Linux Commands For Shared Library Management and Debugging Problem

If you are a developer, you will re-use code provided by others. Usually /lib, /lib64, /usr/local/lib, and other directories stores various shared libraries. You can write your own program using these shared libraries. As a sys admin you need to manage and install these shared libraries. Use the following commands for shared libraries management, security, and debugging problems.

What Is a library in Linux or UNIX?

In Linux or UNIX like operating system, a library is noting but a collection of resources such as subroutines / functions, classes, values or type specifications. There are two types of libraries:

  1. Static libraries – All lib*.a fills are included into executables that use their functions. For example you can run a sendmail binary in chrooted jail using statically liked libs.
  2. Dynamic libraries or linking [ also known as DSO (dynamic shared object)] – All lib*.so* files are not copied into executables. The executable will automatically load the libraries using or

Linux Library Management Commands

  1. ldconfig : Updates the necessary links for the run time link bindings.
  2. ldd : Tells what libraries a given program needs to run.
  3. ltrace : A library call tracer.
  4. Dynamic linker/loader.

Important Files

As a sys admin you should be aware of important files related to shared libraries:

  1. /lib/* : Execution time linker/loader.
  2. /etc/ : File containing a list of colon, space, tab, newline, or comma separated directories in which to search for libraries.
  3. /etc/ : File containing an ordered list of libraries found in the directories specified in /etc/ This file is not in human readable format, and is not intended to be edited. This file is created by ldconfig command.
  4. lib*.so.version : Shared libraries stores in /lib, /usr/lib, /usr/lib64, /lib64, /usr/local/lib directories.

#1: ldconfig command

You need to use the ldconfig command to create, update, and remove the necessary links and cache (for use by the run-time linker, to the most recent shared libraries found in the directories specified on the command line, in the file /etc/, and in the trusted directories (/usr/lib, /lib64 and /lib). The ldconfig command checks the header and file names of the libraries it encounters when determining which versions should have their links updated. This command also creates a file called /etc/ which used to speed linking.


In this example, you’ve installed a new set of shared libraries at /usr/local/lib/:

$ ls -l /usr/local/lib/

Sample outputs:

-rw-r--r-- 1 root root 878738 Jun 16  2010 libGeoIP.a
-rwxr-xr-x 1 root root    799 Jun 16  2010
lrwxrwxrwx 1 root root     17 Jun 16  2010 ->
lrwxrwxrwx 1 root root     17 Jun 16  2010 ->
-rwxr-xr-x 1 root root 322776 Jun 16  2010
-rw-r--r-- 1 root root  72172 Jun 16  2010 libGeoIPUpdate.a
-rwxr-xr-x 1 root root    872 Jun 16  2010
lrwxrwxrwx 1 root root     23 Jun 16  2010 ->
lrwxrwxrwx 1 root root     23 Jun 16  2010 ->
-rwxr-xr-x 1 root root  55003 Jun 16  2010

Now when you run an app related to, you will get an error about missing library. You need to run ldconfig command manually to link libraries by passing them as command line arguments with the -l switch:

# ldconfig -l /path/to/lib/

Another recommended options for sys admin is to create a file called /etc/ as follows:


Now just run ldconfig to update the cache:

# ldconfig

To verify new libs or to look for a linked library, enter:

# ldconfig -v
# ldconfig -v | grep -i geoip

Sample outputs: -> ->

Troubleshooting Chrooted Jails

You can print the current cache with the -p option:

# ldconfig -p

Putting web server such as Apache / Nginx / Lighttpd in a chroot jail minimizes the damage done by a potential break-in by isolating the web server to a small section of the filesystem. It is also necessary to copy all files required by Apache inside the filesystem rooted at /jail/ directory , including web server binaries, shared Libraries, modules, configuration files, and php/perl/html web pages. You need to also copy /etc/{,} files and /etc/ directory to /jail/etc/ directory. Use the ldconfig command to update, print and troubleshoot chrooted jail problems:

### chroot to jail bash
chroot /jail /bin/bash

###  now update update the cache in /jail ###
###  print the cache in /jail ###
ldconfig -p
### copy missing libs ###
cp /path/to/some.lib /jail/path/to/some.lib
ldconfig -v | grep some.lib

### get out of jail ###
### may be delete bash and ldconfig to increase security (NOTE path carefully) ###
cd /jail
rm sbin/ldconfig bin/bash
### now start nginx jail ###

chroot /jail /usr/local/nginx/sbin/nginx


A rootkit is a program (or combination of several programs) designed to take fundamental control of a computer system, without authorization by the system’s owners and legitimate managers. Usually, rootkit use /lib, /lib64, /usr/local/lib directories to hide itself from real root users. You can use ldconfig command to view all the cache of all shared libraries and unwanted programs:

# /sbin/ldconfig -p | less

You can also use various tools to detect rootkits under Linux.

