fips/docs/design/fips-security.md

FIPS Mesh-Interface Security

This document describes the threat model and design rationale for the operator-facing security posture of the fips0 mesh interface on Linux. The default-deny nftables baseline shipped as /etc/fips/fips.nft is the artifact discussed below; for the operator activation steps and drop-in extension recipes, see enable-mesh-firewall.md.

The baseline is a documented operator conffile, not an auto-loaded package side-effect. Activation is an explicit one-liner. The rationale for that design follows.

Threat Model for fips0

The mesh is a flat layer-3 segment. Every mesh node that can route to you can deliver packets to your fips0 address — your direct peers forward traffic from non-peer mesh nodes onto your fips0 the same way any router forwards transit traffic. Identity on the mesh is the originating node's npub — the FMP link layer authenticates direct peers with Noise XX and the FSP session layer authenticates session endpoints with Noise XX — but identity is not authorization. Knowing who sent a packet does not, by itself, decide whether the local host should accept it.

That means: any service on a mesh host that binds to a wildcard address (0.0.0.0, [::], or any IPv6 address that includes the fips0 interface in its scope) is reachable from every mesh node that can route to you by default, not only from your direct peers. There is no NAT, no perimeter firewall, no "local-only" address space between you and an arbitrary mesh node. The mesh is closer to a shared LAN than to the public internet.

Compare to the corresponding internet trust assumptions:

Surface	Public internet	FIPS mesh (no baseline)
Reachability from arbitrary mesh node	Mediated by NAT, firewalls, ISPs	Direct
Default identity	None	Originating node's npub (authenticated)
Default authorization	None	None
Accidental exposure cost	Low (NAT hides you)	High (every mesh node sees you)

The third row is the gap this document closes. The default-deny baseline removes "accidental exposure" from the failure modes an operator has to think about.

The Default-Deny Baseline

The shipped baseline is /etc/fips/fips.nft. It defines a single nftables table, inet fips, with one chain hooked at input. The chain:

1. Returns immediately for any packet not arriving on fips0. This makes the table a no-op for every other interface — Docker, Tor, the host's main filter table, OPNsense, anything.
2. Accepts packets that conntrack identifies as established or related. Replies to outbound flows initiated from the mesh host come back; ICMPv6 errors related to existing flows (Packet Too Big, Destination Unreachable) come back.
3. Accepts ICMPv6 echo-request, so ping6 reachability tests work.
4. Includes operator drop-ins from /etc/fips/fips.d/*.nft. An empty directory is fine — the include glob simply matches nothing.
5. Falls through to counter drop. Every dropped packet increments the counter, visible via nft list table inet fips.

Outbound from fips0 is unrestricted. The baseline is concerned only with what the mesh host accepts, not what it sends.

The file is a documented dpkg conffile. Operator edits to /etc/fips/fips.nft are preserved across upgrades, the same way edits to /etc/fips/fips.yaml and /etc/fips/hosts are preserved. If the packaged baseline is ever updated upstream, dpkg prompts the operator on upgrade rather than silently overwriting local changes.

The canonical artifact is the file itself; read it for the inline documentation that the rest of this document references.

Why no auto-load on package install

The postinst script does not enable fips-firewall.service. This is deliberate. Quietly mutating host firewall state on package install is hostile on every axis that matters: it surprises operators who already have their own nftables ruleset, it can collide with podman/Docker/OPNsense integrations even though the early-return makes it technically safe, and it converts an explicit security decision into an invisible one. The mesh-interface filter belongs to the operator, not to the package's postinst.

The activation gesture is one short, well-formed command. The rationale is documented in the file's inline header and in this document. That is enough; auto-loading would trade discoverability for no real gain.

Coexistence with other firewalls

The inet fips table only matches packets arriving on fips0. Anything else returns from the chain on the first rule. Specifically:

• Docker / containerd install nftables rules in the ip and ip6 families and operate on docker0, br-*, and veth* interfaces. They do not touch fips0. The two tables coexist without interference.
• Tor runs in user space and does not install firewall rules. The baseline is independent of Tor's onion-service and SOCKS listeners.
• OPNsense is an upstream perimeter device. The baseline runs on the local host and applies only to traffic that has already reached the host's fips0 interface. They do not interact.
• The host's main /etc/nftables.conf typically defines a separate inet filter table. nftables allows multiple tables in the same family to coexist; both run in parallel at hook input/priority 0 and the iifname != "fips0" return rule keeps the inet fips table from interfering with anything outside the mesh interface.
• inet fips_gateway, when fips-gateway is running, manages DNAT/SNAT on the LAN-facing interface to translate virtual IPs to mesh addresses. It is a separate concern owned by the gateway binary and is unrelated to this baseline. See the section below.

