# Audit / log collector (receive side)

An **rsyslog** container that receives logs forwarded by booted diskless hosts
and writes them to durable, per-source-host files. Diskless hosts run on an
immutable erofs root with a **RAM-only volatile overlay**, so their local
`/var/log/audit/audit.log` is lost on every reboot — this gives auditd somewhere
permanent to offload to. It implements the remote-audit-offload that STIG
deviation #6 (`docs/stig-deviations.md`) lists as future hardening.

```
booted host                              serving host
┌───────────────────────────┐           ┌──────────────────────────────┐
│ auditd → audisp-syslog     │  TCP/514  │ audit (this container)        │
│      → rsyslog (forward)   ├──────────►│ rsyslogd → /var/log/remote/   │
│         to ${AUDIT_HOST}   │  syslog   │   <sender-ip>/audit.log       │
└───────────────────────────┘           │   <sender-ip>/messages.log    │
                                         └──────────────────────────────┘
```

## What this provides (the receiver)

- Listens on **TCP/514** (plain syslog). Disk-assisted queue so bursts / brief
  stalls don't drop records.
- Writes per **sender IP** (not the claimed hostname): audit records →
  `audit.log`, everything else → `messages.log`, under
  `./logs/<ip>/` (bind-mounted, inspectable on the host).
- Host-networked, so hosts reach it at `${AUDIT_HOST}` (resolved to the serving
  host by the stack's dnsmasq), matching the boot/NVMe DNS pattern.

Run it with the rest of the stack:

```bash
cd serving
docker compose build audit
docker compose up -d audit
docker compose logs -f audit          # startup + connection messages
tail -F audit-collector/logs/*/audit.log
```

Quick self-test (send a line as if from a host):

```bash
logger -n 127.0.0.1 -P 514 -T -t audispd "test audit record"
ls audit-collector/logs/                 # a 127.0.0.1/ dir appears
```

## Serving-host firewall — open 514/tcp (easy to miss)

This container uses **host networking**, so the **serving host's own firewall
governs port 514** — Docker does *not* manage iptables for host-net containers
(the usual "Docker bypasses the host firewall" behaviour only applies to
*published* bridge ports, which insert rules into the `DOCKER` chain ahead of the
host firewall). The symptom when it's closed is deceptive: the container is
healthy, `ss -tlnp` shows `0.0.0.0:514` listening, DNS/DHCP from the same host
work, but booted hosts sit in `SYN-SENT` and rsyslog logs `cannot connect to
audit.fleet.lab:514 ... Connection timed out` (timed out = **dropped**, not
refused). The logs aren't lost — the sender's `action.resumeRetryCount="-1"`
buffers them and drains once the port opens.

Open 514/tcp **scoped to the boot LAN** (don't expose it globally):

```bash
# ufw (Debian/Ubuntu serving host):
sudo ufw allow from 192.168.50.0/24 to any port 514 proto tcp
sudo ufw reload && sudo ufw status numbered

# firewalld (RHEL/Rocky serving host):
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=192.168.50.0/24 port port=514 protocol=tcp accept'
sudo firewall-cmd --reload
```

Verify SYNs now complete (SYN → SYN-ACK) and a sender dir appears:

```bash
sudo tcpdump -ni any 'tcp port 514 and host <sender-ip>'   # SYN-ACK now returns
ls -R audit-collector/logs/                                 # <sender-ip>/ appears within ~1 min
```

> If you ever move the collector from host networking to **published ports**
> (`514:514`), this firewall rule stops applying — published Docker ports bypass
> ufw/firewalld and must be controlled via Docker (`DOCKER-USER` chain) instead.

## What's still needed (the sender — in the image)

This container only **receives**. To make hosts actually send, the hardened
image needs (not yet wired — keep `no per-host state`, use the generic
`${AUDIT_HOST}` name, never a per-host address):

1. **auditd → syslog**: enable the `audisp-syslog` plugin
   (`/etc/audit/plugins.d/syslog.conf`: `active = yes`), so audit events also go
   to the local syslog socket.
2. **rsyslog forward rule**: a drop-in (e.g. `/etc/rsyslog.d/60-fleet-remote.conf`)
   forwarding to the collector with a disk-assisted action queue, so a network
   blip doesn't drop records:

   ```rsyslog
   *.* action(type="omfwd" target="audit.fleet.lab" port="514" protocol="tcp"
              queue.type="LinkedList" queue.fileName="fwdq"
              queue.maxDiskSpace="256m" queue.saveOnShutdown="on"
              action.resumeRetryCount="-1")
   ```
3. Bake `audit.fleet.lab` (the generic name) — DNS resolves it per environment.

Once the sender is wired, flip STIG deviation #6 in `docs/stig-deviations.md`
from "local-only" to "offloading to ${AUDIT_HOST}" and the
`rsyslog_remote_loghost` rule should pass.

## Production hardening (lab simplifications to revisit)

The lab stack is **plaintext TCP**, mirroring the HTTP-boot/no-TLS posture. For
a secured fleet:

- **TLS**: rsyslog `gtls` driver (mutual auth with the boot-infra CA), or
  **RELP + TLS** (`imrelp`/`omrelp`) for guaranteed, acknowledged delivery.
- **Restrict senders** by subnet (`$AllowedSender`) or firewall :514 to the
  boot LAN.
- Add log rotation/retention (logrotate sidecar or rsyslog `outchannel`) and
  ship to long-term storage / SIEM.