Sitaram Chamarty c3ec349721 sshkeys-lint: new program

run without arguments for usage

2010-01-25 13:17:14 +05:30

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ssh troubleshooting

In this document:

basic ssh troubleshooting
- passphrases versus passwords
- ssh-agent problems
- basic ssh troubleshooting for the main admin
- basic ssh troubleshooting for a normal user
details
- files on the server
- files on client
- why two keys on client
more complex ssh setups
- two gitolite servers to manage?
giving shell access to gitolite users

This document should help you troubleshoot ssh-related problems in accessing gitolite after the install has completed successfully.

In addition, I strongly recommend reading this document -- it's a very detailed look at how gitolite uses ssh's features on the server side. Most people don't know ssh as well as they think they do; even if you dont have any problems right now, it's worth skimming over.

In addition to both these documents, there's now a program called sshkeys-lint that you can run on your client. Run it without arguments to get help on how to run it and what inputs it needs.

Please also note that ssh problems don't always look like ssh problems. One common example: when the remote says the repo you're trying to access "does not appear to be a git repository", and yet you are sure it exists, you haven't mis-spelled it, etc. Another example is being able to access repositories using the full unix path (typically like git@server:repositories/reponame.git, assuming default $REPO_BASE setting, instead of specifying only the part below $REPO_BASE, i.e., git@server:reponame.git).

[Both these errors indicate that you managed to bypass gitolite completely and are using your shell access -- instead of running via /some/path/gl-auth-command <your_username> it is just going to bash and working from there!]

basic ssh troubleshooting

I assume the gitolite server is called "server" and the user hosting all the gitolite repos is "git". I will also be using "sitaram" as the gitolite username of the admin.

Unless specifically mentioned, all these commands are run on the user's or admin's workstation, not on the server.

passphrases versus passwords

When you create an ssh keypair, you have the option of protecting it with a passphrase. When you subsequently use that keypair to access a remote host, your local ssh client needs to unlock the corresponding private key, and ssh will probably ask for the passphrase you set when you created the keypair.

Do not confuse or mistake this prompt (Enter passphrase for key '/home/sitaram/.ssh/id_rsa':) for a password prompt from the remote server!

You have two choices to avoid this prompt every time you try to access the remote. The first is to create keypairs without a passphrase (just hit enter when prompted for one). Be sure to add a passphrase later, once everything is working, using ssh-keygen -p.

The second is to use ssh-agent (or keychain, which in turn uses ssh-agent) or something like that to manage your keys. Other than the next section, further discussion of this is out of scope of this document.

ssh-agent problems

Run ssh-add -l. If this responds with either "The agent has no identities." or "Could not open a connection to your authentication agent.", skip this section.

However, if it lists some keys, like this:

2048 fc:c1:48:1e:06:31:97:a4:8b:fc:37:b2:76:14:c7:53 /home/sitaram/.ssh/id_rsa (RSA)
2048 d2:e0:7f:fa:1a:89:22:41:bb:06:d9:ff:a7:27:36:5c /home/sitaram/.ssh/sitaram (RSA)

then run ls ~/.ssh and make sure that all the keypairs you have there are represented in the ssh-add -l output.

If you find any keypairs in ~/.ssh that are not represented in the ssh-add -l output, add them. For instance, if ssh-add -l showed me only the id_rsa key, but I also had a sitaram (and sitaram.pub) keypair, I'd run ssh-add ~/.ssh/sitaram to add it.

This is because ssh-agent has a quirk: if ssh-add -l shows any keys at all, ssh will only use those keys. Even if you explicitly specify an unlisted key using ssh -i or an identityfile directive in the config file, it won't use it.

basic ssh troubleshooting for the main admin

You're the "main admin" if you're trying to access gitolite from the same workstation and user account where you ran the "easy install" command. You should have two keypairs in your ~/.ssh directory. The pair called id_rsa (and id_rsa.pub) was probably the first one you created, and you used this to get passwordless (pubkey based) access to the server (which was a pre-requisite for running the easy install command).

The second keypair has the same name as the last argument in the easy install command you ran (in my case, sitaram and sitaram.pub). It was probably created by the easy install script, and is the key used for gitolite access.

In addition, you should have a "gitolite" paragraph in your ~/.ssh/config, looking something like this:

host gitolite
     user git
     hostname server
     identityfile ~/.ssh/sitaram

If any of these are not true, you did something funky in your install; email me or hop onto #git and hope for the best ;-)

Otherwise, run these checks:

ssh git@server should get you a command line.

