PMISuite/BackTunnel
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BackTunnel/scripts/backtunnel-share

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#!/usr/bin/env bash
# SPDX-License-Identifier: GPL-3.0-or-later
# Copyright (c) 2025 LUXIM d.o.o., Slovenia
# Author: Matjaž Mozetič
#
# Name: backtunnel-share
# Summary: Time-bounded reverse-SSH tunnel to expose the local SSH service for remote, temporary SFTP-based access.
# Description:
# Sets up a reverse SSH tunnel (ssh -R) from a remote host back to the local sshd, for a fixed duration.
# Prints an optional “invite” command that the remote side can run to mount a given local folder over SFTP
# using a companion tool. Optionally and temporarily adds a restricted public key entry to authorized_keys,
# scoped to localhost and internal-sftp only, and removes it on exit.
#
# Usage:
# backtunnel-share /path/to/folder with remoteuser:remotehost for <duration> [options]
#
# Examples:
# backtunnel-share ~/projects with alice:vps.example.com for 2h
# backtunnel-share ~/projects with alice@vps.example.com for 1d -p 4422 -l 2222
# backtunnel-share ~/projects with @work for 2h -i --qr --allow-known alice
#
# Dependencies:
# - bash >= 4.x
# - ssh, timeout, tail
# - optional: qrencode (for --qr)
#
# Configuration:
# Profiles file precedence (high → low):
# - ${XDG_CONFIG_HOME:-$HOME/.config}/backtunnel/profiles.ini
# - /etc/backtunnel/profiles.ini
# - /usr/share/backtunnel/profiles.ini
# Sections:
# - [default] global defaults
# - [name] referenced via @name → expands to user@host and overrides select defaults
#
# Exit codes:
# 0 success (including duration reached)
# 1 invalid usage/arguments or validation failure
# 124 timeout reached (handled and normalized to 0 when expected)
# 2+ other runtime errors (from commands like ssh/timeout/tail)
#
# Security:
# - Reverse tunnel binds on remote loopback only (127.0.0.1:PORT).
# - Temporary authorized_keys entries (if enabled) are restricted with:
# from="127.0.0.1",command="internal-sftp",restrict
# and are removed on exit via trap.
# - ~/.ssh perms are enforced (700 dir, 600 authorized_keys).
#
# Portability/Assumptions:
# - Uses bash arrays, [[ ]] tests, and bash-specific parameter expansions.
# - Uses awk for INI parsing (best-effort).
#
# Notes:
# - set -euo pipefail: stop on error/undefined vars; treat pipeline errors as failures.
# - Traps ensure SSH child is terminated and temporary keys are cleaned up.
# backtunnel-share: Share a folder using reverse SSH for a limited duration
# Syntax:
# backtunnel-share /path/to/folder with remoteuser:remotehost for 2h [options]
#
# Options:
# -p|--tunnel-port <PORT> Remote port to expose with -R (default: 2222)
# -l|--local-ssh-port <PORT> Local sshd port to forward to (default: 22)
# -i|--invite Print an invite command (and/or QR) for chat
# --invite-mount <PATH> Suggested mount point in invite (default: $HOME/remote-rssh)
# --invite-file <FILE> Also write invite text (with unmount hint) to FILE
# --qr Also render the invite command as a QR code (qrencode)
# --allow-key <FILE> Temporarily authorize accessor's public key for this session
# --allow-known <NAME> Temporarily authorize ~/.config/backtunnel/authorized/NAME.pub
# -h|--help Show help
#
# Profiles: ~/.config/backtunnel/profiles.ini (overrides /etc/backtunnel/profiles.ini then /usr/share/backtunnel/profiles.ini)
