Mastering Python for Networking and Security

Filtering UDP packets

In the following example, we see how we define a function that will be executed every time a packet of type UDP is obtained when making a DNS request:

scapy> a = sniff(filter="UDP and port 53",count=100,prn=count_dns_request)

This function can be defined from the command line in this way. First we define a global variable called DNS_QUERIES, and when scapy finds a packet with the UDP protocol and port 53, it will call this function to increment this variable, which indicates there has been a DNS request in the communications:

>>> DNS_QUERIES=0>>> def count_dns_request(package):>>>    global DNS_QUERIES>>>    if DNSQR in package:>>>        DNS_QUERIES +=1