I want to connect to Unix Domain Socket created by ZeroMQ (IPC model) via command nc. I can connect, but when I sending some messages then, my deamon, which is listening to this socket, is not getting any message...
I'm using nc like:
nc -U /path/to/socket
Very well, here's a longer version.
ZeroMQ implements a message queue transport system over the top of stream connections like sockets, named pipes, etc. To do this it runs a protocol called ZMTP over the top of the stream, which provides all the message demarcation, communication patterns, and so forth. It also has to deal with protocol errors in order to give itself some resiliency.
Comparison to a Web Browser
It's the same idea to a web browser and web server communicating using http over a socket. Http is used to transport html files. If you look at the data flowing over the socket you see the html mixed up with the messages involved in running the http protocol. And because http is a text based protocol, it looks kinda OK to the human eye.
Talking the Same Language
Thus when a program that uses the zmq libraries for communication connects a socket / named pipe / etc, it will be expecting to exchange data over that connection in the way defined by the ZMTP protocol (in the same way a web browser is expecting to talk to a server using http). If the program at the other end is also using zmq, then they're both talking the same protocol and everything is good.
Incompatible Protocols
However, if you connect a program that doesn't of itself use the ZMTP protocol such as a web browser, and that sends a http request, it's unlikely to mean anything. And the zmq library routines will no doubt receive the bytes that make up the http request, attempt to interpret it, fail to understand it, and ultimately reject it as garbage.
Similarly if the program that uses the zmq library wants to send messages, nothing will happen unless the underlying ZMTP protocol driver is content that it is communicating with something else that talks ZMTP. If anything at all emerges from netcap, it won't look anything like the message you were sending (it'll be jumbled up with the bytes that ZMTP uses).
Human Equivalent
The equivalent is an Englishman called Bob picking up the phone and dialling the number for his English friend called Alice living in Paris. However, if a Frenchman called Charlie answers the phone by mistake (wrong number), it'll be very difficult for them to exchange information. Meanwhile Eve, who's tapped the phone line, is laughing her head off at the ineptitude of these two people's failed attempt to communicate. (I make sweeping and partly justifiable generalisations about us Englishmen's poor ability to speak any other language).
Way Forward
There's a ZMQ binding available for almost everything, possibly even bash. Whatever it is you're trying to accomplish it's probably well worth while getting a decent binding of ZMQ for the programming or scripting language your using, and use that to provide a proper ZMQ endpoint.
