redirsniff6: 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/redirsniff6 - from package: thc-ipv6

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

Error in other languages:
redirsniff6: bevel nie gevind nie
redirsniff6: comandă negăsită
redirsniff6: comando não encontrado
redirsniff6: commande introuvable
redirsniff6: command not found
redirsniff6: käsku ei ole
redirsniff6: không tìm thấy lệnh
redirsniff6: komanda nerasta
redirsniff6: Kommando nicht gefunden.
redirsniff6: kommandot finns inte
redirsniff6: komut yok
redirsniff6: nie znaleziono polecenia
redirsniff6: níor aimsíodh an t-ordú
redirsniff6: no se encontró la orden
redirsniff6: no s'ha trobat l'ordre
redirsniff6: opdracht niet gevonden
redirsniff6: parancs nem található
redirsniff6: perintah tidak ditemukan
redirsniff6: príkaz nenájdený
redirsniff6: příkaz nenalezen
redirsniff6: команда не найдена
redirsniff6: командата не е открита
redirsniff6: 命令找不到
redirsniff6: コマンドが見つかりません