Arturo R Montesinos

DHCPv4 · commercial displays · Linux · Rust

monitor-dhcp

A known address for the monitor that only speaks DHCP.

monitor-dhcp is a deliberately small DHCP server for one dedicated Ethernet link. It gives each known monitor the address you wrote down for it, and every unknown device one predictable default, so you always know where to find the screen your PC controls.

Get started View on GitHub

  • One TOML file is the whole state
  • Same address every time, no lease database
  • Runs as an unprivileged, hardened systemd service
  • MIT licensed

The problem

You have a PC wired straight to a commercial monitor, and you want to control the monitor over the network. The monitor insists on getting its address by DHCP and offers no way to set a static one. There is no router on that cable, and nothing else should be.

The usual answer is a full DHCP server such as dnsmasq, isc-dhcp-server or Kea, or NetworkManager’s connection sharing. They all work, but they are built for networks: address pools, lease files, DNS, and settings that decide which address a device probably gets. For one cable and one screen, you want something smaller that always gives the same answer.

A general-purpose DHCP server

Pools, leases kept on disk, often DNS as well. The monitor's address depends on lease history, and there is a lot of configuration to get right for a single cable.

monitor-dhcp

A short TOML file maps the monitor's MAC to its address. Any other device gets one fixed default. Nothing is remembered between requests, so the answer never drifts.

How it works

monitor-dhcp listens on exactly one interface. When a request arrives, it looks up the client’s MAC address in the [hosts] table of its configuration. A known MAC gets its reserved address; anything else gets default_ip. It answers discovery, requests, renewals and reboots, and keeps no lease state at all: the configuration file is the source of truth.

  1. Address the linkGive the dedicated Ethernet port a static address with no gateway or DNS.
  2. Describe itWrite the interface, the server address and the monitor's MAC in one TOML file.
  3. Run itStart the service. The monitor comes up at the address you chose.

Small on purpose

The design rule is “do the smallest thing that works reliably”, and most of the strengths follow from what monitor-dhcp leaves out.

Predictable by design

No pool and no lease database. The same MAC gets the same address every time, and an unknown device always gets default_ip.

Stays on its own cable

It binds to one named interface and never changes addresses or routes. Your other network connections are left alone.

Safe reloads

A systemctl reload applies a valid new configuration atomically. An invalid one is rejected, and the last good one keeps running.

Check before you start

monitor-dhcp --check validates the configuration without touching the network, so mistakes show up before anything is bound.

Least privilege

The supplied systemd unit runs it as its own unprivileged account, with only the two Linux capabilities needed to bind the DHCP port.

Tested against a real client

A network-namespace test suite runs dhclient against it: acquisition, renewal, reloads, malformed packets and interface isolation.

What it supports today

monitor-dhcp is at version 1.0. It targets Ubuntu 20.04 and newer and other reasonably modern Linux distributions. The release on GitHub is a statically linked x86_64 binary; the build helper can also produce ARMv6 and ARMv7 executables.

Because every unknown device gets the same default_ip, only one unknown client should be active on the link at a time. There is no conflict detection. Pools, DNS, gateways, IPv6, relays, PXE and routing are intentionally out of scope. Devices that need a link-layer unicast reply before they have an address may not work, since initial replies are sent as broadcasts.

Get started

Use a dedicated, isolated link. Never attach monitor-dhcp to a production network or to an interface that already has a DHCP server. If you work over SSH, make sure the monitor's port is not the one carrying your session.

1. Build and install

With a Rust toolchain on Linux:

git clone https://github.com/arturormk/monitor-dhcp-rust.git
cd monitor-dhcp-rust
cargo build --release
sudo install -Dm0755 target/release/monitor-dhcp \
    /usr/bin/monitor-dhcp

Prefer not to build? The releases page has a static x86_64 archive with a SHA-256 checksum.

2. Write the configuration

Find the wired interface with ip link. You also need the monitor’s wired MAC address; the remote validation guide (below) shows how to capture it. Then create /etc/monitor-dhcp/config.toml:

interface = "enp3s0"
server_ip = "192.168.50.1"
subnet_mask = "255.255.255.0"
default_ip = "192.168.50.2"

[hosts]
"00:11:22:33:44:55" = "192.168.50.10"

Check it (this does not touch the network):

monitor-dhcp --config /etc/monitor-dhcp/config.toml --check

3. Try it in the foreground

Give the interface the server address, then run the server and switch the monitor on:

sudo ip addr add 192.168.50.1/24 dev enp3s0
sudo ip link set enp3s0 up
sudo monitor-dhcp --config /etc/monitor-dhcp/config.toml

The log shows DISCOVER, OFFER, REQUEST and ACK, and then ping 192.168.50.10 reaches the monitor.

4. Make it permanent

Set the same static address in Netplan or NetworkManager (without connection sharing, which starts its own DHCP server), then install the hardened unit:

sudo groupadd --system monitor-dhcp
sudo useradd --system --gid monitor-dhcp --no-create-home \
    --shell /usr/sbin/nologin monitor-dhcp
sudo install -Dm0644 packaging/monitor-dhcp.service \
    /etc/systemd/system/monitor-dhcp.service
sudo systemctl daemon-reload
sudo systemctl enable --now monitor-dhcp

Follow it with journalctl -u monitor-dhcp -f. After editing the configuration, sudo systemctl reload monitor-dhcp applies it.

Learn more

monitor-dhcp is MIT licensed. It builds on a vendored copy of the dhcproto library, which is MIT licensed too.