dejima-devbox-hardener/proxmox/README.md

Provisioning a devbox on Proxmox

The node-side half: the VM itself and the firewall applied to it. Everything here is done in the Proxmox web console.

What happens inside the VM is docs/building-a-devbox.md, which is the same on any hypervisor. Do this page first, then go there.

That split is deliberate, and so is the console-only rule. A script that runs qm create and writes into /etc/pve/firewall/ is exactly the kind of artifact people paste into a root shell without reading, and the blast radius is the whole node, including every VM on it that has nothing to do with this one. The steps below are short enough to follow and specific enough to check.

Nothing on this page asks you to edit a file on the node. Where a console path and a file path both exist, this runbook takes the console path even when the file would be tidier, because a hand-edited file cannot be checked as you follow it and puts the hypervisor's configuration outside the tool's reach.

The in-VM half is a script: bin/dejima-devbox-bootstrap.

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What you are building

A machine you cannot afford to lose, treated as one you might.

That sentence is the whole difference from a CI runner. A runner is built to be lost: it is rebuilt from a snapshot on a schedule, so anything that gets into it has until the next rollback and no longer. A devbox holds uncommitted work, so a rollback on a timer destroys the thing the machine exists to hold. Nothing reclaims this VM on a cadence.

What follows from that: containment here is the firewall on this page, plus keeping the box worth little, plus being able to rebuild it. There is no fourth thing, and no scheduled undo.

The firewall goes on the node, not in the guest. Root in the VM can flush any rule set it can see. It cannot reach one applied to its own tap device.

dejima's own controls are not a second copy of this. They bound what a session inside the container reaches, and they do that well: the container's egress is deny-by-default and it is given the Docker host's address rather than the host's subnet, so a session cannot walk your LAN. What they cannot bound is what the VM reaches, because they run inside it. That gap is what this page closes, and it is why this is worth doing even though dejima already firewalls itself.

What this does not prevent. Root on this VM. Nothing here tries to. What it does is make root on this VM worth less: no LAN to move onto, and a machine whose rebuild cost you have already paid once. See docs/threat-model.md for what each layer actually grants.

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What order to do this in

This page and the guest runbook interleave. Doing them strictly in sequence does not work, and the reason is worth knowing before you hit it.

1. §1 and §2 here. Datacenter firewall on, VM created.
2. Install the OS, then come back. That is the guest runbook's §1 and it is the only part of it you need before returning.
3. §3 here. Build the rules and apply them. Both halves of its verification work at this point, including the ones run from inside the VM.
4. The rest of the guest runbook, from its §2 onward.
5. §4 here whenever you want a snapshot.

The rules go on after the OS is installed, not before, and that ordering is load-bearing rather than a matter of taste. The Debian installer takes its address by DHCP whatever you intend to pin it to afterwards, and a lease needs a reply back in. Apply the group first without the DHCP pair from §3 and the installer simply has no network, which presents as a broken mirror or a dead link and not as a firewall you switched on ten minutes earlier.

Ticking the Firewall checkbox in §2 is safe on its own. It is the group and the policies in §3 that start dropping things.

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1. Check the datacenter firewall is on

Do this first. A per-VM firewall with the datacenter switch off is read and enforced nowhere, and nothing about that is visible from the VM — it looks exactly like a working setup.

Datacenter → Firewall → Options → Firewall: Yes

If you have to change it, check that nothing else on the node depended on having no firewall. The datacenter default policy is ACCEPT in and out, so turning it on does not by itself filter anything — but it is worth knowing before you flip it rather than after.

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2. Create the VM

Create VM, and the values that matter:

Tab	Setting	Value
General	Name	devbox, or whatever you prefer
OS	ISO	Debian netinst, current stable
System	Qemu Agent	✓ — and see the note below, the tick alone does nothing
Disks	Size	128 GB. A devbox accumulates checkouts, container images and build caches, and it is not rebuilt on a schedule to clear them
CPU	Cores	4
Memory		16384 MB. dejima builds its image and runs sessions; 8 GB works and is tight
Network	Bridge	your normal bridge
Network	Firewall	✓ — this is the checkbox everything else depends on

The Firewall checkbox is easy to miss and easy to uncheck later without noticing. If it is off, the per-VM rules do not apply to the interface and the VM is simply on your LAN.

