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Questions tagged [sysctl]

sysctl is a mechanism for controlling options and limits in the Unix-like operating system kernels. Sysctls can be used to tune performance or change behaviors of many subsystems.

Linux:
The sysctl command and configuration file use the /proc filesystem's sys directory to read information and apply changes.

The Linux Journal has an article about using sysctl, but most for the most complete information look to this description of /proc/sys's contents.

BSD-derived:
The sysctl command is used to read and configure certain system settings. The Handbook describes its general use in more detail, and the FreeBSD Performance Tuning Community Wiki has some ideas.

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Overwhelmed by "TCP: time wait bucket table overflow" errors -- What can I do to mitigate?

I've got a legacy system running Debian 7 (proxmox) hosting OpenVZ containers, and I'm seeing a troublesome problem where the system is being overwhelmed by open connections to VZ container running the apache frontend. When this is happening, the…
Ross Messiah
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Unable to access Ubuntu EC2 ssh, Connection to Server-IP Closed

My server was having error due to high traffic and it showed this message often when I access the website Error: EMFILE: too many open files, open '/home/ubuntu/chat2/public/style.css' at Error (native) To fix this, I googled and tried to…
Faizan
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net.inet.tcp.blackhole option on Fedora

This link of freebsdblog.org suggests a configuration option for FreeBSD that tells the system to drop all packages arriving from the network to a closed port and thus somewhat securing your system and also saving some upload bandwidth as well as…
Mark Szente
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SYN flooding on port 443 while nginx reload

While reloading nginx, I started getting errors in messages log "possible SYN flooding on port 443", and it seems like nginx becomes completely irresponsive at that time (quite for a while), cause zabbix reports "nginx is down" with ping 0s. RPS at…
d.ansimov
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sysctl: cannot stat /proc/sys/kernel/exec-shield: No such file or directory

Enable ExecShield protection - ExecShield is security Linux kernel patch to avoid worms and other problems. After Adding to /etc/sysctl kernel.exec-shield = 1 Getting this error sysctl: cannot stat /proc/sys/kernel/exec-shield: No such file or…
Ashish Karpe
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Slow POST upload (Linux kernel Tuning)

I'm working in a Caucho Resin backend with an HAproxy configured in front of them, in a dedicated server. Now I'm dealing with a file upload problem. Our web application allows a file upload for images, if I hit directly to one of my Resin servers…
Masterl1nk
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Recommended TCP keepalive settings for a busy server

We are having some network timeout issues on a Debian server that is quite busy, and maintains multiple connections to a number of other servers on the network. Here are our current TCP keepalive settings in…
UpTheCreek
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How to learn max/min value of a sysctl parameter can be in FreeBSD?

I have a simple question. I want to learn max/min value of a sysctl parameter can be. Where can I find bounds of these parameters? Thanks
ibrahim
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Writeback cache in linux

In linux how do I have a writeback cache on disks? Basically, I'd like to setup a 100MB RAM cache for writing that writes out every 3-5 minutes or when full. Is this possible? Is there a sysctl setting to tweak? Thanks!
user101130
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SYN Cookies on a LAN server - Ok / advisable to disable?

We are looking at tuning our linux tcp stack on one of our lan boxes. We were wondering if it is ok / advisable to disable SYN cookies on a server that doesnt have any sort of external (eg: "internet") access. Is it ok to disable syn cookies on a…
anonymous-one
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ipv6 max_addresses in sysctl.conf (Debian 6)

I'd like to assign any number of IPv6 addresses to a Debian 6 host, but I see the following setting, and want to know what are the consquences of setting higher, or does this setting approach limits by affecting system…
mpbloch
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net.ipv6.conf.all.forwarding = 1 is an unknown key

I have few system with ipv6 addresses. I want to forward request from one system to another. hence I kept the option net.ipv6.conf.all.forwarding = 1 in sysctl.conf but the request is not getting forwarded as it is doing in ipv4, its giving me the…
sanketmlad
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Persistent changes to /proc/sys/sunrpc/tcp_slot_table_entries

I'm trying to make a persistent change to sunrpc.tcp_slot_table_entries on a Linux CentOS 5.5. This value has been found important for the performance of our NFS clients, and must be set before the NFS mounts are done. Simply putting the value in…
David142
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Sysctl config for high performance, large activity

We have some server with a huge amount of visitors (every minute, every day). Basically we had a nice fine-tuned sysctl config for this purpose. (If you don't modify it, it starts to behave strangely). I'd like to ask the community if you could…
Apache
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Disable ipv4 with /etc/sysctl.conf | GNU/LINUX Debian 11

I can disable ipv6 as su in /etc/sysctl.conf on my GNU/LINUX Debian 11 with net.ipv6.conf.all.disable_ipv6 = 1 net.ipv6.conf.default.disable_ipv6 = 1 and restart with /sbin/sysctl -p when i try to disable ipv4 in /etc/sysctl.conf and restart…
Z0OM
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