Introduced in OpenSSH 9.0 it became the part of the default kexAlgorithm
selection, visibile in sshd_config(5).
It is also enabled by default in the OpenSSH client, as can be seen from
$ ssh -Q KexAlgorithms
Also clarifies that we use the referenced documents as the lower bound,
given that they haven't been updated for 5-7y.
In a previous PR [1], the conditional to generate a new host key file
was changed to also include the case when the file exists, but has zero
size. This could occur when the system is uncleanly powered off shortly
after first boot.
However, ssh-keygen prompts the user before overwriting a file. For
example:
$ touch hi
$ ssh-keygen -f hi
Generating public/private rsa key pair.
hi already exists.
Overwrite (y/n)?
So, lets just try to remove the empty file (if it exists) before running
ssh-keygen.
[1] https://github.com/NixOS/nixpkgs/pull/141258
Add a new type, inheriting 'types.str' but checking whether the value
doesn't contain any newline characters.
The motivation comes from a problem with the
'users.users.${u}.openssh.authorizedKeys' option.
It is easy to unintentionally insert a newline character at the end of a
string, or even in the middle, for example:
restricted_ssh_keys = command: keys:
let
prefix = ''
command="${command}",no-pty,no-agent-forwarding,no-port-forwarding,no-X11-forwarding
'';
in map (key: "${prefix} ${key}") keys;
The 'prefix' string ends with a newline, which ends up in the middle of
a key entry after a few manipulations.
This is problematic because the key file is built by concatenating all
the keys with 'concatStringsSep "\n"', with result in two entries for
the faulty key:
''
command="...",options...
MY_KEY
''
This is hard to debug and might be dangerous. This is now caught at
build time.
When startWhenNeeded is enabled, a brute force attack on sshd will cause
systemd to shut down the socket, locking out all SSH access to the machine.
Setting TriggerLimitIntervalSec to 0 disables this behavior.
In case of a power loss shortly after first boot,
the host keys gernerated by ssh-keygen could exist
in the file system but have zero size, preventing
sshd from starting up.
This commit changes the behaviour to generate host
keys if the file either does not exist or has zero
size, fixing the problem on the next boot.
Thanks to @SuperSandro2000 for figuring this out.
The previous justification for using "VERBOSE" is incorrect,
because OpenSSH does use level INFO to log "which key was used
to log in" for sccessful logins, see:
6247812c76/auth.c (L323-L328)
Also update description to the wording of the sshd_config man page.
`fail2ban` needs, sshd to be "VERBOSE" to work well, thus
the `fail2ban` module sets it to "VERBOSE" if enabled.
The docs are updated accordingly.
The `curve25519-sha256` key exchange method is defined in RFC 8731 that
is identical to curve25519-sha256@libssh.org. OpenSSH supports the
method since version 7.4, released on 2016-12-19. It is literally a
violation of the "both in Secure Secure Shell and Mozilla guidelines"
rule, but it provides essentially the same but a future-proof default.
Also, links to the Mozilla OpenSSH guidelines are updated to refer to
the current place.
Signed-off-by: Masanori Ogino <167209+omasanori@users.noreply.github.com>
A centralized list for these renames is not good because:
- It breaks disabledModules for modules that have a rename defined
- Adding/removing renames for a module means having to find them in the
central file
- Merge conflicts due to multiple people editing the central file
With `sshd -t` config validation for SSH is possible. Until now, the
config generated by Nix was applied without any validation (which is
especially a problem for advanced config like `Match` blocks).
When deploying broken ssh config with nixops to a remote machine it gets
even harder to fix the problem due to the broken ssh that makes reverts
with nixops impossible.
This change performs the validation in a Nix build environment by
creating a store path with the config and generating a mocked host key
which seems to be needed for the validation. With a broken config, the
deployment already fails during the build of the derivation.
The original attempt was done in #56345 by adding a submodule for Match
groups to make it harder screwing that up, however that made the module
far more complex and config should be described in an easier way as
described in NixOS/rfcs#42.
