You must decide whether to use automatic yum updates on each of your machines. There are a number of arguments both for and against automatic updates to consider. However, there is no single answer to this question: It is up to the system administrator or owner of each machine to decide whether automatic updates are desirable or not for that machine. One of the things which makes one a good system administrator is the ability to evaluate the facts and other people's suggestions, and then decide for onesself what one should do.
A general rule that applies in most cases is as follows:
If the machine is a critical server, for which unplanned downtime of a service on the machine can not be tolerated, then you should not use automatic updates. Otherwise, you may choose to use them.
Even the general rule above has exceptions, or can be worked around. Some issues might be resolved through a special setup on your part. For example, you could create your own yum repository on a local server, and only put in it tested or trusted updates. Then use the automatic updates from only your own repository. Such setups, while perhaps more difficult to setup and maintain, can remove a large amount of risk otherwise inherent in automatic updates.
How are automatic updates done?
You can use a service to automatically download and install any new updates (for example security updates). The yum-cron RPM package provides a service which is started automatically. Though, you have to change a configuration file. In order to do this, run as the root user (or become root via su -) from a terminal window. On a fresh install of Fedora 20 with default options the yum-cron RPM is not installed, the first command below installs this RPM.
yum install -y yum-cron su -c "gedit /etc/yum/yum-cron.conf"
and enter your password. After, change the line
apply_updates = no
apply_updates = yes
Save the file. You are now done. Yum-cron updates your system every time when there are new updates available.
Can we trust yum updates?
Yum in Fedora has the GPG key checking enabled by default. Assuming that you have imported the correct GPG keys, and still have gpgcheck=1 in your /etc/yum.conf, then we can at least assume that any automatically installed updates were not corrupted or modified from their original state. Using the GPG key checks, there is no way for an attacker to generate packages that your system will accept as valid (unless they have a copy of the *private* key corresponding to one you installed) and any data corruption during download would be caught.
However, the question would also apply to the question of update quality. Will the installation of the package cause problems on your system? This we can not answer. Each package goes through a QA process, and is assumed to be problem free. But, problems happen, and QA can not test all possible cases. It is always possible that any update may cause problems during or after installation.
Why use Automatic updates?
The main advantage of automating the updates is that machines are likely to get updated more quickly, more often, and more uniformly than if they updates are done manually. We see too many compromised machines on the internet which would have been safe if the latest updates where installed in a timely way.
So while you should still be cautious with any automated update solution, in particular on production systems, it is definitely worth considering, at least in some situations.
Reasons FOR using automatic updates
While no one can determine for you if your machine is a good candidate for automatic updates, there are several things which tend to make a machine a better candidate for automatic updates.
Some things which might make your machine a good candidate for automatic updates are:
- You are unlikely to apply updates manually for whatever reason(s).
- The machine is not critical and occasional unplanned downtime is acceptable.
- You can live without remote access to the machine until you can get to its physical location to resolve problems.
- You do not have any irreplaceable data on the machine, or have proper backups of such data.
If all of the above apply to your machine(s), then automatic updates may be your best option to help secure your machine. If not all of the above apply, then you will need to weigh the risks and decide for yourself if automatic updates are the best way to proceed.
Reasons AGAINST using automatic updates
While no one can determine for you if your machine is a bad candidate for automatic updates, there are several things which tend to make a machine a worse candidate for automatic updates.
Some things which might make your machine be a bad candidate for automatic updates are:
- It provides a critical service that you don't want to risk having unscheduled downtime.
- You installed custom software, compiled software from source, or use third party software that has strict package version requirements.
- You installed a custom kernel, custom kernel modules, third party kernel modules, or have a third party application that depends on kernel versions (this may not be a problem if you exclude kernel updates, which is the default in Fedora yum.conf files). (But see also bug #870790 - you may need to modify /etc/yum/yum-cron.conf to exclude=kernel*.)
- Your enviroment requires meticulous change-control procedures.
