#!/usr/bin/env bash

# This script uses openssl, gnutls, or stunnel to secure an rsync daemon connection.

# By default this script takes rsync args and hands them off to the actual
# rsync command with an --rsh option that makes it open an SSL connection to an
# rsync daemon.  See the rsync-ssl manpage for usage details and env variables.

# When the first arg is --HELPER, we are being used by rsync as an --rsh helper
# script, and the args are (note the trailing dot):
#
#    rsync-ssl --HELPER HOSTNAME rsync --server --daemon .
#
# --HELPER is not a user-facing option, so it is not documented in the manpage.

# The first SSL setup was based on:  http://dozzie.jarowit.net/trac/wiki/RsyncSSL
# Note that an stunnel connection requires at least version 4.x of stunnel.

function rsync_ssl_run {
    case "$*" in
    *rsync://*) ;;
    *::*) ;;
    *)
	echo "You must use rsync-ssl with a daemon-style hostname." 1>&2
	exit 1
	;;
    esac

    exec rsync --rsh="'$0' --HELPER" "${@}"
}

function rsync_ssl_helper {
    if [[ -z "$RSYNC_SSL_TYPE" ]]; then
	found=`path_search openssl stunnel4 stunnel` || exit 1
	if [[ "$found" == */openssl ]]; then
	    RSYNC_SSL_TYPE=openssl
	    RSYNC_SSL_OPENSSL="$found"
	elif [[ "$found" == */gnutls-cli ]]; then
	    RSYNC_SSL_TYPE=gnutls
	    RSYNC_SSL_GNUTLS="$found"
	else
	    RSYNC_SSL_TYPE=stunnel
	    RSYNC_SSL_STUNNEL="$found"
	fi
    fi

    case "$RSYNC_SSL_TYPE" in
	openssl)
	    if [[ -z "$RSYNC_SSL_OPENSSL" ]]; then
		RSYNC_SSL_OPENSSL=`path_search openssl` || exit 1
	    fi
	    optsep=' '
	    ;;
	gnutls)
	    if [[ -z "$RSYNC_SSL_GNUTLS" ]]; then
		RSYNC_SSL_GNUTLS=`path_search gnutls-cli` || exit 1
	    fi
	    optsep=' '
	    ;;
	stunnel)
	    if [[ -z "$RSYNC_SSL_STUNNEL" ]]; then
		RSYNC_SSL_STUNNEL=`path_search stunnel4 stunnel` || exit 1
	    fi
	    optsep=' = '
	    ;;
	*)
	    echo "The RSYNC_SSL_TYPE specifies an unknown type: $RSYNC_SSL_TYPE" 1>&2
	    exit 1
	    ;;
    esac

    if [[ -z "$RSYNC_SSL_CERT" ]]; then
	certopt=""
	gnutls_cert_opt=""
    else
	certopt="-cert$optsep$RSYNC_SSL_CERT"
	gnutls_cert_opt="--x509certfile=$RSYNC_SSL_CERT"
    fi

    if [[ -z "$RSYNC_SSL_KEY" ]]; then
	keyopt=""
	gnutls_key_opt=""
    else
	keyopt="-key$optsep$RSYNC_SSL_KEY"
	gnutls_key_opt="--x509keyfile=$RSYNC_SSL_KEY"
    fi

    if [[ -z ${RSYNC_SSL_CA_CERT+x} ]]; then
	# RSYNC_SSL_CA_CERT unset - default CA set AND verify:
	# openssl:
	caopt="-verify_return_error -verify 4"
	# gnutls:
	# gnutls-cli has no reliable "use the system trust store AND make
	# verification fatal" switch across versions, so the gnutls path
	# refuses below unless RSYNC_SSL_ALLOW_INSECURE_GNUTLS is set.
	gnutls_opts=""
	# stunnel:
	# Since there is no way of using the default CA certificate collection,
	# we cannot do any verification. Thus, stunnel should really only be
	# used if nothing else is available.
	cafile=""
	verify=""
    elif [[ "$RSYNC_SSL_CA_CERT" == "" ]]; then
	# RSYNC_SSL_CA_CERT set but empty -do NO verifications:
	# openssl:
	caopt="-verify 1"
	# gnutls:
	gnutls_opts="--insecure"
	# stunnel:
	cafile=""
	verify="verifyChain = no"
    else
	# RSYNC_SSL_CA_CERT set - use CA AND verify:
	# openssl:
	caopt="-CAfile $RSYNC_SSL_CA_CERT -verify_return_error -verify 4"
	# gnutls:
	gnutls_opts="--x509cafile=$RSYNC_SSL_CA_CERT"
	# stunnel:
	cafile="CAfile = $RSYNC_SSL_CA_CERT"
	verify="verifyChain = yes"
    fi

    port="${RSYNC_PORT:-0}"
    if [[ "$port" == 0 ]]; then
	port="${RSYNC_SSL_PORT:-874}"
    fi

