- sysctl: name={{ item.name }} value={{ item.value }} sysctl_set=yes
  with_items:
    - { name: 'kernel.domainname', value: '{{ ansible_domain }}' }

      # Networking. See
      # https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt

      # Enable Spoof protection (reverse-path filter). Turn on Source
      # Address Verification in all interfaces to prevent some spoofing
      # attacks.
    - { name: 'net.ipv4.conf.default.rp_filter', value: 1 }
    - { name: 'net.ipv4.conf.all.rp_filter',     value: 1 }

      # Disable SYN cookies and improve SYN backlog handling, see tcp(7) and
      # https://levelup.gitconnected.com/linux-kernel-tuning-for-high-performance-networking-high-volume-incoming-connections-196e863d458a
      # TODO bullseye: remove net.core.somaxconn as 5.4 and later default to 4096
    - { name: 'net.ipv4.tcp_syncookies',      value: 0     }
    - { name: 'net.ipv4.tcp_synack_retries',  value: 1     }
    - { name: 'net.ipv4.tcp_max_syn_backlog', value: 32768 }
    - { name: 'net.core.somaxconn',           value: 4096  }

      # We rate-limit not only the default ICMP types 3, 4, 11 and 12
      # (0x1818), but also types 0 and 8. See icmp(7).
    - { name: 'net.ipv4.icmp_ratemask',   value: 6425 }
    - { name: 'net.ipv4.icmp_ratelimit',  value: 1000 }

      # Disable packet forwarding between interfaces (we are not a router).
    - { name: 'net.ipv4.ip_forward',          value: 0 }
    - { name: 'net.ipv6.conf.all.forwarding', value: 0 }

      # Do not accept ICMP redirects (prevent MITM attacks).
    - { name: 'net.ipv4.conf.all.accept_redirects', value: 0 }
    - { name: 'net.ipv6.conf.all.accept_redirects', value: 0 }

      # Do not send ICMP redirects (we are not a router).
    - { name: 'net.ipv4.conf.default.send_redirects', value:  0 }
    - { name: 'net.ipv4.conf.all.send_redirects',     value:  0 }

      # Do not accept IP source route packets (we are not a router).
    - { name: 'net.ipv4.conf.all.accept_source_route', value:  0 }
    - { name: 'net.ipv6.conf.all.accept_source_route', value:  0 }

      # Log Martian Packets.
    - { name: 'net.ipv4.conf.all.log_martians', value: 1 }

      # Ignore ICMP broadcasts.
    - { name: 'net.ipv4.icmp_echo_ignore_broadcasts', value: 1 }

      # Ignore bogus ICMP errors.
    - { name: 'net.ipv4.icmp_ignore_bogus_error_responses', value: 1 }