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.

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52 views

Why isn't net.ipv4.tcp_rfc1337 enabled by default?

The tcp_rfc1337 setting seems to have a solution for TIME-WAIT Assassination. The first problem is that old duplicate data may be accepted erroneously in new connections, leading to the sent data ...
0
votes
1answer
47 views

Irresponsive nginx while doing “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 ...
0
votes
1answer
51 views

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 ...
2
votes
0answers
23 views

running or building docker results in net.ipv4.ip_forward being reset

this is on a rhel6 system (docker 1.7.1 from epel). basically, everytime i do a build or a run, i can see that the net.ipv4.ip_forward sysctl gets reset to 0 - which results in the container loosing ...
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0answers
666 views

sysctl: cannot stat /proc/sys/net/ipv4/netfilter/ip_conntrack_max: No such file or directory

I try to change net.ipv4.netfilter.ip_conntrack_max by editing /etc/sysctl.conf net.ipv4.netfilter.ip_conntrack_max = 65535 and sysctl -p But i receive error: sysctl: cannot stat /proc/sys/net/...
0
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0answers
49 views

What could cause system failure in sysctl and how to test values safely?

On AWS ec2, ubuntu t2.micro. I tried implement suggested improvements in sysctl listed here: https://gist.github.com/devster31/5900ab23b50a5a57f025#file-sysctl-conf and also added from other ...
0
votes
1answer
128 views

increase max_connections in mysql on ubuntu wily

I have ubuntu wily installed on my server. I'm trying to increase max_connections of mysql server to 1237 but when running mysql client and executing show variables like 'max_connections'; i get 214. ...
0
votes
1answer
50 views

Redis and Postgresql on same machine: vm.overcommit_memory=?

In the Redis documentation it clearly states that vm.overcommit_memory should be set to 1 to ensure background saving newer fails: http://redis.io/topics/faq HOWEVER In postgresql documentation it ...
3
votes
1answer
44 views

enabling ufw disables some of the settings in sysctl.conf

When I add rules to ufw and then enable it, some of my settings in sysctl.conf become void so I have to do sysctl -p after doing ufw enable. So for example one of my settings in sysctl.conf is to not ...
0
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0answers
38 views

nginx monitoring failed to load status page under load

Nginx monitoring script so-called ztc failing to load nginx test page (mostly under highest load to nginx about 2000 rps, which used as proxy), causing errors like "nginx is down" on zabbix, and, in a ...
0
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0answers
78 views

Unusually high dentry cache usage leads to PHP performance degradation

We have lots of servers serving lots of requests with apache and php accessing remote memcacheds. But we are experiencing degradation from php side accessing memcached and obtaining lots of memcached ...
0
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1answer
290 views

NGINX not responding during DDoS attacks

I'm going to get straight to the point, NGINX doesn't seem to handle any HTTP requests during a DDoS attack using XML-RPC. The server only uses about 1% of the CPU during an XML-RPC DDoS attack. The ...
0
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0answers
474 views

centos 7: sysctl -p /etc/sysctl.d/50-default.conf returns error

I have rented container from OVH a while ago but last night all of a sudden my machine stopped responding on IP level... after reboot I have noticed the following systemd-sysctl.service - Apply ...
0
votes
0answers
166 views

UFW - Set correct kernel params

As every system administrator does, one of the first things i did right after booting up my web server was install ufw and tune/set kernel parameters in /etc/sysctl.conf. By the way, this is my /etc/...
0
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0answers
126 views

systemd difference between start and restart

In /etc/sysctl.d folder we have config files are going to be applied at boot time. I use debian 8 with systemd. And now procps is managed by systemd. Please, help me understand, why service procps ...
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0answers
2k views

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 ...
5
votes
3answers
389 views

OOM invoked with plenty of free swap

I am running an ARM router with i2p and tor on it - a Netgear R7000. Of course I've added a full 512 MB of SWAP to it to prevent OOMs, understanding that it could slow down the system... But then I ...
0
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0answers
563 views

