Linux System Administration Recipes A Problem-Solution Approach

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CHAPTER 6 ■ SECURING YOUR SYSTEMSwhere you have this key set up; see recipe 6-1) with one of these keys. You won’t need to use yourpassphrase.Now, log out, and then log back in again. This time, you shouldn’t be challenged for yourpassphrases, because the ssh-agent session will still be running. Again, you should be able to log in tothat suitable machine without typing your passphrase. The ssh-agent session should keep running untilthe next time you reboot.■ Note This means that if you’re away from your computer, even for just a few minutes, then someone withphysical access to your machine will also be able to access any machines with keys managed by keychain/sshagent.Always lock your screen whenever you leave your machine! This is good security practice anyway.This long-term running may be seen as a security issue. If someone managed to log in as you, they’dautomatically be able to use your keys, as well. If you use the --clear option for keychain, then everytime you log in, your passphrases will be cleared, and you will be asked for them again. Substitute thisline for the earlier line 02:02 /usr/bin/keychain --clear ~/.ssh/id_rsa ~/.ssh/id_dsa > /dev/null■ Note The passphrases are cleared when you log in, not when you log out. This feature means that you can setyour cronjobs, and so on, to get the passphrases via ssh-agent, and it won’t matter if you’ve logged out overnight.6-3. Limiting rsync Over sshThere may be circumstances in which you want to be able to allow users to rsync to a server but not tossh into it, but you’d still rather run rsync over ssh (which is a good security practice). In general, it’s abad idea to allow users to log on to servers. Use the AllowUsers directive in /etc/ssh/sshd_config torestrict this:AllowUsersjkemp, manager, adminHowever, let’s say you want to allow users to back up their own laptops to a RAID array or other bigdisk. rsync by default these days runs over ssh, which is great in many obvious ways but means that ifyou restrict ssh login to the machine, rsync won’t work either. You could mount the disk over NFS from agateway machine, but that would mean the user transferring their data over two connections: ssh to thegateway machine and then NFS to the RAID array.There is a solution, however, which works like this:1. ssh is allowed, but only for a subset of users who have asked for backup access,using the AllowUsers directive in /etc/ssh/sshd_config:AllowUsersjkemp, admin, user1, user3138Download at WoweBook.Com
CHAPTER 6 ■ SECURING YOUR SYSTEMS2. These users are then added to /etc/password, with the shell set as/bin/nologin (thus overriding the LDAP data):user1:x:3434:102:User Name:/disk/raid/userdir:/bin/nologinThe second field points to /etc/shadow for the password; the third and fourthfields are the userID and groupID, which you’ll need to get out of LDAP withldapsearch "(uid=user1)". The sixth field sets their home directory, which inthis case is their user directory on the RAID array (or whatever other big disk isbeing used for laptop backup purposes). The seventh field is the one that setsthe shell to /bin/nologin.3. They’re also added to /etc/shadow, with *K* (meaning “use Kerberos”) as thepassword. This avoids maintaining two sets of passwords, which is a bad idea:user1:*K*::::(Since you’re getting password information from Kerberos, you can leave allthe fields in /etc/shadow other than the username and password blank—youneed four colons after the *K*.)4. /bin/nologin looks like this:01 #!/bin/sh02 # Script to disallow remote login - set as shell in /etc/passwd03 # JK 03.04.200904 command=$205 if (expr "$command" : 'rsync ..server .* .raid' > /dev/null)06 then07 if (expr "$command" : '.*;' > /dev/null)08 then09 exit10 else11 /bin/sh "$@"12 fi13 else14 echo "***********************************"15 echo "* No login allowed! *"16 echo "***********************************"17 exit18 fiLines 04 and 05 test to see whether the command being used is one that looksroughly like rsync directory/ server:/raid. The trick I used for finding thecommand that’s being sent (which is not the same as the command that youtype on the command line when you use rsync!) was to first set up/bin/nologin simply as this:#!/bin/shecho $@ > /tmp/commandexitand then examine /tmp/command on the rsync server. $2 is the second argumenton the line in /tmp/command.139Download 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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Page 191 and 192: C H A P T E R 8■ ■ ■Making Be
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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 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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