#!/bin/sh

argc=$#
if [ $argc -lt 5 -o $((argc % 2)) -ne 1 ]; then
{
  echo "tcplb.sh 20260917 - TCP loadbalancing shell script"
  echo
  echo "Usage:"
  echo "  tcplb.sh <progname> <facility> <timeout>     \\"
  echo "           <remote host 1> <remote port 1> ... \\"
  echo "           <remote host N> <remote port N>"
  echo
  echo "  <progname>    tag each line written to syslog with <progname>"
  echo "  <facility>    write syslog messages in the given <facility>"
  echo "  <timeout>     disconnect after being idle for <timeout> seconds"
  echo "  <remote host X> <remote port X> ..."
  echo "                backend or real server address"
  echo
  echo "Example:"
  echo "  To start tcplb.sh from inetd, add this line to /etc/inetd.conf:"
  echo
  echo "  http stream tcp nowait nobody /usr/local/libexec/tcplb.sh \\"
  echo "    tcplb.sh webproxy daemon 5 10.0.0.1 8081 10.0.0.2 8080"
  echo
  echo "  Each connection to port 80 on the local machine will randomly be"
  echo "  connected to 10.0.0.1:8081 or 10.0.0.2:8080. If one backend service"
  echo "  is down, then another one is chosen."
  echo "  In the above example, tcplb.sh runs as unprivileged user 'nobody',"
  echo "  each syslog message is tagged as 'webproxy' and is written to the"
  echo "  'daemon' facility. If a connection has been inactive for at least"
  echo "  5 seconds, it will be terminated."
  exit 1
} >&2
fi

progname="$1"
facility="$2"
timeout="$3"
shift 3

# "host port" per line, lines shuffled
hosts=`while [ -n "$2" ]; do
  echo "$1 $2"
  shift 2
done | random -f -`

cntavail=$(((argc-3)/2))
retval=1

# retry until connection succeeds, or no more hosts available
while [ $cntavail -ge 1 -a $retval -ne 0 ]; do
  # split host and port
  addr=`echo "$hosts" | head -n $cntavail | tail -n 1`
  host="${addr%% *}"
  port="${addr##* }"

  # the actual proxy connection
  # connect fd 3 to stdout outside the command group, i.e. to the client who
  # connected to inetd
  # inside the command group, connect stderr to stdout, captured by errmsg
  # connect stdout of nc to fd 3, opened outside the command group
  {
    errmsg=`nc -v -w "$timeout" "$host" "$port" 2>&1 >&3 3>&-`
  } 3>&1
  retval=$?

  # log to syslog
  if [ $retval -eq 0 ]; then
    logger -p "$facility.info" -t "$progname[$$]" \
              "host=$host port=$port retval=$retval"
  else
    logger -p "$facility.warn" -t "$progname[$$]" \
              "host=$host port=$port retval=$retval errmsg=$errmsg"
  fi

  # start at bottom line, then work up to first entry of hosts
  cntavail=$((cntavail-1))
done

exit $retval