Книга: Fedora™ Unleashed, 2008 edition

Kernel

Kernel

As the Linux kernel developed over time, developers sought a way to fine-tune some of the kernel parameters. Before sysctl, those parameters had to be changed in the kernel configuration and then the kernel had to be recompiled.

The sysctl command can change some parameters of a running kernel. It does this through the /proc file system, which is a "virtual window" into the running kernel. Although it might appear that a group of directories and files exist under /proc, that is only a representation of parts of the kernel. You can read values from and write values to those "files," referred to as variables. You can display a list of the variables as shown in the following. (An annotated list is presented because roughly 250 items — or more — exist in the full list.)

# sysctl -A
net.ipv4.tcp_max_syn_backlog = 1024
net.ipv4.tcp_rfc1337 = 0
net.ipv4.tcp_stdurg = 0
net.ipv4.tcp_abort_on_overflow = 0
net.ipv4.tcp_tw_recycle = 0
net.ipv4.tcp_syncookies = 0
net.ipv4.tcp_fin_timeout = 60
net.ipv4.tcp_retries2 = 15
net.ipv4.tcp_retries1 = 3
net.ipv4.tcp_keepalive_intvl = 75
net.ipv4.tcp_keepalive_probes = 9
net.ipv4.tcp_keepalive_time = 7200
net.ipv4.ipfrag_time = 30

The items shown are networking parameters, and actually tweaking these values is beyond the scope of this book. If you want to change a value, however, the -w command is used:

# sysctl -w net.ipv4.tcp_retries 2=20

This increases the value of that particular kernel parameter.

If you find that a particular setting is useful, you can enter it into the /etc/sysctl.conf file. The format is as follows, using the earlier example:

net.ipv4.tcp_retries 2=20

Of more interest to kernel hackers than regular users, sysctl is a potentially powerful tool that continues to be developed and documented.

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