# dhcpcd

The uplink seat's module for a machine whose own network is dhcpcd's (novox/hq ADR 0117). It
asks two things of dhcpcd, and nothing else: leave the resolver file to the mesh, and leave the
private network's interface alone. It never declares an interface, an address, a route, a
wireless network or its credentials — the link dhcpcd keeps is the only channel the mesh reaches
the machine over.

## What it writes

Two lines into `/etc/dhcpcd.conf`, as the mesh's marked region (`into: block`) — dhcpcd reads no
drop-in directory, so the mesh writes into its one file rather than over it (ADR 0102):

- `nohook resolv.conf` — dhcpcd's resolv.conf hook rewrites `/etc/resolv.conf` on every lease it
  takes or renews, which would silently replace the resolver `resolv-conf` names.
- `denyinterfaces mesh0` — dhcpcd never asks for a lease on the private network's interface, and
  never takes it down. dhcpcd leaves a point-to-point interface alone by default; this says so
  rather than relying on it.

**At the start of the file** (`at: start`). Both are global options, and dhcpcd reads every line
after an `interface` or `ssid` line as that interface's own. A configured machine's file ends in
exactly such a block (the interface, its static address), so appended at the end these two would
quietly apply to one interface only.

## Why it declares no service

dhcpcd is the machine's, not the mesh's. The mesh never starts, stops or enables it: stopping it
drops the address the machine is reached at, and a module unassigned by mistake must not be able
to do that. And there is nothing to reload it with — `dhcpcd.service` reports `CanReload=no`, and
a restart drops the lease. So the two lines take effect at **dhcpcd's next start**.

On an adopted machine that is normally no gap: the predecessor wrote the same `nohook` line, and
it is already in force. **On a machine that was not adopted, it is one:** until dhcpcd next
starts (a reboot, or the operator restarting it in a window of their choosing), a lease renewal
still rewrites `/etc/resolv.conf`, and `resolv-conf` puts it back at the next push. Assign this
module before `resolv-conf` on such a machine, and restart dhcpcd once, by hand, when losing the
link for a moment is acceptable.

## One manager per machine

It claims `the-uplink`: a machine runs one network manager, and assigning a second module that
claims the seat is refused. Assigning this one to a machine whose network is NetworkManager's
installs the package and writes the two lines, and starts nothing.