dos-new-ip6: command not found

Have You tried accessing the binary with the full path?

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/dos-new-ip6 - from package: thc-ipv6

You may install the required package with command yum install {package}
Error in other languages:
dos-new-ip6: bevel nie gevind nie
dos-new-ip6: comandă negăsită
dos-new-ip6: comando não encontrado
dos-new-ip6: commande introuvable
dos-new-ip6: command not found
dos-new-ip6: käsku ei ole
dos-new-ip6: không tìm thấy lệnh
dos-new-ip6: komanda nerasta
dos-new-ip6: Kommando nicht gefunden.
dos-new-ip6: kommandot finns inte
dos-new-ip6: komut yok
dos-new-ip6: nie znaleziono polecenia
dos-new-ip6: níor aimsíodh an t-ordú
dos-new-ip6: no se encontró la orden
dos-new-ip6: no s'ha trobat l'ordre
dos-new-ip6: opdracht niet gevonden
dos-new-ip6: parancs nem található
dos-new-ip6: perintah tidak ditemukan
dos-new-ip6: príkaz nenájdený
dos-new-ip6: příkaz nenalezen
dos-new-ip6: команда не найдена
dos-new-ip6: командата не е открита
dos-new-ip6: 命令找不到
dos-new-ip6: コマンドが見つかりません