Cybersecurity on TECHNERDONLINE

Bastion Host On DigitalOcean

Sun, 23 May 2021 21:17:12 +0600

What is a Bastion Host?

A bastion host is a special-purpose computer on a network specifically designed and configured to withstand attacks. There are two common network configurations that include bastion hosts and their placement.

The first requires two firewalls, with bastion hosts sitting between the first “outside world” firewall, and an inside firewall, in a DMZ. Often, smaller networks do not have multiple firewalls, so if only one firewall exists in a network, bastion hosts are commonly placed outside the firewall.

Getting Started

You can use the referral badge below to get started with a $100 credit from Digital Ocean or use this link to DigitalOcean.

DigitalOcean VPC

On 7 April, 2020, the VPC service replaced the Private Networking service on DigitalOcean.

A Virtual Private Cloud (VPC) is a private network interface for collections of DigitalOcean resources. VPC networks provide a more secure connection between resources because the network is inaccessible from the public internet and other VPC networks. Traffic within a VPC network doesn’t count against bandwidth usage.

VPC are available at no additional cost and are enabled by default. They serve the same function as VLANs do. You have two options, you can either manual create a VPC network or if you don’t have a VPC network DigitalOcean will create it for you when you build a new VPS.

DigitalOcean Cloud Firewalls

Cloud Firewalls affect both public and VPC network traffic. Rules specific to either must specify the public or private IP range.

We will be creating two cloud firewall rules, one named public-network and the other named private-network. These will be used as Access Control Lists to help protect our VPC network.

Both the Public-Network and Private-Network cloud firewalls should be added to the bastion-host, while only the Private-Network cloud firewall should be added to all other members of your VPS.

PUBLIC-NETWORK

INBOUND RULES :

Type	Protocol	Port Range	Sources
ICMP	ICMP	None	All IPv4, All IPv6
SSH	TCP	22	All IPv4, All IPv6
HTTP	TCP	80	All IPv4, All IPv6
HTTPS	TCP	443	All IPv4, All IPv6
CUSTOM	UDP	51820	All IPv4, All IPv6

OUTBOUND RULES :

Type	Protocol	Port Range	Sources
ICMP	ICMP	None	All IPv4, All IPv6
All TCP	TCP	All Ports	All IPv4, All IPv6
All UDP	UDP	All Ports	All IPv4, All IPv6

PRIVATE-NETWORK

INBOUND RULES :

Type	Protocol	Port Range	Sources
ICMP	ICMP	None	10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
All TCP	TCP	All Ports	10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
All UDP	UDP	All Ports	10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16

OUTBOUND RULES :

Type	Protocol	Port Range	Sources
ICMP	ICMP	None	All IPv4
All TCP	TCP	All Ports	All IPv4
All UDP	UDP	All Ports	All IPv4

I also recommend creating new ssh keys to add to your bastion-host.

ssh-keygen -b 4096  -a 1000 -t rsa -f ~/.ssh/id_rsa

Let’s lock down your ssh service.

$ sudo mv /etc/ssh/sshd_config /etc/ssh/sshd_config.bak

# Harden SSH Settings
$ sudo cat <<-EOF > /etc/ssh/sshd_config
HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
AcceptEnv LANG LC_*
AllowGroups root sudo
Banner /etc/issue.net
ChallengeResponseAuthentication no
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr
ClientAliveCountMax 0
ClientAliveInterval 300
Compression no
HostbasedAuthentication no
IgnoreUserKnownHosts yes
KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256
LoginGraceTime 20
LogLevel VERBOSE
Macs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-256
MaxAuthTries 3
MaxSessions 3
MaxStartups 10:30:60
PermitEmptyPasswords no
PermitRootLogin no
PubkeyAuthentication yes
PasswordAuthentication no
PermitUserEnvironment no
PrintLastLog yes
PrintMotd no
StrictModes yes
Subsystem sftp internal-sftp
UseDNS no
UsePAM yes
X11Forwarding no
AllowTcpForwarding yes
EOF

Create a new set of ssh host keys.