Fast Ethernet UDP optimize receive buffers

I have a Linux computer which must receive streaming data from several devices (up to 30 or so) during long terms. This computer is connected to a Fast Ethernet local area network (100 Mbps) where ...
4
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4answers
5k views

Kernel Tuning with Privileged Docker Container

I'm building a container to tune kernel settings for a load balancer. I'd prefer to deploy those changes to the host in an image using a single privileged container. For example: docker run --rm --...
1
vote
1answer
312 views

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 ...
0
votes
2answers
2k views

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 ...
22
votes
1answer
22k views

How does vm.overcommit_memory work?

When I use the default settings: vm.overcommit_memory = 0 vm.overcommit_ratio = 50 I can read these values from /proc/meminfo file: CommitLimit: 2609604 kB Committed_AS: 1579976 kB But ...
4
votes
2answers
3k views

Permission denied for root to change inotify max_user_watches, how solve this?

As root in a CentOs 6.4 server I got this error in an application: Fri May 16 01:45:23 2014 Error: Terminating since out of inotify watches. Consider increasing /proc/sys/fs/inotify/max_user_watches ...
2
votes
2answers
3k views

How to enable core files on CentOS 6

I am trying to enable core files on a machine running CentOS 6; however, nothing I have tried has produced core files…here is what I have done: Added the following two lines to /etc/security/...
0
votes
1answer
2k views

TCP Zero windows size and full windows size

The problem that we are facing is that some of the http connection have response time > 60s (about 5%). I discover that the problem should between the web server and loadbalancer. Here is my finding, ...
8
votes
1answer
1k views

TCP packet being retransmitted 7 times when sysctl tcp_retries1 is set to 3 - why?

Ubuntu 12.04 I am trying to better understand how many times TCP will attempt to retransmit a packet when it does not receive confirmation the destination received it. After reading the tcp man page ...
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vote
0answers
67 views

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
2
votes
1answer
206 views

How do you change tcp_congestion_control on OpenVZ?

I have a network of OpenVZ machines that I am testing various congestion control methods on. I need to know how to change the congestion control. Currently I get nothing but permission denied. Is it ...
0
votes
1answer
256 views

Permission denied when setting values in sysctl on Ubuntu 12.04

I am trying to configure my Ubuntu 12.04 VPS (OpenVZ) to reboot when it runs out of memory. To do this I have added following lines to my "/etc/sysctl.conf" file. vm.panic_on_oom = 1 kernel.panic = ...
0
votes
2answers
408 views

nginx or kernel queueing traffic

We have a setup that looks like this: nginx->haproxy->app servers We are terminating SSL with nginx and it sits in front of everything. During our peak load times, we are experiencing about a 2x ...
0
votes
2answers
1k views

Should rp_filter sysctl parameter be set on bonding master or member slaves?

I have a server running RHEL6.4. I need to configure .rp_filter sysctl parameter for my server's private interconnect. This server has 2 NICs bonded together. I looked through the documentation and ...
-1
votes
1answer
92 views

available file handles is 0, how to increase [closed]

Since some days, we're getting "Too Many Open Files" within an Java application while loading files. So i start searching on Google, and increased fs.file-max to 200000 in my /etc/sysctl.conf. After ...
6
votes
2answers
5k views

Netstat -s showing (and growing) “packets pruned from receive queue” and “packets collapsed in receive queue”

we are seeing the following: [root@primary data]# netstat -s | grep buffer ; sleep 10 ; netstat -s | grep buffer 20560 packets pruned from receive queue because of socket buffer overrun ...
2
votes
1answer
3k views

No buffer space available and tuning with sysctl

Not really a network guy so please bear with me. I'm currently hosting a web application that sends UDP to all IP addresses in it's subnet periodically. That's 2048 IPs. My problem is it throws No ...
5
votes
1answer
3k views

Linux networking port exhaustion

I've done as much research as I could on this without digging right off in the kernel source. There seems to be a large amount of disinformation/incorrect info on the subject, so I hope this answers ...
9
votes
1answer
317 views

Difference between ulimit, launchctl, sysctl?

