纠正错误 添加实例

# sysctl

时动态地修改内核的运行参数

## 补充说明

sysctl命令 被用于在内核运行时动态地修改内核的运行参数，可用的内核参数在目录`/proc/sys`中。它包含一些TCP/ip堆栈和虚拟内存系统的高级选项， 这可以让有经验的管理员提高引人注目的系统性能。用sysctl可以读取设置超过五百个系统变量。

### 语法

    sysctl(选项)(参数)

### 选项

    -n：打印值时不打印关键字；
    -e：忽略未知关键字错误；
    -N：仅打印名称；
    -w：当改变sysctl设置时使用此项；
    -p：从配置文件“/etc/sysctl.conf”加载内核参数设置；
    -a：打印当前所有可用的内核参数变量和值；
    -A：以表格方式打印当前所有可用的内核参数变量和值。

### 参数

变量=值：设置内核参数对应的变量值。

### 实例

查看所有可读变量：

sysctl -a

读一个指定的变量，例如`kern.maxproc`：

sysctl kern.maxproc kern.maxproc: 1044

要设置一个指定的变量，直接用`variable=value`这样的语法：

    sysctl kern.maxfiles=5000
    kern.maxfiles: 2088 -> 5000

您可以使用sysctl修改系统变量，也可以通过编辑sysctl.conf文件来修改系统变量。sysctl.conf看起来很像rc.conf。它用`variable=value`的形式来设定值。指定的值在系统进入多用户模式之后被设定。并不是所有的变量都可以在这个模式下设定。

sysctl变量的设置通常是字符串、数字或者布尔型。（布尔型用 1 来表示'yes'，用 0 来表示'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

编辑此文件：`/etc/sysctl.conf`

如果该文件为空，则输入以下内容，否则请根据情况自己做调整：

    # 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

如果希望屏蔽别人 ping 你的主机，则加入以下代码：

    # Disable ping requests
    net.ipv4.icmp_echo_ignore_all = 1

编辑完成后，请执行以下命令使变动立即生效：

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