## Update ssh_host keys
rm /etc/ssh/ssh_host_*
ssh-keygen -t ecdsa -b 521 -f /etc/ssh/ssh_host_ecdsa_key -N ""
ssh-keygen -t rsa -b 4096 -f /etc/ssh/ssh_host_rsa_key -N ""
ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N ""

awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduli

We need to edit the netplan for both the bastion-host and the webproxy so that the bastion-host handles all routing.

sudo nano /etc/netplan/50-cloud-init.yaml

You can read more details on editing your netplan on DigitalOcean.

network:
    version: 2
    ethernets:
        eth0:
            addresses:
            - xxx.xxx.xxx.xxx/20
            - 26xx:xxxx:x:xxx::xx:xxxx/64
            gateway4: xxx.xxx.xxx.1
            gateway6: 26xx:xxxx:x:xxx::1
            match:
                macaddress: ab:ab:ab:ab:ab:ab
            nameservers:
                addresses:
                - 1.1.1.1
                - 1.0.0.1
                - 2606:4700:4700::1111
                - 2606:4700:4700::1001
                search: [technerdonline.com]
        eth1:
            addresses:
            - 10.128.0.2/20
            match:
                macaddress: ba:ba:ba:ba:ba:ba
            nameservers:
                addresses:
                - 10.128.0.2
                search: [local]
            routes:
            -   to: 10.128.0.0/20
                via: 10.128.0.2

On the proxy server make the following netplan change.

network:
    version: 2
    ethernets:
        eth0:
            addresses:
            - xxx.xxx.xxx.xxx/20
            - 26xx:xxxx:x:xxx::xx:xxxx/64
            #gateway4: xxx.xxx.xxx.1
            #gateway6: 26xx:xxxx:x:xxx::1
            match:
                macaddress: ab:ab:ab:ab:ab:ab
            nameservers:
                addresses:
                - 1.1.1.1
                - 1.0.0.1
                - 2606:4700:4700::1111
                - 2606:4700:4700::1001
                search: [technerdonline.com]
        eth1:
            addresses:
            - 10.128.0.3/20
            match:
                macaddress: ba:ba:ba:ba:ba:ba
            nameservers:
                addresses:
                - 10.128.0.2
                search: [local]
            routes:
            -   to: 0.0.0.0/0
                via: 10.128.0.2

Apply the netplan changes.

sudo netplan apply

We will need to create a set of IPTABLES rules for both IPv4 and IPv6 but first we need to load some Kernel modules.

sudo nano /etc/modules-load.d/iptables.conf

overlay
br_netfilter
ip_vs
ip_vs_rr
ip_vs_wrr
ip_vs_sh
nf_conntrack
iptable_nat
iptable_filter
iptable_mangle
ip_nf_target_redirect
ip_set
ip_vs_nfct
ip_vs_proto_tcp
ip_vs_proto_udp
veth
bridge
bridge_netfilter
ip_nf_filter
ip_nf_target_masquerade
netfilter_xt_match_addrtype
netfilter_xt_match_conntrack
netfilter_xt_match_ipvs
nf_nat

Copy and paste the following content, replacing PUBLIC_IP with the public IP address of the bastion-host, WEBPROXY_PRIVATE_IP with the VPC IP address for the webproxy, and BASTION_PRIVATE_IP with the VPC IP address with the bastion host private IP.

