The sysctl preload/configuration file can be created at /etc/sysctl.d/99-sysctl.conf. For systemd, /etc/sysctl.d/ and /usr/lib/sysctl.d/ are drop-in directories for kernel sysctl parameters. The naming and source directory decide the order of processing, which is important since the last parameter processed may override earlier ones. For example, parameters in a /usr/lib/sysctl.d/50-default.conf will be overriden by equal parameters in /etc/sysctl.d/50-default.conf and any configuration file processed later from both directories.
Files found under the /etc/sysctl.d directory that end with .conf are parsed within sysctl at boot time. If you want to set kernel variables you can either edit /etc/sysctl.conf or make a new file. The filename isn't important, but don't make it a package name as it may clash with something the package builder needs later. The file name must end with .conf, or it will not be read. The recommended location for local system settings is /etc/sysctl.d/local.conf but as long as you follow the rules for the name of the file, anything will work.
To load all configuration files manually, execute
# sysctl --system
which will also output the applied hierarchy. A single parameter file can also be loaded explicitly with
# sysctl -p filename.conf
The parameters available are those listed under /proc/sys/. For example, the kernel.sysrq parameter refers to the file /proc/sys/kernel/sysrq on the file system. The sysctl -a command can be used to display all currently available values.
Settings can be changed through file manipulation or using the sysctl utility. For example, to temporarily enable the magic SysRq key:
# sysctl kernel.sysrq=1
# echo "1" > /proc/sys/kernel/sysrq
To preserve changes between reboots, add or modify the appropriate lines in /etc/sysctl.d/99-sysctl.conf or another applicable parameter file in /etc/sysctl.d/.
3.1 Improving performance
# reuse/recycle time-wait sockets net.ipv4.tcp_tw_reuse = 1 net.ipv4.tcp_tw_recycle = 1
3.2 TCP/IP stack hardening
The following specifies a parameter set to tighten network security options of the kernel for the IPv4 protocol and related IPv6 parameters where an equivalent exists.
For some usecases, for example using the system as a router, other parameters may be useful or required as well.
#### ipv4 networking and equivalent ipv6 parameters #### ## TCP SYN cookie protection (default) ## helps protect against SYN flood attacks ## only kicks in when net.ipv4.tcp_max_syn_backlog is reached net.ipv4.tcp_syncookies = 1 ## protect against tcp time-wait assassination hazards ## drop RST packets for sockets in the time-wait state ## (not widely supported outside of linux, but conforms to RFC) net.ipv4.tcp_rfc1337 = 1 ## sets the kernels reverse path filtering mechanism to value 1(on) ## will do source validation of the packet's recieved from all the interfaces on the machine ## protects from attackers that are using ip spoofing methods to do harm net.ipv4.conf.all.rp_filter = 1 net.ipv6.conf.all.rp_filter = 1 ## tcp timestamps ## + protect against wrapping sequence numbers (at gigabit speeds) ## + round trip time calculation implemented in TCP ## - causes extra overhead and allows uptime detection by scanners like nmap ## enable @ gigabit speeds net.ipv4.tcp_timestamps = 0 #net.ipv4.tcp_timestamps = 1 ## log martian packets net.ipv4.conf.all.log_martians = 1 ## ignore echo broadcast requests to prevent being part of smurf attacks (default) net.ipv4.icmp_echo_ignore_broadcasts = 1 ## ignore bogus icmp errors (default) net.ipv4.icmp_ignore_bogus_error_responses = 1 ## send redirects (not a router, disable it) net.ipv4.conf.all.send_redirects = 0 ## ICMP routing redirects (only secure) #net.ipv4.conf.all.secure_redirects = 1 (default) net.ipv4.conf.default.accept_redirects=0 net.ipv4.conf.all.accept_redirects=0 net.ipv6.conf.default.accept_redirects=0 net.ipv6.conf.all.accept_redirects=0
4 Virtual memory
There are several key parameters to tune the operation of the virtual memory (VM) subsystem of the Linux kernel and the writeout of dirty data to disk. See the Linux kernel documentation for more information.
# Contains, as a percentage of total system memory, the number of pages at which # a process which is generating disk writes will start writing out dirty data. vm.dirty_ratio = 3 # Contains, as a percentage of total system memory, the number of pages at which # the background kernel flusher threads will start writing out dirty data. vm.dirty_background_ratio = 2
As noted in the comments, one needs to consider the total amount of RAM when setting these values.
- vm.dirty_ratio defaults to 10 (percent of RAM). Consensus is that 10% of RAM when RAM is say half a GB (so 10% is ~50 MB) is a sane value on spinning disks, but it can be MUCH worse when RAM is larger, say 16 GB (10% is ~1.6 GB), as that's several seconds of writeback on spinning disks. A more sane value in this case is 3 (16*0.03 ~ 491 MB).
- vm.dirty_background_ratio similarly, 5 (% of RAM) by default may be just fine for small memory values, but again, consider and adjust accordingly for the amount of RAM on a particular system.
When the kernel performs a resync operation of a software raid device it tries not to create a high system load by restricting the speed of the operation. Using sysctl it is possible to change the lower and upper speed limit.
# Set maximum and minimum speed of raid resyncing operations dev.raid.speed_limit_max = 10000 dev.raid.speed_limit_min = 1000
If mdadm is compiled as a module md_mod, the above settings are available only after the module has been loaded. If the settings shall be loaded on boot via /etc/sysctl.d, the module md_mod may be loaded beforehand through /etc/modules-load.d.
6.1 Small periodic system freezes
Set dirty bytes to small enough value (for example 4M):
vm.dirty_background_bytes = 4194304 vm.dirty_bytes = 4194304
Try to change kernel.io_delay_type (x86 only):
- 0 - IO_DELAY_TYPE_0X80
- 1 - IO_DELAY_TYPE_0XED
- 2 - IO_DELAY_TYPE_UDELAY
- 3 - IO_DELAY_TYPE_NONE
7 See also
- sysctl(8) and sysctl.conf(5)
- Linux kernel documentation for /proc/sys/
- Kernel Documentation: IP Sysctl
- SysCtl.conf Tweaked for Security and Cable Speed[dead link 2015-12-14]
- Kernel network parameters for sysctl