Don't ever fix your `ssh_config` by appending stuff to the end of the file. The configuration syntax allows for block constructs without an explicit end marker (like `Match` and `Host`). Appending will cause all kinds of sadness.

It's from the linked page.

Indeed. The page gives bad advise. If your config has `Host` blocks, like often in people's personal configs, or if you have a `Match` block, the new directive only applies to the last of those blocks in the config file.

Sufficient to place it at the top of ~/.ssh/config? Or does it also need a block header of its own, like:

    Host *
    UseRoaming no

I confirmed that it does not need to be placed into a Host block, as long as it is above all of your blocks.

For instance:

   UseRoaming no

   Host *
      Blah yes

Test with: ssh -v remote.ssh.host.com uptime 2>&1 | grep -i roaming

If it returns nothing, the config fix is active. If it isn't active, you'll see 'debug1: Roaming not allowed by server'

Which is why they're appending a newline and Host *?

Note that the example was changed as a result of my initial comment.

Presumably you would have to connect to a malicious host to be effected? Or perhaps a MITM on your connection to a legit host can exploit you somehow.

From the updated OP:

> The authentication of the server host key prevents exploitation by a man-in-the-middle, so this information leak is restricted to connections to malicious or compromised servers.

Malicious, compromised, or new servers. Because really, how many people check the host key for a newly spun up EC2 instance?

> Presumably you would have to connect to a malicious host to be effected?

Malicious or compromised.

> roaming code in the ssh client could be tricked by a hostile sshd server, potentially leaking key material.

Would this work too?

  $ echo "UseRoaming no" >> ~/.ssh/config

Yes, but it'll only affect that user.

And only the last `Host` entry in that config, if present.

Yes, but it would not apply to other users.

I'm emailing the one-liner to folks and found it helpful to just include the root subshell:

    sudo bash -c 'echo -e "Host *\n\tUseRoaming no\n" >> /etc/ssh/ssh_config'