nano ipv4.conf

*mangle
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
COMMIT

*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
-A PREROUTING -i eth0 -d {PUBLIC_IP} -p tcp -m tcp --dport 80 -j DNAT --to-destination {WEBPROXY_PRIVATE_IP}:80
-A PREROUTING -i eth0 -d {PUBLIC_IP} -p tcp -m tcp --dport 443 -j DNAT --to-destination {WEBPROXY_PRIVATE_IP}:443
-A POSTROUTING -d {WEBPROXY_PRIVATE_IP} -o eth1 -p tcp -m tcp --dport 80 -j SNAT --to-source {BASTION_PRIVATE_IP}
-A POSTROUTING -d {WEBPROXY_PRIVATE_IP} -o eth1 -p tcp -m tcp --dport 443 -j SNAT --to-source {BASTION_PRIVATE_IP}
-A POSTROUTING -s {BASTION_PRIVATE_IP} -o eth0 -j SNAT --to-source {PUBLIC_IP}
-A POSTROUTING -s {PRIVATE_SUBNET} ! -d {PRIVATE_SUBNET} -j MASQUERADE
COMMIT

*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]
:FILTERS - [0:0]
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -m conntrack --ctstate INVALID -j DROP
-A INPUT -p icmp -m icmp --icmp-type 8 -m limit --limit 5/sec -j ACCEPT
-A INPUT -i eth1 -m conntrack --ctstate NEW -s {PRIVATE_SUBNET} -j ACCEPT
-A INPUT -j FILTERS
-A INPUT -j DROP
-A FORWARD -o eth1 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i eth1 -o eth1 -m conntrack --ctstate NEW -s {PRIVATE_SUBNET} -j ACCEPT
-A FORWARD -o eth0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i eth1 -o eth0 -m conntrack --ctstate NEW -s {PRIVATE_SUBNET} -j ACCEPT
-A FORWARD -i eth0 -o eth1 -j FILTERS
-A OUTPUT -o lo -j ACCEPT
-A OUTPUT -o eth0 -j ACCEPT
-A OUTPUT -o eth1 -j ACCEPT
-A FILTERS -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FILTERS -p tcp -m conntrack --ctstate NEW -m tcp --syn --dport 22 -j ACCEPT
-A FILTERS -p tcp -m conntrack --ctstate NEW -m tcp --syn --dport 80 -j ACCEPT
-A FILTERS -p tcp -m conntrack --ctstate NEW -m tcp --syn --dport 443 -j ACCEPT
-A FILTERS -p udp -m conntrack --ctstate NEW -m udp --dport 51820 -j ACCEPT
-A FILTERS -p udp -m conntrack --ctstate NEW -m udp --dport 51821 -j ACCEPT
-A FILTERS -m conntrack --ctstate INVALID -j DROP
-A FILTERS -j REJECT
COMMIT

Now create IPTABLES Rules for IPv6.

nano ipv6.conf

*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
COMMIT

*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]
:FILTERS - [0:0]
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -m conntrack --ctstate INVALID -j DROP
-A INPUT -i eth0 -p ipv6-icmp -m icmp6 --icmpv6-type 128 -m limit --limit 5/sec -j ACCEPT
-A INPUT -i eth0 -p ipv6-icmp -m icmp6 --icmpv6-type 133 -m limit --limit 5/sec -j ACCEPT
-A INPUT -i eth0 -p ipv6-icmp -m icmp6 --icmpv6-type 134 -m limit --limit 5/sec -j ACCEPT
-A INPUT -i eth0 -p ipv6-icmp -m icmp6 --icmpv6-type 135 -m limit --limit 5/sec -j ACCEPT
-A INPUT -j FILTERS
-A INPUT -j DROP
-A FILTERS -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FILTERS -p tcp -m conntrack --ctstate NEW -m tcp --dport 80 --tcp-flags FIN,SYN,RST,ACK SYN -j ACCEPT
-A FILTERS -p tcp -m conntrack --ctstate NEW -m tcp --dport 443 --tcp-flags FIN,SYN,RST,ACK SYN -j ACCEPT
-A FILTERS -m conntrack --ctstate INVALID -j DROP
-A FILTERS -j REJECT
-A OUTPUT -o lo -j ACCEPT
-A OUTPUT -o eth0 -j ACCEPT
-A OUTPUT -o eth1 -j ACCEPT
-A FORWARD -o eth1 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i eth1 -o eth1 -j FILTERS
-A FORWARD -o eth0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i eth1 -o eth0 -m conntrack --ctstate NEW -j ACCEPT
-A FORWARD -i eth0 -o eth1 -j FILTERS
-A FORWARD -j REJECT
COMMIT