Qemu Agent is two halves, and this tick is one of them. It opens a virtio-serial channel on the host side; answering on it takes the qemu-guest-agent package inside the guest, which dejima-devbox-bootstrap installs and which tells you if this tick is missing. Leave it on: without the channel the agent has nothing to attach to, the Summary pane shows no address, qm shutdown falls back to hoping ACPI is acted on, and a snapshot of a running VM stays crash-consistent.

What a moving address does not break. Not the firewall. Every rule in the next section keys on a destination, except the two inbound rules, which key on your client subnets — and Proxmox applies the whole set to this VM's tap device, identified by VMID rather than by IP. Nothing anywhere names this VM by its own address, so a rebuilt VM that comes up on a different one is still fully firewalled.

That is a real difference from the sister project, where the inbound rule names a single management host. Scoping inbound to subnets rather than to one address means a changed lease breaks nothing in the rule set.

What it breaks is you finding the machine: ssh <you>@<vm-ip>, and every later visit. That is reason enough to pin it.

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Now install the OS, then come back

The VM exists and has no operating system. Go to docs/building-a-devbox.md §1, install Debian, and return here for §3.

Nothing else from the guest runbook is needed yet. Coming back with a booted VM is what makes the rest of this page checkable rather than assumed: §3 ends by testing the rules from inside, and there is nothing to test from until an OS is running.

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3. The firewall rules

Twelve rules, built once as a security group and applied to each devbox. Even with one VM, build the group: hand-entered copies are two chances to get the ordering wrong and one of them to drift later.

This is the primary control. Everything else in this repository exists to verify it or to explain it.

What each part is for

The outbound half is short and does the work. Six DROPs cover every address a neighbour of this VM can hold, and above them sit exceptions for the two hosts it genuinely needs: your forge and your resolver. That is the bound this whole project states, and it does not change however long the inbound list gets.

The inbound half is the part that grows, because unlike a CI runner a devbox offers services you want to reach. Keep it legible rather than short: every subnet named here, every port stated, and no rule added anywhere else.

Build the group once

Datacenter → Firewall → Security Group → Create, named dejima-devbox-hardener. Then Add these to it, in this order:

#	Direction	Action	Protocol	Dest. port	Source / Dest	Why
1	in	ACCEPT	tcp	22	Source: admin subnet	SSH, from wherever you administer
2	in	ACCEPT	tcp	front door	Source: front-door subnet	Remote access, narrower on purpose
3	out	ACCEPT	tcp	443	Dest: forge IP	Clone, fetch and push over HTTPS
4	out	ACCEPT	tcp	22	Dest: forge IP	Push over SSH
5	out	ACCEPT	udp	53	Dest: nameserver IP	The forge's name is internal
6	out	ACCEPT	tcp	53	Dest: nameserver IP	Same, over TCP
7	out	DROP	—	—	Dest: 10.0.0.0/8
8	out	DROP	—	—	Dest: 172.16.0.0/12
9	out	DROP	—	—	Dest: 192.168.0.0/16	The three that matter
10	out	DROP	—	—	Dest: 169.254.0.0/16	Link-local, incl. metadata services
11	out	DROP	—	—	Dest: 100.64.0.0/10	Carrier-grade NAT, where a tailnet lives
12	out	DROP	—	—	Dest: ::/0	IPv6, the whole family

Rules 1 and 2 are added once per client subnet. Phones and laptops sit on different wireless networks often enough that assuming one is the wrong default. Add the rule again for each.

Do not widen the mask instead. A range chosen to span two subnets is not two subnets, it is every subnet between them, and it will keep being right for years before it is suddenly very wrong.

Rule 4 is the one a CI runner never needed. A devbox pushes over SSH. Miss it and git push hangs against rule 7 rather than failing, which reads as the forge being down.

Rule 11 is for a tailnet. Tailscale numbers its hosts out of 100.64.0.0/10, which is none of the three RFC 1918 ranges, so without this rule a devbox on your tailnet reaches every machine you own anywhere — which is a wider network than the one rules 7 to 9 were drawn around, not a narrower one. Add it whether or not you run Tailscale today: it costs nothing on a devbox that has no overlay, and the day someone installs one is not a day anyone revisits the firewall.

Rule 12, and why IPv6 is dropped whole

Rules 7 to 11 work because the protected set is enumerable. RFC 1918 named three ranges, your LAN is inside them, and the list is short enough to read.

IPv6 has no RFC 1918. A v6 network is numbered out of a globally routable prefix delegated by your ISP, so there is no equivalent list to write down. Dropping fc00::/7 catches unique-local addresses most networks do not use and misses the prefix your actual neighbour is on. Since the set cannot be enumerated, the family goes.