If it asks you for a password, then your id_rsa keypair changed after you ran the easy install, or someone fiddled with the ~/.ssh/authorized_keys file on the server.

If it prints gitolite version and access info, you managed to overwrite the id_rsa keypair with the sitaram keypair, or something equally weird.
ssh gitolite info should print some gitolite version and access info. If you get the output of the GNU info command instead, you probably reused your id_rsa keypair as your sitaram keypair, or overwrote the sitaram keypair with the id_rsa keypair.

There are many ways to fix this, depending on where and what the damage is. The most generic way (and therefore time-taking) is to re-install gitolite from scratch:

make a backup of your gitolite-admin repo clone somewhere (basically your "keydir/*.pub" and your "conf/gitolite.conf"). If necessary get these files from the server's ~/.gitolite directory.
log on to the server somehow (using some other account, using a password, su-ing in, etc) and delete ~/.ssh/authorized_keys. Rename or move aside ~/.gitolite so that also looks like it is missing.
back on your workstation, make sure you have 2 keypairs (id_rsa and sitaram, along with corresponding .pub files). Create them if needed. Also make sure they are different and not a copy of each other :-)
install gitolite normally:
- run ssh-copy-id -i ~/.ssh/id_rsa git@server to get passwordless access to the server. (Mac users may have to do this step manually)
- make sure ssh git@server pwd prints the $HOME of git@server without asking for a password. Do not proceed till this works.
- run easy install again, (in my case: cd gitolite-source; src/gl-easy-install -q git server sitaram)
go to your gitolite-admin repo clone, and copy conf/gitolite.conf and keydir/*.pub from your backup to this directory
copy (be sure to overwrite!) ~/.ssh/sitaram.pub also to keydir
now git add keydir; git commit; git push -f

That's a long sequence but it should work.

basic ssh troubleshooting for a normal user

For a normal user, life is much simpler. They should have only one pubkey, which was previously sent to the gitolite admin to add into the admin repo's keydir as "user.pub", and then "user" given permissions to some repo.

ssh git@server info should get you gitolite version and access info. If it asks you for a password, your pubkey was not sent to the server properly. Check with your admin.

If it gets you the GNU info command output, you have shell access. This means you had command line access to the server before you were added as a gitolite user. If you send that same key to your gitolite admin to include in the admin repo, it won't work. For reasons why, see below.

details

Here's how it all hangs together.

files on the server

the authkeys file; this contains one line containing the pubkey of each user who is permitted to login without a password.

Pubkey lines that give shell access look like this:
```
ssh-rsa AAAAB3NzaC[snip]uPjrUiAUew== /home/sitaram/.ssh/id_rsa
```
On a typical server there will be only one or two of these lines.

Note that the last bit (/home/sitaram/.ssh/id_rsa) is purely a comment field and can be anything. Also, the actual lines are much longer, about 400 characters; I snipped 'em in the middle, as you can see.

In contrast, pubkey lines that give access to git repos hosted by gitolite look like this:
```
command="[some path]src/gl-auth-command sitaram",[some restrictions] ssh-rsa AAAAB3NzaC[snip]s18OnB42oQ== sitaram@sita-lt
```
You will have many more of these lines -- one for every pubkey file in keydir/ of your gitolite-admin repo, with the corresponding username in place of "sitaram" in the example above.

The "command=" at the beginning ensures that when someone with the corresponding private key logs in, they don't get a shell. Instead, the gl-auth-command program is run, and (in this example) is given the argument sitaram. This is how gitolite is invoked, (and is told the user logging in is "sitaram").

files on client

default keypair; used to get shell access to servers. You would have copied this pubkey to the gitolite server in order to log in without a password. (On Linux systems you may have used ssh-copy-id to do that). You would have done this before you ran the easy install script, because otherwise easy install won't run!
```
~/.ssh/id_rsa
~/.ssh/id_rsa.pub
```
gitolite keypair; the "sitaram" in this is the 3rd argument to the src/gl-easy-install command you ran; the easy install script does the rest
```
~/.ssh/sitaram
~/.ssh/sitaram.pub
```
config file; this file has an entry for gitolite access:
```
~/.ssh/config
```
To understand why we need that, let's step back a bit. Normally, you might expect to access gitolite repos like this:
```
ssh://git@server/reponame.git
```
But this won't work, because this ends up using the default keypair (normally), which gives you a command line. Which means it won't invoke the gl-auth-command program at all, and so none of gitolite's access control will work.

You need to force ssh to use the other keypair when performing a git operation. With normal ssh, that would be
```
ssh -i ~/.ssh/sitaram git@server
```
but git does not support putting an alternate keypair in the URL.

