sysctl.d - Configure kernel parameters at boot



  • SYSCTL.D(5)                        sysctl.d                        SYSCTL.D(5)
    
    NAME
           sysctl.d - Configure kernel parameters at boot
    
    SYNOPSIS
           /etc/sysctl.d/*.conf
    
           /run/sysctl.d/*.conf
    
           /usr/lib/sysctl.d/*.conf
    
    DESCRIPTION
           At boot, systemd-sysctl.service(8) reads configuration files from the
           above directories to configure sysctl(8) kernel parameters.
    
    CONFIGURATION FORMAT
           The configuration files contain a list of variable assignments,
           separated by newlines. Empty lines and lines whose first non-whitespace
           character is "#" or ";" are ignored.
    
           Note that either "/" or "."  may be used as separators within sysctl
           variable names. If the first separator is a slash, remaining slashes
           and dots are left intact. If the first separator is a dot, dots and
           slashes are interchanged.  "kernel.domainname=foo" and
           "kernel/domainname=foo" are equivalent and will cause "foo" to be
           written to /proc/sys/kernel/domainname. Either
           "net.ipv4.conf.enp3s0/200.forwarding" or
           "net/ipv4/conf/enp3s0.200/forwarding" may be used to refer to
           /proc/sys/net/ipv4/conf/enp3s0.200/forwarding.
    
           The settings configured with sysctl.d files will be applied early on
           boot. The network interface-specific options will also be applied
           individually for each network interface as it shows up in the system.
           (More specifically, net.ipv4.conf.*, net.ipv6.conf.*, net.ipv4.neigh.*
           and net.ipv6.neigh.*).
    
           Many sysctl parameters only become available when certain kernel
           modules are loaded. Modules are usually loaded on demand, e.g. when
           certain hardware is plugged in or network brought up. This means that
           systemd-sysctl.service(8) which runs during early boot will not
           configure such parameters if they become available after it has run. To
           set such parameters, it is recommended to add an udev(7) rule to set
           those parameters when they become available. Alternatively, a slightly
           simpler and less efficient option is to add the module to modules-
           load.d(5), causing it to be loaded statically before sysctl settings
           are applied (see example below).
    
    CONFIGURATION DIRECTORIES AND PRECEDENCE
           Configuration files are read from directories in /etc/, /run/, and
           /usr/lib/, in order of precedence. Each configuration file in these
           configuration directories shall be named in the style of filename.conf.
           Files in /etc/ override files with the same name in /run/ and
           /usr/lib/. Files in /run/ override files with the same name in
           /usr/lib/.
    
           Packages should install their configuration files in /usr/lib/. Files
           in /etc/ are reserved for the local administrator, who may use this
           logic to override the configuration files installed by vendor packages.
           All configuration files are sorted by their filename in lexicographic
           order, regardless of which of the directories they reside in. If
           multiple files specify the same option, the entry in the file with the
           lexicographically latest name will take precedence. It is recommended
           to prefix all filenames with a two-digit number and a dash, to simplify
           the ordering of the files.
    
           If the administrator wants to disable a configuration file supplied by
           the vendor, the recommended way is to place a symlink to /dev/null in
           the configuration directory in /etc/, with the same filename as the
           vendor configuration file. If the vendor configuration file is included
           in the initrd image, the image has to be regenerated.
    
    EXAMPLES
           Example 1. Set kernel YP domain name
    
           /etc/sysctl.d/domain-name.conf:
    
               kernel.domainname=example.com
    
           Example 2. Apply settings available only when a certain module is
           loaded (method one)
    
           /etc/udev/rules.d/99-bridge.rules:
    
               ACTION=="add", SUBSYSTEM=="module", KERNEL=="br_netfilter", \
                     RUN+="/usr/lib/systemd/systemd-sysctl --prefix=/net/bridge"
    
           /etc/sysctl.d/bridge.conf:
    
               net.bridge.bridge-nf-call-ip6tables = 0
               net.bridge.bridge-nf-call-iptables = 0
               net.bridge.bridge-nf-call-arptables = 0
    
           This method applies settings when the module is loaded. Please note
           that, unless the br_netfilter module is loaded, bridged packets will
           not be filtered by Netfilter (starting with kernel 3.18), so simply not
           loading the module is sufficient to avoid filtering.
    
           Example 3. Apply settings available only when a certain module is
           loaded (method two)
    
           /etc/modules-load.d/bridge.conf:
    
               br_netfilter
    
           /etc/sysctl.d/bridge.conf:
    
               net.bridge.bridge-nf-call-ip6tables = 0
               net.bridge.bridge-nf-call-iptables = 0
               net.bridge.bridge-nf-call-arptables = 0
    
           This method forces the module to be always loaded. Please note that,
           unless the br_netfilter module is loaded, bridged packets will not be
           filtered with Netfilter (starting with kernel 3.18), so simply not
           loading the module is sufficient to avoid filtering.
    
    SEE ALSO
           systemd(1), systemd-sysctl.service(8), systemd-delta(1), sysctl(8),
           sysctl.conf(5), modprobe(8)
    
    systemd 239                                                        SYSCTL.D(5)
    

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