    # If the user specified USER@HOSTNAME::module, then rsync passes us
    # the -l USER option too, so we must be prepared to ignore it.
    if [[ "$1" == "-l" ]]; then
	shift 2
    fi

    hostname="$1"
    shift

    if [[ -z "$hostname" || "$1" != rsync || "$2" != --server || "$3" != --daemon ]]; then
	echo "Usage: rsync-ssl --HELPER HOSTNAME rsync --server --daemon ." 1>&2
	exit 1
    fi
    validate_ssl_hostname "$hostname"

    if [[ $RSYNC_SSL_TYPE == stunnel && -z ${RSYNC_SSL_CA_CERT+x} && "$RSYNC_SSL_ALLOW_INSECURE_STUNNEL" != 1 ]]; then
	echo "stunnel requires RSYNC_SSL_CA_CERT for server certificate validation (or set RSYNC_SSL_ALLOW_INSECURE_STUNNEL=1 to opt out)" 1>&2
	exit 1
    fi

    # Bind the server certificate to the requested host (openssl -verify_hostname,
    # stunnel checkHost) so a cert that is CA-valid for a *different* name can't be
    # used to MITM. Set RSYNC_SSL_SKIP_HOSTNAME_CHECK=1 to keep CA chain
    # verification but skip the identity check (e.g. bare-IP or internal-CA setups).
    # checkHost only applies while the chain is being verified.
    openssl_host_opt="-verify_hostname $hostname"
    stunnel_host_opt=""
    [[ "$verify" == "verifyChain = yes" ]] && stunnel_host_opt="checkHost = $hostname"
    if [[ "$RSYNC_SSL_SKIP_HOSTNAME_CHECK" == 1 ]]; then
	openssl_host_opt=""
	stunnel_host_opt=""
    fi

    if [[ $RSYNC_SSL_TYPE == gnutls && -z ${RSYNC_SSL_CA_CERT+x} && "$RSYNC_SSL_ALLOW_INSECURE_GNUTLS" != 1 ]]; then
	echo "gnutls-cli requires RSYNC_SSL_CA_CERT for server certificate validation (or set RSYNC_SSL_ALLOW_INSECURE_GNUTLS=1 to opt out)" 1>&2
	exit 1
    fi

    if [[ $RSYNC_SSL_TYPE == openssl ]]; then
	exec "$RSYNC_SSL_OPENSSL" s_client $caopt $certopt $keyopt -quiet -verify_quiet -servername $hostname $openssl_host_opt -connect $hostname:$port
    elif [[ $RSYNC_SSL_TYPE == gnutls ]]; then
	exec "$RSYNC_SSL_GNUTLS" --logfile=/dev/null $gnutls_cert_opt $gnutls_key_opt $gnutls_opts $hostname:$port
    else
	# devzero@web.de came up with this no-tmpfile calling syntax:
	exec "$RSYNC_SSL_STUNNEL" -fd 10 11<&0 <<EOF 10<&0 0<&11 11<&-
foreground = yes
debug = crit
connect = $hostname:$port
client = yes
TIMEOUTclose = 0
$verify
$stunnel_host_opt
$certopt
$cafile
EOF
    fi
}

function validate_ssl_hostname {
    local host="$1"
    if [[ -z "$host" || "$host" == -* || "$host" =~ [^A-Za-z0-9._:-] ]]; then
	echo "invalid rsync-ssl hostname: $host" 1>&2
	exit 1
    fi
}

function path_search {
    IFS_SAVE="$IFS"
    IFS=:
    for prog in "${@}"; do
	for dir in $PATH; do
	    [[ -z "$dir" ]] && dir=.
	    if [[ -f "$dir/$prog" && -x "$dir/$prog" ]]; then
		echo "$dir/$prog"
		IFS="$IFS_SAVE"
		return 0
	    fi
	done
    done

    IFS="$IFS_SAVE"
    echo "Failed to find on your path: $*" 1>&2
    echo "See the rsync-ssl manpage for configuration assistance." 1>&2
    return 1
}

if [[ "$#" == 0 ]]; then
    echo "Usage: rsync-ssl [--type=SSL_TYPE] RSYNC_ARG [...]" 1>&2
    echo "The SSL_TYPE can be openssl or stunnel"
    exit 1
fi

if [[ "$1" = --help || "$1" = -h ]]; then
    exec rsync --help
fi

if [[ "$1" == --HELPER ]]; then
    shift
    rsync_ssl_helper "${@}"
fi

if [[ "$1" == --type=* ]]; then
    export RSYNC_SSL_TYPE="${1/--type=/}"
    shift
fi

rsync_ssl_run "${@}"
