town/cmd/welcome/README.md

# welcome command

this command is used to exchange a town invite token for a user account. it is
responsible for:

1. accepting and validating an invite token generated by the `review` command
2. accepting and validating a new user's username choice (ie enforcing rules and checking for dupes)
3. accepting and validating a user's email for use in account recovery (defaulting to an email embedded in the invite token)
4. accepting and validating a display name
5. asking what shell they'd like
6. accepting and validating a user's public ssh key

upon receipt of these things a user account is created. if it fails, the user
is told about the failure and told to email root@tilde.town for guidance; us
admins get a local mail about the problem.

upon successful creation, `welcome` prints a message on STDOUT suggesting how to log in then quits.

It is risky to let `welcome` create users but no riskier at a high level than the Django admin we had. I can re-use the sudoers trick I did there for the `welcome` user.

## an invite token

an invite token consists of two pieces that are then base64 encoded. the first piece is a random string of 30 characters (alphanumeric and symbols except space) and the second is an email address the invite was sent to; they are separated by a space.

## sudoers config

something like:

```
welcome ALL=(ALL)NOPASSWD:/usr/sbin/adduser,/usr/sbin/usermod,/bin/mkdir,/town/bin/generate_welcome_present.sh,/town/bin/create_keyfile
```

though I will likely move welcome_present generation inline to `welcome` itself.

## user creation flow

once we accept what we need from the user accepting an invite, the flow looks like:

1. create user account
  a. run `adduser`, set shell and displayname
  b. add user to town group
2. write authorized keys
  a. create `~/.ssh`
  b. write `~/.ssh/authorized_keys2` and put their key in there
  c. write blank `~/.ssh/authorized_keys` with note about adding custom keys
3. generate welcome gift
4. alert hooks (more of a future idea; but it would be nice to have a "WELCOME NEW USER!" in the mailing list / IRC / etc)

## creating keyfiles

A frustrating hurdle is that `welcome`, just like `ttadmin`, has to write a keyfile that is perms 600 for the new user. This is annoying as shit and requires running `sudo` as the new user. In the old python code:

```python
def write_authorized_keys(self):
    # Write out authorized_keys file
    # Why is this a call out to a python script? There's no secure way with
    # sudoers to allow this code to write to a file; if this code was to be
    # compromised, the ability to write arbitrary files with sudo is a TKO.
    # By putting the ssh key file creation into its own script, we can just
    # give sudo access for that one command to this code.
    #
    # We could put the other stuff from here into that script and then only
    # grant sudo for the script, but then we're moving code out of this
    # virtual-env contained, maintainable thing into a script. it's my
    # preference to have the script be as minimal as possible.
    with TemporaryFile(dir="/tmp") as fp:
        fp.write(self.generate_authorized_keys().encode('utf-8'))
        fp.seek(0)
        error = _guarded_run(['sudo',
                              '--user={}'.format(self.username),
                              '/town/src/tildetown-admin/scripts/create_keyfile.py',
                              self.username],
                             stdin=fp)
        if error:
            logger.error(error)

```

this warrants porting `create_keyfile.py` to a new Go program that can live at `/town/bin/create_keyfile` or wherever.