One-way SOCK_STREAM - c++

Question

We're creating a socket using SOCK_STREAM over AF_UNIX (a local file). We have many processes on the system which are clients, and we are only interested in broadcasting messages to those clients. OS is Ubuntu 14.04.5 LTS.
This all works if the file permissions are 666, however as a security measure we want to change it to 644. However, this causes a permission exception when clients try to connect using the below code:
int XyzClient::establish_connection() {
int fd;
if ((fd = socket(AF_UNIX, SOCK_STREAM, 0)) == -1) {
throw XyzException(strerror(errno));
}
sockaddr_un addr = {};
addr.sun_family = AF_UNIX;
strncpy(addr.sun_path, DEFAULT_SOCKET_PATH, sizeof(addr.sun_path)-1);
if (connect(fd, (sockaddr*)&addr, sizeof(addr)) == -1) {
// connect failed, close fd and throw exception
close(fd);
throw XyzException(strerror(errno));
}
return fd;
}
Presumably the issue is the above code tries to establish read-write access to the file and which is causing the permission error.
Is it possible to do a one-way SOCK_STREAM such that we can lock down the file to be read-only by everyone except the owner/broadcaster?

Answer 1

It is not possible to make a read-only unix domain socket and enforce this with file permissions. From the relevant man page unix(7)
On Linux, connecting to a stream socket object requires write permission on that socket; sending a datagram to a datagram socket likewise requires write permission on that socket. POSIX does not make any statement about the effect of the permissions on a socket file, and on some systems (e.g., older BSDs), the socket permissions are ignored. Portable programs should not rely on this feature for security.