![]() On the other hand, if the HTTP server resides on the host of the SSH client and the HTTP client resides on the host of the SSH server, then one can use remote port forwarding to access the local HTTP server from the remote side. example of local port forwarding: ssh -L 8000:localhost:80 sshd-host If the HTTP server resides on host of the SSH server and the HTTP client resides on host of the SSH client, then one can use local port forwarding to access the remote HTTP server from the local side. Consider the previous example involving the HTTP server and client. Correspondingly, ssh-host will refer to the host of the SSH client.Įxample. HOSTNAME), while the name appd-host will be used to refer to the host that provides the service of interest. For this reason, the name sshd-host will be used to refer to the host of the SSH server (i.e. The naming of the arguments here is rather unfortunate as the word “host” is overloaded to mean several things. By default, this only binds to the loopback interface. ![]() HOSTNAME: the SSH server (unrelated to HOST or HOSTPORT).īIND_ADDRESS: this is an optional argument that specifies the address that PORT should be associated with. HOSTPORT: the port on HOST listened by the service of interest. In the case of remote port forwarding, HOST is on the side of the SSH client. In the case of local port forwarding, HOST is on the side of the SSH server. HOST: the host that provides the service of interest. In the case of remote port forwarding, PORT is bound to the host of the SSH server. In the case of local port forwarding, PORT is bound to the host of the SSH client. SSH will listen to this port and forward connections made to this port to the other side (i.e. # local port forwarding ssh -L PORT:HOST:HOSTPORT HOSTNAME # remote port forwarding ssh -R PORT:HOST:HOSTPORT HOSTNAME In other words, the type of port forwarding depends on the location of the service of interest (in the example earlier, the httpd server) with respect to the SSH server.įrom the manual pages of SSH, the argument syntax for port forwarding is: … in remote port forwarding, the port that is being forwarded resides on the remote end, i.e. the host of the SSH server. In local port forwarding, the port that is being forwarded resides on the local end, i.e. the host of the SSH client, whereas … local port forwarding (There is also the so-called dynamic port forwarding, which won’t be discussed here.) The first and most important question here is: what is the difference between local and remote forwarding? SSH supports two kinds of port forwarding: local and remote port forwarding. (Some systems, usually the Unix-like ones, would also reserve port numbers below 1024 so that one would require superuser privileges to forward them.) Port forwarding wget connects to httpd via a forwarded portįorwarded ports are generally temporary, so conventionally one would use a large port number to avoid conflicts with the more frequently used ports. In this case, the client would connect to, say, port 8000, which is then forwarded to port 80 on the server. However, if the firewall permits SSH connections, port forwarding can be used to bypass the barrier. … unless of course, there is a firewall that blocks port 80. Normally, given the correct port, the client can simply connect directly to the server… wget connects to httpd directly They are not shown in any of these diagrams. This port, however, is ephemeral and generally irrelevant here. Note: In the process of establishing a connection to a server, the clients themselves also obtain a port on their end. Hence, HTTP clients such as web browsers or downloaders would typically have a target port of 80. An HTTP web server such as httpd would typically listen to port 80 (one can also say the service httpd “runs on” port 80). Clients can then connect to the target port that corresponds to the one chosen by the service.Įxample. The port numbers for many services are specified by conventions. Services can listen to a particular port, waiting for clients to initiate connections to the port. Each service is allowed to pick its own port, denoted by a 16-bit unsigned integer (0‒65535). This is where the notion of a port comes in. Hence, there needs to be a way to distinguish between them. The address suffices to identify the network interface and hence the host, but a single host can provide a variety of services (e.g. web server, mail server, SSH server, etc). Each host can have multiple network interfaces, each of which is assigned an (IP) address. In networking lingo, a host generally refers to a single computer. To add security to an unencrypted connection, or There are two common reasons for doing this: SSH port forwarding can be a very useful tool for working with remote systems.
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