Tag: bash

Fixing Ubuntu 17.04 DNS problems

Ryanbash, ubuntu

I recently upgraded my Ubuntu box to 17.04.

Much to my surprise DNS starting behaving strangely, so I checked my DNS server … worked fine if I queried it directly, so I checked if DHCP was giving out the wrong DNS IP … nope, that was fine too. I checked /etc/nsswitch.conf , and that looked fine too so I checked what was ending up in /etc/resolv.conf and was surprised that it contains nameserver 127.0.0.53  instead of the “real” DNS server.

After a bit of research I found out that Ubuntu switched over to using systemd-resolved, which shoves itself between user land and the DNS servers and (at least in Ubuntu 17.04) has problems with servers that support DNSSEC. Very frustrating when you know everything is OK and worked in the past, just systemd messing with stuff and breaking it.

My workaround was to turn of DNSSEC validation. Not pretty but better than no DNS at all, until Ubuntu get’s their problems sorted out.

 

How to fetch IP ranges/entries from SPF records in bash

Ryanbash, SPF

Recently I needed to fetch IP ranges from SPF records. After looking at different python/ruby/perl modules I came to the conclusion that a fancy module (sometimes with wonky dependencies) was overkill just to parse a simple SPF record. So I threw together a simple bash script that is mainly just fetching the SPF record with dig and grep:

It iterates through the options (it currently recognizes a, mx, ip4, ip6, include, and redirect), and then sorts the output by ipv4, then ipv6.

Download URL: fetch_spf.sh

How to compare package version strings in bash

Ryanbash

This is a little function I use to compare package version strings. Sometimes they can get complex with multiple different delimiters or strings in them. I cheated a bit by using sort –version-sort for the actual comparison. If you are looking for a pure bash version to compare simpler strings (e.g. compare 1.2.4 with 1.10.2), I’d suggest this stackoverflow posting.

The function takes three parameters (the version strings and the comparison you want to apply) and uses the return code to signal if the result was valid or not. This gives the function a somewhat natural feel, for example compare_version 3.2.0-113.155 “<” 3.2.0-130.145 would return true. Aside from < and > you can also use a few words like bigger/smaller, older/newer or higher/lower for comparing the strings.

List of return codes and meanings:

 

 

How to easily switch between ansible versions

Ryanansible, bash

Lately I’ve run into issues with different versions of ansible (1.9 handling async better, 2.x having more modules and handling IPv6 better) and having to test playbooks and roles against different versions to make sure they work. TO make life easier I put this little function in my .bashrc to switch back and forth between ansible versions. It checks out the specified version from github if it needs to, and switches over to it (just for that terminal, not the system). Usage is straight forward ansible_switch <branch> , i.e. ansible_switch 2.1  (or whatever branch you want, here is a list of all branches).

It is currently limited to stable branches, but you can change line 6 from stable- to whatever you want (or remove the prefix completely). If you have a github account you also may want to change from https to ssh by using the git@github.com:ansible/ansible.git checkout URL.

 

A script to diff files/directories on two different servers

Ryanbash, linux, scripting

Ok,  short one today. This is a straightforward script that simplifies comparing directories on different servers. There is no magic in it, it just rsyncs the directories to a local temp directory and runs diff against them (then deletes the directory afterwards). Mainly intended for config files, I wouldn’t recommend trying to diff gigabytes of binaries with it.