If the connection succeeds, its socket is assigned to the server’s channel for both reading and writing. Then, the server connects to the remote address and port which were specified in the client’s payload.This way the two sides exchange channel IDs for It immediately “attaches” this channel to theĬlient’s one, using the received identifier, and sends its own identifier to the client. When the server sees this special message, it creates a new channel.Identifier (index), the local address & port and the remote address & port to which the server should connect later on. The message includes the client’s channel The client then sends a message to the server, informing it of the new channel.Unlike sockets, channels are part of the SSH protocol and are not operating-system objects. Once a connection is accepted on the socket, the client creates a channel with the socket’s file descriptor.Listen() is called on the created, bound socket. If no localaddr is specified (which is usually the case for me), the client will create a socket for localhost or all network interfaces (implementation-dependent). The client creates a socket and binds it to localaddr and localport (actually, it binds a socket for eachĪddress resolved from localaddr, but let’s keep things simple).If you wish to understand how local port forwarding works in SSH, without going into any specific implementation, This section summarizes the process without quoting any line of code. Research process here, because there’s already enough information to share. Implementation and also stick to local (as opposed to remote) port forwarding. In this blog post, I’ll focus on the DropBear SSH What happens in the SSH client and server when they respond to this port forwarding instruction? You can read more about SSH port forwarding (also known as “tunneling”) here.īut what happens behind the scenes when the above-mentioned command is executed? This allows me, for example, to communicate with a host that is only accessible to the SSH server. I often use the command ssh server_addr -L localport:remoteaddr:remoteport to create an SSH tunnel.
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