# [default] tunnel_port=2222 local_ssh_port=22 invite_mount=$HOME/remote-rssh invite=true qr=false
# [alice] user=alice host=vps.example.com tunnel_port=4422
set -euo pipefail
# ----------------------------
# Config discovery
# Purpose: choose the highest-precedence profiles.ini available.
# ----------------------------
CONFIG_USER="${XDG_CONFIG_HOME:-$HOME/.config}/backtunnel/profiles.ini"
CONFIG_SYS="/etc/backtunnel/profiles.ini"
CONFIG_PKG="/usr/share/backtunnel/profiles.ini"
if [[ -f "$CONFIG_USER" ]]; then
CONFIG_FILE="$CONFIG_USER"
elif [[ -f "$CONFIG_SYS" ]]; then
CONFIG_FILE="$CONFIG_SYS"
else
CONFIG_FILE="$CONFIG_PKG"
fi
# ----------------------------
# INI helpers
# ----------------------------
# shellcheck disable=SC2317
# ini_get: read a value from CONFIG_FILE INI [SECTION] key=value
# Arguments:
# $1: section name
# $2: key
# Env:
# CONFIG_FILE (read)
# Returns:
# prints the value on success; empty string if not found
# Notes:
# - Best-effort parsing; trims spaces around '=' and the value.
ini_get() { # ini_get SECTION KEY -> value
local sec="$1" key="$2"
awk -v s="[""$sec""]" -v k="$key" '
$0==s {ok=1; next}
/^\[/ {ok=0}
ok && $0 ~ /^[[:alnum:]_.-]+[[:space:]]*=/ {
line=$0
sub(/[[:space:]]*=[[:space:]]*/, "=", line)
split(line,a,"=")
if (a[1]==k) {
val=substr(line, index(line,"=")+1)
gsub(/^[[:space:]]+|[[:space:]]+$/,"",val)
print val
exit
}
}' "${CONFIG_FILE}" 2>/dev/null || true
}
# shellcheck disable=SC2317
# profile_expand_remote: expand @name -> user@host based on profiles.ini, else pass through.
# Arguments:
# $1: input remote spec (e.g., @work or user@host or user:host)
# Returns:
# prints expanded remote (user@host) if @name is resolvable; otherwise returns input unchanged
profile_expand_remote() { # "@name" -> user@host, otherwise pass through
local in="$1"
if [[ "$in" == @* ]]; then
local name="${in#@}" user host
user="$(ini_get "$name" user)"
host="$(ini_get "$name" host)"
if [[ -n "$user" && -n "$host" ]]; then
printf '%s@%s\n' "$user" "$host"
return 0
fi
fi
printf '%s\n' "$in"
}
# shellcheck disable=SC2317
# profile_apply_defaults: set global defaults from [default], then override from named profile.
# Arguments:
# $1: profile name (without '@'), may be empty
# Side effects:
# - Modifies global variables: TUNNEL_PORT, LOCAL_SSH_PORT, INVITE_MOUNT, INVITE, QR, DURATION
# Notes:
# - Only applies default values if globals still hold built-in defaults.
profile_apply_defaults() { # set globals if unset; named overrides default
local name="$1" v
# defaults
v="$(ini_get default tunnel_port)"; [[ -n "$v" && "${TUNNEL_PORT}" == "2222" ]] && TUNNEL_PORT="$v"
v="$(ini_get default local_ssh_port)"; [[ -n "$v" && "${LOCAL_SSH_PORT}" == "22" ]] && LOCAL_SSH_PORT="$v"
v="$(ini_get default invite_mount)"; [[ -n "$v" && "${INVITE_MOUNT}" == "$HOME/remote-rssh" ]] && INVITE_MOUNT="$v"
v="$(ini_get default invite)"; [[ "${v,,}" == "true" ]] && INVITE=true
v="$(ini_get default qr)"; [[ "${v,,}" == "true" ]] && QR=true
if [[ -z "$DURATION" ]]; then
v="$(ini_get default duration)"; [[ -n "$v" ]] && DURATION="$v"
fi
# named section
if [[ -n "$name" ]]; then
v="$(ini_get "$name" tunnel_port)"; [[ -n "$v" ]] && TUNNEL_PORT="$v"
v="$(ini_get "$name" local_ssh_port)"; [[ -n "$v" ]] && LOCAL_SSH_PORT="$v"
v="$(ini_get "$name" invite_mount)"; [[ -n "$v" ]] && INVITE_MOUNT="$v"
