linux-cloud-router/README.md

# vpn-router

A Debian package that configures a Linux host or virtual machine as a VPN router:

- site-to-site IKEv2 IPSec with pre-shared key authentication (strongSwan swanctl)
- road-warrior (P2S) access using IKEv2 EAP-TLS, with a bundled certificate authority
- optional WireGuard endpoint
- routing and NAT for a protected subnet, TCP MSS clamping, and UFW firewall rules

Nothing in the package is specific to a cloud provider. It is built on the external and
internal NIC names, and a `mode` setting says how much of the rest you are supplying and
how much is read from the system. Detection happens only in the mode that asks for it, so
a router never quietly guesses which interface to NAT out of unless you told it to.
Platform-specific additions live in separate modules and are selected explicitly.

Installing the package configures nothing by default. The shipped platform is `none`,
which means the files are laid down and the machine is left alone until you say
otherwise.

## Installing

The package can be installed by hand or as a step in automated provisioning. Both are
supported equally, and both end up with the same two things: a configuration file and a
service that applies it.

```
/etc/vpn-router/vpn-router.conf      the configuration
systemctl restart vpn-router-setup   apply it
```

### By hand

```sh
apt-get install vpn-router          # answer "none" to the platform question
$EDITOR /etc/vpn-router/vpn-router.conf
systemctl restart vpn-router-setup
```

Answering `none` to the platform question defers everything: no further questions are
asked and nothing on the machine is configured. The only change an install makes is the
package's own `/etc/sysctl.d/` drop-in, which enables IP forwarding and relaxes
`rp_filter` as a router needs. Set `platform` in the file when you are ready.

### Automated

Preseed the answers and the router comes up configured, with no follow-up command. Note
that `platform` must be something other than `none`, and `mode` says how much you are
supplying:

```shell
debconf-set-selections <<'EOF'
vpn-router vpn-router/platform         select generic
vpn-router vpn-router/mode             select manual
vpn-router vpn-router/external_interface string eth0
vpn-router vpn-router/internal_interface string eth1
vpn-router vpn-router/int_addr         string 10.1.1.4
vpn-router vpn-router/int_gateway_ip   string 10.1.1.1
vpn-router vpn-router/local_fqdn       string router.example.com
vpn-router vpn-router/local_cidrs      string 10.0.0.0/24
vpn-router vpn-router/remote_addrs     string peer.example.net
vpn-router vpn-router/remote_id        string peer.example.net
vpn-router vpn-router/remote_cidrs     string 192.168.0.0/24
vpn-router vpn-router/psk              password s3cr3t
EOF
DEBIAN_FRONTEND=noninteractive apt-get install -y vpn-router
```

With `mode = interfaces` the `int_addr` and `int_gateway_ip` lines are dropped and read
from the internal NIC instead; with `mode = auto` the interface names go too.

Alternatively write `/etc/vpn-router/vpn-router.conf` directly before or after installing.
A configuration-management tool should do that and then restart `vpn-router-setup`.

See [examples/](examples/) for a cloud-init template and a shell installer.

## Configuration

`/etc/vpn-router/vpn-router.conf` is INI, mode 0600, and is created on first install. It
is managed with `ucf`, so it is yours to edit: a later `dpkg-reconfigure` applies what
changed and asks before touching anything you altered by hand. A fully commented
reference is installed at `/usr/share/doc/vpn-router/vpn-router.conf.example`.

Lists are comma-separated, booleans are `true`/`false`, an empty value means unset, and a
`_b64` suffix means the value is base64-encoded.

| Section | Setting | Meaning |
|---|---|---|
| `general` | `platform` | `none`, `generic`, `azure` or `gcp`. `none` defers configuration entirely |
| `general` | `mode` | How much you supply: `manual`, `interfaces` or `auto` |
| `interfaces` | `external` | Name of the NIC facing the untrusted network |
| `interfaces` | `internal` | Name of the NIC facing the protected network |
| `wan` | `local_fqdn` | This router's FQDN |
| `wan` | `local_id_mode` | IKE identity source: `fqdn`, `public_ip` or `internal_ip` |
| `local` | `cidrs` | Local subnets advertised into the tunnel |
| `local` | `int_addr` | This host's address on the internal network |
| `local` | `int_gateway_ip` | Next hop on the internal side |
| `remote` | `addrs` | Remote gateway address(es) or FQDN |
| `remote` | `id` | Remote IKE identity, without a leading `@` |
| `remote` | `cidrs` | Remote subnets reachable through the tunnel |
| `remote` | `psk_b64` | Pre-shared key, base64-encoded |
| `remote` | `psk_file` | Path to a file holding the raw key; wins over `psk_b64` |
| `p2s` | `enabled` | Road-warrior access |
| `p2s` | `address_pool` | Pool assigned to road-warrior clients |
| `p2s` | `ca_name` | Common name for a locally created CA |
| `wireguard` | `enabled` | WireGuard endpoint |
| `wireguard` | `address` | Address and prefix for `wg0` |
| `wireguard` | `listen_port` | UDP port, default 51820 |

WireGuard peers are added by hand in `/etc/wireguard/wg0.conf`. The package owns the
`[Interface]` section of that file and rewrites it when the settings above change, but it
carries every `[Peer]` section across untouched.

Encode the pre-shared key with:

```sh
printf %s 'the key' | base64 -w0
```

Every feature is independent. Leaving a group empty simply means that feature is not
configured, and its generated files are removed.

