sysctl是一個允許您改變正在運行中的Linux系統的接口。它包含一些 TCP/IP 堆棧和虛擬內存系統的高級選項， 這可以讓有經驗的管理員提高引人注目的系統性能。用sysctl可以讀取設置超過五百個系統變量。基于這點，sysctl(8) 提供兩個功能：讀取和修改系統設置。

常用

查看所有可讀變量：
[xt@butbueatiful ~] sysctl -a

讀一個指定的變量，例如 kernel.msgmnb：
[xt@butbueatiful ~]$ sysctl kernel.msgmnb
kern.maxproc: 65536

要設置一個指定的變量，直接用 variable=value 這樣的語法：
[xt@butbueatiful ~]$ sudo sysctl kernel.msgmnb=1024
kernel.msgmnb: 1024

可以使用sysctl修改系統變量，也可以通過編輯sysctl.conf文件來修改系統變量。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用戶模式之后被設定。并不是所有的變量都可以在這個模式下設定。

sysctl 變量的設置通常是字符串、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

[root@butbueatiful ~] vi /etc/sysctl.conf

如果該文件為空，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

執行以下命令使變動立即生效：

/sbin/sysctl -p
/sbin/sysctl -w net.ipv4.route.flush=1

我們常常在 Linux 的 /proc/sys 目錄下,手動設定一些 kernel 的參數或是直接 echo 特定的值給一個 proc下的虛擬檔案,俾利某些檔案之開啟,常見的例如設定開機時自動啟動 IP Forwarding:
echo “1” > /proc/sys/net/ipv4/ip_forward

其實,在 Linux 我們還可以用 sysctl command 便可以簡易的去檢視、設定或自動配置特定的 kernel 設定。我們可以在系統提示符號下輸入「sysctl -a」

sysctl 的表示法乃把目錄結構的「/」以「.」表示,一層一層的連結下去。當然以echo 特定的值給一個 proc下的虛擬檔案也是可以用 sysctl加以表示,例如:

#sysctl –w net.ipv4.ip_forward =”1”

或是直接在 /etc/sysctl.conf 增刪修改特定檔案的 0,1值亦可:　
# Enables packet forwarding

net.ipv4.ip_forward = 1

# Enables source route verification

net.ipv4.conf.default.rp_filter = 1

# Disables the magic-sysrq key

kernel.sysrq = 0

當然如果考慮 reboot 后仍有效, 直接在 /etc/sysctl.conf 增刪修改特定檔案的 0,1值才可使之保留設定(以RedHat 為例,每次開機系統啟動后, init 會執行 /etc/rc.d/rc.sysinit,便會使用 /etc/sysctl.conf 的預設值去執行 sysctl)

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