80 lines
2.7 KiB
Bash
80 lines
2.7 KiB
Bash
#!/usr/bin/env bash
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# SPDX-License-Identifier: GPL-3.0-or-later
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# Copyright (c) 2025 LUXIM d.o.o., Slovenia
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# Author: Matjaž Mozetič
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#
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# Name: backtunnel-auth-setup
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# Summary: Initialize a tunnel-only, SFTP-only SSH key for BackTunnel access via reverse tunnel.
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# Description:
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# Generates (if missing) a dedicated SSH key (~/.ssh/id_ed25519_backtunnel) and installs its public
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# key on the remote account’s authorized_keys, restricted to:
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# from="127.0.0.1",command="internal-sftp",restrict
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# This makes the key usable only through the reverse tunnel (localhost on the remote) and only for SFTP,
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# not for shell or port-forwarding.
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#
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# Usage:
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# backtunnel-auth-setup [-p PORT] user@localhost
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#
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# Examples:
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# backtunnel-auth-setup alice@localhost
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# backtunnel-auth-setup -p 4422 alice@localhost
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#
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# Dependencies:
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# - bash
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# - ssh, ssh-keygen
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#
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# Exit codes:
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# 0 success (key exists/created, restricted entry ensured)
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# 1 invalid usage (missing destination) or other failures
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#
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# Security:
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# - The installed authorized_keys entry is tightly scoped to 127.0.0.1 and internal-sftp with restrict.
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# - Remote authorized_keys is created with 600 permissions; umask 077 enforced during remote script.
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#
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# Notes:
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# - Idempotent: re-running won’t duplicate the restricted line if it is already present.
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# Initialize tunnel-only SSH auth for BackTunnel (Option A)
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# Usage: backtunnel-auth-setup [-p PORT] user@localhost
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set -euo pipefail
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PORT=2222
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while [[ $# -gt 0 ]]; do
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case "$1" in
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-p|--port) PORT="$2"; shift 2;;
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-h|--help)
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echo "Usage: backtunnel-auth-setup [-p PORT] user@localhost"
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exit 0;;
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*) break;;
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esac
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done
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DEST="${1:-}"
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[[ -n "$DEST" ]] || { echo "Missing destination (e.g., user@localhost)."; exit 1; }
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KEY="$HOME/.ssh/id_ed25519_backtunnel"
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PUB="$KEY.pub"
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# 1) Create a dedicated key if missing
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if [[ ! -f "$KEY" ]]; then
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echo "Generating dedicated key at $KEY ..."
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ssh-keygen -t ed25519 -f "$KEY" -N "" -C "backtunnel"
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fi
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# 2) Append restricted key only (idempotent): tunnel-only + SFTP-only
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echo "Installing restricted key (tunnel-only, SFTP-only) via port $PORT ..."
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RESTRICTED_LINE="$(printf 'from=\"127.0.0.1\",command=\"internal-sftp\",restrict '; cat "$PUB")"
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ssh -p "$PORT" "$DEST" bash -lc '
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set -euo pipefail
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umask 077
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mkdir -p ~/.ssh
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touch ~/.ssh/authorized_keys
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chmod 600 ~/.ssh/authorized_keys
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# Only append if not already present
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if ! grep -Fqx -- "$RESTRICTED_LINE" ~/.ssh/authorized_keys 2>/dev/null; then
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printf "%s\n" "$RESTRICTED_LINE" >> ~/.ssh/authorized_keys
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fi
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' _ RESTRICTED_LINE="$RESTRICTED_LINE"
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echo "Done. This key will only work via the reverse tunnel (127.0.0.1) and only for SFTP."
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