. If you see the following error in your log, it means that the compression settings you have in your configuration do not match the server:

Bad compression stub decompression header byte:

Unfortunately these errors can be sporadic, or may not appear at all. Another symptom that your compression settings may not be right is if you are able to connect, but no traffic is passing through the VPN. For example, websites may be timing out while connected to your VPN server.

Troubleshooting

If you are not the administrator of the server you are connecting to, as a first step we highly recommend contacting your VPN Provider for assistance. If you're unsure who this is please see the How Do I Find Out Who My VPN Provider Is? article. They may be able to provide you with an OpenVPN 2.4 configuration which you can simply import into Viscosity and start using, possibly saving you time and the headache of going through options to configure your connection for OpenVPN 2.4 correctly.

If you are the administrator of the server you are connecting to, we recommend updating your server to OpenVPN 2.4 and use the compress option added in this version and remove any comp-lzo options. OpenVPN 2.4 is backwards compatible with older OpenVPN servers in most cases though.

The following are some changes you can try to adapt your configuration to OpenVPN 2.4:

Edit your connection and go to the Advanced tab. If there is a line starting with comp-lzo here, take note of this line (it could be comp-lzo yes, comp-lzo no, comp-lzo adaptive, or just comp-lzo). Then delete this line, Save your connection and try to connect.

Once you save your connection, Viscosity will try to adapt your configuration to OpenVPN 2.4 automatically. If you are able to connect and access resources through your VPN, there is nothing more you need to do. If you are still having problems though, please try the following:

  • Edit your connection and go to the Options tab
  • If you had comp-lzo yes, comp-lzo adaptive, or just comp-lzo, the Compression option should be set to "LZO". If you are unable to use your VPN, change the Compression option to Off, save your connection and try connecting again.
  • If you had comp-lzo no, the Compression option should be set to "Off". If you are unable to use your VPN, change the Compression option to LZO, save your connection and try connecting again.