LIS508 basic system administration Thomas Krichel

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.

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.

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.

basic order logs time processes networks users disks system upgrade

/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.

/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

/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

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

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.

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.

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.

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.

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

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.

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

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”

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.

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”

/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

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.

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

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.

/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 – localhost – foo.domain foo

/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.

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.

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.

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)

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.

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.

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.

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.

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”.

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.

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.

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.

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.

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”

/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.

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.

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.

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.

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.

rsync rsync uses ssh to incementally copy file on one system to the other. Say “rsync -qa /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.

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.

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”.

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.

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.

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.

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.

aptitude remove “aptitude remove foo” remove a package that is called foo. Example: “aptitude remove apache2- common”. This will also remove packages that depend on the removed package. The configuration of the package, in /etc, will be kept.

aptitude search “aptitude search foo” searchs for a package that contains the string foo in the name. Example: “aptitude search view”

aptitude update This will fetch a list of available packages from a package server. Use the file /etc/apt/sources.list to maintain information about where to get packages from. Use the # to make comments in the file.

sources of packages: Debian Debian has a number of mirrors that distribute all the Debian packages. Choose mirrors that are close to you to get fast access to the packages. Have at two Debian mirrors as sources.

aptitude save-upgrade This will look at the packages you have installed and the latest packages version available When a new version of package foo, say depends a package bar that is not installed, package foo will be held in its current state.

aptitude full-upgrade This will look at the packages you have installed and the latest package versions available. When a new version of package foo, say depends a package bar that is not installed, package bar will be installed an package foo will be updated.

aptitude purge “aptitude purge foo” removes a package that is called foo. Example: “aptitude purge apache2- common”. This will also remove packages that depend on the removed package. The configuration of the package, in /etc, will be removed.

sources of packages: others Debian only distirbutes free software, mostly GPL. But anybody can produced.deb files even for non-free software. Such providers usually set up archives that you can also include in your /etc/apt/sources.list file.

Please shutdown the computers when you are done. Thank you for your attention!

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