Server Hardening

Introduction

Security of a system is often overlooked or not considered important enough. Some might think not about the possible consequences, which might result in a hacked or compromised server. Maybe sensitive data gets stolen or encrypted with major consequences.

A collection of tools, techniques and best pratices can help to reduce such vulnerabilities. This process is called system hardening.

We have seen quote a lot of hacks and data breaches over the past years across different big companies, including but not limited to: PayPal, LastPast, T-Mobile, MailChimp, Netflix, Dropbox, OpenSea, Twitter…

As stated earlier, the consequences of a poorly secured system can be disastrous. Your system can be compromised, so how secure is your Linux system? It’s wise to audit the security of your own Linux system.

I split this article in three sections, first hardening tools, then some best pratices on the host OS level and next we focus on several Docker security improvements. Let’s first start with some toolings:

Hardening Tools

There exists many different hardening tools, which will help you identify best practices or other possible vulnerabilities that requires your attention.

Those hardening tools will check your system on most common pitfalls or misconfigurations, which should help you to what needs to be changed.

We will focus on the two tools listed below. And since both tools will give a lot of improvement proposals, we can’t cover all those suggestions the tool reports back in this article.
However, this article will definitely give you a head start.

Docker Bench for Security

Clone and run the Docker Bench Security script:

git clone https://github.com/docker/docker-bench-security.git
cd docker-bench-security
sudo ./docker-bench-security.sh

Lynis

Install Lynis using

cd /opt
sudo wget https://downloads.cisofy.com/lynis/lynis-3.0.8.tar.gz
sudo tar -xf lynis-3.0.8.tar.gz
cd lynis

Or download Lynx from here.

To perform a local security system scan, execute now:

sudo ./lynis audit system

Host Machine

We will first cover the best practices about hardening the Linux host machine.

Enforce secure passwords

We want to enforce secure passwords to users, so it’s harder to brute force passwords. For that, we first need to install: sudo apt install libpam-cracklib

Under Debian based systems we edit the file: /etc/pam.d/common-password.

Finally, we add the following line to the top of the file (at least above the existing line: password [success=1 default=ignore]pam_unix.so):

password requisite pam_cracklib.so retry=3 minlen=15 difok=3 ucredit=-1 lcredit=-1 dcredit=-1 ocredit=-1

Disable su

The sudo group under Linux group giving users access to execute commands as the root user. I don’t like the fact that users who are part of the sudo group can run the su or sudo -l commands to actually become the root user.

Let disable that by editing /etc/pam.d/su file:

# DISALLOW su command, my commenting this line below:
#auth       sufficient pam_rootok.so

Within the same configuration file, we can also do some fancy PAM stuff. For example let’s say the user melroy still want to be able to switch to another user called demo:

# Allow melroy user to switch to another user (demo)
auth  [success=ignore default=1] pam_succeed_if.so user = demo
auth  sufficient                 pam_succeed_if.so use_uid user = melroy

Sudoers file

I just would like to share an alternative approach for the sudo / su groups as well.

Let’s say you want to give a user “apt” execution rights but nothing else that requires the sudo command.

You can edit the /etc/sudoers file and add the following line for a user “demo”:

demo ALL=(root) /usr/bin/apt update, /usr/bin/apt install *, /usr/bin/apt upgrade, /usr/bin/apt-get update, /usr/bin/apt-get install *, /usr/bin/apt-get upgrade

Then you do NOT need to add the demo user to the sudo group. While this demo user is allowed to execute apt commands and nothing else.

Create separate user

If you didn’t already, let’s create a separate user on our server, which will be the replacement of the root user.

Creating a new user, via: useradd -m <user> -s /bin/bash
Set password for user: passwd <user>
Add user to sudo group: usermod -aG sudo <user>

And maybe you want to add the docker group to the user? If so, execute: usermod -aG docker <user>

Disable root user

Disabling the root user prevents unauthorized access with unlimited privileges.

Disabling the root shell login: sudo chsh -s /usr/sbin/nologin root

Next, we disable root user on the machine (remove password and lock the user):
sudo passwd --delete --lock root

SSH Daemon

The default OpenSSH daemon, also known as sshd, configuration is rather unsafe. Let’s not use password authentication for example. Instead we will use our local SSH key pair for authorization towards the SSH serer.

First, we generate new SSH key on your local machine, if you didn’t have this already: ssh-keygen

Next, we will copy the SSH public key to your server using the following command: ssh-copy-id <user>@<server-ip>

We are now ready to change the server SSH daemon config. We will edit the config file /etc/ssh/sshd_config:

LoginGraceTime 2m
PermitRootLogin no
MaxAuthTries 2
PubkeyAuthentication yes
PasswordAuthentication no
DisableForwarding yes
X11Forwarding no
DebianBanner no
ClientAliveCountMax 2

This snippet will not only disable password authentication and enable public key authentication.

But also disable root login (very important!). And we will disable TCP and X11 forwarding as well as lower the amount of retries and alive connections. Last but not least disabling any banner message (for Debian based systems).

