LIS508 basic system administration

1. LIS508basic system administration Thomas Krichel 2010-01-09

2. background • now we have a functional system, what are we doing with it? • What is the basic architecture? • How to keep the box up to date? • Basic troubleshooting.

3. troubleshooting • 95% of all times, you can put an error message into your Google search box and get some meaningful advice • If you don't than you have a rare problem. Rare problem are usually your own fault rather than a systemic issue with the software.

4. learning Debian • Debian is impossible to learn. • With 20,000 packages, you don't know what is in them to satisfy your needs. • The best way to find out is go to a meeting with Debian-knowledgable folks and talk to them.

5. basic order • logs • time • processes • networks • users • disks • system upgrade

6. /etc • This contains the configurations • If a packages is called foo, changes are that the configuration is in /etc/foo.conf or in /etc/foo/ somewhere. • Every package is configured in a different way.

7. /usr/share/doc • This very large directory contains documentation that comes with packages • Much of this documentation is in compressed files. • Compressed files are understood by the extention .gz • “zcat foo.gz” shows the file foo.gz • “zcat foo.gz | less ” shows the file is less

8. /var/log • This directory contains logs. • When a package does not work as expected, looking at the logs is a good idea, usually. • Some daemon programs log to /var/daemon.log

9. log permissions • The current logs are usually readable by a person in the adm group. • It is therefore a good idea to add the person that usually runs the machine to the adm group. • This can be done by editing /etc/group

10. log rotation • Files in logs are rotated by the logrotate package. • This package compresses old logs and puts them into separate files. • Log rotation is fired up by a entry in the system crontab.

11. time keeping • Normally, ntp package will keep correct time on the machine. • Keeping accurate time is very important in digital library application. • You need the ntpd daemon package installed, the client on its own will not do it, I think.

12. dating • To set up a machine for a local time zone, use “dpkg-reconfigure tzdata”. • You will be guided through a menu that • Adjustment for daylight savings time happens automatically.

13. processess • ps is the basic utility for looking at processes. • I most often use it as “ps axf | less”. Note that axf are command flags but strangely enough, there is no minus sign used.

14. process managment • htop is a good program to watch system usage. • if you don't have it, get it with “aptitude install htop”

15. killing • “kill -signal process” sends the signal signal to the process indentifed by the • The most important signal is 9, meaning to kill the process for as sure as you can. • If it still living, you can only get rid of it by a reboot.

16. killalll • killall is a utilty to kill a bunch of processes. • “killall foo” kills all processes that have to string foo in their names.

17. shutdown • This is a ulity to shutdown the system. • “shutdown -h now” brings the system to a halt. Don't use this on your rented machine. • “shutdown -r now” reboots the system. • “reboot” is an alias for “shutdown -r now”

18. uptime • This says how long the system has been up for. • It is useful to find out if a reboot has occurred since a certain time. • Sometime reboots occur because of eletricity failures.

19. starting and stopping daemons • To start a daemon daemon, use “/etc/init.d/daemon start” • To stop a daemon daemon, use “/etc/init.d/daemon stop” • To restart a daemon daemon, use “/etc/init.d/daemon restart”

20. /etc/init.d/networking • This controls the network. It is dealt with in the same way as a daemon would. • The configuration of the network lives in /etc/networking. • The most important file is called interfaces. It contains the interface configuration. • On my laptop, I have many interface configuration files

21. route • route is the command to show the kernel routing table. • It may help you it find out what is going on with the IP configuration.

22. ifconfig • This is the command to find out the state of your interfaces. • This is an essential utility for the network with wired access. • For wireless, there is iwconfig

23. traceroute • This is a utilty to see how packet are traveling between hosts. • Usually between your machine and another as in “traceroute foo” to see the route to host foo. • This is useful to understand at what level your are experiencing a problem with the network connection.

24. /etc/hosts • This contains the basic host configuration. • You can use this to make local host configuration that bypass the DNS. • The exact detail of this file is a mystery to me. It should have lines • 127.0.0.1 localhost • 127.0.0.1 foo.domain foo

25. /etc/resolv.conf • This contains the DNS configuration. • “nameserver ip” says that the machine with IP address ip will be a nameserver.It is good to have several of these lines. • “search domain” will instruct the DNS client to search for “foo.domain” when there is a request to resolve host “foo”. This saves typing time.

26. adding and removing users • adduser is a utity to add physical users to the system. Its password strength checking • /etc/skel contains skeleton home directories • userdel removes a user. It needs the -f flag to remove the home directory as well.

