Difference between revisions of "Sysctl"

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(Configuration)
(Configuration)
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== Configuration ==
 
== Configuration ==
  
The '''sysctl''' preload/configuration file can be created at {{ic|/etc/sysctl.d/99-sysctl.conf}}. For [[systemd]], {{ic|/etc/sysctl.d/}} is ''"a drop-in directory for kernel sysctl parameters, extending what you can already do with /etc/sysctl.conf."'' See [http://0pointer.de/blog/projects/the-new-configuration-files The New Configuration Files] and more specifically [http://0pointer.de/public/systemd-man/sysctl.d.html systemd's sysctl.d man page] for more information.
+
The '''sysctl''' preload/configuration file can be created at {{ic|/etc/sysctl.d/99-sysctl.conf}}. For [[systemd]], {{ic|/etc/sysctl.d/}} and {{ic|/usr/lib/sysctl.d/}} are drop-in directories ''"for kernel sysctl parameters, extending what you can already do with {{ic|/etc/sysctl.conf}}."'' See [http://0pointer.de/blog/projects/the-new-configuration-files The New Configuration Files] and more specifically [http://0pointer.de/public/systemd-man/sysctl.d.html systemd's sysctl.d man page] for more information.
  
 
The parameters available are those listed under {{ic|/proc/sys/}}. For example, the {{Ic|kernel.sysrq}} parameter refers to the file {{ic|/proc/sys/kernel/sysrq}} on the file system. The {{Ic|sysctl -a}} command can be used to display all values currently available.
 
The parameters available are those listed under {{ic|/proc/sys/}}. For example, the {{Ic|kernel.sysrq}} parameter refers to the file {{ic|/proc/sys/kernel/sysrq}} on the file system. The {{Ic|sysctl -a}} command can be used to display all values currently available.

Revision as of 18:25, 13 September 2013

sysctl is a tool (in official repositories package procps-ng) for examining and changing kernel parameters at runtime. sysctl is implemented in procfs, the virtual process file system at /proc/.

Configuration

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, extending what you can already do with /etc/sysctl.conf." See The New Configuration Files and more specifically systemd's sysctl.d man page for more information.

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 values currently available.

Note: If you have the kernel documentation installed (linux-docs), you can find detailed information about sysctl settings in /usr/src/linux-$(uname -r)/Documentation/sysctl/. It is highly recommended reading before changing sysctl settings.

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

or:

# echo "1" > /proc/sys/kernel/sysrq

To preserve changes between reboots, add or modify the appropriate lines in /etc/sysctl.d/99-sysctl.conf.

Tip: After changing settings in /etc/sysctl.d/99-sysctl.conf, you can load them with
# sysctl -p

Security

Preventing link TOCTOU vulnerabilities

See the commit message for when this feature was added for the rationale.

fs.protected_hardlinks = 1
fs.protected_symlinks = 1

Hide kernel symbol addresses

Enabling kernel.kptr_restrict will hide kernel symbol addresses in /proc/kallsyms from regular users, making it more difficult for kernel exploits to resolve addresses/symbols dynamically. This will not help that much on a precompiled Arch Linux kernel, since a determined attacker could just download the kernel package and get the symbols manually from there, but if you're compiling your own kernel, this can help mitigating local root exploits. This will break some perf commands when used by non-root users (but main perf features require root access anyway). See this bug for more information.

kernel.kptr_restrict = 1

Networking

Improving Performance

Warning: This may cause dropped frames with load-balancing and NATs, only use this for a server that communicates only over your local network.
# reuse/recycle time-wait sockets
net.ipv4.tcp_tw_reuse = 1
net.ipv4.tcp_tw_recycle = 1

TCP/IP stack hardening

#### ipv4 networking ####

## TCP SYN cookie protection
## 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

## 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

## source address verification (sanity checking)
## helps protect against spoofing attacks
net.ipv4.conf.all.rp_filter = 1

## disable ALL packet forwarding (not a router, disable it)
net.ipv4.ip_forward = 0

## log martian packets
net.ipv4.conf.all.log_martians = 1

## ignore echo broadcast requests to prevent being part of smurf attacks
net.ipv4.icmp_echo_ignore_broadcasts = 1

## optionally, ignore all echo requests
## this is NOT recommended, as it ignores echo requests on localhost as well
#net.ipv4.icmp_echo_ignore_all = 1

## ignore bogus icmp errors
net.ipv4.icmp_ignore_bogus_error_responses = 1

## IP source routing (insecure, disable it)
net.ipv4.conf.all.accept_source_route = 0

## send redirects (not a router, disable it)
net.ipv4.conf.all.send_redirects = 0

## ICMP routing redirects (only secure)
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.secure_redirects = 1

Troubleshooting

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

See also

  • The sysctl(8) and sysctl.conf(5) man pages
  • Linux kernel documentation (<kernel source dir>/Documentation/sysctl/)
  • SysCtl Config Tutorial
  • Kernel Documentation: IP Sysctl