- You update from other third party yum repositories besides Fedora (core, extras, legacy ) repositories which may conflict in versioning schemes for the same packages.
There are also some other reasons why installing automatic updates without testing may be a bad idea. A few such reasons are:
- The need to back up your configuration files before an update. Even the best package spec files can have mistakes. If you have modified a file which is not flagged as a configuration file, then you might lose your configuration changes. Or an update may have a different format of configuration file, requiring a manual reconfiguration. It is often best to backup your configuration files before doing updates on critical packages such as mail, web, or database server packages.
- Unwanted side effects. Some packages can create annoying side effects, particularly ones which have cron jobs. Updates to base packages like openssl, openldap, sql servers, etc. can have an effect on many other seemingly unrelated packages.
- Bugs. Many packages contain buggy software or installation scripts. The update may create problems during or after installation. Even cosmetic bugs like those found in previous Mozilla updates (causing the user's icons to be removed or break) can be annoying or problematic.
- Automatic updates may not complete the entire process needed to make the system secure. For example, yum can install a kernel update, but until the machine is rebooted (which yum will not do automatically) the new changes won't take effect. The same may apply to restarting daemons. This can leave the user feeling that he is secure when he is not.
Best practices when using automatic updates
If you decide to use automatic updates, you should at least do a few things to make sure you are up-to-date.
Check for package updates which have been automatically performed, and note if they need further (manual) intervention. You can monitor what yum has updated via its log file (usually /var/log/yum.log). You can monitor this automatically by email by modifying the cron job to mail you the last part of the log file. For example, edit /etc/cron.daily/yum.cron so that it looks like the following:
#!/bin/sh if [ -f /var/lock/subsys/yum ] ; then /usr/bin/yum -R 10 -e 0 -d 0 -y update yum /usr/bin/yum -R 120 -e 0 -d 0 -y update /usr/bin/tail /var/log/yum.log | /bin/mail -s yum-report youremail@yourdmain fi
You would replace youremail@yourdomain with a actual email address to which you want to report sent. This change will mean that after yum runs every night, it will email you the tail end of the log file showing what happened. (Note this assumes you have a working mail setup on your machine.)
Alternatives to automatic updates
Instead of automatic updates, yum can alert your via email of available updates which you could then install manually. You could accomplish such a setup with a cron job such as that listed below. Simply put this in /etc/cron.daily with a suitable filename (such as yum-check-updates.cron).
#!/bin/sh /usr/bin/yum check-update 2>&1 | /bin/mail -s "yum check-update output" root
You can of course change the email address it sends to, etc. to meet your own needs.
Another common problem is having automatic updates run when it isn't desired (holidays, weekends, vacations, etc). If there are times that no one will be around to fix any problem arising the from the updates, it may be best to avoid doing updates on those days. One method is to use a crontab entry instead of the /etc/cron.daily/yum.conf provided by default. For example, to only run updates from Monday through Friday mornings (avoiding weekends), you might use a crontab entry such as the following:
0 7 * * 1-5 /usr/bin/yum -y update
If you need more control over when it runs, you could create a file called, for example, /usr/local/etc/no-yum-update.conf, which contains a list of dates not to update on. What dates go in this file is up to you to decide (vacations, holidays, etc). The dates would be in the format YYYY-MM-DD (e.g. 2005-03-31). Then create a /etc/cron.daily/yum-update.cron script something like the following:
#!/bin/sh today=$(date +%Y-%m-%d) while read banned; do [ "$today" == "$banned" ] && exit 0 done < /usr/local/etc/no-yum-update.conf yum -y update
Other methods of protection
Yet another thing to consider if not using automatic updates is to provide your machine with some other forms of protection to help defend any attacks that might occur before updates are in place. This might include an external firewall, a host-based firewall (like iptables, ipchains, and/or tcp wrappers), not performing dangerous tasks on the computer (like browsing the web, reading e-mail, etc), and monitoring the system for instrusions (with system log checkers, IDS systems, authentication or login monitoring, etc).