How to run an 'undetectable' program

Daemons and Processes

What is a daemon? It is a running background process used in Unix to run services. These typically are not attached to a terminal, and will keep running even when users are logged off.

A friend asked me whether if it could be possible to run a background daemon undetected. Now this idea intrigued me. Well, it depends on what you mean by "undetected". Typically, a list of all currently running processes can easily be extracted, either by using the /proc sub directory, or by using a program like htop or ps. Now, if you were to run a daemon on for a long time, you could easily detect such program, for example, run htop then filter (F6) by time. The issue is any long-term daemons will just show up at the very top. Worse, the large times will actually be highlighted in RED text! So much for undetected program. Another limitation is with ulimit. Typically, sysadmins could set the maximum CPU time of any running program to an upper bound so that you don't waste away all their precious CPU clock cycles, especially when others are also using the same server.


Now there is a pretty ingenous solution to these limitations, plus more. Enter fork. Fork is typically implemented as a syscall (well technically, modern Linux systems use another syscall called clone, but still retains fork as a glib library function). According to the manpage, fork() creates a new process by duplicating the calling process. This is all fine and dandy, but what use is this syscall? Well, we can use fork to trick the system into thinking that we are multiple processes.

The idea of fork is that when you call fork, the kernel creates a copy of your currently running program, with all it's memory, file descriptors, signal handlers! This means that you have now two copies of the program, the "parent" process, which was running before you called fork(), and the "child" process, which is the newly created process. The only difference between the child and parent process is simply this: in the child process, fork() returns 0, but in the parent process fork() returns the PID of the child process.

Actually I lied, when fork() is called, there are some stuff that the child does not inherit. For one, it's process resource utilizations get reset to zero (interesting), second, it has a different pid than of its parent (obviously). This comes in handy later on. So when we called fork() we simply have a clean slate program when it comes to resources. We can simply go into a infinite loop, do some work, then fork, kill the parent process, letting the child live, then continuing this cycle (a sly-fork program). This is "undetected" on two-folds: (a) any resource times that count how much resource this process is using is repeatedly resetted to zero, which means the sysadmin will have no clue that this process has actually been running for a long time, hogging a lot of precious CPU time, and (b) if you fork frequently enough, such "sly-fork processes" actually will not show up on htop, (and even if it does, it will only do so for a few frames or so, and then spurriously disappear.)

Here is a brief source code that prints the approximate number of seconds it was alive in total, by using sleep and fork:

int main() {
long i = 0;
while (1) {
// Wait for one second

// Call fork()
pid_t ret = fork();
if (ret == -1) {
//An error occured
} else if (ret == 0) {
// Child process: continues to live for one more cycle.
printf("I am live for %d seconds.\n", i++);
} else {
// Parent exits out of program, child becomes new parent process.

The Dangers (and maybe some ways to curb such a program)

This is actually quite dangerous, because now, you virutally have no way of reliably detecting such a program (if it was written correctly). This process could continue to run in the background, harvesting off of some CPU cycles and run all sorts of menace calculations. This also could have potential for scaling (the user could then run multiple instances of such slyfork programs easily.) Then it comes to killing the program, (if you even know what the program was named). The problem is you cannot use the kill command because the pid of the program will keep on changing, so you are stuck using pkill and specifying a program name. pkill is a little heavy-handed, because it kills all programs with that particular name by default, (maybe there is an option to only kill one such program?). However, the user could get away with pkill by being more creative with their process name: either they can use some unsafe characters (like the - (dash) character in the beginning), or they could create a long program name similar to like a password, to prevent the user from bruteforcing the name.

Of course, the sysadmin could also just restart the server and be done with it. However, it can also be very difficult to detect such program, when they flash in and out of different programs. A possible way to effectively detect such program, could be to write a c program that detects programs that have very short life times, and then report these such programs. If they reoccur with such a large frequency, it should alert the admin to as a possible "sly-fork programs". The easiest way to catch such a program is if such short-lived programs have the same name attached to it. (However, this can be avoided, by calling exec right after a fork, and calling a new program that also continues on with the "sly-fork" program). Therefore, it is still advisable to just print out all such short-lived programs; there might be some flukes and noise within the output, but I think it should be good enough detect such rogue processes. If on the other hand these show up for more than a few seconds, it should more easily be caught by sysadmins and flagged if they keep cropping up in the list of processes.