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fix(ssh): define useful ways for hardening

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Michele Cereda
2024-09-30 19:08:16 +02:00
parent 520807f6cc
commit 59397fd100
3 changed files with 105 additions and 26 deletions
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6
knowledge base/set up port knocking.md
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@@ -1,14 +1,16 @@
# Set up port knocking
Technique where a daemon keeps listening on specific ports for a specific sequence of connections.<br/>
When the correct sequence is used, the daemon issues a configured command, usually to open a defined port for the client only.
When the correct sequence is used, the daemon issues a configured command, usually to open a defined port for the client
only.
This is frequently used to open the SSH port in a server for a specific client.
See [Knockd].
<!--
References
Reference
═╬═Time══
-->
<!-- Knowledge base -->

116
knowledge base/ssh.md
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@@ -8,10 +8,9 @@
1. [Optimize connection handling](#optimize-connection-handling)
1. [Integrate with GnuPG](#integrate-with-gnupg)
1. [Server configuration](#server-configuration)
1. [Change port](#change-port)
1. [Disable password authentication](#disable-password-authentication)
1. [Permit root login](#permit-root-login)
1. [Change listening port](#change-listening-port)
1. [Conditional blocks](#conditional-blocks)
1. [Hardening](#hardening)
1. [SSHFS](#sshfs)
1. [Installation](#installation)
1. [Troubleshooting](#troubleshooting)
@@ -293,35 +292,108 @@ Include /usr/etc/ssh/sshd_config.d/*.conf
This avoids issues from updates overwriting the default file and allows user configurations to override defaults in a
cleaner way.
### Change port
### Change listening port
```ssh-config
Port 2222
```
### Disable password authentication
```ssh-config
PasswordAuthentication no
ChallengeResponseAuthentication no
```
### Permit root login
```ssh-config
PermitRootLogin yes
```
### Conditional blocks
> Only a subset of keywords may be used in a _Match_ block. Check the [`SSHD_CONFIG(5)`][sshd_config man page] man page.
> Only a subset of keywords may be used in a _Match_ block.<br/>
> Check the [`SSHD_CONFIG(5)`][sshd_config man page] man page for more information.
```ssh-config
Match Address 192.168.111.0/24
PasswordAuthentication no
PermitRootLogin no
Match User ashley
AllowUsers ashley@203.0.113.1
```
### Hardening
Don't bother changing the SSH port from the default `22` value unless you require it for other purposes.<br/>
Security through obscurity is utterly ineffective and can create issues with tools expecting standard usage
<sup>
[1][Why I don't change SSH from port 22],
[2][Security Through Obscurity (STO): History, Criticism & Risks]
</sup>.
Suggestions:
- Disable `root` login:
```ssh-config
PermitRootLogin no
```
Or limit it to keys:
```ssh-config
PermitRootLogin prohibit-password
```
- Limit the maximum number of authentication attempts for any login session:
```ssh-config
MaxAuthTries 3
```
- Limit the amount of time user have to complete authentication after the initial connection:
```ssh-config
LoginGraceTime 20
```
- Disable authentication with empty passwords:
```sh
PermitEmptyPasswords no
```
- Disable password authentication altogether:
```ssh-config
PasswordAuthentication no
```
- Restrict authentication options to keys:
```ssh-config
ChallengeResponseAuthentication no
```
- Disable other authentication methods if not used:
```ssh-config
ChallengeResponseAuthentication no
KerberosAuthentication no
GSSAPIAuthentication no
```
- Disable other features if not used:
```ssh-config
AllowAgentForwarding no
AllowTcpForwarding no
PermitTunnel no
X11Forwarding no
