问题
If I have a Java program that listens to multicast data, how can I tell what the remaining TTL of the incoming datagram is?
I know how to set the TTL on the sending multicast socket, but I want to know the remaining TTL of the arriving packet.
回答1:
If you receive the UDP datagrams with a raw socket you can inspect the TTL (or anything else in the IP or UDP headers). There are lots of gotchas, though:
- Raw Sockets are non-native to Java (though at least one multi-platform 3rd party add-on is available: http://www.savarese.org/software/rocksaw/index.html ).
- On most operating systems, you must have root access or administrative privileges to use raw sockets
- If you wanted to use the UDP data also, you would have to do checksums and reassembly of fragmented datagrams yourself ...as well as possibly encountering different TTLs among fragments (as jambjo said).
回答2:
What is the arriving packet? A datagram or UDP packet can actually have been split into several IP packets, which arrive with different TTLs if they are routed differently. In either case (split or not), Java doesn't provide access to the IP header.
回答3:
I looked (as I imagine you did as well), but could not find a nice Java API for doing this.
The TTL is the 9th whole byte in an IPv4 datagram. What happens if you output the 9th byte of the DatagramPacket from an incoming datagram? Does that look like the TTL value? Or, are the headers from the IP datagram not available in DatagramPacket, only the data portion of the IPv4 datagram?
来源：https://stackoverflow.com/questions/1723429/determine-the-remaining-ttl-of-a-multicast-packet-in-java