FIN scan with scapy

[peterkl]

I'm trying to make a FIN scan with python3.5 using scapy, I send a FIN packet and in case the port is closed I should get a RST packet back, in case it is opened the server should ignore my request. The problem is I'm not getting anything at all, nothing in resp part of the sending function. The same thing happens when I try to do a xmas scan (FPU). I've already tried with several hosts but none seems to work. What am I doing wrong ? Here's the code :

...
def TCP_Fin():
    ip_p = IP(dst=host_ip)
    tcp_p = TCP(dport=(1,100),flags='F')
    packets = ip_p/tcp_p
    resp, non_resp = sr(packets,timeout=0.5)
    for item in non_resp :
        print('[-]Port:',item.sport,'closed)
    for sent,recv in resp: # I DO NOt get any recv packets
        if recv[1].flags == 4 : # 4 == RST packet
            print('[+]Port:',sent[1].dport,'closed, but !port service on!')
        if recv[1].flags != 4 :
            print('[+]Port:',sent[1].dport,'opened')
            print(recv[1].flags)