Coexistence with inet fips_gateway

When fips-gateway is running, it manages a separate nftables table, inet fips_gateway, containing the DNAT and masquerade rules that translate between the gateway's virtual-IP pool and mesh addresses on the LAN-facing interface. That table is created and torn down by the gateway binary at runtime and is not an operator artifact in the same sense as inet fips.

The two tables do not interfere:

• inet fips filters inbound on fips0.
• inet fips_gateway performs NAT on the LAN interface.

They operate on different interfaces and at different hook points (input filter vs. prerouting/postrouting NAT). Both can be loaded simultaneously on a gateway host, and that is the intended deployment shape. See fips-gateway.md for the gateway table's structure.

What the Baseline Does Not Cover

The baseline is one half of a defense-in-depth posture. It is explicitly not:

• Outbound filtering. Anything the mesh host originates on fips0 is unrestricted. If you need to constrain what the host can send to the mesh, add rules to a separate chain hooked at output — out of scope for the baseline.
• Application-layer authorization. The baseline decides whether a packet reaches a service. It does not decide whether the originating mesh node's npub is allowed to use that service. That is the application's responsibility (e.g., an authorized_keys file for SSH, an ACL in the application's configuration).
• ACL on the mesh handshake. The FMP Noise XX handshake authenticates the peer's npub and, on both inbound and outbound paths, consults the peer ACL (peers.allow / peers.deny) before promoting the connection. The ACL evaluates in TCP-Wrappers order: an allow match permits, otherwise a deny match rejects, otherwise the connection is permitted. A strict allowlist posture therefore requires an explicit ALL entry in peers.deny; a populated peers.allow alone does not turn the ACL into a strict allowlist. Mesh-level ACLs are a separate concern from the inbound packet filter described here; see the peer ACL section in ../reference/security.md.
• Compromised peers. A peer whose key has been stolen or whose host has been taken over is, by mesh-level identity, still that peer. Source-address filtering in drop-ins operates on the source mesh address of inbound traffic regardless of whether that source is a direct peer or a multi-hop mesh node, and so can limit damage from a known-compromised mesh address; but the baseline cannot revoke trust on its own.

Treat the baseline as removing the "wide-open by default" failure mode. Higher-layer authorization decisions are the operator's and the application's, the same as on any other shared network.

Future Work

The current baseline is Linux-only. Parallel work for other targets:

• macOS PF baseline. macOS uses Packet Filter (PF), inherited from OpenBSD. PF maps cleanly onto the same conceptual model as nftables: stateful inspection (keep state ≈ ct state established,related), default policy, anchor-based modular rule loading. A packaging/macos/fips.pf will land alongside the Linux baseline with the same posture: documented asset, no auto-load, operator opts in via launchd. The macOS interface name is utunN rather than fips0, so the rule template needs runtime substitution or a PF interface group assigned at TUN bring-up; this is being worked through with the macOS port.
• OpenWrt fw4 path. OpenWrt's fw4 already drives nftables under the hood, but rules go into /etc/nftables.d/ includes or UCI entries in /etc/config/firewall, not a free-standing fips.nft. The ipk will ship a layout-compatible variant or document the operator setup separately, decided when the OpenWrt packaging is updated.
• Cross-OS gateway abstraction. fips-gateway is currently Linux-only because src/gateway/nat.rs uses the rustables netlink API directly. macOS gateway support requires a PF-backed equivalent behind a shared backend trait. This is a larger lift than the static baseline and is tracked separately under the same cross-OS thread.

When those land, this document will grow per-OS sections describing each baseline's load mechanism and extension points. The threat model and the operator-extension principle are the same on every OS; only the filter syntax and the activation gesture differ.

See also

• enable-mesh-firewall.md — operator activation steps, drop-in recipes, drop visibility and debugging
• ../reference/security.md — consolidated security reference (cryptographic primitives, peer ACL format, filesystem permissions, default network exposures)
• fips-gateway.md — fips-gateway service and the separate inet fips_gateway table