harden_linux

ansible-role-harden-linux

This Ansible role was mainly created for my blog series Kubernetes the not so hard way with Ansible - Harden the instances. But it can be used also standalone of course to harden Linux. It has the following features (some of them are optional):

Add a regular/deploy user used for administration (e.g. for Ansible or login via SSH)
Adjust APT update intervals
Setup UFW firewall and allow only SSH access by default (add more rules/allowed networks if you like)
Adjust security related sysctl settings
Adjust sshd settings e.g disable sshd password authentication, disable sshd root login and disable sshd PermitTunnel
Install sshguard and adjust whitelist
Change root password
Install/configure Network Time Synchronization (NTP) e.g. openntpd/ntp/systemd-timesyncd
Change systemd-resolved configuration

Versions

I tag every release and try to stay with semantic versioning. If you want to use the role I recommend to checkout the latest tag. The master branch is basically development while the tags mark stable releases. But in general I try to keep master in good shape too.

Changelog

Change history:

See full CHANGELOG.md

Recent changes:

v8.2.0

FEATURE
- add support for Ubuntu 24.04

v8.1.0

OTHER
- update comments about using mkpasswd instead of ansible to create encrypted password
- Ubuntu: add autoremove task
- update Github workflow
MOLECULE
- use alvistack instead of generic Vagrant boxes
- use different IP addresses

v8.0.0

BREAKING/FEATURE
- introduce harden_linux_deploy_group and harden_linux_deploy_group_gid variables. Both are optional. But at least harden_linux_deploy_group must be specified if harden_linux_deploy_user is also set. If harden_linux_deploy_group is set to root nothing will be changed.
- if harden_linux_deploy_user is set to root nothing will be changed.
- harden_linux_deploy_user is now optional. If not set, no user will be setup. Also all variables that start with harden_linux_deploy_user_ are only used if harden_linux_deploy_user is specified. Additionally harden_linux_deploy_user_home variable was added. harden_linux_deploy_user_shell, harden_linux_deploy_user_home, harden_linux_deploy_user_uid and harden_linux_deploy_user_password are now optional. $HOME directory of harden_linux_deploy_user is only created if harden_linux_deploy_user_home is set.
MOLECULE
- update test scenario to reflect deploy user/group changes

Installation

Directly download from Github (Change into Ansible role directory before cloning. You can figure out the role path by using ansible-config dump | grep DEFAULT_ROLES_PATH command): git clone https://github.com/githubixx/ansible-role-harden-linux.git githubixx.harden_linux
Via ansible-galaxy command and download directly from Ansible Galaxy: ansible-galaxy install role githubixx.harden_linux
Create a requirements.yml file with the following content (this will download the role from Github) and install with ansible-galaxy role install -r requirements.yml (change version if needed):

---
roles:
  - name: githubixx.harden_linux
    src: https://github.com/githubixx/ansible-role-harden-linux.git
    version: v8.1.0

Role Variables

The following variables don't have defaults. You need to specify them either in a file in group_vars or host_vars directory. E.g. if this settings should be used only for one specific host create a file for that host called like the FQDN of that host (e.g host_vars/your-server.example.tld) and put the variables with the correct values there. If you want to apply this variables to a host group create a file group_vars/your-group.yml e.g. Replace your-group with the host group name which you created in the Ansible hosts file (do not confuse with /etc/hosts...).

If you want to set or change the password of the root user set harden_linux_root_password variable. This is optional. It expects an encrypted password. Ansible won't encrypt the password for you. How to create an encrypted password is described in the Ansible FAQs. On Linux the following command is most probably the most reliable one:

mkpasswd --method=sha-512

To install a user that can execute commands with sudo without password set the following variables:

harden_linux_deploy_user: "a_username"
harden_linux_deploy_user_password: "a_password"
harden_linux_deploy_user_home: "/home/a_user"
harden_linux_deploy_user_uid: "9999"
harden_linux_deploy_user_gid: "9999"
harden_linux_deploy_user_shell: "/bin/bash"

harden_linux_deploy_user specifies the user we want to use to login at the remote host. As already mentioned the harden_linux role will disable root user login via SSH for a good reason. So a different user is needed. This user will get "sudo" permission which is need for Ansible (and/or yourself of course) to do it's work.