Luckily, ssh has a very convenient way of capturing all the connection information (username, hostname, port number (if it's not the default 22), and keypair to be used) in one "paragraph" of ~/.ssh/config. This is what the para looks like for us (the easy install script puts it there the first time):
```
host gitolite
     user git
     hostname server
     identityfile ~/.ssh/sitaram
```
(The "gitolite" can be anything you want of course; it's like a group name for all the stuff below it). This ensures that typing
```
ssh gitolite
```
is equivalent to
```
ssh -i ~/.ssh/sitaram git@server
```
and therefore this:
```
git clone gitolite:reponame.git
```
now works as expected, invoking the special keypair instead of the default one.

why two keys on client

Why do I (the admin) need two different keypairs?

There are two types of access the admin will make to the server: a normal login, to get a shell prompt, and gitolite access (clone/fetch/push etc). The first access needs an authkeys line without any "command=" restrictions, while the second requires a line with such a restriction.

And we can't use the same key for both because there is no way to disambiguate them; the ssh server will always (always) pick the first one in sequence when the key is offered by the ssh client.

So the next question is usually "I have other ways to get a shell on that account, so why do I need a key for shell access at all?"

The answer to this is that the "easy install" script, being written for the most general case, needs shell access via ssh to do its stuff.

If you really, really, want to get rid of the extra key, here's a transcript that should have enough info to get you going (but it helps to know ssh well):

on "sitaram" user, on my workstation

cd ~/.ssh
cp id_rsa sitaram
cp id_rsa.pub sitaram.pub
cd ~/gitolite-clone
src/gl-easy-install -q git my.git.server sitaram

that last command produces something like the following:

you are upgrading from     (unknown)     to v0.80-6-gdde8c4e
setting up keypair...
    ...reusing /home/sitaram/.ssh/sitaram.pub...
creating gitolite para in ~/.ssh/config...
finding/creating gitolite rc...
installing/upgrading...
Pseudo-terminal will not be allocated because stdin is not a terminal.
[master (root-commit) e717a89] start
 2 files changed, 11 insertions(+), 0 deletions(-)
 create mode 100644 conf/gitolite.conf
 create mode 100644 keydir/sitaram.pub
cloning gitolite-admin repo...
Initialized empty Git repository in /home/sitaram/gitolite-admin/.git/
fatal: 'gitolite-admin.git' does not appear to be a git repository
fatal: The remote end hung up unexpectedly

notice that the final step (the clone of the newly created gitolite-admin repo) failed, as expected

now log on to the git hosting account (git@my.git.server in this example), edit ~/.ssh/authorized_keys, and delete the line with the first occurrence of your key (this should be before the # gitolite start line)

now go back to your workstation and

git clone git@my.git.server:gitolite-admin

That should do it.

more complex ssh setups

What do you need to know in order to create more complex ssh setups (for instance if you have two gitolite servers you are administering)?

two gitolite servers to manage?

they can have the same key; no harm there (example, sitaram.pub)

instead of just one ssh/config para, you now have two (assuming that the remote user on both machines is called "git"):

host gitolite
     user git
     hostname server
     identityfile ~/.ssh/sitaram

host gitolite2
     user git
     hostname server2
     identityfile ~/.ssh/sitaram

now access one server's repos as gitolite:reponame.git and the other server's repos as gitolite2:reponame.git.

giving shell access to gitolite users

We've managed (thanks to an idea from Jesse Keating) to make it possible for a single key to allow both gitolite access and shell access.

This is done by:

(on the server) listing all such users in a variable called $SHELL_USERS in the ~/.gitolite.rc file. For example:
```
$SHELL_USERS = "alice bob";
```
(Note the syntax: a space separated list of users in one string variable).
(on your client) make at least a dummy change to your clone of the gitolite-admin repo and push it.

IMPORTANT UPGRADE NOTE: a previous implementation of this feature worked by adding people to a special group (@SHELL) in the config file. This meant that anyone with gitolite-admin repo write access could add himself to the @SHELL group and push, thus obtaining shell.

This is not a problem for most setups, but if someone wants to separate these two privileges (the right to push the admin repo and the right to get a shell) then it does pose a problem. Since the "rc" file can only be edited by someone who already has shell access, we now use that instead, even though this forces a change in the syntax.

To migrate from the old scheme to the new one, add a new variable $SHELL_USERS to ~/.gitolite.rc on the server with the appropriate names in it. It is best to do this directly on the server before upgrading to this version. (After the upgrade is done and tested you can remove the @SHELL lines from the gitolite config file).

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