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Handy One-Liners

This page is a collection of random one-liners I’ve used and wanted to keep track of.

list contents of all crontabs

for user in $(cut -f1 -d: /etc/passwd); do crontab -u $user -l; done

I ran into two issues when setting up Let’s Encrypt SSL certificates on two of my servers - permission issues for Exim and the certbot cron job supplied by the package doesn’t handle the renew very well for nginx, exim or dovecot.

Resolving Exim’s Permission Problems

1. Create a new group. I named it sslcerts. Add the exim user to that group. If you’re not using Debian, adjust the user in the command below.

I ran into a problem with NGINX failing to start on boot/reboot on my Debian 8 (Jessie) server. After reviewing what seemed like a hundred sites to try to find a fix, I stumbled across one solution that worked, but was incredibly inelegant. This was to add:

RestartSec=30s
Restart=on-failure

to nginx.service in the [Service] section using the override.conf. It worked but didn’t fix the underlying problem.

A quick look using journalctl -u nginx showed that the service was failing because the IPv6 address hadn’t been assigned to the network adaptor yet. This caused nginx to fail because it couldn’t bind to the IPv6 port. Here are the log lines:

merge-ngx-conf.pl is a perl script used to assemble a set of nginx configuration files for one site. It has a number of options. See the bitbucket page or the help documentation in the script itself.

In its simplest form, it’s called by issuing this command:

merge-ngx-conf.pl /path/sites-available/filename

The output is an assembled nginx configuration file with all the includes inserted. Using nginx.conf and domain.conf (or just domain.conf depending on the options selected), the script iterates through the include directives in the files and inserts the text from the referenced file. The script handles wildcard masks and follows include directives down multiple levels (i.e. nested levels). It will also follow referenced files in directories external to the nginx configuration directory.

Here’s a perl script I put together that uses Email::Simple to extract the headers from a message. See link below.

I’m using it to examine spam. It parses all the headers, with a focus on the Received headers. It should be easy to alter it to examine any header you want. As it is currently written, it:

1. finds all the Received headers
2. finds the first Received header that was added to the mail (presumably the header added by the first MTA that received it)
3. extracts the IP from that header
4. does an rDNS lookup
5. if there’s a hostname, it looks up the nameservers for the base domain.

By “base domain” I mean that if the rDNS returns a hostname like “1234.my.example.domain.com”, the base domain would be “domain.com”.