fi
}
# ----------------------------
# Defaults
# Purpose: initialize built-in defaults before applying profiles and flags.
# ----------------------------
TUNNEL_PORT=2222 # remote-side port exposed via -R
LOCAL_SSH_PORT=22 # local sshd port to forward to
DURATION="" # required: e.g. 30m, 2h, 1d
INVITE=false # print a ready-to-copy access command
INVITE_MOUNT="$HOME/remote-rssh"
INVITE_FILE=""
QR=false # render invite as terminal QR (requires qrencode)
# Accessor authorization (session-scoped)
ALLOW_KEY_FILE=""
ALLOW_KNOWN_NAME=""
AUTHORIZED_STORE="${XDG_CONFIG_HOME:-$HOME/.config}/backtunnel/authorized"
ADDED_MARKER_ID="" # unique ID for cleanup removal
usage() {
cat >&2 <<EOF
Usage:
$(basename "$0") /path/to/folder with remoteuser:remotehost for <duration> [options]
Positional (required, in order):
/path/to/folder Informational only (the actual folder is mounted by backtunnel-access)
with Literal keyword
remoteuser:remotehost Or remoteuser@remotehost (or @profile)
for Literal keyword
<duration> e.g. 30m, 2h, 1d (passed to 'timeout')
Options:
-p, --tunnel-port N Remote tunnel port (default: ${TUNNEL_PORT})
-l, --local-ssh-port N Local sshd port to expose (default: ${LOCAL_SSH_PORT})
-i, --invite Print a ready-to-copy access command for the remote side
--invite-mount PATH Mount point suggested in invite (default: ${INVITE_MOUNT})
--invite-file FILE Also write the invite text (with unmount hint) to FILE
--qr Also print a QR code (requires 'qrencode')
--allow-key FILE Authorize this public key for this session (restricted SFTP-only, tunnel-only)
--allow-known NAME Authorize ~/.config/backtunnel/authorized/NAME.pub
-h, --help Show this help
Examples:
$(basename "$0") ~/projects with alice:vps.example.com for 2h
$(basename "$0") ~/projects with alice@vps.example.com for 1d -p 4422 -l 2222
$(basename "$0") ~/projects with @work for 2h -i --qr --allow-known alice
EOF
exit 1
}
# ----------------------------
# Positional parsing
# Purpose: enforce command grammar and capture the five required positionals.
# ----------------------------
[[ $# -lt 5 ]] && usage
FOLDER=$1
KW1=$2
REMOTE=$3
KW2=$4
DURATION=$5
shift 5 || true
[[ "$KW1" != "with" ]] && usage
[[ "$KW2" != "for" ]] && usage
# Apply [default] + named profile (if REMOTE is @name) before flag overrides
profile_name=""
if [[ "$REMOTE" == @* ]]; then
profile_name="${REMOTE#@}"
fi
profile_apply_defaults "$profile_name"
# Expand @name -> user@host for actual ssh use
REMOTE="$(profile_expand_remote "$REMOTE")"
# ----------------------------
# Optional flags
# Purpose: parse options and override derived defaults.
# ----------------------------
while [[ $# -gt 0 ]]; do
case "$1" in
-p|--tunnel-port)
[[ $# -lt 2 ]] && usage
TUNNEL_PORT=$2
shift 2
;;
-l|--local-ssh-port)
[[ $# -lt 2 ]] && usage
LOCAL_SSH_PORT=$2
shift 2
;;
-i|--invite)
INVITE=true
shift
;;
--invite-mount)
[[ $# -lt 2 ]] && usage
INVITE_MOUNT=$2
shift 2
;;
--invite-file)
[[ $# -lt 2 ]] && usage
INVITE_FILE=$2
shift 2
;;
--qr)
QR=true
shift
;;
--allow-key)
[[ $# -lt 2 ]] && usage
ALLOW_KEY_FILE=$2
shift 2
;;
--allow-known)
[[ $# -lt 2 ]] && usage
ALLOW_KNOWN_NAME=$2
shift 2
;;
-h|--help)
usage
;;
*)
echo "Unknown option: $1" >&2
usage
;;
esac
done
# ----------------------------
# Duration validation
# Purpose: accept forms like 30m, 2h, 1d (s/m/h/d).
# ----------------------------
if [[ ! "$DURATION" =~ ^[0-9]+[smhd]$ ]]; then
echo "Invalid duration '$DURATION' (use forms like 30m, 2h, 1d)." >&2
exit 1
fi
# ----------------------------
# Split remote user/host
# Purpose: support user:host or user@host; reject anything else (except @profile pre-expanded).
# ----------------------------
REMOTE_USER="" REMOTE_HOST=""
if [[ "$REMOTE" == *:* ]]; then
REMOTE_USER=${REMOTE%%:*}
REMOTE_HOST=${REMOTE#*:}
elif [[ "$REMOTE" == *"@"* ]]; then
REMOTE_USER=${REMOTE%%@*}
REMOTE_HOST=${REMOTE#*@}
else
echo "Invalid remote format. Use remoteuser:remotehost or remoteuser@remotehost or @profile" >&2
