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Hardening - the Ansible role

An Ansible role to make a CentOS, Debian or Ubuntu server a bit more secure, systemd edition.

Requires Ansible >= 2.9.

Available on Ansible Galaxy.

Do not use this role without first testing in a non-operational environment.

CentOS 8, Debian 10 and Ubuntu 20.04 are supported platforms.

CentOS Stream, Debian 11 and Ubuntu 20.10 are in a testing phase.

Dependencies

None.

Example Playbook

---
- hosts: all
  serial: 50%
    - { role: konstruktoid.hardening, sshd_admin_net: [10.0.0.0/24] }
...

Note regarding Debian family UFW firewall rules

Instead of resetting ufw every run and by doing so causing network traffic disruption, the role deletes every ufw rule without comment: ansible managed task parameter and value.

The role also sets default deny policies, which means that firewall rules needs to be created for any additional ports except those specified in the sshd_port and ufw_outgoing_traffic variables.

Task Execution and Structure

See STRUCTURE.md for a detailed map regarding all tasks and the role structure.

Role testing

See TESTING.md.

Role Variables with defaults

./defaults/main/auditd.yml

auditd_action_mail_acct: root
auditd_admin_space_left_action: suspend
auditd_disk_error_action: suspend
auditd_disk_full_action: suspend
auditd_max_log_file: 8
auditd_max_log_file_action: keep_logs
auditd_mode: 1
auditd_num_logs: 5
auditd_space_left: 75
auditd_space_left_action: email
grub_audit_backlog_cmdline: audit_backlog_limit=8192
grub_audit_cmdline: audit=1

Enable auditd at boot using Grub.

auditd_action_mail_acct should be a valid email address or alias.

auditd_admin_space_left_action defines what action to take when the system has detected that it is low on disk space. suspend will cause the audit daemon to stop writing records to the disk.

auditd_max_log_file_action sets what action to take when the system has detected that the max file size limit has been reached. E.g. the rotate option will cause the audit daemon to rotate the logs. The keep_logs option is similar to rotate except it does not use the num_logs setting. This prevents audit logs from being overwritten.

auditd_space_left_action tells the system what action to take when the system has detected that it is low on disk space. email means that it will send a warning to the email account specified in action_mail_acct as well as sending the message to syslog.

auditd_mode sets auditd failure mode, 0=silent 1=printk 2=panic.

auditd.conf(5)

./defaults/main/compilers.yml

compilers:
  - /bin/as
  - /bin/cc
  - /bin/gcc
  - /bin/make
  - /usr/bin/as
  - /usr/bin/cc
  - /usr/bin/gcc
  - /usr/bin/make

List of compilers that will be restricted to the root user.

./defaults/main/dns.yml

dns: 127.0.0.1 1.1.1.1
fallback_dns: 9.9.9.9 1.0.0.1
dnssec: allow-downgrade
dns_over_tls: opportunistic

IPv4 and IPv6 addresses to use as system and fallback DNS servers. If dnssec is set to "allow-downgrade" DNSSEC validation is attempted, but if the server does not support DNSSEC properly, DNSSEC mode is automatically disabled. If dns_over_tls is true, all connections to the server will be encrypted if the DNS server supports DNS-over-TLS and has a valid certificate. systemd option.

./defaults/main/firewall.yml

ufw_outgoing_traffic:
  - 22
  - 53
  - 80
  - 123
  - 443
  - 853

Open ufw ports, allowing outgoing traffic.

./defaults/main/limits.yml

limit_nofile_hard: 1024
limit_nofile_soft: 512
limit_nproc_hard: 1024
limit_nproc_soft: 512

Maximum number of processes and open files.

./defaults/main/misc.yml

reboot_ubuntu: false
redhat_rpm_key:
  - 567E347AD0044ADE55BA8A5F199E2F91FD431D51
  - 47DB287789B21722B6D95DDE5326810137017186

If reboot_ubuntu: true an Ubuntu node will be rebooted if required.

redhat_rpm_key are RedHat Product Signing Keys

./defaults/main/module_blocklists.yml

fs_modules_blocklist:
  - cramfs
  - freevxfs
  - hfs
  - hfsplus
  - jffs2
  - squashfs
  - udf
  - vfat
misc_modules_blocklist:
  - bluetooth
  - bnep
  - btusb
  - cpia2
  - firewire-core
  - floppy
  - n_hdlc
  - net-pf-31
  - pcspkr
  - soundcore
  - thunderbolt
  - usb-midi
  - usb-storage
  - uvcvideo
  - v4l2_common
net_modules_blocklist:
  - dccp
  - sctp
  - rds
  - tipc

Blocked kernel modules.

