Socat keeps waiting for input from stdin

The author probably never tried it. The manual says this about the fork option:

After establishing a connection, handles its channel in a child process and keeps the parent process attempting to produce more connections, either by listening or by connecting in a loop (example).

That means socat will not fork before starting the main listen loop.

The -d option just increases verbosity (as in debug).

tl;dr: socat can’t do that.

The source tree contains this helper script to launch socat in the background. You can adapt it to your purposes. I did not write this script!

#! /bin/sh
# source: daemon.sh
# Copyright Gerhard Rieger 2001
# Published under the GNU General Public License V.2, see file COPYING

# This script assumes that you create group daemon1 and user daemon1 before.
# they need only the right to exist (no login etc.)

# Note: this pid file mechanism is not robust!

# You will adapt these variables
USER=daemon1
GROUP=daemon1
INIF=fwnonsec.domain.org
OUTIF=fwsec.domain.org
TARGET=w3.intra.domain.org
INPORT=80
DSTPORT=80
#
INOPTS="fork,setgid=$GROUP,setuid=$USER"
OUTOPTS=
PIDFILE=/var/run/socat-$INPORT.pid
OPTS="-d -d -lm"    # notice to stderr, then to syslog
SOCAT=/usr/local/bin/socat

if [ "$1" = "start" -o -z "$1" ]; then

    $SOCAT $OPTS tcp-l:$INPORT,bind=$INIF,$INOPTS tcp:$TARGET:$DSTPORT,bind=$OUTIF,$OUTOPTS </dev/null &
    echo $! >$PIDFILE

elif [ "$1" = "stop" ]; then

    /bin/kill $(/bin/cat $PIDFILE)
fi

Socat keeps waiting for input from stdin

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