ldns-resolver: 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/ldns-resolver - from package: ldns

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

On Debian possible default full paths are:

/usr/bin/ldns-resolver - from package: ldnsutils

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

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