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To better control the resources available to processes started by the shell, the ulimit facility can be used System-wide defaults can be configured using the /etc/security/ limitsconf file ulimit options can be used to control such things as the number of files that may open, how much memory they may use, CPU time they may use, how many processes they may open, etc The settings are read by the PAM (Pluggable Authentication Module) libraries when a user starts up The key to choosing ulimit values is to consider the purpose of the system For example, in the case of an application server, if the application is going to require a lot of processes to run, then the system administrator needs to ensure that ulimit caps don t cripple the functionality of the system Other types of servers, such as a Domain Name System (DNS) server for example, should not need more than a small handful of processes It should be noted that there is a caveat here: PAM has to have a chance to run to set the settings before the user does something If the application starts as root and then drops permissions, PAM is not likely to run From a practical point of view, this means that having individual per-user settings is not likely to do you a lot of good in most server environments What will work are global settings that apply to both root and normal users This detail turns out to be a good thing in the end; having root under control helps keep the system from spiraling away both from attacks and from broken applications
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A common trick that students still play on other students is to log into their workstations and run a fork bomb This is a program that simply creates so many processes that it overwhelms the system and brings it to a grinding halt For a student, this is annoying For a production server, this is fatal A simple shell-based fork bomb using Bourne Again Shell (BASH) is
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[yyang@serverA ~]$ while true; do sh -c sh & done
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If you don t have protections in place, this script will crash your server The interesting thing about fork bombs is that not all of them are intentional Broken applications, systems under denial-of-service attacks, and sometimes just simple typographical errors entering commands can lead to bad things happening By using the limits described in this chapter, you can mitigate the risk of a fork bomb by restricting the maximum number of processes that a single user can invoke While the fork bomb may still cause your system to become highly loaded, it will still likely remain responsive enough to allow you to log in and deal with the situation, all the while hopefully maintaining the services offered It s not perfect, but it is a reasonable balance between dealing with the malicious and not being able to do anything at all
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The format of each line in the /etc/security/limitsconf file is as follows:
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<domain> <type> <item> <value>
Any line that begins with a pound sign (#) is a comment The domain value holds the login of a user or the name of a group; it can also be a wildcard (*) The type refers to the type of limit as soft or hard The item refers to what the limit applies to The following is a subset of items that an administrator might find useful: Item fsize nofile cpu nproc maxlogins Description Maximum file size Maximum number of open files Maximum amount of time (in minutes) a CPU can be used Maximum number of processes that a user can have Maximum number of logins for a user Fedora Defaults Unlimited 1024 Unlimited 4096 (2048 in Ubuntu/Debian) Unlimited
A reasonable setting for most users is to simply restrict the number of processes, unless there is a specific reason to limit the other settings If you need to control total disk usage for a user, you should use disk quotas instead An example for limiting the number of processes to 128 for each user would be
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