Fortunately Nmap provides an option for this: -Pn, which tells Nmap to not bother pinging the host before scanning it. This means that Nmap will always treat the target host(s) as being alive, effectively bypassing the ICMP block;
--data-length (Append random data to sent packets)
Normally Nmap sends minimalist packets containing only a header. So its TCP packets are generally 40 bytes and ICMP echo requests are just 28. Some UDP ports and IP protocols get a custom payload by default. This option tells Nmap to append the given number of random bytes to most of the packets it sends, and not to use any protocol-specific payloads. (Use --data-length 0 for no random or protocol-specific payloads. OS detection (-O) packets are not affected because accuracy there requires probe consistency, but most pinging and portscan packets support this. It slows things down a little, but can make a scan slightly less conspicuous.