In harden_linux_deploy_user_password the user's encrypted password stored. Same applies as for harden_linux_root_password regarding how to create an encrypted password.

The user's $HOME directory is specified in harden_linux_root_password. For the UID and GID set harden_linux_deploy_user_uid and harden_linux_deploy_user_gid. Note: If the user already exists but has a different home directory, UID and/or GID it will be changed according to the settings above! This also applies to harden_linux_deploy_user_shell which specifies the shell the user should use after login e.g.

harden_linux_deploy_user_public_keys specifies a list of public SSH key files you want to add to $HOME/.ssh/authorized_keys of the deploy user on the remote host. If you specify /home/deploy/.ssh/id_rsa.pub e.g. as a value here the content of that local file (present on the Ansible controller node) will be added to $HOME/.ssh/authorized_keys of the deploy user on the remote host.

harden_linux_optional_packages (before version v6.0.0 of this role this variable was called harden_linux_required_packages) specifies additional/optional packages to install on the remote host. By default this variable is not specified. E.g.:

harden_linux_optional_packages:
  - vim

In contrast to the former variable, harden_linux_absent_packages will uninstall OS packages on the remote host. By default this variable is not specified. E.g.:

harden_linux_absent_packages:
  - vim

The following variables below have defaults. So you only need to change them if you need another value for the variable. The role changes some sshd settings by default:

harden_linux_sshd_settings:
  "^PasswordAuthentication": "PasswordAuthentication no"  # Disable password authentication
  "^PermitRootLogin": "PermitRootLogin no"                # Disable SSH root login
  "^PermitTunnel": "PermitTunnel no"                      # Disable tun(4) device forwarding
  "^Port ": "Port 22"                                     # Set sshd port

Personally I always change the default SSH port as lot of brute force attacks taking place against this port (but of course a port scanner will still be able to figure this out quickly). So if you want to change the port setting you can do so e.g.:

harden_linux_sshd_settings_user:
  "^Port ": "Port 22222"

(Please notice the whitespace after ^Port!). The playbook will combine harden_linux_sshd_settings and harden_linux_sshd_settings_user while the settings in harden_linux_sshd_settings_user have preference which means it will override the ^Port setting/key in harden_linux_sshd_settings. As you may have noticed all the key's in harden_linux_sshd_settings and harden_linux_sshd_settings_user begin with ^. That's because it is a regular expression (regex). One of playbook task's will search for a line in /etc/ssh/sshd_config e.g. ^Port (while the ^ means "a line starting with ...") and replaces the line (if found) with e.g Port 22222. This way makes the playbook very flexible for adjusting settings in sshd_config (you can basically replace every setting). You'll see this pattern for other tasks too. So everything mentioned here holds true in such cases.

Next some default settings for firewall/iptables. Firewall/iptables rules and settings are managed by UFW:

harden_linux_ufw_defaults:
  "^IPV6": 'IPV6=yes'
  "^DEFAULT_INPUT_POLICY": 'DEFAULT_INPUT_POLICY="DROP"'
  "^DEFAULT_OUTPUT_POLICY": 'DEFAULT_OUTPUT_POLICY="ACCEPT"'
  "^DEFAULT_FORWARD_POLICY": 'DEFAULT_FORWARD_POLICY="DROP"'
  "^DEFAULT_APPLICATION_POLICY": 'DEFAULT_APPLICATION_POLICY="SKIP"'
  "^MANAGE_BUILTINS": 'MANAGE_BUILTINS=no'
  "^IPT_SYSCTL": 'IPT_SYSCTL=/etc/ufw/sysctl.conf'
  "^IPT_MODULES": 'IPT_MODULES="nf_conntrack_ftp nf_nat_ftp nf_conntrack_netbios_ns"'

