A PostgreSQL server on host db01 listens only on 127.0.0.1 port 5432. From your workstation you want to reach it by connecting to port 15432 on your own machine, tunnelled over SSH. Which command sets that up?
LPIC-1 Exam 102-500, objective 110. Security hard
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The options
Not correct ssh -R 15432:localhost:5432 db01
Wrong direction. -R is remote forwarding: it opens a listening port on db01 that is forwarded back to your workstation. You would be publishing a local service to the server, not reaching the server's service.
Not correct ssh -D 15432 db01
Wrong. -D creates a dynamic SOCKS proxy on port 15432. It is useful for a browser, but the client application must speak SOCKS and no fixed destination is bound, so a plain psql connection to localhost:15432 fails.
Not correct ssh -X db01
Wrong. -X enables X11 forwarding so graphical applications started on db01 can display on your workstation. It forwards no TCP port of your choosing.
Correct ssh -L 15432:localhost:5432 db01
Correct. -L is local forwarding: ssh listens on port 15432 on your workstation and forwards each connection through the tunnel, where db01 opens it to localhost:5432 — the loopback address as seen from db01.
Why
The -L argument reads [bind_address:]port:host:hostport, and the crucial subtlety is that host:hostport is resolved by the far end of the connection, which is why localhost here means db01's own loopback interface. By default the local listener binds only to 127.0.0.1; GatewayPorts and an explicit bind address are needed to expose it to other machines. -R is the mirror image, listening on the remote side, and -D is a SOCKS proxy rather than a point-to-point forward.
Where this comes from
- Cited
- LPI exam objective 110.3
- What it says
- Tunnel a TCP connection through SSH using port forwarding.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 110 and space the ones you get wrong.