发送到 127.0.0.1 的数据包是否完全使用工作连接套接字？

Question

Given a client-server machines system (a MP videogame, for example), server wants for some reasons send a packet to itself (protocol used doesn't matter, I suppose). One can send such packet to server via IP, but this involves actually sending the packet in the internet and then wait for it to come back (also, it's kind of weird to do this). That's why we can send our packet to the address 127.0.0.1.

My question is: when sending a packet to 127.0.0.1, does the packet actually get to the socket used for the estabilished connection in order to be delivered to the same machine, or it is "captured" just before and sent back (because it's a self-sending packet) leaving then the socket free to send/receive packets from the outside?

Answer 1

Operating systems have some form of "loopback device" that's used when a host communicates with itself. On Linux, you can see this explicitly as the lo device:

# ip -4 a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN 
    inet 127.0.0.1/8 scope host lo
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    inet 172.16.5.202/24 brd 172.16.5.255 scope global eth0

When you send a packet to 127.0.0.1, it really does go through this device. It will be processed normally by your sockets API calls, consuming space in the socket buffer until processed, just as if it had come from "the Internet".

Answer 2

First, packets addressed to a locally-assigned IP, even when it is not loopback, are managed internally by the networking stack and not sent to the network.

It seems there is a confusion in the question itself.

The client opens a connection with the server, and when the connection is established, in the server side a new socket is created and returned to the server application. If the connection is opened with a server in 127.0.0.1 (loopback address), the server needs to be listening on address 127.0.0.1 or INADDR_ANY, it will receive the datagrams and the connection can be established. If the server is listening only to 127.0.0.1 it won't receive messages from outside.

Sockets is just the interface provided by the operating system to deal with different protocols. When the client sends the message to the server, it sends it through the socket, it is sent to the kernel networking stack which, at the IP layer looks at the destination address and decides what to do with that datagram. If it goes to loopback or a locally-assigned address it will pass the datagram internally to the appropriate data structure to be received on the other end of the connection: the socket returned by the accept system call in the server, if it is a established connection.

These datagrams never go to the NIC and are never sent out to the network.

Answer 3

Well this really depends upon an implementation - but on Linux - even a packet that originates on a node say 192.168.0.1 and is destined for the same host 192.168.0.1 would not see so much as Ethernet card of the machine. So sending it to the IP address itself is not going to hurt. But if you choose to send it to 127.0.0.1 that's okay too.

Coming back to your question does the packet gets sent to the socket. It depends - if you are listening on INADDR_ANY , then it can get sent to the server. If you are listening only on a specific address (that of your Ethernet Adapter say) you will not receive it if you send to .

For a packet to be delivered to a socket - there has to be a socket that is 'connected' or 'listening' on a given address. That's broadly how it is.

Hope that helps.