rdnssd: 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/sbin/rdnssd - from package: ndisc6

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

On Debian possible default full paths are:

/sbin/rdnssd - from package: rdnssd

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

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