Now let’s restart the daemon: sudo systemctl restart sshd

Allow / Deny

We can leverage the allow and deny hosts files under Linux to even block all access to the sshd service and only allow incoming connections from your local IP network. The example below is only about sshd, but other services can be restricted as well in similar fashion.

To allow SSH on the local network only, edit the /etc/hosts.allow and add the following:

sshd: 192.168.1.\*

And then edit: /etc/hosts.deny:

sshd : ALL

Enable syslog messages via TCP - RSyslogd

Rsyslogd is a powerful logging daemon, most likely present and enabled by default on your installation.

Let’s enable TCP port (port 514) in rsyslogd. So rsyslog allows incoming requests on localhost:514, so other applications can log towards RSyslogd. Which can be used by Docker daemon, see later.

Uncommit the following two lines in /etc/rsyslog.conf:

module(load="imtcp")
input(type="imtcp" port="514")

Then restart the service: sudo systemctl restart rsyslog.service

Linux Audit Daemon - Auditd

We can use auditd if let’s say the system was compromised, then we can can track back and see how its system was compromised.

Install auditd: sudo apt install auditd

The main config file is located in: /etc/audit/rules.d/audit.rules. Here an example of my audit.rules file:

# Delete all previous rules
-D

# Increase the buffers to survive stress events.
# Make this bigger for busy systems
-b 48192

# This determine how long to wait in burst of events
--backlog_wait_time 60000

# Set failure mode to print a failure message
-f 1

# Set rate limit (messages per second)
-r 100

# Add Docker to the audit
-w /usr/bin/dockerd -p rwxa -k docker
-w /run/containerd -p rwxa -k docker
-w /var/lib/docker -p rwxa -k docker
-w /etc/docker -p rwxa -k docker
-w /lib/systemd/system/docker.service -p rwxa -k docker
-w /lib/systemd/system/docker.socket -p rwxa -k docker
-w /etc/default/docker -p rwxa -k docker
-w /etc/docker/daemon.json -p rwxa -k docker
-w /etc/containerd/config.toml -p rwxa -k docker
-w /usr/bin/containerd -p wa -k docker
-w /usr/bin/containerd-shim -p wa -k docker
-w /usr/bin/containerd-shim-runc-v1 -p wa -k docker
-w /usr/bin/containerd-shim-runc-v2 -p wa -k docker
-w /usr/bin/runc -p wa -k docker

Note: The audit roles above are on purpose only focusing on Docker. Be free to extend your audit.rules file with more rules.

Sudoers.d folder

Permissions for directory /etc/sudoers.d where not strict enough according to Lynis, execute:

sudo chmod 750 /etc/sudoers.d

Kernel hardening

I extended my /etc/sysctl.d/10-kernel-hardening.conf config with the following additional kernel configs:

dev.tty.ldisc_autoload = 0
fs.protected_fifos = 2
fs.suid_dumpable = 0
kernel.kptr_restrict = 2
kernel.modules_disabled = 1
kernel.perf_event_paranoid = 3
kernel.sysrq = 0
kernel.unprivileged_bpf_disabled = 1
net.core.bpf_jit_harden = 2
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.log_martians = 1
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.default.log_martians = 1
net.ipv6.conf.all.accept_redirects = 0
net.ipv6.conf.default.accept_redirects = 0

Important note: net.ipv4.conf.all.forwarding needs to be set to: 1, if you disable IP forwarding then Docker containers would be unable to route the packages. Thus you will not be able to access the Internet from Docker. Unless that is really what you want.

And I updated the following two lines in /etc/sysctl.d/10-network-security.conf:

net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.all.rp_filter = 1

Reload the sysctl configs without rebooting: sudo sysctl --system

See more kernel tuning snippet.

I love to go into details about each kernel option, but then this article would be way too long.

Docker

Now let’s move to our Docker setup. Docker Engine is used for running isolated containers, but unlike VMs Docker can reuse the OS kernel resources. The following sections are based on the audit results performed by the Docker Bench for Security script.

Introducing Docker daemon config

We will create or edit the default Docker daemon configuration file /etc/docker/daemon.json, with the following content:

{
  "experimental": false,
  "icc": false,
  "userns-remap": "default",
  "log-driver": "syslog",
  "log-opts": {
    "syslog-address": "tcp://127.0.0.1:514"
  },
  "storage-driver": "overlay2",
  "default-ulimits": {
    "nofile": {
      "Hard": 64000,
      "Name": "nofile",
      "Soft": 64000
    }
  },
  "userland-proxy": false,
  "no-new-privileges": true
}

Note: We will use the rsyslogd configuration above for logging.

Let’s explain that is happening. I disable experimental features, so I set experimental option to false. The icc option to false will disable inter-container communication (on the default bridge network). userns-remap option will use Linux namespaces to map to separate user (the dockremap user and group is created and used for this purpose).

We set the default storage-driver option to overlay2 (which should be the default).

