Using strace against daemons

The strace application not only makes sense for command-line applications but also for daemons. A popular approach to debugging daemons is to start them from the command line, possibly with a specific debug flag, so that the daemon doesn't detach and run in the background. However, this is often not possible on SELinux: the policy will not allow the daemon to run as a command-line foreground process.

How to do it…

The approach to use strace against daemons is similar as with command lines, focusing on the process ID rather than the command:

1. Find out what the process ID of the daemon is:

   ~$ pidof postgres
   2557
   

2. Use strace to attach to the running process:

   ~$ strace -o strace.log -f -s 256 -p 2557
   

3. Specify which system calls to watch out for. For instance, permission issues while binding or connecting to ports or sockets can be filtered as follows:

   ~$ strace -e poll,select,connect,recvfrom,sendto -o strace.log -f -s 256 -p 2557
   

4. Press Ctrl + C to interrupt the strace...