fake_dns6d: 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/fake_dns6d - from package: thc-ipv6

You may install the required package with command yum install {package}
Error in other languages:
fake_dns6d: bevel nie gevind nie
fake_dns6d: comandă negăsită
fake_dns6d: comando não encontrado
fake_dns6d: commande introuvable
fake_dns6d: command not found
fake_dns6d: käsku ei ole
fake_dns6d: không tìm thấy lệnh
fake_dns6d: komanda nerasta
fake_dns6d: Kommando nicht gefunden.
fake_dns6d: kommandot finns inte
fake_dns6d: komut yok
fake_dns6d: nie znaleziono polecenia
fake_dns6d: níor aimsíodh an t-ordú
fake_dns6d: no se encontró la orden
fake_dns6d: no s'ha trobat l'ordre
fake_dns6d: opdracht niet gevonden
fake_dns6d: parancs nem található
fake_dns6d: perintah tidak ditemukan
fake_dns6d: príkaz nenájdený
fake_dns6d: příkaz nenalezen
fake_dns6d: команда не найдена
fake_dns6d: командата не е открита
fake_dns6d: 命令找不到
fake_dns6d: コマンドが見つかりません