How to create a Unix domain socket with specific access rights
have a situation where I intend to communicate with the service through the command interface provided by a UNIX domain socket on the file system. I was able to successfully send commands to it, but for a while I was confused as to why I couldn’t get any response to my query.
It turns out that the service does not have sufficient permissions to write to the address that I (or the operating system) gave it. However, I realized that if I explicitly
bind (bind) to an address on the filesystem, then I can adjust file permissions by utilizing
int mySocket; struct sockaddr_un local_addr; mySocket = socket(AF_UNIX, SOCK_DGRAM, 0); local_addr.sun_family = AF_UNIX; snprintf(local_addr.sun_path, 108 "/path/to/mySocket"); bind(mySocket, (struct sockaddr *) &local_addr, sizeof(struct sockaddr_un)); chmod("/path/to/mySocket", 777);
That is, without the final
chmod step, the service will not be able to write
to mySocket because it does not have proper write permissions. Obviously, without explicitly
binding to a specific address, this is a harder problem to spot, as the underlying OS will implicitly generate this socket for the user – but it still exists and will still have the same access issues.
So, my question is about this last step. Is there a way to allow the OS to implicitly generate a socket for my endpoint (i.e. the address to which the service will respond) but request that it be granted certain permissions?
The reason this problem becomes problematic is that other parts of the program need to be executed as root. So when I try
send to a background service as root, the OS will implicitly create an address to which the reply will be directed. However, this has led to an issue with my socket file, whether implicitly or created with
bind, which will have permissions like
srw- --- --- , so another endpoint can only reply if they also elevate themselves.
So if I
bind first and then
chmod permissions, the problem goes away, as shown above.
Is there a way to allow the OS to implicitly generate the socket for my endpoint (i.e. the address to which the service will respond) but request that it be given certain permissions?
I solved this problem by calling
current_mask = umask(umask_to_be_used_on_afunix_socket_file_system_entry_creation); bind afunix socket here umask(current_umask);