Tag Archives: Linux


Ask someone with headphones and a lanyard in the halls of a datacenter what transport does DNS use, there’s a good chance the answer you’d get back is UDP Port 53.

But not always!

In scenarios where the DNS response is large (beyond 512 bytes) a DNS query will shift over to TCP for delivery.

How does the client know when to shift the request to TCP – After all, the DNS server knows how big the response is, but the client doesn’t.

The answer is the Truncated flag, in the response.

The DNS server sends back a response, but with the Truncated bit set, as per RFC 1035:

TC TrunCation – specifies that this message was truncated due to length greater than that permitted on the transmission channel.

RFC 1035

Here’s an example of the truncated bit being set in the DNS response.

The DNS client, upon receiving a response with the truncated bit set, should run the query again, this time using TCP for the transport.

One prime example of this is DNS NAPTR records used for DNS in roaming scenarios, where the response can quite often be quite large.

If it didn’t move these responses to TCP, you’d run the risk of MTU mismatches dropping DNS. In that half of my life has been spent debugging DNS issues, and the other half of my life debugging MTU issues, if I had MTU and DNS issues together, I’d be looking for a career change…

Installing Yate from Source on Ubuntu 20.04

Here’s my build instructions for compiling and running Yate on Ubuntu 20.04 from source:

apt-get update
apt-get install wget make gcc autoconf subversion libsctp-dev libsctp1 g++ -y
cd /usr/src
svn checkout http://voip.null.ro/svn/yate/trunk yate
cd yate
vi /etc/modprobe.preload

Enable SCTP by adding “sctp” into the file and saving, then we can get on with compilation:

modprobe sctp
sysctl -p
./configure --enable-sctp=yes
make install-noapi
yate -V

And done, Yate installed with SCTP support, for all your SIGTRAN needs!

Soon we’ll be using this in our series investigating SS7 networks…

Ubuntu Cloned VMs getting Duplicate IPs (and yes – the MAC Addresses are unique)

So I run a lot of VMs. It’s not unusual when I’m automating something with Ansible or setting up a complex lab to be running 20+ VMs at a time, and often I’ll create a base VM and clone it a dozen times.

Alas, Ubuntu 20.04 has some very irritating default behaviour, where even if the MAC addresses of these cloned VMs differ they get the same IP Address from DHCP.

That’s because by default Netplan doesn’t use the MAC address as the identifier when requesting a DHCP lease. And if you’ve cloned a VM the identifier it does use doesn’t change even if you do change the MAC address…

Irritating, but easily fixed!

Editing the netplan config:

      dhcp4: true
      dhcp-identifier: mac
  version: 2

Run a netplan-apply and you’re done.

Now you can clone that VM as many times as you like and each will get it’s own unique IP address.

Remember Bash History Forever

History in Bash is a huge time saver.

That beautifully crafted sed command you put together to replace the contents of something a few months ago? Just search through your Bash history and there it is.

Previously I’d been grepping the output of history to find what I was looking for, and now I’ve fallen in love with the search feature, but by default, many Linux distros limit the number of lines in the Bash history to 2,000. If you’re a regular Linux user, this isn’t cutting the mustard.

By default the bashrc file that ships with Ubuntu is limited to 2,000 lines or 1MB,

We can change all this very easily, by editing the ~/.bashrc file (Bash shell script), upping the limit of entries we keep. While you’re at it adding HISTTIMEFORMAT allows you to timestamp the commands you’re running, and the PROMPT_COMMAND below also writes immediately, so you won’t get lost data or missing stuff that you’ve just run in another terminal.

Example contents of ~/.bashrc:

export HISTSIZE=100000
export HISTFILESIZE=200000
export HISTTIMEFORMAT='%d/%m/%y %T '

And you can apply the changes with:

source ~/.bashrc