sysctl是一個允許您改變正在運行中的Linux系統的介面。它包含一些 TCP/IP 堆棧和虛擬記憶體系統的進階選項， 這可以讓有經驗的管理員提高令人信服的系統效能。用sysctl可以讀取設定超過五百個系統變數。基於這點，sysctl(8) 提供兩個功能：讀取和修改系統設定。
查看所有可讀變數：
% sysctl -a
讀一個指定的變數，例如 kern.maxproc：
% sysctl kern.maxproc kern.maxproc: 1044
要設定一個指定的變數，直接用 variable=value 這樣的文法：
# sysctl kern.maxfiles=5000
kern.maxfiles: 2088 -> 5000
您可以使用sysctl修改系統變數，也可以通過編輯sysctl.conf檔案來修改系統變數。sysctl.conf 看起來很像 rc.conf。它用 variable=value 的形式來設定值。指定的值在系統進入多使用者模式之後被設定。並不是所有的變數都可以在這個模式下設定。
sysctl 變數的設定通常是字串、數字或者布爾型。 (布爾型用 1 來表示'yes'，用 0 來表示'no')。 sysctl -w kernel.sysrq=0
sysctl -w kernel.core_uses_pid=1
sysctl -w net.ipv4.conf.default.accept_redirects=0
sysctl -w net.ipv4.conf.default.accept_source_route=0
sysctl -w net.ipv4.conf.default.rp_filter=1
sysctl -w net.ipv4.tcp_syncookies=1
sysctl -w net.ipv4.tcp_max_syn_backlog=2048
sysctl -w net.ipv4.tcp_fin_timeout=30
sysctl -w net.ipv4.tcp_synack_retries=2
sysctl -w net.ipv4.tcp_keepalive_time=3600
sysctl -w net.ipv4.tcp_window_scaling=1
sysctl -w net.ipv4.tcp_sack=1 配置sysctl 編輯此檔案： vi /etc/sysctl.conf 如果該檔案為空白，則輸入以下內容，否則請根據情況自己做調整： # Controls source route verification
# Default should work for all interfaces
net.ipv4.conf.default.rp_filter = 1
# net.ipv4.conf.all.rp_filter = 1
# net.ipv4.conf.lo.rp_filter = 1
# net.ipv4.conf.eth0.rp_filter = 1 # Disables IP source routing
# Default should work for all interfaces
net.ipv4.conf.default.accept_source_route = 0
# net.ipv4.conf.all.accept_source_route = 0
# net.ipv4.conf.lo.accept_source_route = 0
# net.ipv4.conf.eth0.accept_source_route = 0 # Controls the System Request debugging functionality of the kernel
kernel.sysrq = 0 # Controls whether core dumps will append the PID to the core filename.
# Useful for debugging multi-threaded applications.
kernel.core_uses_pid = 1 # Increase maximum amount of memory allocated to shm
# Only uncomment if needed!
# kernel.shmmax = 67108864 # Disable ICMP Redirect Acceptance
# Default should work for all interfaces
net.ipv4.conf.default.accept_redirects = 0
# net.ipv4.conf.all.accept_redirects = 0
# net.ipv4.conf.lo.accept_redirects = 0
# net.ipv4.conf.eth0.accept_redirects = 0 # Enable Log Spoofed Packets, Source Routed Packets, Redirect Packets
# Default should work for all interfaces
net.ipv4.conf.default.log_martians = 1
# net.ipv4.conf.all.log_martians = 1
# net.ipv4.conf.lo.log_martians = 1
# net.ipv4.conf.eth0.log_martians = 1 # Decrease the time default value for tcp_fin_timeout connection
net.ipv4.tcp_fin_timeout = 25 # Decrease the time default value for tcp_keepalive_time connection
net.ipv4.tcp_keepalive_time = 1200 # Turn on the tcp_window_scaling
net.ipv4.tcp_window_scaling = 1 # Turn on the tcp_sack
net.ipv4.tcp_sack = 1 # tcp_fack should be on because of sack
net.ipv4.tcp_fack = 1 # Turn on the tcp_timestamps
net.ipv4.tcp_timestamps = 1 # Enable TCP SYN Cookie Protection
net.ipv4.tcp_syncookies = 1 # Enable ignoring broadcasts request
net.ipv4.icmp_echo_ignore_broadcasts = 1 # Enable bad error message Protection
net.ipv4.icmp_ignore_bogus_error_responses = 1 # Make more local ports available
# net.ipv4.ip_local_port_range = 1024 65000 # Set TCP Re-Ordering value in kernel to ‘5′
net.ipv4.tcp_reordering = 5 # Lower syn retry rates
net.ipv4.tcp_synack_retries = 2
net.ipv4.tcp_syn_retries = 3 # Set Max SYN Backlog to ‘2048′
net.ipv4.tcp_max_syn_backlog = 2048 # Various Settings
net.core.netdev_max_backlog = 1024 # Increase the maximum number of skb-heads to be cached
net.core.hot_list_length = 256 # Increase the tcp-time-wait buckets pool size
net.ipv4.tcp_max_tw_buckets = 360000 # This will increase the amount of memory available for socket input/output queues
net.core.rmem_default = 65535
net.core.rmem_max = 8388608
net.ipv4.tcp_rmem = 4096 87380 8388608
net.core.wmem_default = 65535
net.core.wmem_max = 8388608
net.ipv4.tcp_wmem = 4096 65535 8388608
net.ipv4.tcp_mem = 8388608 8388608 8388608
net.core.optmem_max = 40960 如果希望屏蔽別人 ping 你的主機，則加入以下代碼： # Disable ping requests
net.ipv4.icmp_echo_ignore_all = 1 編輯完成後，請執行以下命令使變動立即生效： /sbin/sysctl -p
/sbin/sysctl -w net.ipv4.route.flush=1 |
