Linux system administration

Vulcan Nerve Pinch

enable it with

#> sysctl -w kernel.sysrq=1

AltSysRqR	use raw keyboard events
AltSysRqK	kill current VT in use
AltSysRqE	terminate all running processes (except `init`)
AltSysRqI	kill all processes (except `init`)
AltSysRqL	kill all processes (including `init`)
AltSysRqB	reboot
AltSysRqS	sync all drives
AltSysRqU	umount all filesystems
AltSysRqO	turn the machine off
AltSysRqP	dump processor’s registers
AltSysRqT	dump current tasks
AltSysRqM	dump some memory info
AltSysRq0 up to AltSysRq9	set kernel log level and redirect it to console

Users/groups

End user
- chsh change login shell
- passwd change password
- chfn change finger information
System administration
- vipw edit the file /etc/passwd
- vigr edit the file /etc/group
- visudo edit the file /etc/sudoers

Network

DNS

`host foobar`	perform a DNS lookup and return the IP address of `foobar`
`host 11.22.33.44`	perform a DNS lookup and return the name of `11.22.33.44`

starting with Fedora Core 2, use system-config-network to configure the network
before this, use redhat-config-network
FTP
- VSFTP
  - /etc/vsftpd.ftpusers contains the list of users that are not allowed to log in via FTP (these users are still prompted for a password)
  - if userlist_enable is YES, /etc/vsftpd.user_list contains the list of users that will not be prompted for a password and, hence, will not be able to log in (this avoid to transmit the passwords in clear text over the network)
- WU-FTP
  - to disallow FTP users, use deny-uid, deny-gid, allow-uid and allow-gid in /etc/ftpaccess instead of using /etc/ftpusers

Hardware analysis

report the temperature, number of errors, lifetime… of an ATA/IDE/SCSI3 hard drive

#> smartctl -a /dev/sda

`scsiinfo`	display/set SCSI device parameters
`scsiinfo -X`	set SCSI device parameters (use `scsi-config` instead)
`scsiinfo -d /dev/sda`	dump defect list from manufacturer table and grown table
`scsiinfo -a /dev/sda`	display all information

hdparm display/set IDE hard disk parameters
scsiformat format a SCSI disk
badblocks list the bad blocks of a device

System analysis

The messages log by the system are in /var/log.

`/var/log/messages`	System log
`/var/log/boot.log`	Service start/stop log
`/var/log/secure`	Security log
`/var/log/maillog`	Sendmail log
`/var/log/httpd/access_log`	Apache access log
`/var/log/httpd/error_log`	Apache error log

a script can log a message in syslog with logger
display the kernel ring buffer

#> dmesg
Modules
- list all the modules currently loaded in the kernel (this list is also in /proc/modules)
  
  #> lsmod
- load a module
  
  #> insmod mymod
- unload a module from the kernel
  
  #> rmmod mymod
display resources used by the processes

#> top
report system activity

#> sar
display the free/used memory in real time

#> watch -n 1 -d free

systat

report processors related statistics

#> mpstat
report CPU and I/O statistics

#> iostat

get the lists of the writing processes

#> service syslog stop # to avoid a mad loop if syslogd tries to write something during the experiment
                                            

