Category Archives: Sysadmin

Using wildcards in ssh configuration to create per-client setups

In my role as a linux consultant, I tend to work with a number of different companies. Obviously they all use ssh for remote access, and many require going through a gateway/bastion server first in order to access the rest of the network. I want to treat these clients as separate and secure as possible so I’ll always create a new SSH key for each client. Most clients would have large numbers of machines on their network and rather than having to cut and paste a lot of different configurations together you can use wildcards in your ~/.ssh/config file.

However this is not amazingly easy – as SSH configuration requires the most general settings to be at the bottom of the file. So here’s a typical setup I might use for an imaginary client called abc:

Percent signs in crontab

As this little-known ‘feature’ of cron has now bitten me several times I thought I should write a note about it both so I’m more likely to remember in future, but also so that other people can learn about it. I remember a few years ago when I was working for Webfusion we had some cronjobs to maintain the databases and had some error message that kept popping up that we wanted to remove periodically. We set up a command looking something like:

but it was not executing. Following on from that, today I had some code to automatically create snapshots of a certain btrfs filesystem (however I recommend that for serious snapshotting you use the excellent (if a bit hard to use) snapper tool):

But it was not executing… Looking at the syslog output we see that cron is running a truncated version of it:

Looking in the crontab manual we see:

D’oh. Fortunately the fix is simple:

I’m yet to meet anyone who is using this feature to pipe data into a process run from crontab. I’m also yet to meet even very experienced sysadmins who have noticed this behaviour making this a pretty good interview question for a know-it-all sysadmin candidate!