Linux – From where does sysctl -a print all kernel parameters

linuxsysctl

We have a Linux machine (Redhat 6)

When we print all parameters from /etc/sysctl.conf we see only ~20
but when we perform sysctl -a we see more than 200.

So from where sysctl -a print all these parameters ?

Or for example when we do

sysctl -w variable=value

how to know where the variable should be saved?

How to know each parameters if it is dynamic or static?

Best Answer

Sysctl settings are stored in the kernel. These settings influence kernel behavior; basically, they're variables of the kernel which programs running on the system can read and write.

When the kernel boots, each sysctl setting has a default value. This value can be changed at any time by a program such as the sysctl command, or, under Linux, by writing to the corresponding file under /proc/sys.

The file /etc/sysctl.conf does not determine the value of the settings at run time. It's only used at boot time, to change some settings from the default value compiled into the kernel. If you've made some changes to /etc/sysctl.conf, you can apply them as a whole by restarting the sysctl “service” — the sysctl service doesn't correspond to a running process, it's a pseudo-service that just loads the settings into the kernel when it starts.

If you want to change a setting so that the value is preserved across reboots, add it to /etc/sysctl.conf. To apply a setting temporarily or to try it out, use sysctl or echo … >/proc/sys/….

Related Solutions

Difference Between ‘all’, ‘default’, and ‘eth*’ in /proc/sys/net/ipv[46]/conf/

I've found the answer while still writing the question. I've decided to post it anyway because others may find this insightful, and then answer it myself; I hope this is not frowned upon :)

The user Philipp Matthias Hahn on the linux-kernel mailing list has figured it out at least partially:

As far as I researched for IPv4 some time ago, the "default" value gets
copied to newly created interfaces only once.
"all" on the other hand allways gets applied in addition to the current
setting, but it depends on the exact setting, if its ORed, ANDed, or
whatevered:
    log_martians         OR
    accept_redirects     AND
    forwarding           ?
    mc_forwarding        AND
    medium_id
    proxy_arp            OR
    shared_media         OR
    secure_redirects     OR
    send_redirects       OR
    bootp_relay          AND
    accept_source_route  AND
    rp_filter            AND
    arp_filter           OR
    arp_announce         MAX
    arp_ignore           MAX
    arp_accept
    app_solicit
    disable_policy
    disable_xfrm
    tag
(see include/linux/inetdevice.h:83 for IN_DEV_{AND,OR,MAX}CONF)

Putting a new value in "all" doesn't change the value you read from
"$interface", but it only gets computed and used internally.

He doesn't cover accept_ra but at least it's clear now how all and default work, or rather, how they do not work as I would have expected.

Change sysctl parameters for all interfaces

Try all and default:

# sysctl -w "net.ipv4.conf.all.rp_filter=0"
# sysctl -w "net.ipv4.conf.default.rp_filter=0"

This is explained in the Kernel documentation:

conf/default/*:    Change the interface-specific default settings.
conf/all/*:        Change all the interface-specific settings.

However note that putting a new value in all doesn't change the value you read from the interface, but it only gets computed and used internally. See this email for more details.

Best Answer

Related Solutions

Difference Between ‘all’, ‘default’, and ‘eth*’ in /proc/sys/net/ipv[46]/conf/

Change sysctl parameters for all interfaces

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