                                            #> echo 1 > /proc/sys/vm/block_dump

perform

#> dmesg

clean up

#> echo 0 > /proc/sys/vm/block_dump
                                            

                                            #> service syslog start

report virtual memory statistics

#> vmstat

SCSI
- return the SCSI devices corresponding to the /dev/sgs
  
  #> sg_map
- list the /dev/sgs bound to a SCSI device
  
  #> sg_scan -i
PCI
- list the PCI devices
  
  #> lspci

/proc files

`/proc/cmdline`	the command line parameters used to start the kernel
`/proc/kcore`	is the image of the physical memory (in the core file format)
`/proc/kmsg`	is the kernel messages log (only a provileged process can read it and syslog must be called by a process)
`/proc/ksyms`	is the kernel symbol table
`/proc/loadavg`	contains load indicators (same as uptime)
`/proc/cpuinfo`	CPU characteristics (only the cpu and bogomips are standard entries)
`/proc/meminfo`	contains the current memory usage (same as free)
`/proc/modules`	lists the modules currently loaded in the kernel
`/proc/uptime`	contains the uptime and idle time
`/proc/version`	contains the versions of the Linux kernel, gcc and Red Hat Linux installed on the system
`/proc/filesystems`	lists the file systems supported by the kernel
`/proc/swaps`	lists the swap spaces and their use
`/proc/stat`	contains some kernel statistics
`/proc/slabinfo`	lists the memory usage on the slab level
`/proc/mounts`	lists all the mount of the system
`/proc/locks`	lists the file currently locked by the kernel
`/proc/devices`	major numbers and device groups
`/proc/ioports`	used IO addresses
`/proc/interrupts`	used interrupts
`/proc/dma`	used DMA channels

/proc/sys

`sysctl`	configure kernel parameters at runtime
`sysctl -a`	list all the parameters currently set
`sysctl -w kernel.sysrq=1`	this is the same as `echo 1 > /proc/sys/kernel/sysrq`

When the system boots, the init program runs the /etc/rc.d/rc.sysinit script which executes sysctl using /etc/sysctl.conf. So any values added to /etc/sysctl.conf will take effect each time the system boots.

/etc/services contains the port/server mapping
the services are defined in /etc/xinetd.d (one file per service)
the configuration file is /etc/xinetd.conf

`chkconfig`	configure system services
`chkconfig --list`	list the current state of the services
`chkconfig rexec on`	enable a service the service will be started only at the nest reboot, use `service rexec start` to start it now
`chkconfig rexec off`	disable a service

`service --status-all`	return the status of all services
`service sshd status`	indicate if `sshd` is running
`service sshd start`	start `sshd`
`service sshd stop`	stop `sshd`

File system analysis

lsof list the open files
shred foobar overwrite a file before deleting it

`/etc/fstab`	description of the filesystems to mount
`/etc/mtab`	tracking of the mounted filesystems

sfdisk -l /dev/sdb display the partition table of disk /dev/sdb

Swap

swapon -s
cat /proc/swaps display swap usage

XFS

`mkfs.xfs /dev/md4` `mkfs -t xfs /dev/md4`	create a XFS filesystem
`mkfs.xfs -f`	do not create the filesystem, only display its parameters
`mkfs.xfs -N`	force the filesystem creation even if the partition already contains one

xfs_growfs -n /dev/md4
                                                        

                                                        xfs_growfs -n /export/home1

display the filesystem parameters

xfs_info /dev/md4
                                                        

                                                        xfs_info /export/home1

same as xfs_growfs -n

xfs_db /dev/md4 launch the XFS debugger on /dev/md4

`xfs_check /dev/md4`	launch the XFS checker on `/dev/md4`
`xfs_check -i 123 /dev/md4`	launch the XFS checker on inode 123 of `/dev/md4`
`xfs_check -l /dev/scsi/sbdc1b0t0u0p5 /dev/md0`	the `-l` option indicates that `/dev/md0` has an external log `/dev/scsi/sbdc1b0t0u0p5`

`xfs_ncheck /dev/md4`	return the list of inodes/pathnames on `/dev/md4`
`xfs_ncheck -i 123 /dev/md4`	return pathname of inode 123 of `/dev/md4`
`xfs_ncheck -l /dev/scsi/sbdc1b0t0u0p5 /dev/md0`	the `-l` option indicates that `/dev/md0` has an external log `/dev/scsi/sbdc1b0t0u0p5`

`xfs_repair /dev/md0`	repair the filesystem mounted on `/dev/md4`
`xfs_repair -l /dev/scsi/sbdc1b0t0u0p5 /dev/md0`	the `-l` option indicates that `/dev/md0` has an external log `/dev/scsi/sbdc1b0t0u0p5`

NFS

`/var/lib/nfs/rmtab`	client information
`/var/lib/nfs/etab`	detailed export information
`/proc/fs/nfs/exports` `/etc/exports`	lists of clients
`/var/lib/nfs/xtab`	explicit machine names

Libraries

ldconfig recreate the links for the shared libraries and rebuild the cache