exit 1
fi
# ----------------------------
# Deps check
# Purpose: fail fast if mandatory tools are missing.
# ----------------------------
command -v ssh >/dev/null 2>&1 || { echo "ssh not found."; exit 1; }
command -v timeout >/dev/null 2>&1 || { echo "timeout not found."; exit 1; }
command -v tail >/dev/null 2>&1 || { echo "tail not found."; exit 1; }
# ----------------------------
# Accessor temporary authorization
# Purpose: optionally add and later remove a restricted authorized_keys entry.
# ----------------------------
# restrict_key_line: produce a hardened authorized_keys line for an OpenSSH public key.
# Arguments:
# $1: public key text (one line, ssh-*)
# Returns:
# prints a restricted options prefix + key; nonzero if key is invalid
restrict_key_line() { # usage: restrict_key_line <pubkey_text>
local pk="$1"
# Normalize
pk="${pk//$'\r'/}"
pk="${pk//$'\n'/}"
if [[ "$pk" != ssh-* ]]; then
echo "Provided key does not look like an OpenSSH public key." >&2
return 1
fi
printf 'from="127.0.0.1",command="internal-sftp",restrict %s' "$pk"
}
# add_temp_authorized_key: append a uniquely marked restricted key block to ~/.ssh/authorized_keys.
# Arguments:
# $1: public key text
# Side effects:
# - Ensures ~/.ssh perms (700) and authorized_keys perms (600)
# - Writes marker lines and sets ADDED_MARKER_ID for later removal
add_temp_authorized_key() {
local pubkey_text="$1"
local ak="$HOME/.ssh/authorized_keys"
# SC2155: Declare and assign separately to avoid masking return values.
# shellcheck disable=SC2155
local marker
marker="BACKTUNNEL-TEMP-$(date +%s)-$$-$RANDOM"
local start="# ${marker} START"
local end="# ${marker} END"
mkdir -p "$HOME/.ssh"
touch "$ak"
chmod 700 "$HOME/.ssh"
chmod 600 "$ak"
local restricted
restricted="$(restrict_key_line "$pubkey_text")" || return 1
{
echo "$start"
echo "$restricted"
echo "$end"
} >> "$ak"
ADDED_MARKER_ID="$marker"
}
# shellcheck disable=SC2317
# remove_temp_authorized_key: delete the previously added marked block from authorized_keys.
# Env:
# ADDED_MARKER_ID (read)
# Notes:
# No-op if no marker or authorized_keys missing. Best-effort; ignores errors.
remove_temp_authorized_key() {
local ak="$HOME/.ssh/authorized_keys"
[[ -z "${ADDED_MARKER_ID:-}" ]] && return 0
[[ -f "$ak" ]] || return 0
awk -v m="$ADDED_MARKER_ID" '
$0 ~ "# " m " START" {skip=1; next}
$0 ~ "# " m " END" {skip=0; next}
!skip {print}
' "$ak" > "${ak}.btnew" && mv "${ak}.btnew" "$ak"
}
ACCESSOR_PUBKEY_TEXT=""
if [[ -n "$ALLOW_KNOWN_NAME" ]]; then
f="${AUTHORIZED_STORE}/${ALLOW_KNOWN_NAME}.pub"
if [[ -f "$f" ]]; then
ACCESSOR_PUBKEY_TEXT="$(cat "$f")"
else
echo "Known accessor key not found: $f" >&2
exit 1
fi
fi
if [[ -n "$ALLOW_KEY_FILE" ]]; then
if [[ -f "$ALLOW_KEY_FILE" ]]; then
ACCESSOR_PUBKEY_TEXT="$(cat "$ALLOW_KEY_FILE")"
else
echo "Given key file not found: $ALLOW_KEY_FILE" >&2
exit 1
fi
fi
if [[ -n "$ACCESSOR_PUBKEY_TEXT" ]]; then
add_temp_authorized_key "$ACCESSOR_PUBKEY_TEXT"
fi
# ----------------------------
# Banner
# Purpose: inform the user what will happen and where to connect from the remote.
# ----------------------------
echo "⏳ Sharing '${FOLDER}' via reverse SSH:"
echo " local sshd port : ${LOCAL_SSH_PORT}"
echo " remote bind port : ${TUNNEL_PORT} (on ${REMOTE_HOST})"
echo " remote user : ${REMOTE_USER}"
echo " duration : ${DURATION}"
if [[ -n "$ACCESSOR_PUBKEY_TEXT" ]]; then
echo " accessor key : temporarily authorized (restricted)"