Apply the iptables rules and install iptables-persistent.

sudo iptables-restore -n ipv4.conf
sudo ip6tables-restore -n ipv6.conf

sudo iptables-save
sudo ip6tables-save

sudo apt install iptables-persistent

Simply install your favorite web server on the proxy droplet (i.e. nginx, caddy, haproxy etc). I would also suggest installing wireguard on the bastion host, it is also pretty easy to get a secure wireguard mesh network setup if you have multiple VPC.

Hardening Your Cloud Server

Sun, 23 May 2021 21:17:12 +0600

Taking the extra steps to protect your Ubuntu or Debian cloud droplet takes only a little effort and time but will have a huge long term impact on your cyber security. You can use the referral badge below to get started with a $100 credit from Digital Ocean or use this link to DigitalOcean.

1. ASSESS & IDENTIFY THE RISK

Undertaking a review to identify potential risks is a important first step. Some useful techniques for identifying risks are:

NMAP is a great tool to help identify potential risks. DigitalOcean has a pretty good guide here.
Tenable offers a great solution that also provides a very friendly report. Although normally a Nessus Professional license isn’t cheap, Tenable does offer a free version as well. You can download the free version here.
Sn1per is a open source solution that puts together a number of great open projects to deliver a very effective and easy to use package. You can check out the project on Github here.
Lynis is a open source tool that audits and grades your linux operating system’s security. You can check out the project on Github here.

2. REDUCE THE RISK

Once you have an idea of what potential cyber security risks you face you should start to take the steps to reduce those risks.

Set the default user profile to “umask 027” it is a good compromise between security and simplicity. A umask setting of 077 causes files and directories created by users to not be readable by any other user on the system. A umask of 027 makes files and directories readable by users in the same Unix group (i.e. “sudo” or “root”), while a umask of 022 makes files readable by every user on the system.

# Setting global umask
sudo echo "umask 027" >> /etc/profile

Restrict at and cron to authorized users only.

# First Remove both at.deny & cron.deny
sudo rm /etc/cron.deny 2> /dev/null
sudo rm /etc/at.deny 2> /dev/null
# Second create both at.allow & cron.allow
sudo echo 'root' > /etc/cron.allow
sudo echo 'root' > /etc/at.allow
# Third set the ownership to root
sudo chown root:root /etc/cron*
sudo chown root:root /etc/at*

Use the hosts.allow and hosts.deny files to help restrict access to services. For example the only IP address that should have access to NRPE on port 5666 is your Nagios server.

# Setup Some Access Control Rules
sudo echo 'sshd : ALL : ALLOW' > /etc/hosts.allow
# Or if this Node should only be accessible via a bastion-host
sudo echo 'sshd: 192.168.0.2' > /etc/hosts.allow
sudo echo 'ALL: LOCAL, 127.0.0.1' >> /etc/hosts.allow
sudo echo 'NRPE: 192.168.0.2' >> /etc/hosts.allow
sudo echo 'ALL: PARANOID' > /etc/hosts.deny
sudo chmod 644 /etc/hosts.allow
sudo chmod 644 /etc/hosts.deny

Limit visibility of running processes to those services that started the process or users in the same group.

# Edit fstab & Hide PID2
sudo echo 'proc /proc proc defaults,hidepid=2 0 0' >> /etc/fstab

Disable Root Recovery console, but make sure you have set a root password first.

# GRUB enable swap & disable root recovery
sudo echo 'GRUB_CMDLINE_LINUX="cgroup_enable=memory swapaccount=1"' >> /etc/default/grub
sudo echo 'GRUB_DISABLE_RECOVERY="true"' >> /etc/default/grub
sudo update-grub

It may seem unimportant, but having the time stamps match your timezone will make things easier later on when you are reviewing your logs and reports.

