validns: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/validns - from package: validns

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/validns - from package: validns

You may install the required package with command apt-get install {package}

Error in other languages:
validns: bevel nie gevind nie
validns: comandă negăsită
validns: comando não encontrado
validns: commande introuvable
validns: command not found
validns: käsku ei ole
validns: không tìm thấy lệnh
validns: komanda nerasta
validns: Kommando nicht gefunden.
validns: kommandot finns inte
validns: komut yok
validns: nie znaleziono polecenia
validns: níor aimsíodh an t-ordú
validns: no se encontró la orden
validns: no s'ha trobat l'ordre
validns: opdracht niet gevonden
validns: parancs nem található
validns: perintah tidak ditemukan
validns: príkaz nenájdený
validns: příkaz nenalezen
validns: команда не найдена
validns: командата не е открита
validns: 命令找不到
validns: コマンドが見つかりません