27. su & sudo • su is a utility to become a certain user. If you give no user name, root is assumed. • sudo is a command to run something as root. It appears to be popular, but I don't use it.

28. user information • Information about all users is in /etc/password. • That file also used to contains passwords. • The passwords are now in /etc/shadow. The passwords are stored in an encrypted form that can not be decrypted (unless you use trial and error)

29. passwd • passwd is a utility to set passwords. • root can set any password directly. • Other users have to enter the old password first. • There is no facility to look up passwords.

30. last • This gives the list of users and when they last logged in. • If you have a machine with a lot of users, it is worth watching this.

31. dmesg • This shows the last messages from the kernel. • This is useful when a new hardware device is plugged in, to find out if the kernel knows about it. • udev, a device manager, will usuall inform the kernel that a new device is there.

32. disks • All spinning disks will break after a while. • Usually the kernel notices this and unmounts the mount point of the disk. • Sometimes you also see a message “input/output error”. • If such an error appears on the root disk, you have a serious problem.

33. df • This utility shows you how full your disks are. • Unfortunately any disk will fill up after some time. You have to keep watching. • Note that inode fill-up can also make a disk unusable, see “df -i”.

34. virtual volumes • There is a way to combine several disks into a virtual disk. • This is called logical volume management. • I only use this to build large disk for backup. I never use it for primary data.

35. fdisk • This is a partition table manipulation utility. • I only used it as “fdisk -l” to list all the devices that the kernel knows about.

36. cfdisk • This is full-screen (curses) utilty to manipulate a partition table. It is self-explanatory. It is invoqued with “cfdisk device” where device is a device name such as /dev/sda. • Remember, changing the partition table on a disk is like erasing all data that is on it.

37. mount • Mounting a disk means making it available at a certain point in the file system. • For example, if you have directory /foo and a disk /dev/sdf, with a partition /dev/sdf1, you say “mount /dev/sdf1 /foo”. • mount has a -t flag to inform the mount about the type of file system used. Most times this can be read from the partition table of the device.

38. umount • umount is a command to a mount point, i.e. a point in the file system where something is mounted. • Thus you say “umount /foo”. • You don't say “umont /dev/sdf1”

39. /etc/fstab • This file contains information about mount points as they are supposed to be mounted at boot time. • “mount -a” will attempt to mount all mount points in that file. • I would not touch this on a rented machine.

40. checking • e2fsck is a utility to check ext2 and ext3 type file systems. • These are the most widely used systems. • You can also add a physical check of every block on the device, but that takes a lot of time. • e2fsck will try to repair the system.

41. broken disks • Disks usually don't break complete. • e2fsk -y will fix errors without you having to confirm all actions. This is useful when you have a lot of errors. • Even if there are few errors a disk has errors, more errors will appear. • You need to replace the disk.

42. broken disk with root file system • If the disk with the root file system is broken, a reboot will usually activate e2fsck. • But if you have only ssh access to the machine, there is nothing you can do because ssh starts after the root disk is mounted. • Some providers allow you something like a virtual console.

43. backups • Already the Beatles knew about the importance of backups. • What to back up is more an art than a science. • I back up /var, /home, /root, and /etc. • I back them up on a remote machine using rsync.

44. rsync • rsync uses ssh to incementally copy file on one system to the other. Say “rsync -qa /etc/ root@remote:/backup/server/etc”. • Here the -a means archival. -q is quite, if you want verbosity say -v. • --delete deletes files on the remote machine that are not on the local. • Note the slash at the end of the source directory, it has to be there.

45. rsync in the digital library • rsync is extremely important in my work because I rely on various machines to process digital library data • I use rsync to transport the result of the output of one system as the input of the other system. • It is fast and efficient, but can strain a network.

46. package management • dpkg is the utility that deals with one package • To maintain packages as a whole, there are three systems. “dselect”, “apt-get” and “aptitude”.

47. dpkg • man dpkg will tell you more about it. • The only time I use it is to say “dpkg -i foo.deb” to install a package contained in a file foo.deb.

48. dselect • dselect is a full-screen package selection system with an unintuitive but simple to learn and well documentented interface. • I used it for many years, but I am told in makes bad suggestion and should be avoided. • I no longer use it.

49. aptitude • Aptitude (apparently) makes better choices than apt-get and dselect • If invoqued on its own, it leads to an interface I don't understand. • I used it now with actions in command line.

50. aptitude install • “aptitude install foo” installs a package that is called foo. • Example: “aptitude install apache2-common”. • This will also install packages that the package to be installed depends on.