# Setup NTP
sudo timedatectl set-ntp true
sudo timedatectl set-timezone America/Los_Angeles
sudo echo 'servers=0.pool.ntp.org 1.pool.ntp.org 2.pool.ntp.org 3.pool.ntp.org' >> /etc/systemd/timesyncd.conf

sudo apt update
sudo apt install apparmor apparmor-profiles apparmor-utils apparmor-easyprof -y
# Enforce apparmor profiles
sudo echo 'session optional pam_apparmor.so order=user,group,default' > /etc/pam.d/apparmor

sudo systemctl start apparmor.service
sudo systemctl enable apparmor.service

sudo apt update
sudo apt install libpam-tmpdir libpam-apparmor libpam-cracklib -y

Disable USB access on your cloud node.

# Install USBGuard
sudo apt update
sudo apt install usbguard -y
# Setting up USBGuard
sudo usbguard generate-policy > /tmp/rules.conf
sudo install -m 0600 -o root -g root /tmp/rules.conf /etc/usbguard/rules.conf

Securing your remote access services isn’t just about disabling root access and enabling authorized keys in your SSH configuration.

sudo mv /etc/ssh/sshd_config /etc/ssh/sshd_config.bak

# Harden SSH Settings
sudo cat <<-EOF > /etc/ssh/sshd_config
HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
AcceptEnv LANG LC_*
AllowGroups root sudo
Banner /etc/issue.net
ChallengeResponseAuthentication no
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr
ClientAliveCountMax 0
ClientAliveInterval 300
Compression no
HostbasedAuthentication no
IgnoreUserKnownHosts yes
KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256
LoginGraceTime 20
LogLevel VERBOSE
Macs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-256
MaxAuthTries 3
MaxSessions 3
MaxStartups 10:30:60
PermitEmptyPasswords no
PermitRootLogin no
PubkeyAuthentication yes
PasswordAuthentication no
PermitUserEnvironment no
PrintLastLog yes
PrintMotd no
StrictModes yes
Subsystem sftp internal-sftp
UseDNS no
UsePAM yes
X11Forwarding no
AllowTcpForwarding no
EOF

sudo systemctl daemon-reload
sudo systemctl restart ssh.service

Now update your host keys and test ssh by starting a 2nd session.

## Switch to root
sudo su -
## Update ssh_host keys
rm /etc/ssh/ssh_host_*
ssh-keygen -t ecdsa -b 521 -f /etc/ssh/ssh_host_ecdsa_key -N ""
ssh-keygen -t rsa -b 4096 -f /etc/ssh/ssh_host_rsa_key -N ""
ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N ""

awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduli

3. MANAGE YOUR RISK

sudo apt update
sudo apt install dbconfig-common dbconfig-sqlite3 sqlite3 fail2ban -y
sudo cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.d/jail.local
sudo systemctl restart fail2ban

sudo apt update
sudo apt install rkhunter -y
sudo dpkg-reconfigure rkhunter

sudo cat <<-EOF > /etc/apt/apt.conf.d/20auto-upgrades
APT::Periodic::Update-Package-Lists "1";
APT::Periodic::Download-Upgradeable-Packages "1";
APT::Periodic::AutocleanInterval "7";
APT::Periodic::Unattended-Upgrade "1";
EOF

4. MONITOR YOUR RISK

Auditd can be a pretty powerful tool once you have the audit rules setup as it will give you valuable insights about your server performance and activities by ensuring that they are written to your logs.

