How to create a public unix domain socket?
Consider /var/run/acpid.socket . At any point I can connect to it and disconnect from it. Compare that with nc :
$ nc -l -U ./myunixsocket.sock Ncat: bind to ./myunixsocket.sock: Address already in use. QUITTING. nc apparently allows only single-use sockets. Question is then, how do I create a socket analogous to /var/run/acpid.socket , for multiple use and reuse ?
The two cases are not equivalent. With nc you are trying to have a «server» listening on the socket while something else is already listening to it. In the first case, a single acpid instance is listening on the socket. In both cases, multiple clients can write to it. (All this written with an appropriate amount of handwaving).
It’s not true that nc allows only single-use sockets. What’s true, is that nc is not a real server (it will not fork a subprocess to handle a connection, then go back to accepting connections while the child is still running, like eg. the X11 server or acpid do); while nc -l is handling a connection, it will not accept() other connections on the same socket. And this is no different from how nc -l works with tcp sockets.
@Kusalananda So what do I do to make it equivalent ? I have to keep an extra listening nc process just to allow multiple other listeners ?
1 Answer 1
You do it with the -k option to nc .
-k Forces nc to stay listening for another connection after its cur- rent connection is completed. It is an error to use this option without the -l option. When used together with the -u option, the server socket is not connected and it can receive UDP data- grams from multiple hosts. $ rm -f /tmp/socket # unlink the socket if it already exists $ nc -vklU /tmp/socket # the server Connection from mack received! yes Connection from mack received! yes . It’s recommended to unlink() the socket after use — but, in fact, most programs check if it exists and remove it before calling bind() on it; if the socket path exists in the filesystem and you try to bind() to it, you will get an EADDRINUSE error even when no program is using it in any way.
One way to avoid this whole mess on linux is to use «abstract» unix sockets, but they don’t seem to be supported by netcat .
Linux create unix socket
NAME
socket - create a TCP or a UNIX domain socket and connect to stdin/out
SYNOPSIS
socket [ -bcfqrvw ] [ -p command ] [ -B local address ] host port socket [ -bcfqrvw ] [ -p command ] /path socket [ -bcfqrvw ] [ -p command ] [ -B local address ] -s [ -l ] port socket [ -bcfqrvw ] [ -p command ] -s [ -l ] /path
DESCRIPTION
Socket creates an Internet domain TCP or a UNIX domain stream socket and connects it to stdin and stdout. The host argument can be an Internet number in dot-notation (like ``130.149.28.10'') or a domain name. In this case it must be possible to resolve the name to a valid Internet address with gethostbyname(3). The port argument can be a port number or a service name which can be mapped to a port number by getservbyname(3). If an UNIX domain socket is wanted to be created instead of an Internet socket, specify the path instead of an internet (canonical domain named or dot-notated) host. The hostname is treated as a pathname if contains at least a single slash. I.e. if one wants to create or connect to a socket in the current directory, use ./filename to specify the connection point.
OPTIONS
-b (background) The program forks itself into the background, detaches from its controlling tty, closes the file descriptors associated with the tty, and changes its current directory to the root directory. -B (local address) This option specifies which local address to binded to when making a connection. -c (crlf) Linefeed characters (LF) are converted to a Carriage Return Linefeed sequence (CRLF) when written to the socket. CRLF sequences read from the socket are converted to a single LF. -f (fork) When a server connection has been accepted, a separate process is forked to handle the connection in background. -l (loop) (only valid with -s) After a connection has been closed, another connection is accepted. -p (program) The specified command is executed for each connection. Its standard input, standard output, and standard error channels are connected to the socket. Command can be any shell command since it is passed to /bin/sh. -q (quit) The connection is closed when an end-of-file condition occurs on standard input. -r (read only) No data is read from standard input and written to the socket. -s (server) A server socket is created. A hostname argument is not required of Internet sockets, only the port number but a pathname is required for UNIX domain sockets. -v (verbose) Messages about connections etc. are issued to stderr. -w (write only) No data is read from the socket and written to the standard output. -version Socket prints its version ID and terminates. This must be the first argument to have an effect.
EXAMPLES
The command socket -v coma.cs.tu-berlin.de nntp connects to the nntp port (port 119) of coma.cs.tu-berlin.de (130.149.28.10). The command socket -sl 3425 creates a server socket on port 3425 on the local host and waits for a connection. After a connection has been closed, a new connection is accepted. The command socket -wslqvp "echo Socket! " 1938 creates a server socket on port 1938 on the local host and waits for a connection. When a connection is accepted, the string "Socket!" is written to the socket. No data is read from the socket and written to the finger program. The connection is closed when an end- of-file condition at the standard output of the program occurs. Then a new connection is accepted.
DIAGNOSTICS
Lots of diagnostics for failed system calls. unknown host host host's address could not be resolved. Signal signal caught, exiting Socket exits on any signal other than SIGTSTP, SIGCONT, SIGCLD, SIGQUIT. A non-zero exit code is returned if socket terminates due to an error condition or a signal.
SEE ALSO
ip(7), tcp(7), unix(7), accept(2), bind(2), listen(2), connect(2), socket(2), gethostbyname(3), getservbyname(3)
BUGS
socket -p terminates due to a SIGPIPE signal when there is more data from the socket available than the executed program wants to read. Please report any other bugs to the author.
VERSION
This manual page describes Socket-1.1.