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4.9 Connecting to a remote host using multiple hops

Multi-hops are methods to reach hosts behind firewalls or to reach the outside world from inside a bastion host. With multi-hops, TRAMP can negotiate these hops with the appropriate user/host authentication at each hop. All methods until now have been the single hop kind, where the start and end points of the connection did not have intermediate check points.

User Option: tramp-default-proxies-alist

tramp-default-proxies-alist specifies proxy hosts to pass through. This variable is list of triples consisting of (host user proxy).

The first match is the proxy host through which passes the file name and the target host matching user@host. host and user are regular expressions or nil, interpreted as a regular expression which always matches.

proxy is a literal TRAMP file name whose local name part is ignored, and the method and user name parts are optional.

The method must be an inline or gateway method (see Inline methods, see Gateway methods). If proxy is nil, no additional hop is required reaching user@host.

For example, to pass through the host ‘bastion.your.domain’ as user ‘bird’ to reach remote hosts outside the local domain:

(add-to-list 'tramp-default-proxies-alist
             '("\\." nil "/ssh:bird@bastion.your.domain:"))
(add-to-list 'tramp-default-proxies-alist
             '("\\.your\\.domain\\'" nil nil))

Note: add-to-list adds elements at the beginning of a list. Therefore, most relevant rules must come last in the list.

Proxy hosts can be cascaded in the alist. If there is another host called ‘jump.your.domain’, which is the only host allowed to connect to ‘bastion.your.domain’, then:

(add-to-list 'tramp-default-proxies-alist
             '("\\`bastion\\.your\\.domain\\'"
               "\\`bird\\'"
               "/ssh:jump.your.domain:"))

proxy can take patterns %h or %u for host or user respectively.

To login as ‘root’ on remote hosts in the domain ‘your.domain’, but login as ‘root’ is disabled for non-local access, then use this alist entry:

(add-to-list 'tramp-default-proxies-alist
             '("\\.your\\.domain\\'" "\\`root\\'" "/ssh:%h:"))

Opening /sudo:randomhost.your.domain: first connects to ‘randomhost.your.domain’ via ssh under your account name, and then perform sudo -u root on that host.

It is key for the sudo method in the above example to be applied on the host after reaching it and not on the local host.

host, user and proxy can also take Lisp forms. These forms when evaluated must return either a string or nil.

To generalize (from the previous example): For all hosts, except my local one, first connect via ssh, and then apply sudo -u root:

(add-to-list 'tramp-default-proxies-alist
             '(nil "\\`root\\'" "/ssh:%h:"))
(add-to-list 'tramp-default-proxies-alist
             '((regexp-quote (system-name)) nil nil))

The above configuration allows TRAMP connection as ‘root’ to remote Ubuntu hosts.

tramp-default-proxies-alist is also used for passing through firewalls or proxy servers.

For example, the local host ‘proxy.your.domain’ on port 3128 serves as HTTP proxy to the outer world. User has access rights to another proxy server on ‘host.other.domain’.1 Then the configuration is:

(add-to-list 'tramp-default-proxies-alist
             '("\\`host\\.other\\.domain\\'" nil
             "/tunnel:proxy.your.domain#3128:"))

Gateway methods in a multiple hop chain can be declared only as the first hop.

Passing through hops involves dealing with restricted shells, such as rbash. If TRAMP is made aware, then it would use them for proxies only.

User Option: tramp-restricted-shell-hosts-alist

An alist of regular expressions of hosts running restricted shells, such as rbash. TRAMP will then use them only as proxies.

To specify the bastion host from the example above as running a restricted shell:

(add-to-list 'tramp-restricted-shell-hosts-alist
             "\\`bastion\\.your\\.domain\\'")

Footnotes

(1)

HTTP tunnels are intended for secure SSL/TLS communication. Therefore, many proxy servers restrict the tunnels to related target ports. You might need to run your ssh server on your target host ‘host.other.domain’ on such a port, like 443 (https). See http://savannah.gnu.org/maintenance/CvsFromBehindFirewall for discussion of ethical issues.

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