dnssort: 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/dnssort - from package: ndisc6

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

On Debian possible default full paths are:

/usr/bin/dnssort - from package: ndisc6

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

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