tramp-default-proxies-alist

tramp-default-proxies-alist is a variable defined in `tramp.el'.
Its value is

nil

Documentation:
Route to be followed for specific host/user pairs.
This is an alist of items (HOST USER PROXY). The first matching
item specifies the proxy to be passed for a file name located on
a remote target matching USER@HOST. HOST and USER are regular
expressions. PROXY must be a Tramp filename without a localname
part. Method and user name on PROXY are optional, which is
interpreted with the default values. PROXY can contain the
patterns %h and %u, which are replaced by the strings matching
HOST or USER, respectively.

HOST, USER or PROXY could also be Lisp forms, which will be
evaluated. The result must be a string or nil, which is
interpreted as a regular expression which always matches.

You can customize this variable.