Linux System Administration Recipes A Problem-Solution Approach

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CHAPTER 6 ■ SECURING YOUR SYSTEMS■ Note In some versions of Debian/Ubuntu, the /etc/hosts file contains the following line:127.0.0.1 localhost localhost.localdomainThis breaks SSH Kerberos. Instead, edit it to read as follows:127.0.0.1 localhostNow (still on the server side), start up kadmin with /usr/bin/kadmin -p krbadm (replace krbadm withyour Kerberos admin user if it’s different), and enter these commands:ank -randkey host/servername.example.com@EXAMPLE.COMktadd -k /etc/krb5.keytab host/servername.example.com@EXAMPLE.COMreplacing servername.example.com and EXAMPLE.COM with the fully qualified domain name of your serverand with your Kerberos domain, respectively.Restart sshd with /etc/init.d/ssh restart.You’ll also need to edit /etc/ssh/ssh_config on the client to contain this line:GSSAPIAuthentication yesNow check that you have a current Kerberos ticket on the client machine with klist, and then ssh tothe server with ssh servername. You should log in without being challenged for your password. Log outagain, delete your ticket with kdelete, and try again. This time you’ll be asked for your password as youwould usually expect.■ Note Bear in mind that Kerberos tickets do expire; if your setup obtains a ticket when you log in via GDM andyou customarily leave yourself logged in overnight, then the next morning you’ll need to log in to Kerberos againusing kinit before you can log into other machines without a password as described here. It’s possible to extendthe default life span of a Kerberos ticket, but this is unwise because it has obvious security implications.6-8. Setting and Enforcing a Password Policy with KerberosThere are two main issues with a password policy: creating it and enforcing it. When setting up apassword policy, it’s important to bear in mind that forcing people to change their passwords too oftencan be counterproductive. If your users have problems remembering stacks of new passwords, they’llresort to writing them down instead.144Download at WoweBook.Com
CHAPTER 6 ■ SECURING YOUR SYSTEMSIf you’re using Kerberos, you can set a minimum length and complexity for your passwords usingthe policy settings. To check what your current default policy is, type kadmin -p krbadm, and then typegetpol default. Your output should look something like this:Policy: defaultMaximum password life: 180 daysMinimum password life: 0Minimum password length: 6Minimum number of password character classes: 2Number of old keys kept: 5Reference count: 40■ Note To show all available policies, use listpols.Most of this is pretty self-explanatory. Number of old keys kept limits the reuse of passwords (sohere, a user can’t reuse a password until they’ve changed it five times). The available password characterclasses are lowercase letters, capital letters, punctuation, numbers, and other characters. Here, thepolicy insists that at least two of these are used, which is a good minimum. The minimum passwordlength here should be increased, as well; eight or ten characters would be better.■ Note If you don’t have a default policy already, you can set one up using addpol default; then use getpoldefault to find out what the current settings are, and modify it using modify_policy as next.To modify the password length for the default policy, use modify_policy (which has the aliasmodpol). Enter this in the kadmin console:modpol -minlength 8 default■ Note Setting the character class value to 3 enforces the nonuse of dictionary words, since you then need tohave at least one nonletter character in your password.modpol -minclasses 3 defaultIf you set the default policy, any principal added who has a password will have this policy applied bydefault. You can also set up named policies. For example, you could have one very tough policy foradmins and a laxer one for regular users:145Download 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 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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