Linux System Administration Recipes A Problem-Solution Approach

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CHAPTER 11 ■ TRACKING DOWN BUGSmemcpy(0x61a6d0, "subversion_repos.html", 22) = 0x61a6d0readdir(0x61ddf0)= 0x61e510memset(0x629e20, '\000', 184)= 0x629e20strlen("mono-samples.png") = 16malloc(17)= 0x61a6f0memcpy(0x61a6f0, "mono-samples.png", 17) = 0x61a6f0readdir(0x61ddf0)= 0x61e538memset(0x629ed8, '\000', 184)= 0x629ed8■ Note As with system calls, library calls are also well documented in <strong>Linux</strong> systems. man memcpy will, forexample, give you information on the memcpy library call.Looking at the output here, the first line is a call to memcpy. This is copying 22 bytes from the memoryaddress containing the string subversion_repos.html to the memory address 0x61a6d0. As with strace,ltrace will dereference certain pointers for you. In this case, instead of giving the pointer to the memoryaddress for the source of the data, it dereferences it to give you the string that’s stored at that address(here it’s subversion_repos.html, which is the name of one of the files that ls is listing). The return valueis just the destination memory address.readdir, on the next line, is a function that returns a pointer to the next directory entry. Theargument (here 0x61ddf0) is a pointer to the directory itself, and the return value is where to look for thenext entry. memset writes a given number (here 184) of bytes of a particular value (here \000) to the stringreferenced by the memory pointer (0x629e20).strlen gives the length of its argument string. Here, it’s the next directory entry (as returned byreaddir), and again, ltrace dereferences the pointer for you to give you the string value instead ratherthan just a memory address. The next line uses a malloc call to deal with memory allocation, setting upsufficient memory (the string length plus one byte) to hold the next directory entry. Then the wholeseries starts over again with memcpy and the next directory entry.Most of the time you probably won’t need to use ltrace to this level of detail, but this shouldprovide you with enough of a guide to be able to use the man pages as a reference to decipher the ltraceoutput for a program that’s causing you problems.■ Note You can also use ldd to check which shared libraries a particular program requires. If a particular librarycan’t be found, ldd will report this.11-5a. Setting ltrace OptionsHere are the ltrace options:• -f: Traces child processes (forks) as they’re created.• -t, -tt, -ttt: Puts the time of day at the start of each line, given to the second,microsecond, or second+microsecond and written as seconds-since-epoch.222Download at WoweBook.Com
CHAPTER 11 ■ TRACKING DOWN BUGS• -T: Shows the time spent in library calls (that is, the time difference between thebeginning and end of each library call).• -r: Prints a relative timestamp at the start of each library call.• -c: Counts the time, calls, and errors for each library call, and summarizes thisinformation when the program exits. This is helpful if you’re debugging andyou’re not sure where to start, because it’ll help you work out where the problemmight lie.• -e CALLNAME: Traces only the specified library calls. Alternatively, -e !CALLNAMEwould trace all calls except for the specified one. You can use this to minimizethe amount of output; it may be best used after you’ve used -c to identify theproblem area.• -p PID: Attaches ltrace to the process with the specified process ID. So if yousuddenly see a process hanging or eating up lots of CPU, you can find out what it’sdoing in real time.• -S: Traces system calls as well.11-6. Logging with syslogdMany of the programs on your system will send their logging messages to syslog, which manages thelogging on most <strong>Linux</strong> systems. syslog can be configured to send particular types of log messages toparticular locations.syslog messages have two pieces of information attached to them: a facility, identifying thesource of the message, and a priority. Both of these are system-defined rather than user-defined;see Tables 11-1 and 11-2 for the available values.Table 11-1. syslog FacilitiesPriority LabelauthauthprivcrondaemonftpkernlprDescriptionAuthorization messages and information (aka security, but this is deprecated)Used for user access messagescronAny daemon that doesn’t have its own facilityFTPKernel messagesPrinter system messages223Download 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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CHAPTER 7 ■ WORKING WITH APACHEAl
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CHAPTER 7 ■ WORKING WITH APACHE
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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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