This costs a devbox nothing. The internet is still reached over IPv4 by the default policy, unbounded and on purpose. Rule 12 removes a second path to the same place, not the only path.

It is worth having even if you are certain your network has no IPv6. Every Linux host brings up an fe80:: address on every interface whether or not anything routes v6, and a daemon bound to :: answers on it. Without rule 12, rules 7 to 11 can all be in force and the machine beside this one is still one ssh fe80::...%eth0 away. That path does not show up in ip -4 route, does not involve your router, and is invisible to anyone checking the v4 rules are right.

Do not do this in the guest instead. net.ipv6.conf.all.disable_ipv6=1 looks equivalent and is not: it is a setting on a machine this whole file assumes an attacker holds root on, and root can set it back. The rule belongs on the node for the same reason every other rule here does.

If the console rejects ::/0, the same coverage is four rules — fe80::/10, fc00::/7, ff00::/8, 2000::/3 — placed together in rule 12's position. Prefer the single line: four entries are four chances to enter three.

dejima-devbox-audit reach checks this one by behaviour, like the rest.

Why the front door gets the narrower list

The intuition that SSH is the more dangerous of the two is backwards.

Both put a terminal in front of whoever reaches them, so they do not differ in capability. They differ in what stands in front. SSH with public-key auth has nothing to guess and nothing to stuff, and is about as attack-resistant as an exposed service gets. A web front door behind HTTP basic auth is a password, and nginx's basic auth brings no rate limiting, no lockout and no second factor unless they are added deliberately.

Same capability behind a weaker gate, so the front door is the higher-risk service and takes the smaller source range. Usage tends to land on that split anyway: you administer over SSH from whatever device is to hand, and you want the front door for a phone, because that is what it is for.

Two cheap improvements to the front door, since it is the weaker side by construction:

• limit_req on that nginx location, which supplies the rate limiting basic auth does not have;
• a client certificate, which phones install as a profile and which brings that gate to roughly the strength of an SSH key.

What this list does not bound

Adding a client subnet widens who can reach this VM. It does not widen what this VM can reach, which is the bound rules 7 to 12 fix and the one this project exists to state. The two are different axes and conflating them leads to refusing a harmless convenience.

What the inbound list does affect is the attack surface of the front door itself. So the number worth knowing is not how many subnets are listed but how many addresses they cover. Work it out once, here, and write it at the top of the group: a /24 is 254 hosts and a /22 is over a thousand, and "three subnets" sounds identical either way.

dejima-devbox-audit cannot do this arithmetic for you, and it is worth saying why rather than leaving it looking like an omission. The audit runs inside the VM and the rules are on the node, so it can no more read your inbound list than it can read the DROPs. It verifies outbound containment by behaviour, which is possible from inside. Inbound exposure can only be checked from outside, by trying to connect, and this tool does not go looking for things to connect to.

One more, if you use dejima's remote access

Skip this if you do not. It is the only rule here that exists for a feature of another project rather than for this VM's containment, and it is the one most likely to be missed, because nothing on this page or the guest-side one goes wrong without it.

#	Direction	Action	Protocol	Dest. port	Source / Dest	Why
6b	out	ACCEPT	tcp	443	Dest: ntfy server IP	Session notifications leave the guest

Place it with the other pinholes, above the DROPs.

Why it is needed at all. dejima-notify-watch is a systemd user unit on this VM — not something inside a container — so the push to ntfy leaves the guest and crosses your LAN. A self-hosted ntfy is a private address, which rules 7 to 9 drop.

How it fails without this rule is the problem. Remote access is configured, enabled, and sends nothing. Nothing errors where you are looking. The phone simply never buzzes, and the cause is three documents away in another repository. If you have ever configured dejima's remote access on a devbox and concluded it did not work, this is why.

What it costs. The ntfy server joins this VM's blast radius: a compromised devbox can reach it and attack whatever it runs. One host, one port — do not widen it to the subnet. What it does not add is a way to phone home, since egress to the internet from this VM is unbounded by design already. This widens lateral reach and nothing else.

If the ntfy server uses access control, scope this VM's token to the topic dejima publishes to and nothing else. A devbox that can write to other topics can send messages that appear to come from your own infrastructure; one that can read a topic carrying alerts about the devbox learns exactly what any monitoring noticed, which is worse.

