feat(ssh): capture attacker-dropped files with session attribution
inotifywait watches writable paths in the SSH decky and mirrors any file close_write/moved_to into a per-decky host-mounted quarantine dir. Each artifact carries a .meta.json with attacker attribution resolved by walking the writer PID's PPid chain to the sshd session leader, then cross-referencing ss and utmp for source IP/user/login time. Also emits an RFC 5424 syslog line per capture for SIEM correlation.
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228
templates/ssh/capture.sh
Executable file
228
templates/ssh/capture.sh
Executable file
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#!/bin/bash
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# DECNET SSH honeypot file-catcher.
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#
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# Watches attacker-writable paths with inotifywait. On close_write/moved_to,
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# copies the file to the host-mounted quarantine dir, writes a .meta.json
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# with attacker attribution, and emits an RFC 5424 syslog line.
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#
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# Attribution chain (strongest → weakest):
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# pid-chain : fuser/lsof finds writer PID → walk PPid to sshd session
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# → cross-ref with `ss` to get src_ip/src_port
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# utmp-only : writer PID gone (scp exited); fall back to `who --ips`
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# unknown : no live session at all (unlikely under real attack)
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set -u
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CAPTURE_DIR="${CAPTURE_DIR:-/var/decnet/captured}"
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CAPTURE_MAX_BYTES="${CAPTURE_MAX_BYTES:-52428800}" # 50 MiB
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CAPTURE_WATCH_PATHS="${CAPTURE_WATCH_PATHS:-/root /tmp /var/tmp /home /var/www /opt /dev/shm}"
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mkdir -p "$CAPTURE_DIR"
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chmod 700 "$CAPTURE_DIR"
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# Filenames we never capture (noise from container boot / attacker-irrelevant).
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_is_ignored_path() {
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local p="$1"
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case "$p" in
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"$CAPTURE_DIR"/*) return 0 ;;
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/var/decnet/*) return 0 ;;
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*/.bash_history) return 0 ;;
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*/.viminfo) return 0 ;;
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*/ssh_host_*_key*) return 0 ;;
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esac
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return 1
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}
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# Resolve the writer PID best-effort. Prints the PID or nothing.
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_writer_pid() {
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local path="$1"
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local pid
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pid="$(fuser "$path" 2>/dev/null | tr -d ' \t\n')"
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if [ -n "$pid" ]; then
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printf '%s' "${pid%% *}"
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return
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fi
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# Fallback: scan /proc/*/fd for an open handle on the path.
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for fd_link in /proc/[0-9]*/fd/*; do
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[ -L "$fd_link" ] || continue
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if [ "$(readlink -f "$fd_link" 2>/dev/null)" = "$path" ]; then
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printf '%s' "$(echo "$fd_link" | awk -F/ '{print $3}')"
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return
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fi
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done
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}
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# Walk PPid chain from $1 until we hit an sshd session leader.
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# Prints: <sshd_pid> <user> (empty on no match).
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_walk_to_sshd() {
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local pid="$1"
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local depth=0
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while [ -n "$pid" ] && [ "$pid" != "0" ] && [ "$pid" != "1" ] && [ $depth -lt 20 ]; do
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local cmd
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cmd="$(tr '\0' ' ' < "/proc/$pid/cmdline" 2>/dev/null)"
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# sshd session leaders look like: "sshd: root@pts/0" or "sshd: root@notty"
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if echo "$cmd" | grep -qE '^sshd: [^ ]+@'; then
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local user
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user="$(echo "$cmd" | sed -E 's/^sshd: ([^@]+)@.*/\1/')"
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printf '%s %s' "$pid" "$user"
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return
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fi
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pid="$(awk '/^PPid:/ {print $2}' "/proc/$pid/status" 2>/dev/null)"
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depth=$((depth + 1))
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done
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}
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# Emit a JSON array of currently-established SSH peers.
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# Each item: {pid, src_ip, src_port}.
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_ss_sessions_json() {
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ss -Htnp state established sport = :22 2>/dev/null \
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| awk '
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{
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peer=$4; local_=$3;
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# peer looks like 198.51.100.7:55342 (may be IPv6 [::1]:x)
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n=split(peer, a, ":");
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port=a[n];
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ip=peer; sub(":" port "$", "", ip);
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gsub(/[\[\]]/, "", ip);
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# extract pid from users:(("sshd",pid=1234,fd=5))
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pid="";
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if (match($0, /pid=[0-9]+/)) {
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pid=substr($0, RSTART+4, RLENGTH-4);
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}
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printf "{\"pid\":%s,\"src_ip\":\"%s\",\"src_port\":%s}\n",
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(pid==""?"null":pid), ip, (port+0);
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}' \
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| jq -s '.'
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}
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# Emit a JSON array of logged-in users from utmp.
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# Each item: {user, src_ip, login_at}.
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_who_sessions_json() {
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who --ips 2>/dev/null \
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| awk '{ printf "{\"user\":\"%s\",\"tty\":\"%s\",\"login_at\":\"%s %s\",\"src_ip\":\"%s\"}\n", $1, $2, $3, $4, $NF }' \
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| jq -s '.'
