sslh service can't be enabled

Description:

When trying to enable sslh service, systemctl returns the following error:

$ sudo systemctl enable sslh                                                                                  
The unit files have no installation config (WantedBy=, RequiredBy=, UpheldBy=,                                         
Also=, or Alias= settings in the [Install] section, and DefaultInstance= for                                           
template units). This means they are not meant to be enabled or disabled using systemctl.                              
                                                                                                                       
Possible reasons for having these kinds of units are:                                                                  
• A unit may be statically enabled by being symlinked from another unit's                                              
  .wants/, .requires/, or .upholds/ directory.                                                                         
• A unit's purpose may be to act as a helper for some other unit which has
  a requirement dependency on it.                                                                                      
• A unit may be started when needed via activation (socket, path, timer,      
  D-Bus, udev, scripted systemctl call, ...).                                                                          
• In case of template units, the unit is meant to be enabled with some        
  instance name specified.

Additional info:

• package version(s): My current version is 2.1.1-1. But this is not a new bug, it has been happening for a long time.

Analysis

Service file installed by Arch Linux doesn't match upstream. Upstream service file has included the section that fixes this issue since 2013:

[Install]
WantedBy=multi-user.target

Workaround

Manually adding this section to the service file fixes the issue.