# PXE NIC guard The PXE server's DHCP/PXE (dnsmasq) must never serve on the GE corporate network. Each server has two NICs: | Role | Network | Address | |---------|-----------------------------|----------------------------------| | imaging | isolated imaging switch | static 172.16.9.1/24, dnsmasq | | ge | GE corporate network | DHCP client (systemd-networkd) | `/etc/dnsmasq.d/pxe-nic-only.conf` pins dnsmasq to the imaging NIC by name. That stops dnsmasq answering on the GE NIC, but not on the imaging NIC when that NIC itself ends up on the wrong network: a cable swap, an imaging switch uplinked into a GE port, or a second PXE server on the same switch. The NIC guard catches those. ## What is installed | File | Purpose | |-----------------------------------------------------------|---------------------------------| | `/usr/local/sbin/pxe-nic-check` | the classifier | | `/etc/pxe-server/nic.conf` | which NIC is which | | `/etc/systemd/system/dnsmasq.service.d/nic-guard.conf` | guard before every dnsmasq start| | `/etc/systemd/system/pxe-nic-check.{service,timer}` | watch every 5 min | | `/usr/local/sbin/pxe-nic-carrier` | re-check on imaging cable moves | | `/etc/networkd-dispatcher/.d/50-pxe-nic-carrier` | calls it on link changes | | `/etc/systemd/system/pxe-nic-carrier.service` | the re-check after carrier gain | | `/run/pxe-nic-status.json` | last result, read by the webapp | Sources: `playbook/pxe-server-helpers/`, installed by `pxe_server_setup.yml` (vars `pxe_nic_mac`, `corp_nic_mac`, `pxe_corp_domain_suffixes`, `pxe_nic_allowed_switches`; `pxe_imaging_mac` / `pxe_ge_mac` are derived from the first two). The same MACs pin the NICs in netplan (`playbook/templates/netplan-pxe.yaml.j2`, names `pxe0` / `corp0`) and in `/etc/dnsmasq.d/pxe-nic-only.conf`. ## How it decides It listens on each NIC (raw socket, sends nothing; it joins the STP, 802.1X, LLDP, CDP and PVST+ multicast groups so a NIC that is not promiscuous still hears them) for: - LLDP and CDP: managed switches announce themselves. GE switches do; the unmanaged imaging switch does not. - 802.1X EAPOL from an authenticator: GE ports ask for port authentication (EAP Request, Success or Failure). EAPOL that clients send is ignored: iPXE sends three EAPOL-Starts every time it opens a NIC and Windows sends Starts and EAP Responses, so a bay PXE-booting on the imaging switch is not mistaken for a GE port. Those frames show up only as "N client EAPOL frames ignored". - DHCP OFFER/ACK from any other server. - STP and the count of distinct ARP hosts: reported, never decisive. On the imaging NIC it also sends one DHCPDISCOVER and collects offers for 6 s. On the GE NIC it reads the networkd lease instead (no traffic). | Verdict | Meaning | |----------------|------------------------------------------------------------| | `isolated` | no foreign DHCP, no LLDP/CDP/EAPOL | | `ge` | LLDP, CDP or EAPOL seen, or a DHCP server whose domain ends in a corp suffix (`geaerospace.net`, `ge.com`; whole labels, so `notgeaerospace.net` does not count) | | `foreign-dhcp` | another DHCP server, no corp domain | | `no-link` | no carrier | | `error` | the check itself failed | Healthy: imaging NIC `isolated` (or `no-link`), GE NIC `ge`. Our own dnsmasq never counts. An offer whose server id is our own address but comes from a MAC that is not ours is a second PXE server and DOES count. If the imaging switch is itself a managed switch that sends LLDP/CDP, put its LLDP system name, CDP device id or chassis MAC in `allowed_switches` (`pxe_nic_allowed_switches`), or the guard will block dnsmasq. 802.1X is never allowed. ## Modes - `pxe-nic-check` - check every NIC (60 s), print a summary. `--json`, `--write`, `--probe` (imaging DHCP probe), `--iface NAME`, `--window S`. - `pxe-nic-check --guard` - dnsmasq `ExecStartPre`. Imaging NIC only, about 6 s. Exit 1 = do not serve: dnsmasq does not start. A bug or no link exits 0 (fail open) with an error line in the journal. - `pxe-nic-check --watch` - the timer. On a bad imaging verdict it runs `systemctl stop dnsmasq` and logs CRIT. It never starts dnsmasq. If dnsmasq was (re)started during the 60 s window (an operator, or the carrier hook, restarted it and its guard passed), the watch re-checks for about 6 s and stops dnsmasq only if that still says blocked; if dnsmasq is already stopped it only logs. - `pxe-nic-carrier` - the networkd-dispatcher hook. Only the imaging NIC (or its bridge ports) counts; GE NIC events are ignored. Carrier lost: dnsmasq is stopped at once if it was running. Carrier back: after 2 s, `pxe-nic-carrier.service` restarts dnsmasq if it was running or the hook stopped it, so the guard checks the new network before dnsmasq serves again; dnsmasq that an operator or the watch stopped stays stopped. A bridge with several ports stops dnsmasq when any one of them loses carrier. Log: `journalctl -t pxe-nic-carrier`. Every start or restart of dnsmasq runs the guard, so `systemctl restart dnsmasq` and the `@reboot` cron restart take about 6 s longer than before. ## Reading it sudo pxe-nic-check # full summary, 60 s cat /run/pxe-nic-status.json # last guard/watch result journalctl -u dnsmasq -b | grep pxe-nic-check journalctl -u pxe-nic-check -b journalctl -t pxe-nic-carrier -b # cable unplug / replug ## dnsmasq will not start 1. `journalctl -u dnsmasq -b | grep pxe-nic-check` shows the evidence, e.g. `REFUSING to start dnsmasq: imaging NIC br-pxe verdict ge: LLDP from WJ-IDF2 port Gi1/0/14`. 