fi
echo
# ----------------------------
# Invite (optional)
# Purpose: print a ready-to-copy command (and QR) for the remote side.
# Notes:
# If a key was not pre-authorized, prepend an auth-setup step executed over the tunnel.
# ----------------------------
if $INVITE; then
INVITE_CMD="backtunnel-access '${FOLDER}' from ${REMOTE_USER}@${REMOTE_HOST} -p ${TUNNEL_PORT} -m '${INVITE_MOUNT}'"
if [[ -z "$ACCESSOR_PUBKEY_TEXT" ]]; then
AUTH_CMD="backtunnel-auth-setup -p ${TUNNEL_PORT} ${REMOTE_USER}@localhost"
INVITE_TEXT=$(
cat <<EOT
# 1) (one-time) install a tunnel-only, SFTP-only key via the reverse tunnel:
${AUTH_CMD}
# 2) mount the share:
${INVITE_CMD}
# Unmount when done:
fusermount -u '${INVITE_MOUNT}' || fusermount3 -u '${INVITE_MOUNT}'
EOT
)
else
AUTH_CMD=""
INVITE_TEXT=$(
cat <<EOT
# Mount the share:
${INVITE_CMD}
# Unmount when done:
fusermount -u '${INVITE_MOUNT}' || fusermount3 -u '${INVITE_MOUNT}'
EOT
)
fi
echo "🔗 Invite (copy to chat):"
echo "------------------------------------------------------------"
[[ -n "$AUTH_CMD" ]] && echo "${AUTH_CMD}"
echo "${INVITE_CMD}"
echo "------------------------------------------------------------"
if [[ -n "${INVITE_FILE}" ]]; then
printf "%s\n" "${INVITE_TEXT}" > "${INVITE_FILE}"
echo "Saved invite to: ${INVITE_FILE}"
fi
if $QR; then
if command -v qrencode >/dev/null 2>&1; then
echo
echo "📱 QR (scan to copy the command):"
printf "%s" "${INVITE_CMD}" | qrencode -t ansiutf8
else
echo "⚠️ 'qrencode' not installed; skipping QR."
fi
fi
echo
fi
echo "Tip: On the remote side, mount with:"
echo " backtunnel-access '${FOLDER}' from ${REMOTE_USER}@${REMOTE_HOST} -p ${TUNNEL_PORT}"
echo "Listener will appear on: ${REMOTE_HOST}:${TUNNEL_PORT} (run access there)."
echo "To stop sharing early: press Ctrl+C in this window."
# ----------------------------
# Pre-flight: warn if remote loopback port already in use (best-effort)
# Purpose: give an actionable warning before attempting the -R bind.
# ----------------------------
if ssh -o BatchMode=yes -o ConnectTimeout=5 "${REMOTE_USER}@${REMOTE_HOST}" \
"command -v nc >/dev/null 2>&1 && nc -z 127.0.0.1 ${TUNNEL_PORT}"; then
echo "⚠️ Port ${TUNNEL_PORT} on remote 127.0.0.1 appears in use; choose another with -p." >&2
# You may 'exit 1' here if you prefer a hard failure
fi
# ----------------------------
# Cleanup & run SSH
# Purpose: start the tunnel, wait bounded by duration, and guarantee cleanup via trap.
# ----------------------------
SSH_PID=""
# shellcheck disable=SC2317
# cleanup: stop the background ssh process and remove any temporary authorized key.
# Notes:
# - Invoked on INT/TERM/EXIT to ensure resources are released.
cleanup() {
# stop ssh child if running
if [[ -n "${SSH_PID:-}" ]] && kill -0 "$SSH_PID" 2>/dev/null; then
echo "⏹️ Stopping share..."
kill -TERM "$SSH_PID" 2>/dev/null || true
wait "$SSH_PID" 2>/dev/null || true
fi
# remove temporary authorized key if we added one
remove_temp_authorized_key
}
trap cleanup INT TERM EXIT
# Start reverse-tunnel ssh in background with keepalives + fast-fail on -R bind
ssh -N \
-o ExitOnForwardFailure=yes \
-o ServerAliveInterval=15 \
-o ServerAliveCountMax=3 \
-R "${TUNNEL_PORT}:localhost:${LOCAL_SSH_PORT}" \
-- "${REMOTE_USER}@${REMOTE_HOST}" &
SSH_PID=$!
# Wait for ssh to exit, but bounded by duration:
# Rationale: use 'timeout … tail --pid=PID -f /dev/null' to avoid subshell/wait-loop complexity.
if timeout "$DURATION" tail --pid="$SSH_PID" -f /dev/null; then
# ssh exited on its own before timeout
exit 0
else
rc=$?
if [[ $rc -eq 124 ]]; then
echo "⏹️ Sharing ended: reached duration (${DURATION})."
# ensure the child is gone
if kill -0 "$SSH_PID" 2>/dev/null; then
kill -TERM "$SSH_PID" 2>/dev/null || true
wait "$SSH_PID" 2>/dev/null || true
fi
exit 0
fi
# other error from timeout/tail
exit "$rc"
fi
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