Open file via SSH and Sudo with Emacs

Question

I want to open a file inside Emacs which is located on a remote server, with sudo powers on the server. I can open local files with sudo via Tramp like this:

C-x C-f /sudo::/home/user/file

But I want to use sudo on the server:

C-x C-f /sudo::user@server/home/user/file

But this gives me sudo powers on my local machine, it asks for my sudo password on the local machine. Is there a way to use sudo on the server?

BTW: Emacs is not installed on the server

Solution 1

As of Emacs 24.3, an analog of the old multi: syntax has been layered on top of the modern tramp-default-proxies-alist approach, meaning that you can once again perform multi-hops without any prior configuration. For details, see:

C-hig (tramp)Ad-hoc multi-hops RET

With the new syntax, each 'hop' is separated by |. The example in the manual is:

C-xC-f /ssh:bird@bastion|ssh:you@remotehost:/path RET

Which connects firstly as bird@bastion, and from there to you@remotehost:/path

/su: or /sudo: on remote hosts

You can also use this syntax to sudo/su to root (or of course any other user) on a remote host:

C-xC-f /ssh:you@remotehost|sudo:remotehost:/path/to/file RET

Important: be sure to specify the hostname explicitly: sudo:remotehost: rather than sudo:: (see below).

As this still uses the proxy mechanism underneath, tramp-default-proxies-alist should now include the value ("remotehost" "root" "/ssh:you@remotehost:")

Meaning that the proxy /ssh:you@remotehost: is going to be used whenever you request a file as root@remotehost.

root is the default user for these methods, but you can of course also change to a non-root user with:

C-xC-f /ssh:you@remotehost|sudo:them@remotehost:/path/to/file RET

Always specify the remote hostname explicitly

You are probably used to using sudo:: or su:: and omitting the hostname. If you are staying on the localhost then this is still fine, but if you are hopping to a remote server then you must specify the hostname for every hop -- even if it is the same as for the previous hop. Always use sudo:hostname: or su:hostname: with remote hosts.

The trap here is that sudo:: does actually appear to work -- however when you do that the HOST for the dynamic proxy entry will be the hostname you originated from rather than the host you connected to. This will not only look confusing (as the wrong host will be displayed in the file paths), but it will also mean that any subsequent attempt to use sudo:: on your localhost will instead be proxied to the remote server! (and the proxy would also presumably be clobbered if you did the same thing on a second server, causing further issues).

In short, don't use :: when you multi-hop!

Emacs 27+

Starting from Emacs 27.1 (or Tramp 2.4.2, if using the GNU ELPA package) the :: case works intuitively, such that /ssh:you@remotehost|sudo:: will re-use remotehost rather than your own local host, and so you won't end up with a bad proxy entry.

In addition, the likes of /ssh:you@remotehost|sudo:localhost: are detected and flagged as user errors.

If you are liable to use a mixture of Emacs versions including versions earlier than 27 (or you are advising someone else who may be using an older version), then it would be safest to continue to treat :: as unsafe when multi-hopping, to avoid potential mishap. (I.e. specifying the correct remote host explicitly will remain the safest approach if the Tramp version is unknown.)

Solution 2

Update: Although this answer solved the original problem, it was written for emacs 20 or 21. For emacs 24, I recommend you use phils's answer because it offers more explanation and is up to date.

---

I think multi-hop filenames in tramp is what you're looking for.

The first hop would be ssh and the second would be sudo.

---

Update: Recent versions of emacs support multiple hops using proxies:

(add-to-list 'tramp-default-proxies-alist ("my-sudo-alias" nil "/ssh:user@ssh-host"))

Then invoke by opening:

/sudo:my-sudo-alias:file-on-ssh-host

Solution 3

I had some troubles with the selected answer. However, it worked when I added this line to .emacs:

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

And then executed the following:

/sudo:ssh-host:file-on-ssh-host

It was slightly confusing because at one point I was prompted for the "root" password, but entering my user's password granted me access. It also universally works on all hosts on the network. Also, I can still do this to not be root:

/ssh:ssh-host:file-on-ssh-host

Solution 4

From the tramp multi-hops configuration webpage

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

Then any

C-x C-f /sudo:remote-host:/file

will open file using sudo after logged with the same username of the user running emacs but on the remote machine.

View more solutions

Comments

[Dan Andreatta]

I tried, but I cannot access files as root with sshfs, when mounting it as regular user. I guess it is a setup problem.

[Dan Andreatta]

As Hassan noted, no_root_squash should be used with care.

[Joe Meyer]

This seems like the solution, but I get: "multi method is no longer supported" Can you point me to an updated manual?

[Dave Bacher]

M-x info, C-s tramp :) You may need to define a fake host as the target of your sudo and add it to tramp-default-proxy-alist.