sudo apt update
sudo apt install auditd audispd-plugins -y
# Setup Auditd Rules and Logging
sudo cat <<-EOF > /etc/audit/rules.d/docker.rules
# Remove any existing rules
-D
# Buffer Size
-b 8192
# Ignore errors
-i
# Failure Mode
-f 1
# Audit the audit logs
-w /var/log/audit/ -k auditlog
# Auditd configuration
-w /etc/audit/ -p wa -k auditconfig
-w /etc/libaudit.conf -p wa -k auditconfig
-w /etc/audisp/ -p wa -k audispconfig
# Monitor for use of audit management tools
-w /sbin/auditctl -p x -k audittools
-w /sbin/auditd -p x -k audittools
# Monitor AppArmor configuration changes
-w /etc/apparmor/ -p wa -k apparmor
-w /etc/apparmor.d/ -p wa -k apparmor
# Monitor usage of AppArmor tools
-w /sbin/apparmor_parser -p x -k apparmor_tools
-w /usr/sbin/aa-complain -p x -k apparmor_tools
-w /usr/sbin/aa-disable -p x -k apparmor_tools
-w /usr/sbin/aa-enforce -p x -k apparmor_tools
# Monitor Systemd configuration changes
-w /etc/systemd/ -p wa -k systemd
-w /lib/systemd/ -p wa -k systemd
# Monitor usage of systemd tools
-w /bin/systemctl -p x -k systemd_tools
-w /bin/journalctl -p x -k systemd_tools
# Special files
-a always,exit -F arch=x86_64 -S mknod -S mknodat -k specialfiles
-a always,exit -F arch=b32 -S mknod -S mknodat -k specialfiles
# Mount operations
-a always,exit -F arch=x86_64 -S mount -F auid>=1000 -F auid!=4294967295 -F key=export
-a always,exit -F arch=b32 -S mount -F auid>=1000 -F auid!=4294967295 -F key=export
# Changes to the time
-a always,exit -F arch=x86_64 -S settimeofday -k audit_time_rules
-a always,exit -F arch=x86_64 -S adjtimex -k audit_time_rules
-a always,exit -F arch=x86_64 -S clock_settime -k audit_time_rules
-a always,exit -F arch=b32 -S settimeofday -k audit_time_rules
-a always,exit -F arch=b32 -S adjtimex -k audit_time_rules
-a always,exit -F arch=b32 -S clock_settime -k audit_time_rules
# Cron configuration & scheduled jobs
-w /etc/cron.allow -p wa -k cron
-w /etc/cron.deny -p wa -k cron
-w /etc/cron.d/ -p wa -k cron
-w /etc/cron.daily/ -p wa -k cron
-w /etc/cron.hourly/ -p wa -k cron
-w /etc/cron.monthly/ -p wa -k cron
-w /etc/cron.weekly/ -p wa -k cron
-w /etc/crontab -p wa -k cron
-w /var/spool/cron/crontabs/ -k cron
# User, group, password databases
-w /etc/group -p wa -k audit_rules_usergroup_modification
-w /etc/passwd -p wa -k audit_rules_usergroup_modification
-w /etc/gshadow -p wa -k audit_rules_usergroup_modification
-w /etc/shadow -p wa -k audit_rules_usergroup_modification
-w /etc/security/opasswd -p wa -k audit_rules_usergroup_modification
# MAC-policy
-w /etc/selinux/ -p wa -k MAC-policy
# Monitor usage of passwd
-w /usr/bin/passwd -p x -k passwd_modification
# Monitor for use of tools to change group identifiers
-w /usr/sbin/groupadd -p x -k group_modification
-w /usr/sbin/groupmod -p x -k group_modification
-w /usr/sbin/addgroup -p x -k group_modification
-w /usr/sbin/useradd -p x -k user_modification
-w /usr/sbin/usermod -p x -k user_modification
-w /usr/sbin/adduser -p x -k user_modification
# Monitor module tools
-w /sbin/insmod -p x -k modules
-w /sbin/rmmod -p x -k modules
-w /sbin/modprobe -p x -k modules
-w /usr/sbin/insmod -p x -k modules
-w /usr/sbin/rmmod -p x -k modules
-w /usr/sbin/modprobe -p x -k modules
# Login configuration and information
-w /etc/login.defs -p wa -k login
-w /etc/securetty -p wa -k login
-w /var/log/faillog -p wa -k login
-w /var/run/faillock/ -p wa -k logins
-w /var/log/lastlog -p wa -k login
-w /var/log/tallylog -p wa -k login
# Network configuration
-w /etc/hosts -p wa -k audit_rules_networkconfig_modification
-w /etc/network/ -p wa -k audit_rules_networkconfig_modification
-w /etc/sysconfig/network -p wa -k audit_rules_networkconfig_modification
# System startup scripts
-w /etc/inittab -p wa -k init
-w /etc/init.d/ -p wa -k init
-w /etc/init/ -p wa -k init
# Library search paths
-w /etc/ld.so.conf -p wa -k libpath
