Ever wonder why repetitive pings fail after a little while in OS X (e.g. those sent via the -f flag)? By default, OS X has an ICMP rate limit of 250 set. You can increase this or disable, using sysctl. To disable, set the value of net.inet.icmp.icmplim
sudo sysctl -w net.inet.icmp.icmplim=0
Happy icmp flooding!
krypted September 20th, 2016
The core dump is a memory image of a processes in-core state that is written to the /cores directory by default and is named core followed by a . and then the pid number. So if a process with pid 87 crashes a file is written by default at /cores/core.87. The feature can be turned off using sysctl, along with the kern.coredump MIB, setting it to 0 (by default it’s 1).
To turn it back on:
Additionally, you can change the path that the files are written into, using sysctl along with the kern.corefile MIB. The corefile is an expression that by defualt writes core.%P where %P is the PID of the process whose memory image is being written. If you wanted to change the path for the corefiles to /cores2, you would run the following:
To change it back to the default:
The sysctl command edits the state but doesn’t persist across reboots, so echo the above lines into /etc/sysctl.conf in order to have them be persistent. To disable core dumps:
echo 'kern.coredump=0' >> /etc/sysctl.conf
Or to change the location:
echo 'kern.corefile=/cores/core2.%P' >> /etc/sysctl.conf
Changing anything with regard to how core dumps would should typically be temporary. If there are no other customizations to sysctl.conf then you can undo these changes by removing the file; otherwise, simply remove the two lines and restart for the kernel to go back to its original state.
krypted August 21st, 2013
“My computer sometimes just runs slow,” “the fan on my laptop won’t turn off sometimes,” and “my network connection keeps dropping.” These are amongst the most annoying off problems to solve for our users because they are intermittent. And to exacerbate things, many of these users have these problems at home or at remote locations, making it difficult for systems administrators to see them.
There is something I use in these cases, though, that has helped isolate these problems from time to time. Simply tell users to Control-Option-Command-Shift-Period when they have these problems. Doing so will run the sysdiagnose command and then open a Finder window with the output of the command. Sysdiagnose takes a quick snapshot of many common logs and performance data, zips it up and opens a Finder window, pointing to where it is (/var/tmp with the filename containing a date stamp of when the command was run). This file contains output from allmemory, lsof, top, netstat, sysctl, spindump, fs_usage, system_profiler, mount, airport, odutil and many others. Each is in its own log and easy to navigate.
When running /usr/sbin/sysdiagnose from the command line there are a couple of options. My favorite is -f (which I think must be short for favorite) which allows me to write to my file to a directory I specify rather than some random object in a tmp directory. You can also get even more output using -t. Verbose logging is obtained using -h and passing a pid will also provide information about the pid. So let’s say that process 10883 is giving me some problems. I could run the following to get some good output on my desktop:
sysdiagnose -h -t -f ~/Desktop
Anyway, hope you enjoy!
krypted May 13th, 2013
I’ve done a number of articles on using Ubuntu 10 as a server recently, but haven’t actually looked at doing the base installation of an Ubuntu 10 host. In this example, I’ll look at using Ubuntu 10.04 Desktop. In many of the previous examples I’ve been looking at Ubuntu 10.10 Server; the reason I’m using 10.04 Desktop here is because I believe there is a smaller learning curve and that inherently Mac OS X Systems Administrators who might be following this thread actually like a GUI. There are a number of aspects of this type of setup that are simply not GUI oriented; however, the base OS can easily be, so here goes.
First up, download the installer of Ubuntu from http://www.ubuntu.com/desktop/get-ubuntu/download. Then, install Fusion. Once installed you’ll be prompted with the welcome screen.
You will then be prompted for credentials that the virtual machine will give the guest operating system when it is installed. Here, type the administrative user name and password that you want to use. You can also choose whether or not you want to make the home folder you use in Mac OS X available to the virtual machine as well as what type of access the virtual machine has to that directory. When you’re satisfied with your settings, click on the Continue button.
At the Finish screen of the New Virtual Machine Assistant, you will be able to review the settings that have been provided to the virtual machine. You can change these later if you see fit. For now, let’s click on the Finish button.
Finally, choose where you want to install the virtual machine at. By default, the virtual machine will be placed in the Virtual Machines folder of your home directory. I usually like to move it to a Virtual Machines directory on the root of the volume that houses my Virtual Machines, but you can place yours wherever you like. When you’ve selected the folder that best fits your needs, click on the Save button.
The virtual machine will then install. This process can take some time, so it’s probably a good chance to grab a bite. When it’s done, you’ll be at the login screen for Ubuntu. Enter the username and password that you provided earlier in the process and then click on the Log In button.
By default, the virtual machine will be sharing the network connection of the Mac. Click on the second radio button (Connect directly to the physical network) and then the indicator light for the interface will go red. Wait for the light to go green, indicating that it’s picked up the correct interface and then close the Settings.
The IP will then need to be set for the guest OS. From Ubuntu, click on the System menu at the top of the screen and then click on Preferences and then Network Connections. Here, click on the Auth eth0 interface and then click on the Edit button.
You should now see the Editing Auth eth0 screen. Here, click on the IPv4 Settings tab and then provide the Address, Subnet mask (Netmask) and Gateway for your environment. You should also take this opportunity to provide a DNS server.
Click on Apply to commit your changes and then reboot the virtual machine so the new network settings are enforced. When Ubuntu comes back online, you should then be able to ping your router or some other device on your network. If you decided to use Ubuntu Server then you will need to go to /etc/network/interfaces and add some lines to bring up the interface using nano or vi, then set the IP to static and then provide your settings. They would appear as follows:
iface lo inet loopback
iface eth0 inet static
Note: Check out ‘man interfaces’ for more information on building out your interfaces file.
You would also need to provide DNS information in your /etc/resolv.conf file:
Note: Check out man resolv.conf for more information on the correct syntax and options if you need more that what we have provided here.
As you can see, doing so in the GUI vs. the command line is almost identical in terms of the amount of time it takes. Next, check the hostname. For this, let’s use the terminal emulator (not as spiffy as the one in Mac OS X, but nice nonetheless). Click on the Applications menu, Accessories and then Terminal. As with Mac OS X Server the forward and reverse names should match. Provided they do, you’re ready to get some services installed; otherwise you will need to set the hostname to be the same as the DNS name. Assuming the DNS name is ubuntu08.krypted.com:
To then make it persistent across a restart, check /etc/hostname and replace the hostname with whatever you see there. Once set, you should see the hostname at the login window. Finally, I ran into an instance a few years back where Debian (not Ubuntu but close enough) wouldn’t change the hostname even after I tweaked the /etc/hosts and /etc/hostname files. Very annoying. The only thing that would work was to do it using sysctl (continuing on with the same example):
Assuming that your Ubuntu box isn’t also acting as your DNS server, you will also need to check the DNS to make sure it’s correctly set. You can use nslookup for this:
krypted November 21st, 2010
Shared Memory Tweaking: 1GB Shared memory (set in /etc/sysctl.conf)
krypted May 25th, 2008
Posted In: Mac OS X
The vnode table represents all file activity in UNIX. There is a unique vnode allocated in the vnode table for each active file, each current directory, each mounted-on file, text file, and the root. To see the number of vnodes available in Mac OS X, look to sysctl for the kern.maxvnodes variable using something similar to the following command:
sysctl -A | grep vnode
krypted January 12th, 2005