Two more, if the VM gets its address by DHCP

Skip both if you pinned the address in the guest with dejima-devbox-static-ip. Add both if you did not, including when the address is reserved for you on the router: reserved or not, the guest still speaks DHCP.

#	Direction	Action	Protocol	Dest. port	Source / Dest	Why
2a	in	ACCEPT	udp	68	—	Let the lease reply back in
6a	out	ACCEPT	udp	67	—	Let the lease request out

Place 2a with the other inbound rules and 6a above the DROPs, as the numbering suggests. Both name a port and no host, which makes them narrower than an exception naming your DHCP server: they add no reachable host to the list above.

Both or neither. Rule 6a alone gets the request out and the VM still never renews, because the reply arrives from the server's address to the VM's, which is not the tuple the request created. Connection tracking is unlikely to match that against Input Policy: DROP, so 2a is what actually lets the lease complete.

Do not count on rebinding to save you. At T2 the client stops unicasting and broadcasts to 255.255.255.255, which none of the DROPs match, so the request does leave without rule 6a — but the reply still needs 2a. Half a working path renews nothing.

This failure is slow and does not look like a firewall. Nothing breaks at reboot or at first boot. The VM simply stops renewing, and somewhere up to a full lease time later it drops off the network.

Rule 2a lets anything on the segment send to the client port, which is a spoofed-reply surface. It is close to free: DHCP on a LAN is trust-on-first-use for every host on that segment whatever the firewall does, so permitting the reply does not create that exposure. Not using DHCP is still narrower, and dejima-devbox-static-ip exists for it.

Then apply it, per VM

A security group carries rules and nothing else. The policies are not part of it, so each VM still needs its own:

VM → Firewall → Options

Firewall	Yes
Input Policy	DROP
Output Policy	ACCEPT

Inbound denies by default — this VM offers no service anyone should reach, so the exceptions are short and knowable. Outbound accepts by default, and the LAN is denied explicitly by the group.

That asymmetry is deliberate, and it is worth resisting the obvious-looking alternative: deny outbound too, then allow tcp/80, tcp/443 and udp/123. That breaks things — apt during a first install, and anything else needing a port nobody listed — and each breakage presents as a network fault rather than as a firewall rule. It also buys almost nothing: this VM is assumed to be owned by an attacker with root, and the guarantee being made is reaches the internet, not your network, not reaches only three ports. Anyone who owns the box tunnels over 443 like everybody else.

Deny-by-default earns its keep where the protected set is enumerable, which is the LAN. The internet is not, and treating it as though it were gets you a rule set that looks strict and only gets in the way.

Then VM → Firewall → Add → Security Group, and select dejima-devbox-hardener. That single entry is the whole rule set, in the order the group defines, and it is the only firewall rule the VM needs.

Adding another devbox later is that one step. The ordering below was got right once, in one place, and cannot drift between hosts.

Order matters within the group, and it is the one thing here that is easy to get wrong. Rules are evaluated top to bottom and the first match wins. Your forge and your resolver are inside the ranges rules 7–10 drop, so if either pinhole ends up below them it is dead and the VM cannot reach the forge or resolve anything.

The console inserts each new rule at the TOP of the list, not the bottom. So entering the rules in the order they are written above leaves you with them in reverse: the four DROPs on top, every pinhole beneath them, and a devbox that cannot reach its forge. Following the table without watching the list is enough to produce exactly the failure this section is warning about.

Two ways through it. Drag each rule to the bottom as you add it, which keeps the list matching the table at every step and is the easier one to check. Or enter them in reverse, from the last row up to the first, and let the insertions stack into the right order — quicker, and you are reading the table backwards while doing it, which is its own way to get one wrong.

Either way, read the finished list top to bottom and compare it against the table before you trust it. Nothing else catches this: a dead pinhole looks exactly like a working firewall until something tries to reach the forge.

Rules 3–6 are the entire remaining LAN exposure, and they are why "no LAN access" is not literally true. Both destinations are pinned to one host, on the ports named.

Nothing follows rule 10. The public internet is reached by the default policy.

Values that must agree

Each of these is entered in two places, and each is silent when the halves disagree. Read the real value off the machine rather than recalling it. That is the whole of the advice, and every one of them is easy to get wrong from memory.

