Linux Component Configuration

Kernel Modules

Linux kernel can be extended with additional modules…

• …without the need to reboot the system
• …modules are extra kernel code …build into .ko object files
• ….executed in kernel-space in the unrestricted mode
• Modules commonly enable…
  • …device drivers for hardware
  • …support for file-systems
  • …additional system calls
• …automatically loaded when needed

/lib/modules

Modules are located in the /lib/modules/$(uname -r)/kernel/$subsystem/ directory…

• …modules can have dependencies to other modules
  • …/lib/modules/$(uname -r)/modules.dep …list of kernel module dependencies
  • …generated with depmod …part of the kmod package
• …modules have parameters to adjust their behaviour

# list all available kernel modules
find /lib/modules/$(uname -r) -type f -name \*.ko

lsmod …list all currently loaded kernel modules

• …first column …names of currently loaded modules
• …second column …amount of memory per module in kilobytes
• …last column …number, and optionally the names of modules that are dependent

modinfo & modprobe

modinfo $module …display information about any kernel module

• …not required that the modules is loaded
• …parm entries show parameters …type of value they expect

modprobe $module …load the relevant kernel module at run-time

• …will not persist after rebooting the system
• modprobe -r …unload the relevant kernel module

systool -v -m <name>                          # list the options that are set for a loaded module
modprobe -c | less                            # comprehensive configuration of all the modules
modprobe --show-depends <name>                # list dependencies of a module
insmod <path> <args>                          # load a module from a file
rmmod <name>                                  # ^^
/etc/modules                                  # list of modules to load at boot
/etc/modules-load.d/*.conf                    # ^^
/etc/modprobe.d/*.conf                        # module parameter configuration

Users

Command-line tools…

id        # user and group information (defaults to $USER)
su        # execute command as different user ID
useradd   # create user configuration
usermod   # modify user configuration
userdel   # delete user configuration

groups    # list groups associated to user (defaults to $USER)
newgrp    # change the current group ID during a login session
sg        # execute command as different group ID
groupadd  # create group configuration
groupmod  # modify group configuration
groupdel  # delete group configuration

Numeric UID/GID ranges…see /etc/login.defs

• 0-200…same on all systems (depending on the distribution)
• 201-999…dynamically allocated system users/groups
• 1000-59999…dynamically allocated user accounts
• 60000-64999…allocated by the distribution
• 65000-65533…reserved
• 65534…user nobody
• 65536-4294967293…dynamically allocated user accounts

# list all the users with a GID of 0-999
cut -d: -f 1,4 /etc/passwd \
      | egrep ":[1-9][0-9]{2}$|:[0-9]{1,2}$" \
      | sort -t':' -k 2

/etc/group

/etc/group group file…man 5 group…

• …defines the groups on the system
• …one entry per line…colon-separated fields…
  • group_name…
  • password…empty (no password needed)…(encrypted) group password
  • GID…numeric group ID
  • user_list…list of the usernames…separated by commas

group_name:password:GID:user_list

/etc/passwd

/etc/passwd password file…man 5 passwd…

• …describes user login accounts for the system
• …x character in the password field…
  • …uses shadow password suit
  • …/etc/shadown substitutes with encrypted passwords
• …each line of the file describes a single user
  • name…login name
  • password…x for shadow file
  • UID…numeric user ID (0 for root)
  • GID…primary group ID
  • GECOS…optional information about user
  • directory…user’s home directory..sets $HOME
  • shell…program to run at login

name:password:UID:GID:GECOS:directory:shell

useradd & groupadd

groupadd…creates a new group account

• …up to 32 characters long
• …lower and upper case letters
• …digits, underscores, or dashes
• …dash not allowed at the beginning

# add a new group
sudo groupadd -g $GID $name
# modify a GID...reassign file to new ID
sudo groupmod -g $GID $name
sudo find / -gid $GID_OLD -exec chgrp $GID_NEW {} \;

useradd …create a new user …/etc/default/useradd default configuration

# create user associated to multiple groups
sudo useradd -u $UID -g $group_primary -G $group_secondary,$group_secondary $name
# modify a UID...reassign file to new ID
sudo usermod -u $UID $name
sudo find / -uid $UID_OLD -exec chown $UID_NEW: {} \;

logrotate

Rotate log files in /var/logs…

• …configuration in /etc/logrotate.conf…
  • …default configuration for log rotation
  • …weekly rotation
  • …create new (empty) files after rotation
  • …dateext use date as suffix
• …includes /etc/logrotate.d/ sub-directory
  • …service specific configuration files
  • …notifempty rotate unless empty
  • …missingok error if the log file is missing
  • …copytruncate…close file…copy…rename…zero file
  • …olddir PATH move rotation files to directory

# manually...
logrotate /etc/logrotate.conf
logrotate /etc/logrotate.d/$service
# dry-run...
logrotate --debug /etc/logrotate.d/$service