When managing the limits of the operating system (specifically OSX), what is the difference and overlap between the functionality and history of these three tools: ulimit launchctl, launchd sysctl ...
2
votes
1answer
5k views

nf_conntrack: table full, dropping packet

Mar 24 03:29:26 kernel: [1557411.243821] TCP: time wait bucket table overflow (CT0) Mar 24 03:29:26 kernel: [1557411.243828] TCP: time wait bucket table overflow (CT0) Mar 24 03:29:26 kernel: [1557411....
9
votes
3answers
5k views

Where is the correct place to set net.netfilter.nf_conntrack_buckets?

I'm currently trying to set net.netfilter.nf_conntrack_buckets on boot. I initially assumed that this could be done through sysctl.conf, but net.netfilter.nf_conntrack_buckets (and other net....
4
votes
4answers
15k views

sysctl -p /etc/sysctl.conf returns error

In a fresh installation of CenotOS 6 in a VPS after running sysctl -p /etc/sysctl.conf I got this errors: error: "net.bridge.bridge-nf-call-ip6tables" is an unknown key error: "net.bridge.bridge-nf-...
2
votes
2answers
2k views

Xserve 10.6.8: Why do `net.inet.ip.forwarding` & `net.inet.ip.fw.enable` kernel sysctl settings stay enabled even disabling?

Very bizarre situation. I have inherited a few Xservers from a previous admin that I have been charged with cleaning up on various levels. I am familiar with settings in Ubuntu & am very ...
0
votes
2answers
529 views

Networking (dhcpcd) under LXC

I would like to boot an Arch Linux instance using LXC. Just about everything works, but I'm encountering the following error when trying to configure an interface using dhcpcd: # dhcpcd eth0 dhcpcd[...
0
votes
2answers
425 views

yum not updating /etc/rc.d/init.d/iptables

I have been hit by the bug https://bugzilla.redhat.com/show_bug.cgi?id=493226 (restarting iptables ignores values in /etc/sysctl.conf). This bug has been fixed by RH quite a while ago. The server ...
3
votes
1answer
2k views

How to make sysctl network bridge settings persist after a reboot?

I am setting up a notebook for software demo purpose. The machine has 8GB RAM, a Core i7 Intel CPU, a 128GB SSD, and runs Ubuntu 12.04 LTS 64bit. The notebook is used as a KVM host and runs a few KVM ...
1
vote
2answers
2k views

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 ...
6
votes
2answers
8k views

How to reset sysctl to it's OS defaults?

I couldn't find an answer in the man pages for sysctl. My sysctl.conf file is still at it's default state, I'm testing several values and loaded new settings via sysctl -p newsettings.conf. I though ...
1
vote
1answer
3k views

Linux webhost security settings in /etc/sysctl.conf

While searching for more ways to secure Linux servers, I found the following /etc/sysctl.conf configuration. It came as is, without much explanation. Before using it on production environment (using ...
5
votes
2answers
11k views

Higher rmem_max value leading to more packet loss

The rmem_max Linux setting defines the size of the buffer that receives UDP packets. When traffic becomes too busy, packet loss starts occurring. I made a graph showing how packet loss increases ...
1
vote
2answers
2k views

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"...
7
votes
2answers
5k views

Modifying kernel shared memory settings on a lion install

What's the location of the sysctl.conf file on lion? In Snow Leopard it was in /etc/sysctl.conf but now that folder doesn't contain it anymore. Searching for the file in spotlight yields no results. ...
7
votes
4answers
5k views

Suggestions for making sysfs parameters persist across reboots

I'm experimenting with large changes to Linux system runtime parameters exposed through the sysfs virtual file system. What is the most efficient way to maintain these parameters so that they persist ...