./defaults/main/ntp.yml

fallback_ntp: 2.ubuntu.pool.ntp.org 3.ubuntu.pool.ntp.org
ntp: 0.ubuntu.pool.ntp.org 1.ubuntu.pool.ntp.org

NTP server host names or IP addresses. systemd option.

./defaults/main/packages.yml

system_upgrade: 'yes'
packages_blocklist:
  - apport*
  - autofs
  - avahi*
  - avahi-*
  - beep
  - git
  - pastebinit
  - popularity-contest
  - rpcbind
  - rsh*
  - rsync
  - talk*
  - telnet*
  - tftp*
  - whoopsie
  - xinetd
  - yp-tools
  - ypbind
packages_debian:
  - acct
  - aide-common
  - apparmor-profiles
  - apparmor-utils
  - apt-show-versions
  - audispd-plugins
  - auditd
  - cracklib-runtime
  - debsums
  - gnupg2
  - haveged
  - libpam-apparmor
  - libpam-modules
  - libpam-pwquality
  - libpam-tmpdir
  - needrestart
  - openssh-server
  - postfix
  - rkhunter
  - rsyslog
  - sysstat
  - tcpd
  - vlock
packages_redhat:
  - aide
  - audispd-plugins
  - audit
  - gnupg2
  - haveged
  - libpwquality
  - openssh-server
  - needrestart
  - postfix
  - psacct
  - rkhunter
  - rsyslog
  - vlock
packages_ubuntu:
  - fwupd
  - secureboot-db

system_upgrade: 'yes' will run apt upgrade or dnf update if required.

Packages to be installed depending of distribution and packages to be removed (packages_blocklist).

./defaults/main/password.yml

crypto_policy: FIPS

Set cryptographic policies if /etc/crypto-policies/config exists.

./defaults/main/sshd.yml

sshd_accept_env: LANG LC_*
sshd_admin_net:
  - 192.168.0.0/24
  - 192.168.1.0/24
sshd_allow_agent_forwarding: 'no'
sshd_allow_groups: sudo
sshd_allow_tcp_forwarding: 'no'
sshd_authentication_methods: any
sshd_banner: /etc/issue.net
sshd_challenge_response_authentication: 'no'
sshd_ciphers: chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr
sshd_client_alive_count_max: 3
sshd_client_alive_interval: 200
sshd_compression: 'no'
sshd_gssapi_authentication: 'no'
sshd_hostbased_authentication: 'no'
sshd_ignore_user_known_hosts: 'yes'
sshd_kerberos_authentication: 'no'
sshd_kex_algorithms: curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256
sshd_log_level: VERBOSE
sshd_login_grace_time: 20
sshd_macs: hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-256
sshd_max_auth_tries: 3
sshd_max_sessions: 3
sshd_max_startups: 10:30:60
sshd_password_authentication: 'no'
sshd_permit_empty_passwords: 'no'
sshd_permit_root_login: 'no'
sshd_permit_user_environment: 'no'
sshd_port: 22
sshd_print_last_log: 'yes'
sshd_print_motd: 'no'
sshd_rekey_limit: 512M 1h
sshd_strict_modes: 'yes'
sshd_subsystem: sftp internal-sftp
sshd_tcp_keep_alive: 'no'
sshd_use_dns: 'no'
sshd_use_pam: 'yes'
sshd_x11_forwarding: 'no'

For a explanation of the options not described below, please read https://man.openbsd.org/sshd_config.

Only the network(s) defined in sshd_admin_net are allowed to connect to sshd_port. Note that additional rules need to be set up in order to allow access to additional services.

OpenSSH login is allowed only for users whose primary group or supplementary group list matches one of the patterns in sshd_allow_groups.

sshd_allow_agent_forwarding specifies whether ssh-agent(1) forwarding is permitted.

sshd_allow_tcp_forwarding specifies whether TCP forwarding is permitted. The available options are yes or all to allow TCP forwarding, no to prevent all TCP forwarding, local to allow local (from the perspective of ssh(1)) forwarding only or remote to allow remote forwarding only.

sshd_authentication_methods specifies the authentication methods that must be successfully completed in order to grant access to a user.

sshd_log_level gives the verbosity level that is used when logging messages.

sshd_max_auth_tries and sshd_max_sessions specifies the maximum number of SSH authentication attempts permitted per connection and the maximum number of open shell, login or subsystem (e.g. sftp) sessions permitted per network connection.

sshd_password_authentication specifies whether password authentication is allowed.

sshd_port specifies the port number that sshd(8) listens on.

