update..

roles/common/files/etc/sysctl.d/10-ddos.conf

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+# ==========
+# Anti-DDoS Kernel Settings
+# ==========
+
+# see:
+#    https://javapipe.com/blog/iptables-ddos-protection/
+
+# These sysctl.conf settings help to maximize the performance of your server
+# under DDoS as well as the effectiveness of the iptables rules that we’re going
+# to provide in this guide.
+kernel.printk = 4 4 1 7
+kernel.panic = 10
+kernel.sysrq = 0
+kernel.shmmax = 4294967296
+#kernel.shmall = 4194304
+kernel.shmall = 134217728
+kernel.core_uses_pid = 1
+kernel.msgmnb = 65536
+kernel.msgmax = 65536
+vm.swappiness = 20
+vm.dirty_ratio = 80
+vm.dirty_background_ratio = 5
+fs.file-max = 2097152
+net.core.netdev_max_backlog = 262144
+net.core.rmem_default = 31457280
+net.core.rmem_max = 67108864
+net.core.wmem_default = 31457280
+net.core.wmem_max = 67108864
+net.core.somaxconn = 65535
+net.core.optmem_max = 25165824
+net.ipv4.neigh.default.gc_thresh1 = 4096
+net.ipv4.neigh.default.gc_thresh2 = 8192
+net.ipv4.neigh.default.gc_thresh3 = 16384
+net.ipv4.neigh.default.gc_interval = 5
+net.ipv4.neigh.default.gc_stale_time = 120
+net.netfilter.nf_conntrack_max = 10000000
+net.netfilter.nf_conntrack_tcp_loose = 0
+net.netfilter.nf_conntrack_tcp_timeout_established = 1800
+net.netfilter.nf_conntrack_tcp_timeout_close = 10
+net.netfilter.nf_conntrack_tcp_timeout_close_wait = 10
+net.netfilter.nf_conntrack_tcp_timeout_fin_wait = 20
+net.netfilter.nf_conntrack_tcp_timeout_last_ack = 20
+net.netfilter.nf_conntrack_tcp_timeout_syn_recv = 20
+net.netfilter.nf_conntrack_tcp_timeout_syn_sent = 20
+net.netfilter.nf_conntrack_tcp_timeout_time_wait = 10
+net.ipv4.tcp_slow_start_after_idle = 0
+net.ipv4.ip_local_port_range = 1024 65000
+net.ipv4.ip_no_pmtu_disc = 1
+net.ipv4.route.flush = 1
+net.ipv4.route.max_size = 8048576
+net.ipv4.icmp_echo_ignore_broadcasts = 1
+net.ipv4.icmp_ignore_bogus_error_responses = 1
+net.ipv4.tcp_congestion_control = htcp
+net.ipv4.tcp_mem = 65536 131072 262144
+net.ipv4.udp_mem = 65536 131072 262144
+net.ipv4.tcp_rmem = 4096 87380 33554432
+net.ipv4.udp_rmem_min = 16384
+net.ipv4.tcp_wmem = 4096 87380 33554432
+net.ipv4.udp_wmem_min = 16384
+net.ipv4.tcp_max_tw_buckets = 1440000
+net.ipv4.tcp_tw_recycle = 0
+net.ipv4.tcp_tw_reuse = 1
+net.ipv4.tcp_max_orphans = 400000
+net.ipv4.tcp_window_scaling = 1
+net.ipv4.tcp_rfc1337 = 1
+net.ipv4.tcp_syncookies = 1
+net.ipv4.tcp_synack_retries = 1
+net.ipv4.tcp_syn_retries = 2
+net.ipv4.tcp_max_syn_backlog = 16384
+net.ipv4.tcp_timestamps = 1
+net.ipv4.tcp_sack = 1
+net.ipv4.tcp_fack = 1
+net.ipv4.tcp_ecn = 2
+net.ipv4.tcp_fin_timeout = 10
+net.ipv4.tcp_keepalive_time = 600
+net.ipv4.tcp_keepalive_intvl = 60
+net.ipv4.tcp_keepalive_probes = 10
+net.ipv4.tcp_no_metrics_save = 1
+net.ipv4.ip_forward = 0
+net.ipv4.conf.all.accept_redirects = 0
+net.ipv4.conf.all.send_redirects = 0
+net.ipv4.conf.all.accept_source_route = 0
+net.ipv4.conf.all.rp_filter = 1

roles/common/files/etc/sysctl.d/30-enable-ipv6.conf

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+
+# Enable packet forwarding for IPv6
+#
+net.ipv6.conf.all.forwarding = 1

roles/common/files/etc/sysctl.d/50-dovecot.conf

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+# Setting need by mailserver (dovecot) 
+# -
+# - Warning messages from dovecot:
+# - Warning: Inotify instance limit for user 5000 (UID vmail) exceeded, disabling.
+# -          Increase /proc/sys/fs/inotify/max_user_instances
+# -
+fs.inotify.max_user_instances = 2048

roles/common/files/etc/sysctl.d/50-redis.conf

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+# -----
+# Running Redis Service (on LX-Container)
+#
+# See also changes at file /etc/rc.local
+# -----
+
+# Got this warning on redis logfile at LX-Container
+#
+#    # WARNING: The TCP backlog setting of 511 cannot be enforced because /proc/sys/net/core/somaxconn
+#    is set to the lower value of 128
+#
+net.core.somaxconn = 65535
+
+# Redis Server says:
+#
+# WARNING overcommit_memory is set to 0! Background save may fail under
+# low memory condition. To fix this issue add 'vm.overcommit_memory = 1'
+# to /etc/sysctl.conf and then reboot or run the command
+# 'sysctl vm.overcommit_memory=1' for this to take effect.
+vm.overcommit_memory = 1
+
+# increase Linux TCP buffer limits
+net.core.rmem_max = 67108864
+net.core.wmem_max = 67108864
+net.core.netdev_max_backlog = 262144
+
+# increase Linux autotuning TCP buffer limits
+# min, default, and max number of bytes to use
+net.ipv4.tcp_rmem = 4096 87380 33554432
+net.ipv4.tcp_wmem = 4096 87380 33554432

roles/common/files/etc/sysctl.d/50-swappiness.conf

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+vm.swappiness = 5

roles/common/files/etc/sysctl.d/60-elasticsearch.conf

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+
+# Needed by ElasticSearch Installation on virtual guest
+# systems (LX-Containers)
+#
+# The error message there was:
+#    max virtual memory areas vm.max_map_count [65530] is too low, increase to at least [262144]
+#
+vm.max_map_count = 524288