This settings are basically changing the values in /etc/defaults/ufw. To override one or more of the default settings you can so by specifying the same key (which is a regex) as above e.g. ^DEFAULT_FORWARD_POLICY and simply assign it the new value:

harden_linux_ufw_defaults_user:
  "^DEFAULT_FORWARD_POLICY": 'DEFAULT_FORWARD_POLICY="ACCEPT"'

As already mentioned above this playbook will also combine harden_linux_ufw_defaults and harden_linux_ufw_defaults_user while the settings in harden_linux_ufw_defaults_user have preference.

Next we can specify some firewall rules with harden_linux_ufw_rules. The default is to allow SSH traffic on port 22 which uses protocol tcp:

harden_linux_ufw_rules:
  - rule: "allow"
    to_port: "22"
    protocol: "tcp"

The following parameters are available with defaults (if any):

rule (no default)
interface (default '')
direction (default 'in')
from_ip (default 'any')
to_ip (default 'any')
from_port (default '')
to_port (default '')
protocol (default 'any')
log (default 'false')
delete (default 'false')

A rule can have the values allow, deny, limit and reject. interface specify interface for the rule. The direction (in or out) used for the interface depends on the value of direction. from_ip specifies the source IP address and from_port the source port. to_ip specifies the destination IP address and to_port the destination port. protocol is any by default. Possible values are tcp, udp, ipv6, esp, ah, gre and igmp. log can either be false (default) or true and specifies if new connections that matched this rule should be logged. delete specifies if a rule should be deleted. This is important if a previously added rule should be removed. Just removing a rule from harden_linux_ufw_rules isn't enough! You must use delete to delete that rule.

You can also allow hosts to communicate on specific networks (without port restrictions) e.g.:

harden_linux_ufw_allow_networks:
  - "10.3.0.0/24"
  - "10.200.0.0/16"

Next harden_linux role also changes some system variables (sysctl.conf / proc filesystem). This settings are recommendations from Google which they use for their Google Compute Cloud OS images (see Google Cloud - Requirements to build custom images and Configure your imported image for Compute Engine). These are the default settings (if you are happy with this settings you don't have to do anything but I recommend to verify if they work for your setup):

harden_linux_sysctl_settings:
  "net.ipv4.tcp_syncookies": 1                    # Enable syn flood protection
  "net.ipv4.conf.all.accept_source_route": 0      # Ignore source-routed packets
  "net.ipv6.conf.all.accept_source_route": 0      # IPv6 - Ignore ICMP redirects
  "net.ipv4.conf.default.accept_source_route": 0  # Ignore source-routed packets
  "net.ipv6.conf.default.accept_source_route": 0  # IPv6 - Ignore source-routed packets
  "net.ipv4.conf.all.accept_redirects": 0         # Ignore ICMP redirects
  "net.ipv6.conf.all.accept_redirects": 0         # IPv6 - Ignore ICMP redirects
  "net.ipv4.conf.default.accept_redirects": 0     # Ignore ICMP redirects
  "net.ipv6.conf.default.accept_redirects": 0     # IPv6 - Ignore ICMP redirects
  "net.ipv4.conf.all.secure_redirects": 1         # Ignore ICMP redirects from non-GW hosts
  "net.ipv4.conf.default.secure_redirects": 1     # Ignore ICMP redirects from non-GW hosts
  "net.ipv4.ip_forward": 0                        # Do not allow traffic between networks or act as a router
  "net.ipv6.conf.all.forwarding": 0               # IPv6 - Do not allow traffic between networks or act as a router
  "net.ipv4.conf.all.send_redirects": 0           # Don't allow traffic between networks or act as a router
  "net.ipv4.conf.default.send_redirects": 0       # Don't allow traffic between networks or act as a router
  "net.ipv4.conf.all.rp_filter": 1                # Reverse path filtering - IP spoofing protection
  "net.ipv4.conf.default.rp_filter": 1            # Reverse path filtering - IP spoofing protection
  "net.ipv4.icmp_echo_ignore_broadcasts": 1       # Ignore ICMP broadcasts to avoid participating in Smurf attacks
  "net.ipv4.icmp_ignore_bogus_error_responses": 1 # Ignore bad ICMP errors
  "net.ipv4.icmp_echo_ignore_all": 0              # Ignore bad ICMP errors
  "net.ipv4.conf.all.log_martians": 1             # Log spoofed, source-routed, and redirect packets
  "net.ipv4.conf.default.log_martians": 1         # Log spoofed, source-routed, and redirect packets
  "net.ipv4.tcp_rfc1337": 1                       # Implement RFC 1337 fix
  "kernel.randomize_va_space": 2                  # Randomize addresses of mmap base, heap, stack and VDSO page
  "fs.protected_hardlinks": 1                     # Provide protection from ToCToU races
  "fs.protected_symlinks": 1                      # Provide protection from ToCToU races
  "kernel.kptr_restrict": 1                       # Make locating kernel addresses more difficult
  "kernel.perf_event_paranoid": 2                 # Set perf only available to root