This	Must equal this	Read the truth from
Rules 5–6's destination	The VM's nameserver	grep nameserver /etc/resolv.conf on the VM
Rules 3–4's destination	The forge's address	getent hosts <forge-hostname>
Rule 1's source	The subnets you administer from	your own devices
Rule 2's source	The subnets a phone is on	your own devices
The forge's hostname	forge_host in /etc/dejima-devbox/config	dejima-devbox-bootstrap asks and writes it
Rule 6b's destination	The host in ntfy_url, [remote-access] in ~/.config/dejima/config	getent hosts <that hostname> on the VM

Rules 5–6 want the nameserver, not the gateway. They are the same address on plenty of networks, which is exactly what makes this worth saying: using the gateway works for some people and, for everyone else, blocks DNS while leaving every other check passing. ip route gives you the gateway; /etc/resolv.conf gives you the nameserver, and it is the second one that belongs in these rules.

The forge pair is the one the audit depends on. dejima-devbox-audit uses the forge as its positive control: if the forge answers, packets are moving, and a timeout to the gateway means the firewall rather than a broken network. Point rules 3–4 at one address and forge_host at a name that resolves elsewhere and the audit reports INDETERMINATE, which is correct but unhelpful.

Nothing checks rule 6b, and that is worth knowing rather than discovering. dejima-devbox-audit verifies containment — that things which should be unreachable are — and a missing pinhole is the opposite shape: something that should be reachable and is not. The audit passes either way. The only symptom is a phone that stops buzzing, which looks like dejima being broken rather than like a firewall rule, so check this one by sending a notification and watching for it.

Check it, rather than assuming it

On the node, with the VM running — Proxmox builds these chains around the VM's tap device, so a stopped VM has none and their absence would tell you nothing:

# what is actually loaded right now
sudo iptables -S | grep "tap<vmid>i0"

# the same, for the other address family. Rule 12 lands here and nowhere else
sudo ip6tables -S | grep "tap<vmid>i0"

# what Proxmox would generate from the current config
sudo pve-firewall compile | grep -B2 -A20 "tap<vmid>i0"

Run both, not just the first. A rule with an IPv6 destination is compiled into ip6tables and is absent from iptables -S — so checking only the v4 table shows rule 12 missing whether it is there or not, and shows it identically either way.

What you are looking for. Proxmox names the per-VM chains after the tap interface — tap<vmid>i0-IN and tap<vmid>i0-OUT for the first NIC — and your rules appear inside them, in the order you added them. In the v4 -OUT chain that means the four pinhole ACCEPTs first, then the five LAN DROPs, then whatever Proxmox appends for the policy. The v6 -OUT chain holds rule 12 and none of the pinholes, because every one of them names an IPv4 address.

What each failure looks like.

What you see	What it means
No tap<vmid>i0 chains at all	The firewall is not applied to this VM. The NIC's Firewall checkbox, the VM's Firewall option, or the datacenter switch — check all three, in that order
Chains exist but hold none of your rules	The rules are on a different VM id, or were added at datacenter or node level rather than to this VM
A LAN DROP above a pinhole ACCEPT	The ordering trap. The VM will not reach the forge or resolve anything; drag the pinholes above the DROPs
Rules present and correct, DNS still failing	The address in rules 5–6 is not the VM's nameserver. The gateway is the usual wrong answer — see "Values that must agree"
v4 chain right, ip6tables -S empty	Rule 12 was not entered, or the console rejected ::/0 and the error was missed. Nothing about the v4 half will look wrong

pve-firewall status is the quicker sanity check of whether the service is running and enabled at all, but it says nothing about these rules — it is the datacenter switch it reports on, not your twelve lines.

Both commands describe configuration. They are worth reading, and they are not proof: a rule that is loaded and a rule that does what you meant are different claims. The checks from inside the VM below are the ones that test behaviour, which is why they follow rather than replace this.

and from inside the VM, once it is up. Three of these must succeed and two must fail, which is what makes the set diagnostic rather than reassuring:

try() { timeout 5 bash -c "exec 3<>/dev/tcp/$1/$2" 2>/dev/null \
        && echo "  CONNECTED  $1:$2" || echo "  blocked    $1:$2"; }

gw=$(ip -4 route show default | awk '{print $3; exit}')
dev=$(ip -4 route show default | awk '{for (i=1;i<NF;i++) if ($i=="dev") print $(i+1); exit}')

try "$gw" 443                  # must be blocked - your own first hop
try <forge-ip> 443             # must connect
try deb.debian.org 80          # must connect - the public internet
getent hosts deb.debian.org && echo "  resolved" || echo "  DNS FAILED"

# must find nobody but this VM - rule 12
sudo ping -6 -n -c 2 -W 1 -I "$dev" ff02::1

Nothing here needs installing: bash opens a TCP connection on its own and getent is part of libc, which matters on a machine you have not finished setting up yet. ping is iputils-ping, which Debian has by default.

