This is a per-connection (per-destination) automatic TTL adjusting feature.
Basically a --set-ttl mode where you don't need to set specific TTL value.
It works as follows:
1. All incoming SYN/ACKs (the response to client's SYN) are intercepted
2. TTL value is extracted from SYN/ACK
3. New TTL is calculated with the simple formula:
128 > extracted_ttl > 64: // Server is running Windows
fakepacket_ttl = 128 - extracted_ttl - decrement
64 > extracted_ttl > 34: // Server is running Linux/FreeBSD/other
fakepacket_ttl = 64 - extracted_ttl - decrement
4. Fake packet is sent
To comply with the multi-path multi-hop server connections
where 1 hop dispersion is not rare, decrement should be at least of
value "2", which is the default.
The patch does not process "too strange" TTL values (bigger than 128,
less than 34).
ValdikSS