You can override every single setting e.g. by creating a variable called harden_linux_sysctl_settings_user:

harden_linux_sysctl_settings_user:
  "net.ipv4.ip_forward": 1
  "net.ipv6.conf.default.forwarding": 1
  "net.ipv6.conf.all.forwarding": 1

One of the playbook tasks will combine harden_linux_sysctl_settings and harden_linux_sysctl_settings_user while again harden_linux_sysctl_settings_user settings have preference. Have a look at defaults/main.yml file of the role for more information about the settings.

If you want UFW logging enabled set:

harden_linux_ufw_logging: 'on'

Possible values are on,off,low,medium,high and full.

Next we've the "sshguard" settings. "sshguard" protects from brute force attacks against SSH. To avoid locking out yourself for a while you can add IPs or IP ranges to a whitelist. By default it's basically only "localhost":

harden_linux_sshguard_whitelist:
  - "127.0.0.0/8"
  - "::1/128"

Also NTP packages can be installed/configured. This is optional. By default I'd recommend to use systemd-timesyncd. You can also use ntp package. But openntpd and systemd-timesyncd have the advantage that they don't listen on any ports by default. If you just want to keep the hosts clock in sync this is absolutely sufficient. Having the same time on all your hosts is critical for some services. E.g. for certificate validation, for etcd, databases, cryptography, and so on.

Valid options for harden_linux_ntp are:

openntpd
ntp
systemd-timesyncd

openntpd and systemd-timesyncd have the advantage that they don't listen on any ports by default as already mentioned. If you just want to keep the hosts clock in sync one of those two should do the job. systemd-timesyncd is already installed if a distribution uses systemd (which is basically true for most Linux OSes nowadays). So no additional packages are needed in this case. To enable openntpd set harden_linux_ntp accordingly e.g.:

harden_linux_ntp: "openntpd"

Settings for openntpd, ntpd or systemd-timesyncd (see next paragraph). For further options see man page: man 5 ntpd.conf for ntp and openntpd and man 5 timesyncd.conf for systemd-timesyncd.

The "key" here is a regular expression of a setting you want to replace and the value is the setting name + the setting value. E.g. we want to replace the line servers 0.debian.pool.ntp.org with servers 1.debian.pool.ntp.org. The regex (the key) would be ^servers 0 which means:

"search for a line in the configuration file that begins with server 0 and replace the whole line with servers 1.debian.pool.ntp.org". This way every setting in the configuration file can be replaced by something else. Some examples:

harden_linux_ntp_settings:
  "^servers 0": "servers 0.debian.pool.ntp.org"
  "^servers 1": "servers 1.debian.pool.ntp.org"
  "^servers 2": "servers 2.debian.pool.ntp.org"
  "^servers 3": "servers 3.debian.pool.ntp.org"

Please note: systemd-timesyncd comes with reasonable compile time defaults. So there is normally no need to change the configuration (not even the NTP servers). So the following are just examples but you don't really need to specify harden_linux_ntp_settings for systemd-timesyncd.