The last one asks the whole LAN who is there, over IPv6. ff02::1 is the all-nodes link-local multicast address and every IPv6 host answers it — and every Linux host is an IPv6 host, because the fe80:: address exists whether or not the network routes v6. Replies from an address that is not this VM's own mean rule 12 is not in force and the segment is reachable over a family none of the v4 checks above touch.

Like the gateway probe, it writes down no target: ff02::1 is the same address on every network there has ever been, so naming it describes nothing about yours.

The first probe targets your gateway, deliberately, and not some other host on your LAN. Naming another host would mean writing down something worth attacking on the machine this page assumes will be owned. The gateway avoids that entirely: its address is already in the routing table, so it reveals nothing; it is guaranteed to be alive, which an arbitrary address is not; and it sits inside the ranges rules 7 to 10 drop, so with the rules applied it must be unreachable as a destination while traffic through it to the internet is untouched.

Read them together, because the pattern says which thing is wrong:

• First one connects → the firewall is not in force at all. Check the Network device's Firewall checkbox and the datacenter switch.
• Second or third fails while the fourth works → a pinhole is below the LAN DROPs. Check the rule order.
• Only the last one finds neighbours → the v4 half is right and rule 12 is missing. This is the failure that looks like success from every other angle, which is why it has its own probe.
• Everything fails → the VM has no working route or DNS, which is a networking problem rather than a firewall one. Note that this also makes the first probe's "blocked" meaningless: silence proves nothing when nothing is getting through. That ambiguity is exactly why dejima-devbox-audit reports three outcomes rather than two.

Do this once by hand, then stop doing it by hand. bin/dejima-devbox-audit runs the same checks, tells the three outcomes apart properly, and a timer re-runs it. Rules get edited, and a rule widened by accident has no symptom on this VM at all.

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The node side is done. Go back to the guest.

Everything above is what a hypervisor does and a guest cannot. The rest is inside the VM: pinning the addressing, installing the audit and its timer, keeping credentials off the box, and patching.

Continue at docs/building-a-devbox.md §2.

Come back here only for §4, when you want a snapshot. If you added the DHCP rules and later pin the address in the guest, you can drop them then - that is the one thing on this page the guest runbook can change.

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4. Snapshots, and why there is no cadence

Take a snapshot before anything you would not want to undo by hand: a kernel upgrade, a change to the container runtime, the first run of new tooling.

VM → Snapshots → Take Snapshot. Leave Include RAM unchecked. You want a disk state you can return to, not a resumed process tree, and the RAM image costs the VM's whole memory in storage for nothing you will use.

There is no scheduled rollback here, and that is the design

This is the single largest difference from a CI runner, and it is worth being explicit about because the sister project's runbook devotes a long section to automating exactly what this page refuses to do.

A runner is rebuilt on a timer because nothing on it is worth keeping. Its whole containment story is that anything which gets in has until the next rollback. A devbox holds uncommitted work, so a rollback on a timer destroys the thing the machine exists to hold. There is no cadence that is both safe for your work and useful against an intruder.

So containment here comes from §3 and from keeping the box worth little. Not from being able to throw it away.

What replaces it

Push more often than you would on a machine you trusted. The forge is the only copy a rollback cannot eat, and it is also the only copy an intruder on this VM cannot quietly alter without it showing up as a push you did not make.

Know your rebuild cost, by having paid it once. On a box you cannot roll back, rebuild time is the real measure of containment: it is what "recover from a compromise" actually costs you. A rebuild you have never done is an estimate, and estimates of this are always low. Doing it once, deliberately, converts the number from a guess into a fact.

Keep the credentials off it, so the rebuild is the whole of the recovery and not the start of it. That part is in docs/building-a-devbox.md, because it is guest work.

Two hazards no rule covers

Agent forwarding. ssh -A into this VM lends it your keys for the life of the session, and this is a VM whose threat model says it may be owned. Put ForwardAgent no in your SSH config for this host, and reach anything else directly rather than through it. This is easy to leave on by accident, because it is often set globally and never thought about again.

Exception creep. A NAS today, a registry next month, a printer after that. Each is reasonable on its own and the set is nobody's decision. Every LAN exception belongs in §3's table and nowhere else, so the list can be read in one place and pruned in one place. A rule added directly to the VM rather than to the group is the one that gets forgotten.