```
- Restrict login to specific IP addresses:
```ssh-config
AllowUsers*@203.0.113.1 *@203.0.113.2 *@192.0.2.0/24 *@172.16.*.1 sammy@203.0.113.1 alex@203.0.113.2
Match User ashley
AllowUsers ashley@203.0.113.1
```
- Temporarily block IPs after repeated authentication failures.<br/>
See [fail2ban].
- [Set up port knocking].
## SSHFS
Notable options:
@@ -388,6 +460,7 @@ Solution: update the SSH server.
- [`sshd_config` example][sshd_config example]
- [ssh-agent]
- [Use GPG keys for SSH authentication]
- [Security Through Obscurity (STO): History, Criticism & Risks]
### Sources
@@ -402,6 +475,8 @@ Solution: update the SSH server.
- [Restrict SSH login to a specific IP or host]
- [Stick with security: YubiKey, SSH, GnuPG, macOS]
- [How to check if an RSA public / private key pair match]
- [Why I don't change SSH from port 22]
- [How To Harden OpenSSH on Ubuntu 20.04]
<!--
Reference
@@ -410,6 +485,7 @@ Solution: update the SSH server.
<!-- Knowledge base -->
[use gpg keys for ssh authentication]: gnupg.md#use-gpg-keys-for-ssh-authentication
[set up port knocking]: set%20up%20port%20knocking.md
<!-- Files -->
[ssh_config example]: ../examples/ssh/ssh_config
@@ -422,14 +498,18 @@ Solution: update the SSH server.
[sshd_config man page]: https://man.openbsd.org/sshd_config
<!-- Others -->
[fail2ban]: https://github.com/fail2ban/fail2ban
[get started with openssh for windows]: https://learn.microsoft.com/en-us/windows-server/administration/openssh/openssh_install_firstuse?tabs=gui
[how to check if an rsa public / private key pair match]: https://serverfault.com/questions/426394/how-to-check-if-an-rsa-public-private-key-pair-match#426429
[how to enable ssh access using a gpg key for authentication]: https://opensource.com/article/19/4/gpg-subkeys-ssh
[how to harden openssh on ubuntu 20.04]: https://www.digitalocean.com/community/tutorials/how-to-harden-openssh-on-ubuntu-20-04
[how to list keys added to ssh-agent with ssh-add?]: https://unix.stackexchange.com/questions/58969/how-to-list-keys-added-to-ssh-agent-with-ssh-add
[how to perform hostname canonicalization]: https://sleeplessbeastie.eu/2020/08/24/how-to-perform-hostname-canonicalization/
[how to reuse ssh connection to speed up remote login process using multiplexing]: https://www.cyberciti.biz/faq/linux-unix-reuse-openssh-connection/
[multiple similar entries in ssh config]: https://unix.stackexchange.com/questions/61655/multiple-similar-entries-in-ssh-config
[restrict ssh login to a specific ip or host]: https://docs.rackspace.com/support/how-to/restrict-ssh-login-to-a-specific-ip-or-host/
[security through obscurity (sto): history, criticism & risks]: https://www.okta.com/uk/identity-101/security-through-obscurity/
[stick with security: yubikey, ssh, gnupg, macos]: https://evilmartians.com/chronicles/stick-with-security-yubikey-ssh-gnupg-macos
[use sshfs to mount a remote directory as a volume on osx]: https://benohead.com/mac-os-x-use-sshfs-to-mount-a-remote-directory-as-a-volume/
[using the ssh config file]: https://linuxize.com/post/using-the-ssh-config-file/
[why i don't change ssh from port 22]: https://www.youtube.com/watch?v=Fzt5dqaIMYc

9
knowledge base/turris os.md
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@@ -464,12 +464,9 @@ uci commit 'dhcp' && reload_config && luci-reload
## Hardening
Suggestions:
See:
- [SSH]:
- Change the SSH port from the default `22` value.
- Restrict login to specific IP addresses.
- Restrict authentication options to keys.
- [SSH hardening].
## The SFP+ caged module
@@ -577,7 +574,7 @@ All the references in the [further readings] section, plus the following:
[openwrt]: openwrt.md
[opkg]: opkg.md
[pi-hole]: pi-hole.md
[ssh]: ssh.md
[ssh hardening]: ssh.md#hardening
[uci]: uci.md
<!-- Upstream -->