sockdown: 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

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 Debian possible default full paths are:

/usr/bin/sockdown - from package: netpipes

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