Common errors

You may see errors as follows:

Dynamic linker error in foo
Can’t map cache file cache-file
Cache file cache-file foo

All of the above errors means the linker cache file /etc/ is corrupt or does not exists. To fix these errors simply run the ldconfig command:

# ldconfig

Can’t find library xyz Error

The executable required a dynamically linked library that or cannot find. It means a library called xyz needed by the program called foo not installed or path is not set. To fix this problem install xyz library and set path in /etc/ file or create a file in /etc/ directory.

#2: ldd command

ldd (List Dynamic Dependencies) is a Unix and Linux program to display the shared libraries required by each program. This tools is required to build and run various server programs in a chroot jail. A typical example is as follows to list the Apache server shared libraries, enter:

# ldd /usr/sbin/httpd

Sample outputs: => /lib64/ (0x00002aff52a0c000) => /lib64/ (0x00002aff52c8f000) => /lib64/ (0x00002aff52eab000) => /usr/lib64/ (0x00002aff530c4000) => /lib64/ (0x00002aff532de000) => /usr/lib64/ (0x00002aff53516000) => /usr/lib64/ (0x00002aff53751000) => /lib64/ (0x00002aff5395f000) => /lib64/ (0x00002aff53c55000) => /usr/lib64/ (0x00002aff53e78000) => /lib64/ (0x00002aff5409f000) => /lib64/ (0x00002aff542ba000) => /lib64/ (0x00002aff544bf000) => /lib64/ (0x00002aff54816000)
	/lib64/ (0x00002aff527ef000) => /lib64/ (0x00002aff54a5c000) => /lib64/ (0x00002aff54c61000) => /usr/lib64/ (0x00002aff54e76000) => /lib64/ (0x00002aff5508f000) => /lib64/ (0x00002aff552dc000) => /usr/lib64/ (0x00002aff5562d000) => /usr/lib64/ (0x00002aff5585c000) => /lib64/ (0x00002aff55af1000) => /usr/lib64/ (0x00002aff55cf3000) => /usr/lib64/ (0x00002aff55f19000) => /usr/lib64/ (0x00002aff5612d000) => /lib64/ (0x00002aff56335000)

Now, you can copy all those libs one by one to /jail directory

# mkdir /jail/lib

# cp  /lib64/ /jail/lib
# cp /lib64/ /jail/lib

You can write a bash script to automate entire procedure:

        local d="$1"            # JAIL ROOT
        local pFILE="$2"        # copy bin file libs

        local files=""
	## use ldd to get shared libs list ###
        files="$(ldd $pFILE |  awk '{ print $3 }' | sed  '/^$/d')"
        for i in $files

          dcc="${i%/*}" # get dirname only
          [ ! -d ${d}${dcc} ] && mkdir -p ${d}${dcc}

          ${_cp} -f $i ${d}${dcc}
        # Works with 32 and 64 bit ld-linux

        sldl="$(ldd $pFILE | grep 'ld-linux' | awk '{ print $1}')"
        [ ! -f ${d}${sldl} ] && ${_cp} -f ${sldl} ${d}${sldlsubdir}


You can call it as follows:

cp_support_shared_libs "/jail" "/usr/local/nginx/sbin/nginx"

Report Missing Functions

Type the following command:

$ ldd -d /path/to/executable

Report Missing Objects

Type the following command:

$ ldd -r /path/to/executable

Determine If Particulate Feature Supported Or Not

TCP Wrapper is a host-based Networking ACL system, used to filter network access to Internet. TCP wrappers was original written to monitor and stop cracking activities on the UNIX / Linux systems. To determine whether a given executable daemon supports TCP Wrapper or not, run the following command:

$ ldd /usr/sbin/sshd | grep libwrap

Sample outputs: => /lib64/ (0x00002abd70cbc000)

The output indicates that the OpenSSH (sshd) daemon supports TCP Wrapper.

