libnet-config: 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/libnet-config - from package: libnet-devel

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

On Debian possible default full paths are:

/usr/bin/libnet-config - from package: libnet1-dev

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

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