30 lines
1.8 KiB
Markdown
30 lines
1.8 KiB
Markdown
---
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date: "2014-12-22T19:45:00Z"
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description: Choosing a small number of newer ciphers prevents SSH bots from connecting.
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tags:
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- ssh
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- ops
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- english
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slug: ssh-bots-do-not-know-about-my-ciphers
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title: SSH Bots Don't Know about My Ciphers
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---
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Last weekend I decided to update my `sshd_config` to include a very limited set of ciphers, MACs and key exchange algorithms. I did this to tighten the security of my `sshd` and not because I wanted to prevent bots from trying (and failing) to log in to my servers. I'm already using [fail2ban](http://www.fail2ban.org) for that.
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However, after I updated my configuration I noticed failed login attempts basically dropped to zero, because all these bots do not support my very restrictive set of ciphers.
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{{< highlight text >}}
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Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com
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MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com
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KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256
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{{< / highlight >}}
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In order to use these settings, you need a recent version of [OpenSSH](http://www.openssh.com). I'm running 6.6 locally and on my servers, the minimum version that supports these settings is 6.4.
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After these changes your `auth.log` will probably contain this line rather often:
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{{< highlight text >}}
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fatal: no matching cipher found: client aes128-ctr,aes192-ctr,aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-cbc,aes128-cbc,blowfish-cbc,arcfour128,arcfour,cast128-cbc,3des-cbc server chacha20-poly1305@openssh.com,aes256-gcm@openssh.com [preauth]
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{{< / highlight >}}
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Please keep in mind that this will not prevent bots from attacking you (in the future) and that you have to mitigate these attacks by other means. I only wrote this post because I thought it was kind of interesting that SSH bots do not support these settings *right now*.
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