> For the complete documentation index, see [llms.txt](https://book.ice-wzl.xyz/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://book.ice-wzl.xyz/tool-guides/ssh.md).

# ssh

* TCP 22, OpenSSH is the most common implementation
* Six authentication methods: password, public-key, host-based, keyboard, challenge-response, GSSAPI

## Config

* Default config: `/etc/ssh/sshd_config`

```
cat /etc/ssh/sshd_config | grep -v "#" | sed -r '/^\s*$/d'
```

## Dangerous Settings

| Setting                    | Description                  |
| -------------------------- | ---------------------------- |
| PasswordAuthentication yes | Password auth enabled        |
| PermitEmptyPasswords yes   | Empty passwords allowed      |
| PermitRootLogin yes        | Root login allowed           |
| Protocol 1                 | Outdated encryption standard |
| X11Forwarding yes          | GUI forwarding               |
| AllowTcpForwarding yes     | Port forwarding              |
| PermitTunnel               | Tunneling                    |
| DebianBanner yes           | Shows specific OS banner     |

## SSH-Audit

```
git clone https://github.com/jtesta/ssh-audit.git && cd ssh-audit
./ssh-audit.py 10.129.14.132
```

## Change Auth Method

```
ssh -v cry0l1t3@10.129.14.132
ssh -v cry0l1t3@10.129.14.132 -o PreferredAuthentications=password
```

## Hostkey Algorithm Fix

* No matching `hostkey` error:

```
no matching host key type found. Their offer: ssh-rsa,ssh-dss
```

* Append to `/etc/ssh/ssh_config`:

```
HostKeyAlgorithms +ssh-rsa,ssh-dss
```

* If the error references a different algorithm, substitute accordingly

## Forced Command Keys and Legacy SCP

If a recovered private key authenticates but the server closes the shell with a forced command, inspect the matching `authorized_keys` entry. A restricted key may look like:

```
no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty,command="/home/USER/scp_wrapper.sh" ssh-rsa ...
```

scp error message too long

```
scp: Received message too long
scp: Ensure the remote shell produces no output for non-interactive sessions.
```

Fix this error with scp legacy mode `-O`

```bash
scp -O -i id_rsa ~/.ssh/id_rsa.pub USER@TARGET:/home/USER/.ssh/authorized_keys
ssh USER@TARGET -i ~/.ssh/id_rsa
```

The `-O` flag forces the legacy SCP protocol instead of SFTP, which can be needed when the remote wrapper expects `scp` and otherwise returns:


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://book.ice-wzl.xyz/tool-guides/ssh.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.