2. Check the imaging cabling: the imaging NIC must go to the imaging switch only, and that switch must not be uplinked anywhere. 3. `sudo pxe-nic-check` until the imaging NIC reads `isolated`. 4. `sudo systemctl start dnsmasq` (runs the guard again). ## Floor tests (run with the floor idle) All three tests stop imaging for a few minutes. Have console or GE-side SSH access to the server; do not rely on the imaging network for the session. ### (a) Rogue DHCP on the imaging switch: expect the alarm 1. Confirm the baseline: `sudo pxe-nic-check --probe` shows the imaging NIC `isolated` and `dnsmasq_bound` true. 2. Plug a device that runs a DHCP server into a free port of the IMAGING switch (a spare home router with DHCP on, LAN port only, nothing in its WAN port; or a laptop running dnsmasq). 3. Run the watch now instead of waiting up to 5 min: `sudo systemctl start pxe-nic-check.service` 4. Expect: - `journalctl -u pxe-nic-check -n 20` has a CRIT line `imaging NIC is NOT isolated, stopping dnsmasq: ... verdict foreign-dhcp: DHCP from 192.168.x.1, ...` (or `ge` if the router sends LLDP). - `systemctl is-active dnsmasq` is `inactive`. - `/run/pxe-nic-status.json` has `"dhcp_guard": {"state": "blocked", ...}`. 5. Prove the guard too: `sudo systemctl start dnsmasq` must FAIL, and `journalctl -u dnsmasq -n 20` shows `REFUSING to start dnsmasq`. 6. Unplug the rogue device. `sudo pxe-nic-check --probe` shows `isolated`. `sudo systemctl start dnsmasq` starts. PXE-boot one client to confirm. ### (b) Cable swap into a GE port: expect the guard to refuse dnsmasq dnsmasq must be stopped BEFORE the cable moves, so it never serves on GE. 1. `sudo systemctl stop dnsmasq`; `systemctl is-active dnsmasq` must say `inactive`. Do not reboot during the test (the `@reboot` cron restarts dnsmasq; the guard would still block it, but do not test that way). 2. Move the IMAGING NIC's cable from the imaging switch into a live GE network port. Wait for link: `ip -br link` shows the imaging NIC `UP`. The carrier hook sees the link come up and runs the guard once (one DHCPDISCOVER) to refresh the status file; with dnsmasq stopped it does not start it. `journalctl -t pxe-nic-carrier -n 5` shows `dnsmasq was not running, left stopped`. 3. Run the guard by hand first: `sudo pxe-nic-check --guard; echo "exit $?"` Expect `exit 1` and a `REFUSING to start dnsmasq` line naming LLDP/CDP, 802.1X, or `DHCP from 10.x.x.x, domain ....geaerospace.net`. This sends one DHCPDISCOVER on the GE port, the same as any PC being plugged in. To send nothing, use `--no-probe --window 60` instead and rely on LLDP/CDP/802.1X (switches announce every 30-60 s). **If it exits 0, STOP: do not start dnsmasq. Move the cable back and send the output of `sudo pxe-nic-check --probe --json`.** 4. Only after step 3 exited 1: `sudo systemctl start dnsmasq`. Expect `Job for dnsmasq.service failed`; `systemctl is-active dnsmasq` says `failed`; `journalctl -u dnsmasq -n 20` shows `REFUSING`. 5. Move the cable back to the imaging switch. `sudo pxe-nic-check --probe` shows the imaging NIC `isolated`. `sudo systemctl start dnsmasq` starts. PXE-boot one client to confirm. ### (c) Imaging cable unplugged and replugged: expect stop, re-check, start 1. dnsmasq running, imaging NIC `isolated`. 2. Unplug the imaging NIC's cable. Within a second `journalctl -t pxe-nic-carrier -n 5` shows `carrier lost on imaging NIC ... stopped dnsmasq` and `systemctl is-active dnsmasq` is `inactive`. 3. Plug it back into the imaging switch. About 10 s later the journal shows `guard passed, dnsmasq (re)started` and dnsmasq is `active`. Had it gone into a GE port instead, the guard refuses: the journal shows `dnsmasq NOT started` and dnsmasq stays down. ## Status file `/run/pxe-nic-status.json`, mode 0644, replaced atomically: {"checked_at": "2026-10-14T16:47:09-04:00", "duration_s": 60, "dhcp_guard": {"state": "allowed"|"blocked"|"unknown", "reason": "..."}, "interfaces": [{"iface": "br-pxe", "mac": "aa:bb:..", "role": "imaging"|"ge"|"other", "link": "up"|"down", "speed_mbps": 5000, "addrs": ["172.16.9.1/24"], "verdict": "isolated"|"ge"|"foreign-dhcp"|"no-link"|"error", "evidence": ["..."], "dhcp_servers": [{"server": "10.1.2.3", "domain": "x.geaerospace.net", "own": false}], "switch": {"lldp": [{"system": "...", "port": "...", "vlan": null}], "cdp": [{"device": "...", "port": "..."}], "eapol": false, "stp": false}, "arp_hosts": 3, "dnsmasq_bound": true}]} `dnsmasq_bound` is null when it cannot be told (no `ss`). A bridge (br-pxe) is reported as one interface; its ports are listened on too, because LLDP and 802.1X frames stop at the port and never reach the bridge.