Revealing Hidden characters in a string

Recently we faced an issue, where there were some hidden characters inside the hostname of a configuration store of our backend services.

Because of this, we were getting continously getting DNS resolution errors.

One quick way to reveal such hidden characters is to run the command

echo "string" | cat -v

For example try copying the belwo string into dig or a whois website.

madhur.​co.​in

The above string contains hidden characters which can be reveled as follows

echo "madhur.co.in" | command cat -v
madhur.M-bM-^@M-^Kco.M-bM-^@M-^Kin

Sometimes these characters are not visible even in terminals, because modern terminals tend to be very good “renderers” and would render these hidden characters.