# Local time zone
-w /etc/localtime -p wa -k localtime
# Time zone configuration
-w /etc/timezone -p wa -k audit_time_ruleszone
# Kernel parameters
-w /etc/sysctl.conf -p wa -k sysctl
# Modprobe configuration
-w /etc/modprobe.conf -p wa -k modprobe
-w /etc/modprobe.d/ -p wa -k modprobe
-w /etc/modules -p wa -k modprobe
# Module manipulations
-a always,exit -F arch=x86_64 -S init_module -S delete_module -F key=modules
-a always,exit -F arch=x86_64 -S init_module -F key=modules
-a always,exit -F arch=b32 -S init_module -S delete_module -F key=modules
-a always,exit -F arch=b32 -S init_module -F key=modules
# PAM configuration
-w /etc/pam.d/ -p wa -k pam
-w /etc/security/limits.conf -p wa -k pam
-w /etc/security/pam_env.conf -p wa -k pam
-w /etc/security/namespace.conf -p wa -k pam
-w /etc/security/namespace.init -p wa -k pam
# Postfix configuration
-w /etc/aliases -p wa -k mail
-w /etc/postfix/ -p wa -k mail
# SSH configuration
-w /etc/ssh/sshd_config -k sshd
# Changes to hostname
-a always,exit -F arch=x86_64 -S sethostname -S setdomainname -k audit_rules_networkconfig_modification
-a always,exit -F arch=b32 -S sethostname -S setdomainname -k audit_rules_networkconfig_modification
# Changes to issue
-w /etc/issue -p wa -k audit_rules_networkconfig_modification
-w /etc/issue.net -p wa -k audit_rules_networkconfig_modification
# Capture all unauthorized file accesses
-a always,exit -F arch=x86_64 -S creat -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=x86_64 -S creat -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=x86_64 -S openat -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=x86_64 -S openat -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=x86_64 -S open_by_handle_at -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=x86_64 -S open_by_handle_at -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=x86_64 -S truncate -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=x86_64 -S truncate -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=x86_64 -S ftruncate -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=x86_64 -S ftruncate -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=x86_64 -S open -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=x86_64 -S open -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=b32 -S open -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=b32 -S open -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=b32 -S creat -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=b32 -S creat -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=b32 -S openat -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=b32 -S openat -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=b32 -S open_by_handle_at -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=b32 -S open_by_handle_at -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=b32 -S truncate -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=b32 -S truncate -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=b32 -S ftruncate -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -F key=access
-a always,exit -F arch=b32 -S ftruncate -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -F key=access
# Monitor for use of process ID change (switching accounts) applications
-w /bin/su -p x -k actions
-w /usr/bin/sudo -p x -k actions
-w /etc/sudoers -p wa -k actions
-w /etc/sudoers.d -p wa -k actions
# Make the configuration immutable
-e 2
EOF

sudo systemctl start auditd
sudo systemctl enable auditd

You can then install logcheck or logwatch to get more detailed reports emailed to you. In addition you can use projects like mkcert or cfssl to enable TLS for internal communications between software like nrpe and nagios for example.

Follow this link for setting up a bastion screening host here or for my git project go here.