feat(wpcarro/blog): SSH Oddities r/4314

Another short post about strange encounters in sysadmin-land.

Change-Id: Ie71ca36553440d706ff808af91bed8e09008f909
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5944
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>

1 files changed, 59 insertions, 0 deletions

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+## Background
+
+I was trying to debug a service over `ssh` that offered password-only
+authentication, but I couldn't seem to get the `ssh` client to prompt me for the
+password.
+
+It looked something like this (skip ahead to the conclusion if you're pressed
+for time):
+
+## Troubleshooting
+
+```shell
+λ ssh root@[redacted]
+Unable to negotiate with [redacted] port 22: no matching host key type found. Their offer: ssh-rsa
+```
+
+But the same command was working just fine for my coworker.
+
+I took a closer look with `ssh -v root@[redacted]`, but nothing jumped-out at
+me. Maybe it's something with *my* `ssh` configuration; let's remove that
+variable:
+
+```shell
+λ ssh -F /dev/null root@[redacted]
+Unable to negotiate with [redacted] port 22: no matching host key type found. Their offer: ssh-rsa
+```
+
+> Ah it looks like there's a way to set my preferred authentication method...
+> -- me
+
+```shell
+λ ssh -F /dev/null -o PreferredAuthentications=password root@[redacted]
+Unable to negotiate with [redacted] port 22: no matching host key type found. Their offer: ssh-rsa
+```
+
+## Conclusion
+
+Well it turns-out that newer SSH clients disable the `ssh-rsa` public key
+signature algorithm because it depends on SHA-1, which is considered insecure.
+
+```shell
+λ ssh -V
+OpenSSH_9.0p1, OpenSSL 1.1.1p  21 Jun 2022
+```
+
+...and according to the `ssh -v` output, the server is running an pre-COVID(!!!)
+version of `ssh`:
+
+```
+debug1: Remote protocol version 2.0, remote software version dropbear_2018.76
+```
+
+So if you don't have time to upgrade the SSH server, and you just want to
+connect, the following should work because we're *opting-into* the less secure
+option:
+
+```shell
+λ ssh -o HostKeyAlgorithms=+ssh-rsa root@[redacted]
+```