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+===== How to fix “nf_conntrack: table full, dropping packet.” issue =====
+during high traffic time, web site become much slower while low traffic is ok. a typical case is a download server, when there are many concurrent connections to download, many or all users feel much slower. one cause may be the linux conntrack table is full.
+==== Check ====
+  #dmesg | tail
+  nf_conntrack: table full, dropping packet.
+  nf_conntrack: table full, dropping packet.
+  nf_conntrack: table full, dropping packet.
+if you see above, then the issue of conntrack table full is confirmed.
+  #sysctl -a | grep conntrack
+  ...
+  net.netfilter.nf_conntrack_max = 65536
+  net.netfilter.nf_conntrack_count = 68999
+  net.netfilter.nf_conntrack_buckets = 16384
+  ...
+  #cat /sys/module/nf_conntrack/parameters/hashsize
+if nf_conntrack_count is close to nf_conntrack_max or even larger, the problem will occur.
+==== Action ====
+  #sysctl -w net.netfilter.nf_conntrack_max=655360
+  net.netfilter.nf_conntrack_max = 655360
+  #echo 163840 > /sys/module/nf_conntrack/parameters/hashsize
+==== Permanent Change ====
+edit /etc/sysctl.conf, add following line:
+  net.netfilter.nf_conntrack_max=655360
+edit /etc/rc.local, add following line:
+  echo 163840 > /sys/module/nf_conntrack/parameters/hashsize