Linux System Administration Recipes A Problem-Solution Approach

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CHAPTER 6 ■ SECURING YOUR SYSTEMSYou can run one of these options at a time (in which case, try “single crack” mode first) or run all ofthem consecutively with this:john /tmp/password.dbTo show the results, use this:john --show /tmp/password.dbYou can then insist on a password change if any of your users have failed the test!■ Note Also remember that you can limit cracking attempts through measures such as locking out specific IPaddresses after multiple failed ssh attempts (see recipe 6-13) or limiting the number of times a user can get apassword wrong when logging on.6-11. Limiting sudoThe /etc/sudoers file sets up who can use sudo (that is, who can masquerade as root) and under whatcircumstances. The basic usage is just to permit a user to have all the privileges that root does, but youcan also do far more than that.It’s best to edit /etc/sudoers (you’ll need to be root to do this or use sudo with a username that’salready in the file) with the command visudo, because that will check that the file can be parsed correctlybefore it’s saved.■ Note visudo won’t necessarily use vi as the editor; it’ll use whatever is the default editor for the user you’relogged in as.The most basic sudoers line would be to permit a particular user to do absolutely anything withsudo:jkempALL = (ALL) ALLALL is an automatically defined alias, which means what it says on the tin. This is the format of thisline:user Host = (Runas) commanduser is the username that you’re running sudo from. Host is the machine that you’re running sudo onor trying to access. Runas is the user that you want to use sudo to log in as. command is the command youwant to run.148Download at WoweBook.Com
CHAPTER 6 ■ SECURING YOUR SYSTEMSSo, in that first example, jkemp can use sudo on any machine to authenticate as any user and run anycommand. Basically, they can do anything they want! (If you look at the /etc/sudoers file, you’ll see thatthis is the same privilege level as root has.)To make life a little easier (and more maintainable), you can set up aliases in the /etc/sudoers file.So, instead of specifying your user directly, you can set up an alias for them:User_AliasSYSADMINSSYSADMINS = jkemp, pgillinALL = (ALL) ALLThis gives both jkemp and pgillin the same full privileges as before. (This group could just as wellcontain only jkemp.)■ Note You can also use groups that exist on the system:%adminALL = (ALL) ALLThis would give all members of the admin group all privileges.Similarly, you can limit the commands that people are able to run. Let’s say that one of your userswants to be able to stop and start their printer. You can allow them to do this with this setup:User_AliasLOCAL = ksmithCommand_Alias PRINTER = /etc/init.d/cupsdLOCAL client3.example.com PRINTING■ Note The (Runas) section is not required; if it’s left blank, root is assumed.ksmith will now be able to run sudo /etc/init.d/cupsd restart on their desktop, client3, whenrequired. The Command_Alias can also be a list of commands, and more can be added in the specificationline. You should always give the full path to a command (to make sure that it’s the command you think itis or at least that someone would have to hack the box first to change the command rather than justwriting their own version in their own directory). Also, you shouldn’t allow commands that are shells(/bin/bash, /bin/tcsh, and so on) or that you can shell out from, because this is effectively the same asallowing any command.6-12. sudo: Figuring Out Which Password to UseThe previous recipe discusses some of the basic ways of setting up sudo access. You can also set a varietyof options having to do with the use of passwords.149Download at WoweBook.Com
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Contents■About the Author .......
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■ CONTENTS3-7. Writing a Nagios P
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■ CONTENTS■Chapter 8: Using the
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■ CONTENTSAbout the Author■Juli
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■ CONTENTSAcknowledgmentsMany tha
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■ CONTENTSIntroductionThis book i
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■ INTRODUCTIONDownloading the Cod
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C H A P T E R 1■ ■ ■Saving Yo
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CHAPTER 1 ■ SAVING YOURSELF EFFOR
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C H A P T E R 2■ ■ ■Centraliz
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CHAPTER 2 ■ CENTRALIZING YOUR NET
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C H A P T E R 3■ ■ ■Monitorin
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CHAPTER 3 ■ MONITORING AND UPDATI
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C H A P T E R 4■ ■ ■Taking Ba
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CHAPTER 9 ■ WORKING WITH TEXT IN
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CHAPTER 9 ■ WORKING WITH TEXT IN
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C H A P T E R 10■ ■ ■Things G
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CHAPTER 10 ■ THINGS GO IN, THINGS
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C H A P T E R 11■ ■ ■Tracking
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CHAPTER 11 ■ TRACKING DOWN BUGSTh
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CHAPTER 11 ■ TRACKING DOWN BUGS01
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CHAPTER 11 ■ TRACKING DOWN BUGS
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CHAPTER 11 ■ TRACKING DOWN BUGSTh
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CHAPTER 11 ■ TRACKING DOWN BUGSAn
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CHAPTER 11 ■ TRACKING DOWN BUGS#
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C H A P T E R 12■ ■ ■Managing
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CHAPTER 12 ■ MANAGING TIME AND PE
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APPENDIX■ ■ ■Perl TipsSee rec
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APPENDIX ■ PERL TIPS• Date::Par
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APPENDIX ■ PERL TIPSPerl Syntax N
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■ ■ ■Index■Symbols$CDPATH,
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■ INDEXenscript option (printing)
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■ INDEX■NNagiosadding hosts to,
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■ INDEXsyslogcentralized logging
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Linux System Administration Recipes A Problem-Solution Approach

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