Title: How to account systemd services bandwidth usage on NixOS

	Title: How to account systemd services bandwidth usage on NixOS
	Author: Solène
	Date: 20 July 2022
	Tags: nixos bandwidth monitoring
	Description: This article shows how to easily record the bandwidth
	usage of specific systemd services on NixOS, but this can be applied to
	Linux in general

	# Introduction

	Did you ever wonder how many bytes a system service is daily receiving
	from the network? Thanks to systemd, we can easily account this.

	This guide targets NixOS, but the idea could be applied on any Linux
	system using systemd.

	NixOS project website

	In this article, we will focus on the nix-daemon service.

	# Setup

	We will enable the attribute IPAccounting on the systemd service
	nix-daemon, this will make systemd to account bytes and packets that
	received and sent by the service. However, when the service is
	stopped, the counters are reset to zero and the information logged into
	the systemd journal.

	In order to efficiently gather the network information over time into a
	database, we will run a script just before the service stops using the
	preStop service hook.

	The script checks the existence of a sqlite database
	/var/lib/service-accounting/nix-daemon.sqlite, creates it if required,
	and then inserts the received bytes information of the nix-daemon
	service about to stop. The script uses the service attribute
	InvocationID and the current day to ensure that a tuple won't be
	recorded more than once, because if we restart the service multiple
	times a day, we need to distinguish all the nix-daemon instances.

	Here is the code snippet to add to your `/etc/nixos/configuration.nix`
	file before running `nixos-rebuild test` to apply the changes.

	```nix
	systemd.services.nix-daemon = {
	serviceConfig.IPAccounting = "true";
	path = with pkgs; [ sqlite busybox systemd ];
	preStop = ''
	#!/bin/sh

	SERVICE="nix-daemon"
	DEST="/var/lib/service-accounting"
	DATABASE="$DEST/$SERVICE.sqlite"

	mkdir -p "$DEST"

	# check if database exists
	if ! dd if="$DATABASE" count=15 bs=1 2>/dev/null \| grep -Ea "^SQLite format.[0-…
	then
	cat <<EOF \| sqlite3 "$DATABASE"
	CREATE TABLE IF NOT EXISTS accounting (
	id TEXT PRIMARY KEY,
	bytes INTEGER NOT NULL,
	day DATE NOT NULL
	);
	EOF
	fi

	BYTES="$(systemctl show "$SERVICE.service" -P IPIngressBytes \| grep -oE "^[0-9]…
	INSTANCE="'$(systemctl show "$SERVICE.service" -P InvocationID \| grep -oE "^[a-…

	cat <<EOF \| sqlite3 "$DATABASE"
	INSERT OR REPLACE INTO accounting (id, bytes, day) VALUES ($INSTANCE, $BYTES, d…
	EOF
	'';
	};

	```

	If you want to apply this to another service, the script has a single
	variable SERVICE that has to be updated.

	# Display the information from the database

	You can use the following command to display the bandwidth usage of the
	nix-daemon service with a day-by-date report:

	```shell
	$ echo "SELECT day, sum(bytes)/1024/1024 AS Megabytes FROM accounting group by …
	day Megabytes
	---------- ---------
	2022-07-17 173
	2022-07-19 3018
	2022-07-20 84
	```

	Please note this command requires the sqlite package to be installed in
	your environment.

	# Enhancement

	I have some ideas to improve the setup:

	* The script could be improved to support multiple services within the
	database by using a new field
	* The command to display data could be improved and turned into a
	system package to make it easier to use
	* Provide an SQL query for monthly summary

	# Conclusion

	Systemd services are very flexible and powerful thanks to the hooks
	provided to run script at the right time. While I was interested into
	network usage accounting, it's also possible to achieve a similar
	result with CPU usage and I/O accesses.