Systemd Failed to Enable Unit?

When I attempt to enable the service using sytemd, I encounter an error where systemd complains that the server does not exists. For example, this is what I observe when I try to enable the OpenVPN service.

$ systemctl enable openvpn@server
Failed to enable unit: Unit file openvpn@server does not exist

This means that I miss the service configuration file. But which one and where should the file be? Although I don't know much about systemd, I resolve this issue by using strace. Using the OpenVPN service as an example, I do the following,

1. Run strace on the systemd process, i.e.,
   

   
       sudo sh -c 'strace -p 1 2>&1 | tee strace_systemd.log'
       

2. Next, run systemctl enable for the OpenVPN service,
   

   
   	sudo systemctl enable openvpn@server
   	

3. Examine strace_systemd.log. For this, we search the service name, e.g., openvpn, in it. In this example, we see,
   

   
   ......
   recvmsg(25, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="/org/freedesktop/systemd1\0\0\0\0\0\0\0"..., iov_len=200}], msg_iovlen=1, msg_controllen=0, msg_flags=MSG_CMSG_CLOEXEC}, MSG_DONTWAIT|MSG_CMSG_CLOEXEC) = 200 
   getuid()                                = 0   
   openat(AT_FDCWD, "/etc/systemd/system.control/openvpn@server.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/run/systemd/system.control/openvpn@server.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/run/systemd/transient/openvpn@server.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/run/systemd/generator.early/openvpn@server.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/etc/systemd/system/openvpn@server.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/run/systemd/system/openvpn@server.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/run/systemd/generator/openvpn@server.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/usr/local/lib/systemd/system/openvpn@server.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/usr/lib/systemd/system/openvpn@server.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/run/systemd/generator.late/openvpn@server.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/etc/systemd/system.control/openvpn@.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/run/systemd/system.control/openvpn@.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/run/systemd/transient/openvpn@.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/run/systemd/generator.early/openvpn@.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   openat(AT_FDCWD, "/etc/systemd/system/openvpn@.service", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = -1 ENOENT (No such file or directory)
   ...
   
   sendmsg(25, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="l\3\1\0015\0\0\0\1\0\0\0g\0\0\0\5\1u\0\1\0\0\0\4\1s\0#\0\0\0"..., iov_len=120}, {iov_base="0\0\0\0Unit file openvpn@server.ser"..., iov_len=53}], msg_iovlen=2, msg_controllen=0, msg_flags=0}, MSG_DONTWAIT|MSG_NOSIGNAL) = 173
       

   where systemd complains that it cannot find the configuration file of the service at these locations.
4. Luckily, I do locate a configuration file in the system, e.g.,
   $ sudo find /etc -name openvpn\* /etc/systemd/system/multi-user.target.wants/openvpn@server.service /etc/openvpn $
5. Thus, to fix this issue is to create a link to the OpenVPN service configuration file in the /etc/systemd/system/ directory where systemd is searching for the file based on the output of the strace command,
   sudo ln -s /usr/lib/systemd/system/openvpn-server\@.service /etc/systemd/system/openvpn@server.service

The systemctl enable is successful after the above.

Tracing System Calls and System Events

I found strace and ltrace were good tools on Linux to diagnose many problems.

On Windows, similar tools actually exist. The equivalent to strace, perhaps is, Process Monitor and that to ltrace, Logger and LogViewer.

 In addition, Windows' event instrumentation is useful.

A good article is http://jbremer.org/intercepting-system-calls-on-x86_64-windows/ . The post is based on the discussion at http://stackoverflow.com/questions/3847745/systrace-for-windows.