SSH: Provide Additional Pipe FDs Besides stdin, stdout, stderr

file-descriptorspipessh

When connecting to a host with SSH, usually three "pipes" are provided between host and guest, for stdin, stdout, and stderr.

Is there a command-line option to create forwards for additional file descriptors (3 and onward)?

For example, I'd like to do

ssh --forwardfd=10:3 remotehost 'echo test >&3'

which would print 'test' to the locally-opened file descriptor 10.

Best Answer

You can do this using socket forwarding, which is available since openssh-6.7. This is some kind of pipe. This technique is described for example here: http://www.25thandclement.com/~william/projects/streamlocal.html

You will gain two-direction route for your data. There is example with mysql:

Proxy MySQL client connections on a remote server to your local instance:
ssh -R/var/run/mysql.sock:/var/run/mysql.sock \
    -R127.0.0.1:3306:/var/run/mysql.sock somehost 

Related Solutions

POSIX IO – How Portable are /dev/stdin, /dev/stdout, and /dev/stderr

It's been available on Linux back into its prehistory. It is not POSIX, although many actual shells (including AT&T ksh and bash) will simulate it if it's not present in the OS; note that this simulation only works at the shell level (i.e. redirection or command line parameter, not as explicit argument to e.g. open()). That said, it should be available on most commercial Unix systems, one way or another (sometimes it's spelled /dev/fd/N for various integers N, but most systems with that will provide symlinks as Linux and *BSD do).

What sets a child’s STDERR, STDOUT, and STDIN

Stdin, stdout and stderr are inherited from the parent process. It's up to the child process to change them to point to new files if that is needed.

From the fork(2) man page:

   *  The  child inherits copies of the parent's set of open file descrip‐
      tors.  Each file descriptor in the child refers  to  the  same  open
      file  description (see open(2)) as the corresponding file descriptor
      in the parent.

Best Answer

Related Solutions

POSIX IO – How Portable are /dev/stdin, /dev/stdout, and /dev/stderr

What sets a child’s STDERR, STDOUT, and STDIN

Related Question