Other usage of ldd command

You can use the ldd command when an executable is failing because of a missing dependency. Once you found a missing dependency, you can install it or update the cache with ldconfig command as mentioned above.

#3: ltrace command

The ltrace command simply runs the specified command until it exits. It intercepts and records the dynamic library calls which are called by the executed process and the signals which are received by that process. It can also intercept and print the system calls executed by the program. Its use is very similar to strace command.

# ltrace /usr/sbin/httpd
# ltrace /sbin/chroot /usr/sbin/httpd
# ltrace /bin/ls

Sample outputs:

__libc_start_main(0x804fae0, 1, 0xbfbd6544, 0x805bce0, 0x805bcd0 
strrchr("/bin/ls", '/')                                                                                                                            = "/ls"
setlocale(6, "")                                                                                                                                   = "en_IN.utf8"
bindtextdomain("coreutils", "/usr/share/locale")                                                                                                   = "/usr/share/locale"
textdomain("coreutils")                                                                                                                            = "coreutils"
__cxa_atexit(0x8052d10, 0, 0, 0xbfbd6544, 0xbfbd6498)                                                                                              = 0
isatty(1)                                                                                                                                          = 1
getenv("QUOTING_STYLE")                                                                                                                            = NULL
getenv("LS_BLOCK_SIZE")                                                                                                                            = NULL
getenv("BLOCK_SIZE")                                                                                                                               = NULL
getenv("BLOCKSIZE")                                                                                                                                = NULL
getenv("POSIXLY_CORRECT")                                                                                                                          = NULL
getenv("BLOCK_SIZE")                                                                                                                               = NULL
getenv("COLUMNS")                                                                                                                                  = NULL
ioctl(1, 21523, 0xbfbd6470)                                                                                                                        = 0
getenv("TABSIZE")                                                                                                                                  = NULL
getopt_long(1, 0xbfbd6544, "abcdfghiklmnopqrstuvw:xABCDFGHI:"..., 0x0805ea40, -1)                                                                  = -1
__errno_location()                                                                                                                                 = 0xb76b8694
malloc(40)                                                                                                                                         = 0x08c8e3e0
memcpy(0x08c8e3e0, "", 40)                                                                                                                         = 0x08c8e3e0
output truncated
free(0x08c8e498)                                                                                                                                   = 
free(NULL)                                                                                                                                         = 
free(0x08c8e480)                                                                                                                                   = 
__fpending(0xb78334e0, 0xbfbd6334, 0xb78876a3, 0xb78968f8, 0)                                                                                      = 0
fclose(0xb78334e0)                                                                                                                                 = 0
__fpending(0xb7833580, 0xbfbd6334, 0xb78876a3, 0xb78968f8, 0)                                                                                      = 0
fclose(0xb7833580)                                                                                                                                 = 0
+++ exited (status 0) +++

The ltrace command is a perfect debugging utility in Linux to monitor the library calls used by a program and all the signals it receives. It is a perfect tool for tracking the execution of processes. It can also show system calls, used by a program.

ltrace Example

Consider the following c program:

#include <stdio.h>
int main(){
	printf("Hello world\n");
	return 0;


Compile and run it as follows:

$ cc  hello.c   -o hello
$ ./hello

Now use the ltrace command to trace all used tracking the execution of processes and libs:

$ ltrace -S -tt ./hello 

Sample outputs:

