sysctl命令被用於在核心執行時動態地修改核心的執行引數，可用的核心引數在目錄/proc/sys中。它包含一些TCP/ip堆疊和虛擬記憶體系統的高階選項，用sysctl可以讀取設定超過五百個系統變數。

sysctl命令

用法：
sysctl [options] [variable[=value] …]

常用選項：
-n：列印時只打印值，不列印引數名稱；
-e：忽略未知關鍵字錯誤；
-N：列印時只打印引數名稱，不列印值；
-w：設定引數的值（不過好像不加這個選項也可以直接設定）；
-p：從配置檔案“/etc/sysctl.conf”載入核心引數設定；
-a：列印所有核心引數變數；
-A：以表格方式列印所有核心引數變數。

示例
1、 設定socket接收緩衝最大值（rmem_max）為212992位元組

sudo sysctl -w net.core.rmem_max=212992

2、檢視socket接收緩衝最大值（rmem_max）

sysctl net.core.rmem_max

sysctl可以設定檢視的核心引數都在目錄/proc/sys中，每個檔案都記錄了一個引數，sysctl命令中其實也記錄了引數的相對路徑，只要將 . 換成 / 就是檔案路徑，比如記錄net.core.rmem_max的檔案就是/proc/sys/net/core/rmem_max，所以也可以直接檢視檔案內容來讀取引數的值

cat
 
 /proc/sys/net/core/rmem_max

如果不知道引數路徑也可以這麼做：

sysctl -a | grep rmem_max

配置sysctl

編輯此檔案vi /etc/sysctl.conf
如果該檔案為空，則輸入以下內容，否則請根據情況自己做調整（複製自http://man.linuxde.net/sysctl）：

# 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

如果希望遮蔽別人 ping 你的主機，則加入以下程式碼：

# 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

TCP/IP引數

所有的TCP/IP引數都位於/proc/sys/net目錄下（請注意，對/proc/sys/net目錄下內容的修改都是臨時的，任何修改在系統重啟後都會丟失），例如下面這些重要的引數：