dnsfilter: 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/dnsfilter - from package: ndjbdns

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

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