./defaults/main/suid_sgid_blocklist.yml

suid_sgid_permissions: true
suid_sgid_blocklist:
  - /bin/awk
  - /bin/base32
  - /bin/base64
  - /bin/bash
  - /bin/busctl
  - /bin/busybox
  - /bin/cat
  - /bin/chage
  - /bin/chmod
  - /bin/chown
  [...]

If suid_sgid_permissions: true loop through suid_sgid_blocklist and remove any SUID/SGID permissions.

A complete file list is available in defaults/main/suid_sgid_blocklist.yml.

./defaults/main/sysctl.yml

sysctl_dev_tty_ldisc_autoload: 0
sysctl_net_ipv6_conf_accept_ra_rtr_pref: 0
sysctl_settings:
  fs.protected_hardlinks: 1
  fs.protected_symlinks: 1
  fs.suid_dumpable: 0
  kernel.core_uses_pid: 1
  kernel.dmesg_restrict: 1
  kernel.kptr_restrict: 2
  kernel.panic: 60
  kernel.panic_on_oops: 60
  kernel.perf_event_paranoid: 2
  kernel.randomize_va_space: 2
  kernel.sysrq: 0
  kernel.unprivileged_bpf_disabled: 1
  kernel.yama.ptrace_scope: 2
  net.core.bpf_jit_harden: 2
  net.ipv4.conf.all.accept_redirects: 0
  net.ipv4.conf.all.accept_source_route: 0
  net.ipv4.conf.all.log_martians: 1
  net.ipv4.conf.all.rp_filter: 1
  net.ipv4.conf.all.secure_redirects: 0
  net.ipv4.conf.all.send_redirects: 0
  net.ipv4.conf.all.shared_media: 0
  net.ipv4.conf.default.accept_redirects: 0
  net.ipv4.conf.default.accept_source_route: 0
  net.ipv4.conf.default.log_martians: 1
  net.ipv4.conf.default.rp_filter: 1
  net.ipv4.conf.default.secure_redirects: 0
  net.ipv4.conf.default.send_redirects: 0
  net.ipv4.conf.default.shared_media: 0
  net.ipv4.icmp_echo_ignore_broadcasts: 1
  net.ipv4.icmp_ignore_bogus_error_responses: 1
  net.ipv4.ip_forward: 0
  net.ipv4.tcp_challenge_ack_limit: 2147483647
  net.ipv4.tcp_invalid_ratelimit: 500
  net.ipv4.tcp_max_syn_backlog: 20480
  net.ipv4.tcp_rfc1337: 1
  net.ipv4.tcp_syn_retries: 5
  net.ipv4.tcp_synack_retries: 2
  net.ipv4.tcp_syncookies: 1
  net.ipv4.tcp_timestamps: 0
  net.ipv6.conf.all.accept_ra: 0
  net.ipv6.conf.all.accept_redirects: 0
  net.ipv6.conf.all.accept_source_route: 0
  net.ipv6.conf.all.forwarding: 0
  net.ipv6.conf.all.use_tempaddr: 2
  net.ipv6.conf.default.accept_ra: 0
  net.ipv6.conf.default.accept_ra_defrtr: 0
  net.ipv6.conf.default.accept_ra_pinfo: 0
  net.ipv6.conf.default.accept_ra_rtr_pref: 0
  net.ipv6.conf.default.accept_redirects: 0
  net.ipv6.conf.default.accept_source_route: 0
  net.ipv6.conf.default.autoconf: 0
  net.ipv6.conf.default.dad_transmits: 0
  net.ipv6.conf.default.max_addresses: 1
  net.ipv6.conf.default.router_solicitations: 0
  net.ipv6.conf.default.use_tempaddr: 2
  net.netfilter.nf_conntrack_max: 2000000
  net.netfilter.nf_conntrack_tcp_loose: 0

Configure sysctl.

Recommended Reading

Center for Internet Security Linux Benchmarks

Common Configuration Enumeration

DISA Security Technical Implementation Guides

SCAP Security Guides

Security focused systemd configuration

Contributing

Do you want to contribute? Great! Contributions are always welcome, no matter how large or small. If you found something odd, feel free to submit a issue, improve the code by creating a pull request, or by sponsoring this project.

License

Apache License Version 2.0

Author Information

https://github.com/konstruktoid

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Ansible role to apply a security baseline. Systemd edition.

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