dns-helper: 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/dns-helper - from package: gnome-libs

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

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