We increase some default ulimits for Docker, thus we increase the maximum number of open files/file descriptors. You might want to increase the nofile soft and hard limits in /etc/security/limits.conf configuration file as well:

root             soft    nofile          500000
root             hard    nofile          500000

We want to keep live-restore to false, since this might trigger a bug to appear 😢. Since I noticed that host.docker.internal host mapping doesn’t seems to work when I enabled live restore, see also this GitHub issue.

We don’t want to use the default userland proxy, we want to use hairpin NAT for for forwarding when Docker ports are exposed. For that we set userland-proxy option to false once again.

Next, we want to prevent a Docker container processes from gaining additional privileges, so we set no-new-privileges option to true.

Read more about: the Docker Daemon config.

Did you know?

Docker containers are running with several Linux kernel capabilities enabled by default. You can disable the Linux capabilities using the option during container start-up: --cap-drop=all.

Once you disabled all capabilities, you might want to enable the Linux capabilities using: --cap-add flag, for the Linux capabilities you really need on that container. Here is an example:

docker run -d --cap-drop=all --cap-add=setuid --cap-add=setgid

Docker containers not running as root-user

Next, we want to prevent running a container as a root user. We can leverage the Dockerfile you will be able to create a new user and then switch to this new user by using the USER option. If you want to have write access to the files, you can copy the files via --chown= flag as well. Here is an example:

RUN useradd -ms /bin/bash worker

WORKDIR /home/worker

COPY --chown=worker:worker requirements.txt ./
RUN pip3 install --user --no-cache-dir -r requirements.txt

COPY --chown=worker:worker . .

USER worker

Do don’t need to copy the files with --chown, if the user does not need to have write access to the files, as long as you place USER <username> in this case at the bottom of the Dockerfile.

Did you know?

The official NodeJS Docker image has already a user called “node”, which you can use and set after you copied the files to the Docker image:

USER node

In the case you don’t want to change or adapt a Dockerfile, you can also specify a user or user id (UID) in a Docker compose file. Using the YAML format: user: <username | user-id>, like below:

services:
  db:
    restart: always
    image: postgres:15-alpine
    user: postgres

Bind network port only to the localhost interface

By default Docker port mapping will be mapped to all interfaces (0.0.0.0). However, it’s a good practice to add Nginx in front of the service, which allows you to add a SSL/TLS certificate as well as load balancing.

In the case you have a reverse proxy, like Nginx, in front of your Docker containers, you only want to map the Docker ports to localhost (127.0.0.1). This can easily done by changing your Docker compose file (or docker run command line). An example of Docker compose, change from:

ports:
  - "8080:8080"

To only listen on localhost / 127.0.0.1:

ports:
  - "127.0.0.1:8080:8080"

Then within Nginx config you will proxy the incoming web requests from a specific (sub) domain name to 127.0.0.1:8080. Example snippet for Nginx:

location / {
   proxy_pass http://127.0.0.1:8080;
}

Of course these are just examples, your actually Nginx config might differ.

Set Docker container Limits

Using Docker Compose v3.8 . Example compose.yaml file:

version: "3.8"
services:
  sevice-name:
    image: ...
    deploy:
      resources:
        limits:
          cpus: "1.5"
          memory: 400M

I use the docker stats command to see what the current memory usage is, and estimate what the maximum might be for each container.

If you do NOT want to use Docker swarm that comes with Compose v3, use the --compatibility flag to start the container: docker compose --compatibility up -d.

Docker Restart Policy

You actually do not want to use the obsolete restart statement anymore. So let’s move to restart_policy with a max attempts of 5 with the on-failure condition.

version: "3.8"
services:
  sevice-name:
    image: ...
    deploy:
      restart_policy:
        condition: on-failure
        delay: 5s
        max_attempts: 5
        window: 120s

You are now free to combine the deploy.resources, from the previous paragraph, together with this deploy.restart_policy section in YAML.

Docker write only file system

Docker Bench for Security will also warn you when your Docker filesystem is writable, which is a bad practice. It’s not always easy to solve, since some applications might need to write to disk.

In Docker compose you can create a read-only filesystem using the read_only: true option:

services:
  sevice-name:
    image: ...
    read_only: true
    volumes:
      - /data:/app/data_folder
    tmpfs:
      - /run:mode=1777,uid=1000,gid=1000

Since everything is now read-only, you might to create volume mounts (or binds) to your host which will be writable. And/or create a tmpfs.

Conclusion

I hope you have come to understand the importance of good security.
I also hope that I managed to get you excited to also delve into your own (server) security.

Don’t forget to run the security benchmarking tools during hardening process to discover remaining vulnerabilities and validate that your hardening actions are indeed improving the benchmark scores. Also execute a regular audit to ensure no new vulnerabilities are introduced.

While we’ve covered quite a bit in this article, it shouldn’t be the end. But rather the start of further hardening your server. Other topics might include: Automatically install security patches and keep your software up-to-date, enable a firewall and limit the incoming and outgoing network traffic, securing DNS traffic, creating alerting on Auditd or RSyslog, securing other services like Postfix, Nginx or rpcbind to name a few topics.

• Hardening
• Security
• Benchmark
• Docker Compose
• RSyslog
• Linux
• GNU
• Bash
• Terminal
• Docker