Date: Wed Dec 26 15:04:55 EST 2018

Logging, Firewall and Backups -- A Void Linux VPS Part II

In part one I covered the installation of Void Linux on a Digital Ocean VPS. In this second part I’m going to cover the process of setting up basic logging, firewall and backups.

Logging

When we installed Void Linux in part one we didn’t setup any logging.

Although rsyslog is in the repositories, we don’t need all of its features, socklog can take care of logging for us instead.

# xbps-install socklog{,-void}

For those unfamiliar, the above bracket expansion is supported in both mksh and bash and will translate to two arguments being passed, socklog and socklog-void.

Then go ahead and enable the daemons

# ln -s /etc/sv/socklog-unix /var/service/
# ln -s /etc/sv/nanoklogd /var/service/

This should take care of logging and put put them /var/log/socklog. Unlike unlike the other logging utilities out there these ones don’t have as much or as easily accessible documentation.

The author has some documentation as well as some more on configuration here. You can gather a little bit more from the socklog-void package here.

The default configuration should be adequate for our purposes. You can easily tail the logs with svlogtail, and then the class of logs. For instance, you can do svlogtail secure to see the logs for authentication and sshd.

Firewall

Most tutorials out there on the internet are going to point you towards iptables for your Linux firewall needs. Typically using some sort of bash script, or similar. Such a setup is sub-optimal for many reasons--even if it is simple. I’m not going to focus on that here. Instead we’re going to setup and use nftables which has been in the mainline Linux kernel since 3.13 in 2014!

Void Linux packages nftables and it’s as simple as:

# xbps-install nftables

From there we can create a simple configuration file:

/etc/nftables.conf

#!/usr/sbin/nft -f

# This is somewhat important, otherwise it will just append to your existing
# rules. This can be somewhat confusing unless you run `nft list table inet
# filter` or similar
flush ruleset

table inet filter {
	chain input     {
		type filter hook input priority 0;

		# Allow all input on loopback
		iif lo accept

		# Accept stateful traffic
		ct state established,related accept

		# Accept SSH
		tcp dport 22 accept

		# Accept HTTP and HTTPs
		tcp dport { 80, 443 } accept

		# Allow some icmp traffic for ipv6
		ip6 nexthdr icmpv6 icmpv6 type {
			nd-neighbor-solicit, echo-request,
			nd-router-advert, nd-neighbor-advert
		} accept

		counter drop
	}
	chain forward   {
		type filter hook forward priority 0;
	}
	chain output    {
		type filter hook output priority 0;
	}
}

That’s about the bare minimum for a webserver firewall. On my systems I specifically block all output except from root, select groups or to the hosts and services I’ve approved.

Outbound filtering ( Optional )

chain output    {
	type filter hook output priority 0;

	# Allow SSH outbound
	tcp sport 22 accept

	# Allow HTTP and HTTPs outbound
	tcp sport { 80, 443 } accept

	oif lo accept

	# Allow some icmp traffic for ipv6
	ip6 nexthdr icmpv6 icmpv6 type {
		nd-neighbor-solicit, echo-request,
			nd-router-advert, nd-neighbor-advert
	} accept

	meta skuid 0 accept
	meta skgid users accept
	meta skgid wheel accept

	counter drop
}

Note that the way skgid works is with the originating group. This is usually the users’s default group as set in /etc/passwd unless an application is specifically sgid. If you don’t know what you’re doing here it’s safe to skip the outbound filtering.

Enabling the firewall on boot

It’s as simple as:

# ln -sv /etc/sv/nftables /var/service/

Installing a cron daemon

For my purposes I’m going to use scron because it’s simple, and in the repositories. I don’t need all of the advanced features of other cron implementations and I have my backups checked on the backup server so I don’t need email on failed cron jobs.

There are other cron implementations in the repositories though:

# xbps-query -Rs cron
[-] cronie-1.5.2_1  Runs specified programs at scheduled times
[-] cronutils-1.9_2 Set of tools to assist the reliable running periodic and batch jobs
[-] dcron-4.5_32    Dillon's lightweight cron daemon
[-] fcron-3.3.0_3   Feature-rich cron implementation
[-] incron-0.5.12_1 A daemon that executes commands due to inotify events
[-] kcron-18.12.0_1 KDE Configure and schedule tasks
[*] scron-0.4_3     Simple cron daemon
[-] tinycron-0.4_12 A very small replacement for cron

Simply:

# xbps-install scron
# ln -sv /etc/sv/crond /var/service/

Backups

We’re going to use restic for our backups. It’s simple straightforward and allows easy backup to s3 compatible data stores. I’m using Minio for my backup server. Something I can cover at a later point if there is interest.

Backup script, I’ve placed mine in /root/bin/backup.sh

#!/bin/sh

export AWS_ACCESS_KEY_ID=$YOUR_AWS_ACCESS_KEY_ID
export AWS_SECRET_ACCESS_KEY=$YOUR_AWS_ACCESS_KEY_ID
# You can change the following line to your AWS endpoint or other S3 compatible
# server
export RESTIC_REPOSITORY="s3:https://minio.example.com/servers"
# The file contains a single line with the repository password.
export RESTIC_PASSWORD_FILE="/root/.restic-passwd"

/usr/bin/restic backup -q --exclude-file /.exclude -x / /var /home

Don’t forget to make it executable with chmod +x /root/bin/backup.sh

Exclude files easily recovered via other means

Excluding modules and firmware mean that you will have to force re-install kernel and firmware when you restore from a backup. This isn’t the end of the world though especially since you’re going to have to re-install the boot loader anyway.

The cache directories shouldn’t cause any harm, other than perhaps some re-downloading if you re-install the kernel, etc.

/root/.cache/*
/usr/lib/modules/*
/usr/lib/firmware/*
/var/cache/*
/var/tmp/*
/home/*/.cache/*

Running on a schedule

Simply adjust /etc/crontab

30 0 * * 0 /root/bin/backup.sh > /var/log/backup.log 2>&1

Note on this setup

If you’re running multiple file systems ( as suggested in Part I) then you will need to specify all of them above as the -x flag prevents restic from crossing file system boundaries.

Note on databases

If you’re running a database such as MySQL/MariaDB or PostgreSQL you’ll need to use an LVM snapshot or similar to complete a proper backup. Many databases also have options that need to be set to ensure consistency. Most enable these by default so they’re crash-tolerant. For Postgres this is usually the WAL and for MySQL/MariaDB using innodb is usually sufficient. Still test your backups.

An example snippet to add your backups to handle MySQL:

#!/bin/sh
set -e

volume_group="vg"
fs="mysql"
mountpoint="/mnt/${fs}-snap"

# How much of the base dataset can change before the snapshot goes away?
overwrite_percentage="50"

mysql <<EOF
flush tables with read lock;
EOF
lvm lvcreate "vg/$fs" --name "${fs}-snap" -s -l "${overwrite_percentage}%ORIGIN"
mysql <<EOF
unlock tables;
EOF

if ! [ -d "$mountpoint" ] ; then mkdir "$mountpoint" fi
mount -o nouuid,ro "/dev/${volume_group}/${fs}-snap" "$mountpoint"

# Then run the backups as normal, but specify the snapshot filesystem instead
# of the current writable version

/usr/bin/restic backup -x / /var /home /mnt/mysql-snap

umount -lf "$mountpoint" # Just in-case something is still accessing it
rmdir "$mountpoint"
lvm lvremove "$vg/${fs}-snap" -y

Testing database backups is left as an exercise to the reader.

End of Part II, and the future

Not a particularly exciting set of tasks but necessary for any system that you care about. In Part III we’ll setup Nginx and PHP.