mosh-with-jump

#!/usr/bin/env bash

set -o errexit -o pipefail -o nounset

orig_args=("$@")
mosh_server_args=()
proxy=
proxy_port=
target=

while [[ $# -gt 0 ]]; do
  case "$1" in
    --jump-server=*)
      proxy=${1#--jump-server=}
      ;;
    -J | --jump-server)
      shift
      proxy=$1
      ;;
    --proxy-port=*)
      proxy_port=${1#--proxy-port=}
      ;;
    -P | --proxy-port)
      shift
      proxy_port=$1
      ;;
    -p)
      mosh_server_args+=("$1" "$2")
      shift
      ;;
    -*)
      mosh_server_args+=("$1")
      ;;
    *)
      if [[ -z $target ]]; then
        target=$1
      else
        mosh_server_args+=("$1")
      fi
      ;;
  esac
  shift
done

# if we're not using a jump server, let's just call mosh directly
if [[ -z $proxy ]]; then
  exec mosh "${orig_args[@]}"
fi

# we may be dealing with host aliases, so let's have SSH resolve that first so
# that we can get the host. however, we'll continue using $proxy and $target for
# SSH itself.
proxy_host=$(ssh -G "$proxy" | perl -anE '/^hostname (.+)/ && print "$1"')
proxy_ip=$(dig "$proxy_host" +short)
target_host=$(ssh -G "$target" | perl -anE '/^hostname (.+)/ && print "$1"')

echo "proxy: $proxy"
echo "proxy host: $proxy_host"
echo "proxy ip address: $proxy_ip"
echo "target: $target"
echo "target host: $target_host"

# start up the mosh server via the proxy
mosh_server_out=$(ssh -J "$proxy" "$target" -- mosh-server new -c 256 -s -l LANG=en_US.UTF-8 -l LANGUAGE=en_US "${mosh_server_args[@]}")

# parse out the port and key; we'll need the port for proxying
connect_line=$(grep -m1 '^MOSH CONNECT ' <<< "$mosh_server_out")
target_port=$(perl -anE 'print $F[2]' <<< "$connect_line")
key=$(perl -anE 'print $F[3]' <<< "$connect_line")

if [[ -z $proxy_port ]]; then
  # we'll need find a free port on the proxy, which might be different from the target
  find_free_port_cmd=$(
    cat << EOF
for (( p=$target_port ; p < $target_port + 10; p++ )); do
  if [[ -z \$(ss -Hlu sport = \$p) ]]; then
    echo "\$p"
    exit
  fi
done
exit 1
EOF
  )
  proxy_port=$(ssh -n "$proxy" bash -c "${find_free_port_cmd@Q}")
fi

echo "proxy port: $proxy_port"
echo "target port: $target_port"

# set up UDP proxying
# overall, we have client <--> proxy:$proxy_port <--> target:$target_port
ssh -tt -n "$proxy" socat udp-l:"$proxy_port",fork udp:"$target_host":"$target_port" 2>&1 &
trap 'kill -TERM -$$' EXIT

MOSH_KEY=$key mosh-client "$proxy_ip" "$proxy_port"