### What `mode` decides

`mode` states how much of the network configuration you are supplying, and therefore how
much the package reads from the system:

| `mode` | You provide | The package works out |
|---|---|---|
| `manual` | both interface names, `int_addr`, `int_gateway_ip` | nothing |
| `interfaces` | both interface names | `int_addr` and `int_gateway_ip`, from those interfaces |
| `auto` | nothing | the interface names too, from the routing table |

`manual` is the only mode in which no detection code runs at all. `auto` is the
convenient one and can pick the wrong interface, which is a trade you make deliberately
by choosing it rather than something the package does behind your back.

The addresses strongSwan binds on always come from the external NIC, in every mode, so
they are never configured twice and cannot drift out of step with the host. If a named
interface does not exist the service stops and says so. In `interfaces` and `auto` mode
the internal gateway is taken from the routing table, falling back to the first host
address of the connected subnet with a warning when the routing table has nothing to say.

## Applying changes

```sh
systemctl restart vpn-router-setup
```

The service also runs at boot. With `platform = none` it does nothing and says so.
Otherwise it regenerates the configuration files, resolves the interfaces and their
addresses as `mode` directs, applies routes, generates WireGuard keys and injects
firewall rules. It then starts `strongswan` and `wg-quick@wg0` if the corresponding
feature is configured, reloading or restarting them only when their inputs changed, and
restarts `systemd-resolved` only when the road-warrior DNS drop-in changed. Turning a
feature off in the configuration stops its service and removes its files. Every step is
idempotent, so restarting with no changes does nothing.

To regenerate the configuration files and PKI without starting, stopping or reloading
anything:

```sh
/usr/lib/vpn-router/configure
```

This does still write to the system: it renders into `/etc/swanctl/`,
`/etc/systemd/resolved.conf.d/` and `/etc/wireguard/`, and if road-warrior access is
enabled with an empty PKI directory it creates the certificate authority and adds it
to the host trust store. What it leaves alone is routes, firewall rules and services -
that is what `systemctl restart vpn-router-setup` adds.

## Certificates

`/etc/vpn-router/pki` is the PKI directory, laid out the way `simple-ca` expects:

```
ca_cert.pem  ca_key.pem  ca_bundle.pem  <label>_cert.pem  <label>_key.pem  crl.pem
```

`<label>` is the first component of the router FQDN, so `router.example.com` gives
`router_cert.pem`.

If road-warrior access is enabled and this directory is empty, a certificate authority and
a server certificate are created there automatically. To use your own instead, place the
files in that layout before enabling the feature; nothing is generated or overwritten when
material is already present.

Either way, the package copies what each consumer needs into place: the CA into
`/etc/swanctl/x509ca/`, the server certificate and key into `/etc/swanctl/x509/` and
`/etc/swanctl/private/`, a CRL into `/etc/swanctl/x509crl/`, and the CA into the system
trust store.

Issue a client certificate with the bundled tool:

```sh
/usr/lib/vpn-router/simple-ca make-cert --ca-dir /etc/vpn-router/pki alice
/usr/lib/vpn-router/simple-ca make-pfx  --ca-dir /etc/vpn-router/pki \
    --password s3cr3t /etc/vpn-router/pki/alice_cert.pem
```

`simple-ca` comes from a separate project, <https://gitea.koszewscy.waw.pl/slawek/simple-ca>,
and also supports `make-crl` and `revoke-cert`.

## Platform modules

Platform-specific values do not belong in the core, so they are Python modules in
`/usr/lib/vpn-router/vpnrouter_platforms/`, imported at run time and selected by
`general.platform`. The platform is never probed: the caller already knows where it is
deploying.

`none` is not a module. It is the shipped default and means "configure nothing at all",
which is how a deferred install is expressed. `generic` is the module to choose when the
router should be configured but the platform adds nothing.

The shipped modules currently add nothing. Road-warrior clients are given the router
itself as their DNS server and the router forwards through its own `systemd-resolved`, so
no provider resolver has to be advertised or routed through the tunnel on any platform.
The modules exist as the documented place for platform specifics if that changes.

A module may define any of the following, and takes the default for anything it omits:

| Name | Type | Effect |
|---|---|---|
| `EXTRA_P2S_DNS` | `str` | added to the road-warrior pool `dns` list |
| `EXTRA_LOCAL_TS` | `str` | added to `local_ts` on both connections |
| `apply(ctx)` | function | called once the system is configured, to apply platform-specific state; must be idempotent |

`ctx` is the template context, so a module reads what it needs from it -
`ctx['wan_iface']`, `ctx['int_iface']`, `ctx['local_cidrs']` and the rest.

Adding a platform is adding one file. Modules are discovered by listing the package, so
nothing in the core needs editing:

```python
# /usr/lib/vpn-router/vpnrouter_platforms/example.py
"""Example provider."""

EXTRA_P2S_DNS = '192.0.2.53'
EXTRA_LOCAL_TS = '192.0.2.53/32'
```

Set `platform = example` and restart the service. The debconf question offers only the
modules shipped with the package; the configuration file accepts any module installed.

## Removal

`apt-get remove` terminates the tunnels and takes `wg-quick@wg0` down, but leaves the
configuration in place. `strongswan` is left enabled, since it is a shared service that
may be in use by something else.

`apt-get purge` additionally removes the generated files and the firewall rules, restoring
`/etc/ufw/before.rules` to its original content, and hands `vpn-router.conf` back to `ucf`
before deleting it so a later reinstall starts clean. Key material is never deleted:
`/etc/vpn-router/pki`, the WireGuard key pair and everything under `/etc/swanctl` are left
alone.

## Building

```sh
cd debian-package
./build.sh
```

The build runs in a container and writes the package to `debian-package/out/`.
`./publish.sh` uploads it to the configured repository.