ldns-update: 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-update - from package: ldns

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

On Debian possible default full paths are:

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

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

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