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}
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_capture_one() {
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local src="$1"
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[ -f "$src" ] || return 0
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_is_ignored_path "$src" && return 0
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local size
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size="$(stat -c '%s' "$src" 2>/dev/null)"
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[ -z "$size" ] && return 0
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if [ "$size" -gt "$CAPTURE_MAX_BYTES" ]; then
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logger -p user.info -t decnet-capture "file_skipped size=$size path=$src reason=oversize"
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return 0
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fi
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# Attribution first — PID may disappear after the copy races.
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local writer_pid writer_comm writer_cmdline writer_uid writer_loginuid
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writer_pid="$(_writer_pid "$src")"
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if [ -n "$writer_pid" ] && [ -d "/proc/$writer_pid" ]; then
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writer_comm="$(cat "/proc/$writer_pid/comm" 2>/dev/null)"
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writer_cmdline="$(tr '\0' ' ' < "/proc/$writer_pid/cmdline" 2>/dev/null)"
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writer_uid="$(awk '/^Uid:/ {print $2}' "/proc/$writer_pid/status" 2>/dev/null)"
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writer_loginuid="$(cat "/proc/$writer_pid/loginuid" 2>/dev/null)"
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fi
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local ssh_pid ssh_user
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if [ -n "$writer_pid" ]; then
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read -r ssh_pid ssh_user < <(_walk_to_sshd "$writer_pid" || true)
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fi
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local ss_json who_json
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ss_json="$(_ss_sessions_json 2>/dev/null || echo '[]')"
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who_json="$(_who_sessions_json 2>/dev/null || echo '[]')"
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# Resolve src_ip via ss by matching ssh_pid.
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local src_ip="" src_port="null" attribution="unknown"
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if [ -n "${ssh_pid:-}" ]; then
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local matched
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matched="$(echo "$ss_json" | jq -c --argjson p "$ssh_pid" '.[] | select(.pid==$p)')"
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if [ -n "$matched" ]; then
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src_ip="$(echo "$matched" | jq -r '.src_ip')"
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src_port="$(echo "$matched" | jq -r '.src_port')"
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attribution="pid-chain"
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fi
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fi
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if [ "$attribution" = "unknown" ] && [ "$(echo "$who_json" | jq 'length')" -gt 0 ]; then
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src_ip="$(echo "$who_json" | jq -r '.[0].src_ip')"
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attribution="utmp-only"
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fi
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local sha
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sha="$(sha256sum "$src" 2>/dev/null | awk '{print $1}')"
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[ -z "$sha" ] && return 0
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local ts base stored_as
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ts="$(date -u +%Y-%m-%dT%H:%M:%SZ)"
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base="$(basename "$src")"
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stored_as="${ts}_${sha:0:12}_${base}"
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cp --preserve=timestamps,ownership "$src" "$CAPTURE_DIR/$stored_as" 2>/dev/null || return 0
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local mtime
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mtime="$(stat -c '%y' "$src" 2>/dev/null)"
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local decky="${HOSTNAME:-unknown}"
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jq -n \
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--arg captured_at "$ts" \
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--arg orig_path "$src" \
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--arg stored_as "$stored_as" \
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--arg sha "$sha" \
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--argjson size "$size" \
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--arg mtime "$mtime" \
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--arg decky "$decky" \
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--arg attribution "$attribution" \
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--arg writer_pid "${writer_pid:-}" \
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--arg writer_comm "${writer_comm:-}" \
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--arg writer_cmdline "${writer_cmdline:-}" \
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--arg writer_uid "${writer_uid:-}" \
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--arg writer_loginuid "${writer_loginuid:-}" \
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--arg ssh_pid "${ssh_pid:-}" \
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--arg ssh_user "${ssh_user:-}" \
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--arg src_ip "$src_ip" \
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--arg src_port "$src_port" \
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--argjson concurrent "$who_json" \
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--argjson ss_snapshot "$ss_json" \
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'{
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captured_at: $captured_at,
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orig_path: $orig_path,
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stored_as: $stored_as,
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sha256: $sha,
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size: $size,
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mtime: $mtime,
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decky: $decky,
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attribution: $attribution,
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writer: {
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pid: ($writer_pid | if . == "" then null else tonumber? end),
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comm: $writer_comm,
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cmdline: $writer_cmdline,
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uid: ($writer_uid | if . == "" then null else tonumber? end),
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loginuid: ($writer_loginuid | if . == "" then null else tonumber? end)
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},
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ssh_session: {
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pid: ($ssh_pid | if . == "" then null else tonumber? end),
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user: (if $ssh_user == "" then null else $ssh_user end),
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src_ip: (if $src_ip == "" then null else $src_ip end),
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src_port: ($src_port | if . == "null" or . == "" then null else tonumber? end)
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},
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concurrent_sessions: $concurrent,
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ss_snapshot: $ss_snapshot
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}' > "$CAPTURE_DIR/$stored_as.meta.json"
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logger -p user.info -t decnet-capture \
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"file_captured orig_path=$src sha256=$sha size=$size stored_as=$stored_as src_ip=${src_ip:-unknown} ssh_user=${ssh_user:-unknown} attribution=$attribution"
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}
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# Main loop.
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# shellcheck disable=SC2086
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inotifywait -m -r -q \
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--event close_write --event moved_to \
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--format '%w%f' \
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$CAPTURE_WATCH_PATHS 2>/dev/null \
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| while IFS= read -r path; do
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_capture_one "$path" &
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done
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