[gaizka]

This was not working for me. It looks that in Ubuntu, at least with version 23.2.1 of Emacs and version 2.1.18-23.2 of tramp this does not work. This works, though: info.solomonson.com/content/…

[gaizka]

(set-default 'tramp-default-proxies-alist (quote ((".*" "\`root\\'" "/ssh:%h:"))))

[ang mo]

I couldn't get it running in my configuration (error 255?), but the following line in .emacs works: (set-default 'tramp-default-proxies-alist (quote (("my-sudo-alias" nil "/ssh:user@ssh-host:"))))

[SabreWolfy]

I am not able to get either of the solutions above to work (add-to-list or set-default). The first causes Emacs to choke on startup and the second gives me "Host abc.xyz.com' looks like a remote host, sudo' can only use the local host" as soon as I enter the second colon in "/sudo:abc.xyz.com:". Ideas? Emacs 23.1.1 on Ubuntu 10.04 LTS.

[Michael Wolf]

The updated answer doesn't define "recent", has invalid elisp, and doesn't work even when you correct it. I'm on a stable version of emacs released over a year after the updated answer.

[Ehvince]

and now added to wikemacs as well wikemacs.org/index.php/TRAMP. Thanks phils, it works great.

[manu]

I had to use tramp-default-proxies-alist for this; but when I try to visit a file sudo:my-sudo-alias:/etc/anything Emacs complains about "Method `scpc' is not supported for multi-hops.". Any ideas?

[TomRoche]

The real answer as of 2014/24.3 is the following answer, using "ad-hoc multi-hops"

[rogerl]

I just found this answer, and it works great...except that I have hostname shorthand defined in an .ssh/config file. When I type /sudo:hostname:/etc/hosts, I get what you would expect, but when I type /sudo:abbrev:/etc/hosts, I get the message "Host abbrev looks like a remote host, sudo can only use the local host". Is this fixable?

[phils]

rogerl: I also have .ssh/config entries, and I don't have any problems using /ssh:abbrev|sudo:abbrev:/etc/hosts for my hostname abbreviations. It looks like you're not hopping to the host first.

[rogerl]

@phils Perhaps I should post this as a new question. However: my config file looks like Host r User rlipsett Hostname odlinux IdentityFile ~/.ssh/id-rsa.pub and my tramp-default-proxies-alist appears to be correct. Yet the behavior I described above persists. Is my config file somehow wrong?

[phils]

It's still not clear to me whether you are actually trying to use /sudo:abbrev:/etc/hosts as you put in your previous comment (which is wrong, as mentioned), or if you're using the correct /ssh:abbrev|sudo:abbrev:/etc/hosts. Your ssh config file is presumably fine, as you can evidentially use it otherwise.

[phils]

If you do have the correct tramp-default-proxies-alist config already in place, then you can likely use /sudo:root@abbrev:/etc/hosts as (slight) shorthand; but I'd suggest using the longer form in general.

[rogerl]

@phils The only reason I was trying that shorthand syntax at all was because of your answer here. It's really unfortunate that there is no decent shorthand, since I use emacs to manage files on a remote server regularly.

[phils]

Ah, I see. That wasn't my answer, though -- and I didn't spot the issue when I was commenting on it, because I only ever manually type the paths using the longer syntax. I'll grant you that it's awfully odd that Michael Albinus would have written something incorrect about tramp, but it rather seems that way. You should follow up that answer and query it with him.

[ceving]

-1 for not answering the question. -1 for suggesting NFS instead of SSH. -1 for suggesting no_root_squash. -1 for suggesting Emacs client, which uses usually Unix domain sockets, for a remote communication problem. -1 for blethering about Emacs without using it.

[Startec]

@phils root ssh login has to be enabled for though correct?

[phils]

@Startec, I wouldn't have thought so, but if you've tested and found that to be the case then I can't nay-say it.

[Startec]

@phils could you give just a little bit of explanation as to what the | does? For instance, at the end of it, am I logged in as user or root? I it as though I ssh'd as the root user? Currently this is not working as this "times out".

[Startec]

@phils this does not work. Emacs just hangs forever. It asks me for the password for the root user, and here is the error log: pastebin.com/M20ezpgi

[phils]

I can't really help, sorry. I can see that "Tramp: Waiting for prompts from remote shell...failed" has timed out after password entry, but the reason is not clear to me. I trust you do have sudo access in the first place? Note that the password you're asked for is commonly the original user's password, not the root user's password (although it may vary between systems and configs), in which case the user needs to have been granted permission to use sudo. I suggest that you confirm your sudo access outside of Emacs, if you have not done so already.

[Startec]

@phils this was my issue / solution!

[phils]

Quick update -- As of Emacs 27, the :: case will work the way you want it to, such that /ssh:you@remotehost|sudo:: will re-use remotehost rather than your own local hostname, so you won't end up with a bad proxy entry. In addition, the likes of /ssh:you@remotehost|sudo:localhost: are detected and flagged as user errors. (Of course, if you are liable to use a mixture of Emacs versions, you should continue to treat :: as unsafe when multi-hopping in general, to avoid potential mishap.)