Reaching a remote host through a gateway using Emacs and Tramp

3
votes

I am attempting to edit a remote file in Emacs, and I'm having trouble getting from the documentation and previous SO questions to doing the thing I want.

I'm working remotely, from a variety of locations, and I want to edit files on server Foo. Server Foo isn't directly reachable from the Internet, but server Bar is, and Foo accepts connections from Bar. I can count on reaching Bar, and Bar can count on reaching Foo.

The Tramp documentation tells me that I need to add to Tramp's proxy list to do this, and that it should look like the following:

(add-to-list 'tramp-default-proxies-alist
    ("foo_host" "seanm" "/ssh:seanm@bar_host"))

However, when I try that, it doesn't work, and I get inscrutable error messages.

  • How can I unambiguously refer to Foo? There is no DNS server that knows how to find Foo from its hostname, and Foo's IP address is in the 10.0.0.0/8 private space. I don't want to refer to Foo by that IP address, because that risks colliding with hosts in other 10.0.0.0/8 networks I may visit. As a kludge, I added Foo to Bar's /etc/hosts file, but that didn't seem to work. Is there a better solution?

  • What quoting rules do I need to adhere to? The examples that I'm seeing use both "double quotes" and `tick-and-single' quotes, the latter apparently requiring double-backslash escaping. I don't understand what's going on there - it seems like there are multiple layers of parsing that this string is going to be shoved through.

  • While trying variations on this, I've gotten error messages that amount to "you can't use that remote-access method for that scenario." How can I set up a second proxy method that will let me do the equivalent of C-x C-f /su::/path/to/file ?

It all seems very painful compared to the previous multi-hop syntax.

Later: I was able to get the many layers of escaping right so that emacs correctly 'hears' the names of hosts. However, I can't get any hops beyond a second to work correctly, which is a very disappointing lapse in functionality.

1
emacssshelisptramp
While that's possible IIRC, another way would be to configure ssh to reach Foo through Bar. With OpenSSH, use a Host alias with a ProxyCommand option; you'll find plenty of examples on Super User, Unix or Server Fault. – Gilles 'SO- stop being evil'
That's super helpful for the first and second problems, but it doesn't look like it'll help with duplicating the functionality of /su::/path/to/file. – Brighid McDonnell
In principle I should be able to do this entire thing in elisp. – Brighid McDonnell
Why didn't you include the "inscrutable error messages"? – Ross Patterson

1 Answers

1
votes

The correct answer turned out to be modifying my SSH config with Host, HostAlias, and ProxyCommand directives.