For systemd-timesyncd the configuration file is a little bit different. In this case a systemd drop-in for /etc/systemd/timesyncd.conf will be created at /etc/systemd/timesyncd.conf.d/.

Example for Debian:

harden_linux_ntp_settings:
  "^#NTP=": "NTP=0.debian.pool.ntp.org 1.debian.pool.ntp.org 2.debian.pool.ntp.org 3.debian.pool.ntp.org"

For Ubuntu:

harden_linux_ntp_settings:
  "^#NTP=": "NTP=ntp.ubuntu.com"

For Archlinux:

harden_linux_ntp_settings:
  "^#NTP=": "NTP=0.arch.pool.ntp.org 1.arch.pool.ntp.org 2.arch.pool.ntp.org 3.arch.pool.ntp.org"

With harden_linux_files_to_delete a list of files can be specified that should be absent on the target host e.g.:

harden_linux_files_to_delete:
  - "/root/.pw"

If systemd-resolved is used for DNS resolution its behavior can be adjusted with harden_linux_systemd_resolved_settings. By default this variable is not specified. A systemd drop-in configuration will be created in /etc/systemd/resolved.conf.d/99-override.conf and the settings specified added there.

Note: If a setting in /etc/systemd/resolved.conf is already set (e.g. DNS=8.8.8.8) then setting DNS=9.9.9.9 below will add up. That means the final setting will be DNS=8.8.8.8 9.9.9.9. If you don't what that you need to "unset" the value first and then add the value you want to have. E.g.:

harden_linux_systemd_resolved_settings:
  - DNS=
  - DNS=9.9.9.9

While the Google DNS server (8.8.8.8, 8.8.4.4) offer speedy DNS lookups it's of course another possibility Google can spy on you. So using some other DNS servers should be at least something to think about. But there is one more thing and that's encrypting DNS requests. One way that systemd-resolved supports is DNSOverTLS. Quad9 (9.9.9.9/149.112.112.112) and Cloudflare (1.1.1.1/1.0.0.1) support DNSOverTLS.
So the following systemd-resolved settings configure Quad9 and Cloudflare DNS for IPv4 and IPv6. The setting DNSOverTLS=opportunistic uses DNSOverTLS if the DNS server supports it and falls back to regular unencrypted DNS if not supported (also see resolved.conf.5):

harden_linux_systemd_resolved_settings:
  - DNS=
  - DNS=9.9.9.9 1.1.1.1 2606:4700:4700::1111 2620:fe::fe
  - FallbackDNS=
  - FallbackDNS=149.112.112.112 1.0.0.1 2620:fe::9 2606:4700:4700::1001
  - DNSOverTLS=
  - DNSOverTLS=opportunistic

Also the package manager caching behavior can be influenced. E.g. for Ubuntu:

# Set to "false" if package cache should not be updated
harden_linux_ubuntu_update_cache: true

# Set package cache valid time
harden_linux_ubuntu_cache_valid_time: 3600

For Archlinux:

# Set to "false" if package cache should not be updated
harden_linux_archlinux_update_cache: true

Example Playbook

If you installed the role via ansible-galaxy install githubixx.harden_linux then include the role into your playbook like in this example:

- hosts: webservers
  roles:
    - githubixx.harden_linux

Testing

This role has a small test setup that is created using Molecule, libvirt (vagrant-libvirt) and QEMU/KVM. Please see my blog post Testing Ansible roles with Molecule, libvirt (vagrant-libvirt) and QEMU/KVM how to setup. The test configuration is here.

Afterwards molecule can be executed:

molecule converge

This will setup a few virtual machines (VM) with different supported Linux operating systems and setup harden_linux role accordingly. A small verification step is also included: