Getting Comfortable With SSH | Hacker News Reader

Getting Comfortable With SSH

jcsalterego.github.com

100 pointsjcsalterego44 comments

Good article overall, but there is one major problem with the section on public-key auth; it's rarely a good idea to use blank pass phrases on your private keys, for two good reasons:

1. The security implications. Without a passphrase, anyone who manages to snare a copy of your private key can use it to access any servers where you've authorized it with no further credentials needed. Not good. Using a passphrase turns SSH auth into a poor man's two factor system: what you have (the key file), and what you know (the passphrase).

2. It breaks agent-forwarding, which is the system that gives you the ability to SSH onwards from the remote machine to other machines without (again) being asked for your password. This has practical implications when using SSH to drive things like application deployment over git, when you have to ssh onward from the server to your git repository. Doing things this way sidesteps the awkward use of deploy keys, and is actually easier to set up and use as well as being more secure. With agent forwarding, there's no need to keep your private key anywhere except your 'home' machine, and yet you get the same benefits your would if you had it everywhere.

The annoyance of having to type in your passphrase every time is greatly lessened by the use of a key agent on your 'home' machine. OS X has provided a key agent since 10.5 I believe, and stores the passphrase in your Keychain (you know you're doing things right if an OS X dialog box prompts you for your passphrase when running SSH in the terminal). Gnome has one built in as well, and ssh-agent is always around for people who want to roll their own.

If there's interest, I'll write up a blog post about how to get this stuff set up. It's not really easily discoverable stuff, but it _is_ really simple and useful once you get it up and running.

Ironically, GitHub warns against using unprotected private keys in their setup guide:

http://help.github.com/working-with-key-passphrases/

Your mention of Gnome's built in key-ring is rather interesting. I just setup keys on my desktop for my work servers and was met with a foreign prompt for this key. I was startled and unclear about what to do, until I did some digging around. I'm sure an article would be appreciated!

Cheers for pointing that out.

I haven't taken the time to figure out key agent -- for shame! -- although I would be interested in a write-up.

If you are going to go the terminal way with ssh-agent I highly recommend using keychain (http://www.funtoo.org/en/security/keychain/intro/ ) which is quite easy to setup and use, and much easier than having to do ssh-add and so on every time you want to use a specific key.

Keychain is the best. It also works with gpg-agent, if you're old and cranky enough to still think gpg is worth using.

I didn't realize I was old and cranky...

I've been using gpg-agent with a smartcard for ssh authentication for the past six months or so. It's the way, the truth, and the light.

Real two-factor authentication. The key can't be compromised without trying to physically take apart the chip on the card and somehow access the internals. Three wrong password attempts and the thing locks. Three wrong attempts with the admin unlock code and the card self-destructs. So even if I lose the card or someone jacks it, it can't be brute forced.

(Yes I don't need that level of security, and I'm not even being paranoid; it's just neat. Makes me feel like James Bond or Batman or some shit like that every time I ssh somewhere. Yes I'm that lame... )

And since it's still using a standard RSA key for ssh authentication, I don't need to install anything special like experimental pam modules, on the host machines. Just copy the public key into authorized_keys. That's nice since I don't have full admin rights on a lot of my host machines.

Sounds interesting? Got a reference? Thanks.

There is a howto on the gnupg site, but frankly it's a little out of date. It focuses enough on hardware to scare people away, but these days most readers are libccid compatible, so it's a non issue. Setup is actually pretty simple.

Basically you either get a card and reader:

http://shop.kernelconcepts.de/product_info.php?cPath=1_26...

Or get an all-in-one cryptostick:

http://www.privacyfoundation.de/crypto_stick/crypto_stick_en...

And setup your gpg keys on there, either by generating them directly on the card or transferring existing keys. These are simple commands documented elsewhere. In addition to the normal signing and encryption keys, you also generate an authentication key.

Then 'ssh-add -L' will spit out your public key in ssh format to copy on the host machines as usual.

After that you just make sure that you'll use gpg-agent instead of ssh-agent. The man page for gpg-agent shows you what you'll want to add to .bashrc.

Then when you ssh into a machine, gpg-agent will take over, pop up a little dialog called pinentry, you enter your code, and you're good. When you go to lunch, remove card, and ssh authentication with that key no longer works.

What kind of smartcard are you using?

There also exists the ControlMaster configuration option which treats multiple sessions to the same host as a single connection. With this enabled, you only have to enter your SSH passphrase once per host. Aside from the convenience of not having to repeatedly enter your passphrase, subsequent sessions are initiated much faster (due to not having to renegotiate) and there is (theoretically) less strain put on your network adapter. Note, however, that if there are no active sessions to a host, the connection to that host will be dropped, and you will have to re-enter your passphrase upon reconnection. See the ControlPersist option for dealing with connection persistence.

If you're like me, however, you'll use both keychain and ControlMaster (for the increased negotiation speed) :)

I would definitely be interested in learning more about this stuff from a blog post.

Me too.

As promised:

http://www.longbeard.org/2011/02/05/Montell-Jordan-Has-Nuthi...

It breaks agent-forwarding

How does it break agent forwarding?

I haven't tried but don't see why that should require a password on the private key.

The key is in the name: agent-forwarding. ssh doesn't pass the key itself through to onward connections, it passes a connection to your local ssh-agent instead.

ssh (the command) can access the material in your private keys in one of two ways, either by direct file access itself (in which case it will always ask you for a passphrase if there is one protecting the key in question), or else via an intermediating ssh-agent process. Roughly speaking, ssh does something like the following when looking for key material:

  if private key file exists and has no passphrase on it
     use it directly (and implicitly do not forward the agent)
  else if ssh-agent is running
     ask the agent for the key (and forward the agent)
  else 
     ask the user for a passphrase ourselves (and implicitly do not forward the agent)
  end

The end result of all this is that if you have an unprotected key, ssh doesn't talk to your local agent at all, and so obviously can't forward it.

I liked the article but I thought he spent too much time on getting the hostname short. Just install bash_completion and then all ssh/network stuff starts completing based on your known_hosts. His "annoying.hostname.example.com" becomes "ssh ann<tab>" which is still shorter than anything he had there.

I'm glad he did mention my favorite config file option for setting the username:

    Host example.com
      User = someweirduser

That way I don't have to remember to "ssh someweirduser@example.com" because some client set up a weird user for me--I just "ssh example.com" instead.

Yeah, the example is more for hyperbolic illustration than anything else. My actual use case is that I've got a lot of IP addresses, different usernames and non-standard ports across various servers, so it's incredibly convenient to have my .ssh/config all set up.

With aliases (and zsh completion support for aliases), I can do "ann<tab>" which saves me even more time.

It won't be long before all I have to type is tab :)

Small point: the syntax of .ssh/config doesn't use equal signs; the actual format of that stanza would be

  Host example.com
  User someweirduser

Another subtle point about SSH config -- only the first matching entry (in top-to-bottom order) for a given parameter is taken from .ssh/config. If you have multiple Host entries that match for a given hostname, the first matching User, Port, etc, entry will be the one that ssh uses.

Modern versions of ssh support the "HashKnownHosts" file which prevents people from discovering what hosts you have ssh'd to in the past - and, as a side effect, prevents tab-completion from discovering what hosts you have ssh'd to as well. ssh_config(5) says the option is disabled by default, but Ubuntu at least enables it in /etc/ssh/ssh_config.

One of the most impressive SSH tricks I've seen is the ControlMaster option. It saves your authentication information on a per-(user, host) basis, which lets you share one connection among multiple ssh/scp/rsync commands and TRAMP sessions. This is less useful if you use passwordless keys or ssh-agent (please use ssh-agent), but becomes much more useful if the host you connect to doesn't support key-based auth.

    # multiplex connections to one host
    ControlMaster auto
    # where is that file
    ControlPath ~/.ssh/master-%r@%h:%p

Other useful things in my .ssh/config, check the manpages before you use them:

    # use compression
    Compression yes
    # really really compress it
    CompressionLevel 9
    # forward X stuff
    ForwardX11 yes
    # yes, really forward X stuff
    ForwardX11Trusted yes

The only issue I had with ControlMaster, IIRC, was that when I killed the original connection, it would take everything else down with it.

There's a good chance I was just doing it wrong™ though.

The behavior I've seen is that when you log out, the process doesn't end until all other clients to the connection have exited. I've observed what you describe if I then ^C it.

The way I usually do it is by opening something over TRAMP before opening any other ssh windows. That way, Emacs will handle the connection maintenance and I don't really need to see that it's up. If it dies, I just try saving something and it'll re-establish the connection.

If you don't use Emacs, you can always just start a dummy connection first and minimize/disown it.

That sounds about right.

Good tip about TRAMP, although I tend to exit out of my emacs sessions frequently. For my long running ssh tunnels, I just screen them.

Ahh, then you want to learn about emacs --daemon and emacsclient. :-)

For reasons others on this thread have outlined, ControlMaster is often more trouble than it's worth for general-purpose SSH usage. However for use in automated tools, it's a lifesaver. I use it extensively in doo for that exact reason:

http://github.com/mtrudel/doo

I have a pretty awesome script that uses control-master to allow me to type the password in once and then preform a bunch of actions. The script specifically is for boot strapping ssh authorized keys on boxes that can't be on puppet.

I'd rather use ssh-agent for day to day work but this is a useful trick for scripting.

> CompressionLevel 9 Just one note regarding the above line: this only works with SSH version 1. This functionality was removed in version 2 of the protocol (in the newer version there is only a single default compression level).

A lot of guides online make the 'mistake'(a minor one) of still using DSA instead of RSA. The patent has expired. I assume a lot of those guides are out of date.

So, thanks for using RSA in the article!

EDIT: I just did some digging. Apparently DSA cannot be used to encrypt. One more reason to keep using RSA, I suppose.

Technically your keys are only used in the handshake. After that you'll get the same strength encryption regardless of the algo used for the original handshake. So using DSA doesn't mean that your connections are now unencrypted.

The problem with DSA is that you can only have a key size of 1024 bits per the original standard. That's starting to get a little weak these days. (Keep that in mind if youi're using PuttyGen to make your keys!) This has since been changed, some systems use "dsa2" and higher bit counts, but it seems OpenSSH doesn't directly support that, at least according to the man page.

Anyway, the new version of OpenSSH just got ECC, which seems to be the future of public/private key encryption. So soon people will be talking trash on RSA. (I kid...)

Another tip:

An ssh session's escape sequence is "<Enter><tilde>", which if you refer as "<X>", you can do the following:

1. Kill an unresponsive ssh session: <X><period>.

2. Enter an ssh command line, to open new forwarding ports among others: <X><C> Then enter <question-mark> for help.

I use this all the time. It's amazing.

found a couple of cool tricks in the article, mainly using the -t flag to set up a chain of servers to pass through when machines are NATed or firewalled.

Seems kind of weird in the tunnel section that the -D flag was not mentioned which creates a dynamic tunnel that can go to multiple hosts when set up as a socks proxy in a web browser.

Shame on me for not reading that section in closer detail. The -t method of forwarding SSH through has a number of limitations, and isn't really the golden path for getting past gateway servers. What you instead want to do is add the following as a config option for the host you're trying to reach:

  "ProxyCommand ssh <gateway_machine> nc %h %p"

You can do this by adding the above line in the relevant place in ~/.ssh/config, or on the command line like so:

  ssh -o "ProxyCommand ssh <gateway_machine> nc %h %p" user@destination_machine

In fact what you really want is not to use nc but ssh's -W option:

    ProxyCommand ssh -W %h:%p <gateway_machine

Interesting -- I did not know about this one. Looks like it was implemented in OpenSSH 5.4, which isn't yet in Ubuntu LTS or OS X (the two environments where I spend most of my days), so it may be a little bleeding edge for general use just yet. Good to know, however.

If you learned something from this, be sure to read http://www.jedi.be/blog/2010/08/27/ssh-tricks-the-usual-and-...

I used this video guide to setting up PuTTY w/ Pageant on Windows: http://vimeo.com/4856534

It's from a series on setting a a Linus VPS for noobs, and I found it indispensable. My only problem is that my Linode runs so trouble free, that I rarely need to go in and mess with it, so even though I've been running a VPS for a year, I still have a limited knowledge of Linux administration.

Any halfway decent tech team manager or sysadmin should make a very clear company-wide policy against passphraseless SSH keys for developers.

Github compares it to leaving your password in a file on your computer for attackers to find, but it's MUCH worse than that. Such a password file could be anywhere on your computer and an attacker would have to search for it and may never actually find it unless it was clearly labelled.

SSH keys are defined in your SSH config file and attackers know exactly where that lives. Using a passphraseless ssh key is more like leaving your password on a sticky note attached to your monitor.

This article is spreading bad advice here to users who don't understand why, and the author should update his post and retract it.

Help me out here: if an attacker can get into my account on my OS X or Linux box, he can run sudo, which means he can replace my ssh client with one that will capture my passphrase the next time I use it.

Can you give a concrete example of a scenario in which an attacker will be able to read my SSH config file but not be able to get into my account? Suppose the SSH config file is not backed up.

I am not saying you are wrong about the need for a passphrase, BTW.

On Linux you need to enter your password to run sudo. If someone can read all the files in your home directory but not make you enter sudo, then they can see your ssh keys but be unable to replace /usr/bin/ssh

Thanks. I have been using OS X where sudo does not ask admin accounts for a password.

You could use plink for setting up reverse ssh tunneling. The documentation on putty and plink has more details.

+5 AGILITY.. nice touch XD

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