Linux System Administration Recipes A Problem-Solution Approach

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CHAPTER 11 ■ TRACKING DOWN BUGSThis will show only the lines that differ, so there’s less clutter on the screen, and it’s easier to read.Here’s some example output showing a single line (plus one blank line) added in hosts-client1:> 10.0.0.10 extrahost>However, this way you don’t get any context for the changed lines (the -C switch is not compatiblewith -y output), which may or may not be problematic, depending on what file you’re looking at andwhat you’re looking for.■ Note Another alternative is to use vimdiff or gvimdiff; both use syntax highlighting to show you thedifferences in the files. This is rather easier to read, as a rule, than the standard diff output, and because it opensup a vim editor window showing both files side by side, it also allows you to navigate through the files and editthem as required while highlighting the differences. In particular, if you’re using a graphical interface, gvimdiffallows you to change the sizes of the windows on the fly, which can be handy for looking more closely at what’sgoing on in the file. ediff mode in GNU emacs works similarly.11-4. Running strace to Look at System Callsstrace is a useful little program and is installed by default on most Linux systems. It allows you to take alook at the system calls being made by a program.■ Note It doesn’t provide a stack trace, name notwithstanding. It just traces system calls.It’s very useful as a basic debugging tool, helping you work out where a program is crashing or whatproblems it’s having. In fact, it’s also interesting to use even if you’re not tracing a problem, because itcan help you find out more about what your system is doing.It’s important to remember that user programs on a modern system run in a little circumscribedarea within the system. They’re not permitted to interact directly with the computer (so you can’t justshove numbers into registers and have Things Happen, unless you’re root). Instead, everything goesthrough the kernel, and it’s these messages to the kernel that strace is tracking. So, if there’s no straceoutput for a while, don’t assume that your program is stuck and that that’s where the bug is. It might justbe working within its own sandbox and not need to talk to the kernel for a while. Having said that, if thestrace output does pause and you can’t obviously see why from eyeballing the program, it’s probablyworth taking a look at the last few system calls to see whether anything looks problematic.The basic strace command is strace program, but strace -o output.txt program is a better bet.This will send the (copious quantities of) output to output.txt rather than straight to the screen, makingit easier to deal with.218Download at WoweBook.Com
CHAPTER 11 ■ TRACKING DOWN BUGS■ Note Some editors (for example, vim) are able to do syntax highlighting of strace output. This is very useful inmaking sense of the output because it helps your eye jump to the correct parts.Here are a couple of lines of output from running strace on a “Hello World” Perl program:execve("./helloworld.pl", ["./helloworld.pl"], [/* 20 vars */]) = 0uname({sys="Linux", node="client1.example.com", ...}) = 0The basic structure of each line is the same. Each line is a single system call, with the call name atthe start of the line, its arguments in brackets, and the return value after the = at the end of the line.The uname line here demonstrates another feature of strace. Some system calls involve a pointer to aparticular data structure. Here, uname returns a pointer to the system information data. stracedereferences the pointer for you and fills in the data. By default, it will print only a little of that datastructure; to get the whole thing, use the -v option to strace.Here’s some more sample output:access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)open("/etc/ld.so.cache", O_RDONLY) = 3fstat(3, {st_mode=S_IFREG|0644, st_size=43270, ...}) = 0mmap(NULL, 43270, PROT_READ, MAP_PRIVATE, 3, 0) = 0x2b8740901000close(3) = 0The access call shows what file the program is trying to access (for example, /etc/ld.so.preload)and whether it was successful. A value of –1 means unsuccessful, and then there’s an error code (E*****:in this case, ENOENT), which is translated for you. ENOENT means “no such file or directory,” but stracehelpfully looks up the plain-English meaning of the error code, so you shouldn’t need to do so. access, ifthe file does exist, checks whether the program is allowed to access it (for example, if the permissions arecorrectly set).■ Note The ld files are related to static and dynamic linking. Static linking means that a program creates its ownlibraries and uses those. Dynamic linking means that a program uses the system libraries, so you need only onecopy of all the standard libraries, not a copy per program. Nearly all Linux programs now use dynamic linking, soany program you run strace on will look for various ld files before doing anything else. Failure to find one of theld files isn’t necessarily a problem. See recipe 11-5 for comments on ltrace and ldd.open does exactly what you’d expect. It tries to open the specified file. If that file exists, the returnvalue is a positive number. This is the file descriptor, which is the “handle” that the system will use torefer to this file for subsequent system calls. Problems with accessing or opening a particular file are oneof the most likely causes of a bug, so pay particular attention to these calls. However, be aware thatconfig files may be looked for in multiple locations, which will generate a cluster of failure-to-open callsthat are absolutely normal. Use the -e option (see recipe 11-4a) to trace only particular types of call.219Download 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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CHAPTER 7 ■ WORKING WITH APACHE
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Page 191 and 192: C H A P T E R 8■ ■ ■Making Be
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Page 223 and 224: C H A P T E R 10■ ■ ■Things G
Page 225 and 226: CHAPTER 10 ■ THINGS GO IN, THINGS
Page 233 and 234: 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
Page 253 and 254: CHAPTER 12 ■ MANAGING TIME AND PE
Page 267 and 268: APPENDIX■ ■ ■Perl TipsSee rec
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Page 271 and 272: APPENDIX ■ PERL TIPSPerl Syntax N
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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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