Linux System Administration Recipes A Problem-Solution Approach

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CHAPTER 11 ■ TRACKING DOWN BUGS■ Note You may have heard of the idea of “teddy bear programming.” There’s a story told in The Practice ofProgramming (among other places) of a university computing support department that had a teddy bear sitting bythe door. Students were allowed to take their problem to a human only if they’d already explained it to the bear. Itwas fairly common that the act of explaining the problem was sufficient to give the person doing the explaininganother idea to try or a realization of what might be going wrong. A similar thing sometimes occurs, in myexperience, when writing an e-mail to a mailing list and trying to describe the problem clearly and succinctly andanticipating possible queries. Explaining your problem to a co-worker can also work in the same way. Theimportant point is that the act of talking it through can help you come to a better understanding of what’s going on,or even to a solution.Once you have a clear understanding of the problem, the next thing to do is to try to narrow downthe problem space. Generate a test case from your initial bug report, and then shrink the test case downas far as you can while still demonstrating the problem. When your increasingly small test eventuallystops showing the problem, that’s more valuable information in tracking down the bug.You can also run your test case as another user, as root, as the same user on another machine, andas root or another user on another machine. All these tests will help establish a space where the problemmight be. It’s well worth running as many tests as you can before you start actually doing any work inchanging anything; otherwise, you won’t necessarily be able to tell what it is you did that fixed theproblem (see Chapter 1).If the machine you’re working on is a desktop machine, if the problem is a config or software-relatedone, and if it’s showing up only on this machine, consider how much time you’re prepared to spend ontrying to fix it. It’s good practice to have your desktops basically interchangeable and to have an installsetup that’s as straightforward and hands-off as you can possibly make it. My own rule of thumb is that ifsuch a problem is still unsolved after an hour, it’s time to kick off a reinstall. (All data partitions areindependent from all root and boot partitions in my work setup, so this doesn’t cause any dataproblems.) The exact amount of time may vary for you, but chances are, there’s a point at which itdoesn’t make much sense to keep throwing your time and energy at a problem that will often be fixed bya quick, clean reinstall. This may seem like a bit of a cheat (surely on <strong>Linux</strong> you should always be able tofind and fix a bug “properly”!), but sometimes cheating is the most pragmatic thing to do if your aim is tokeep your systems running and your users happy. An alternative, if you just can’t bear to cheat, is to havespare desktops around that you can switch in (so the user is happy) and keep the problem box in youroffice while you beat your head against the issue.■ Note Obviously, not all problems will be solved by this! Anything that’s showing up on multiple machines or foronly one user is probably not going to be fixed by a reinstall, at least not permanently.214Download at WoweBook.Com
CHAPTER 11 ■ TRACKING DOWN BUGSThe rest of this chapter discusses some useful tools and strategies for bug fixing. My final noteon the matter of saving time is that sometimes the quickest way to solve a problem is to step back fromit for a while. Make a cup of tea, go for a walk around the block, or go play NetHack for a few minutes(preferably not for the whole day...); just do anything that gives you a little mental space from theproblem. Several times I’ve been hit by the solution for a particularly nasty bug while cycling homefrom work and concentrating on the traffic rather than on the problem. It’s possible to focus so hardon an issue that you can’t see a valuable but slightly sideways solution to it. Giving your subconscioussome time to chew on things can be incredibly helpful and save you hours of banging your head againstthe desk.11-2. Knowing What to Check FirstHere’s a quick list of things to check for any bug before you start doing any serious problem solving:• Run df -h: Are any of the disks full? / or /var in particular can cause peculiarproblems, but the /home partition filling up can also give rise to errors (as can/tmp).■ Note Sometimes a process may create a large log file that is then deleted but not closed by the process, so itremains there until the process drops it (in other words, the disk space won’t actually be released). If you think thismight have happened, you can use lsof to take a look:lsof | grep /varThis will show you any files that are open in the /var/ directories. Here’s an example:innd 3360 news mem REG 254,6 155656 2129922 /var/spool/news/overview/group.indexmutt 26671 juliet 3r REG 254,1 2694918 1425422 /var/mail/julietThe seventh column shows the file size, and the second column shows the PID of the process. If you find adubious entry, you can try restarting the relevant process to clear the file properly.• Run ifconfig: Check that the network interface still has a correct-looking addressand hasn’t taken itself down.• Run ping gateway; ping nfs.example.com; ping ldap.example.com; pingkerberos.example.com; ping machine.external.com: Check that all your localservers are still contactable and then that the outside world is contactable.215Download 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 4 ■ TAKING BACKUPS AND MA
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C H A P T E R 5■ ■ ■Working w
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CHAPTER 5 ■ WORKING WITH FILESYST
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C H A P T E R 6■ ■ ■Securing
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CHAPTER 6 ■ SECURING YOUR SYSTEMS
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C H A P T E R 7■ ■ ■Working w
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CHAPTER 7 ■ WORKING WITH APACHEOn
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Page 233: C H A P T E R 11■ ■ ■Tracking
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Page 251 and 252: C H A P T E R 12■ ■ ■Managing
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Page 267 and 268: APPENDIX■ ■ ■Perl TipsSee rec
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Page 275 and 276: ■ INDEXenscript option (printing)
Page 277 and 278: ■ INDEX■NNagiosadding hosts to,
Page 279 and 280: ■ INDEXsyslogcentralized logging
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