15:20:38.561616 SYS_brk(NULL)                                                                                                                      = 0x08f42000
15:20:38.561845 SYS_access("/etc/", 00)                                                                                               = -2
15:20:38.562009 SYS_mmap2(0, 8192, 3, 34, -1)                                                                                                      = 0xb7708000
15:20:38.562155 SYS_access("/etc/", 04)                                                                                               = -2
15:20:38.562336 SYS_open("/etc/", 0, 00)                                                                                                = 3
15:20:38.562502 SYS_fstat64(3, 0xbfaafe20, 0xb7726ff4, 0xb772787c, 3)                                                                              = 0
15:20:38.562629 SYS_mmap2(0, 76469, 1, 2, 3)                                                                                                       = 0xb76f5000
15:20:38.562755 SYS_close(3)                                                                                                                       = 0
15:20:38.564204 SYS_access("/etc/", 00)                                                                                               = -2
15:20:38.564372 SYS_open("/lib/tls/i686/cmov/", 0, 00)                                                                                    = 3
15:20:38.564561 SYS_read(3, "\177ELF\001\001\001", 512)                                                                                            = 512
15:20:38.564694 SYS_fstat64(3, 0xbfaafe6c, 0xb7726ff4, 0xb7705796, 0x8048234)                                                                      = 0
15:20:38.564822 SYS_mmap2(0, 0x1599a8, 5, 2050, 3)                                                                                                 = 0xb759b000
15:20:38.565076 SYS_mprotect(0xb76ee000, 4096, 0)                                                                                                  = 0
15:20:38.565209 SYS_mmap2(0xb76ef000, 12288, 3, 2066, 3)                                                                                           = 0xb76ef000
15:20:38.565454 SYS_mmap2(0xb76f2000, 10664, 3, 50, -1)                                                                                            = 0xb76f2000
15:20:38.565604 SYS_close(3)                                                                                                                       = 0
15:20:38.565709 SYS_mmap2(0, 4096, 3, 34, -1)                                                                                                      = 0xb759a000
15:20:38.565842 SYS_set_thread_area(0xbfab030c, 0xb7726ff4, 0xb759a6c0, 1, 0)                                                                      = 0
15:20:38.566070 SYS_mprotect(0xb76ef000, 8192, 1)                                                                                                  = 0
15:20:38.566185 SYS_mprotect(0x08049000, 4096, 1)                                                                                                  = 0
15:20:38.566288 SYS_mprotect(0xb7726000, 4096, 1)                                                                                                  = 0
15:20:38.566381 SYS_munmap(0xb76f5000, 76469)                                                                                                      = 0
15:20:38.566522 __libc_start_main(0x80483e4, 1, 0xbfab04e4, 0x8048410, 0x8048400 

15:20:38.566667 puts("Hello world" 
15:20:38.566811 SYS_fstat64(1, 0xbfab0310, 0xb76f0ff4, 0xb76f14e0, 0x80484c0)                                                                      = 0
15:20:38.566936 SYS_mmap2(0, 4096, 3, 34, -1)                                                                                                      = 0xb7707000
15:20:38.567126 SYS_write(1, "Hello world\n", 12Hello world
)                                                                                                  = 12
15:20:38.567282 <... puts resumed> )                                                                                                               = 12
15:20:38.567348 SYS_exit_group(0 
15:20:38.567454 +++ exited (status 0) +++

You need to carefully monitor the order and arguments of selected functions such as open() [used to open and possibly create a file or device] or chown() [used to change ownership of a file] so that you can spot simple kinds of race conditions or security related problems. This is quite useful for evaluating the security of programs without source code to find out what kind of changes made to your system.

ltrace: Debugging Memory & I/O Usage For HA Based Cluster Computers

The ltrace command can be used to trace memory usage of the malloc() and free() functions in C program. You can calculate the amount of memory allocated as follows:

[node303 ~]$ ltrace -e malloc,free ./simulator arg1 agr2 arg3

The ltrace will start ./simulator program will trace the malloc() and free() functions. You can also pass the -e option to find out I/O problems as follows:

[node303 ~]$ ltrace -e   fopen,fread,fwrite,fclose ./simulator arg1 agr2 arg3

You may need to change function names as your programming languages or UNIX platform may use different memory allocation functions.

#4: Command

The or / used as follows by Linux:

  1. To load the shared libraries needed by a program.
  2. To prepare the program to run, and then runs it.

List All Dependencies and How They Are Resolved

Type the following command:

# cd /lib

For 64 bit systems:

# cd /lib64

Pass the –list option, enter:

#  ./ --list /path/to/executable

Other options

From the man page:

  --verify                   verify that given object really is a dynamically linked object we can handle
  --library-path PATH   use given PATH instead of content of the environment variable LD_LIBRARY_PATH
  --inhibit-rpath LIST    ignore RUNPATH and RPATH information in object names in LIST

Environment Variables

The LD_LIBRARY_PATH can be used to set a library path for finding dynamic libraries using LD_LIBRARY_PATH, in the standard colon seperated format. Please note that these variables are ignored when executing setuid/setgid programs:

$ export LD_LIBRARY_PATH=/opt/simulator/lib:/usr/local/lib

The LD_PRELOAD allow an extra library not specified in the executable to be loaded:

$ export LD_PRELOAD=/home/vivek/dirhard/

coded by nessus