How can I determine the supported MACs, Ciphers, Key length and KexAlogrithms supported by my ssh servers?

I need to create a list for an external security audit. I'm looking for something similar to openssl s_client -connect example.com:443 -showcerts. From my research the ssh uses the default ciphers as listed in man sshd_config. However I need a solution I can use in a script and man sshd_config does not list information about key length. I need to correct myself here: You can specify ServerKeyBits in sshd_config.

I guess that ssh -vv localhost &> ssh_connection_specs.out returns the information I need but I'm not sure if the listed ciphers are the ciphers supported the client or by the server. Also I'm not sure how to run this non interactive in a script.

Is there a convenient way to get SSH connection information?

  • 3
    turned out that sshd -T | grep "\(ciphers\|macs\|kexalgorithms\)" as suggested by @Jakuje works only on RHEL7 hosts, but not RHEL6. I ended up using nmap --script SSH2-hostkey localhost and nmap --script ssh-hostkey localhost – Henrik Pingel Nov 11 '15 at 9:48
  • ssh -vv outputs the supported functionality as client to server (ctos) and server to client (stoc). However, it seems that those outputs are limited to what both sides support, making them less useful for a security audit. – Moshe Dec 19 '19 at 18:21

You miss few points in your question:

  • What is your openssh version? It can differ a bit over the versions.
  • ServerKeyBits is option for protocol version 1, which you have hopefully disabled!

Supported Ciphers, MACs and KexAlgorithms are always available in manual and this doesn't have anything in common with key lengths.

Enabled Chiphers, MACs and KexAlgorithms are the ones that are offered using connection as you point out. But they can be gained also in other ways, for example using sshd -T | grep "\(ciphers\|macs\|kexalgorithms\)"

To get the key length of your server key(s), you can use ssh-keygen: ssh-keygen -lf /etc/ssh/ssh_host_rsa_key.pub

but you will probably want also the moduli sizes that are offered and used during the key exchange, but it really depends on the key exchange method, but it should be also readable from debug output ssh -vvv host.

  • 1
    thanks. It turned out that sshd -T | grep "\(ciphers\|macs\|kexalgorithms\)" worked only my RHEL7 hosts, but not RHEL6. I ended up using nmap --script SSH2-hostkey localhost and nmap --script ssh-hostkey localhost – Henrik Pingel Nov 11 '15 at 9:46
  • 1
    RHEL6 host with latest openssh update should have it fixed as well. – Jakuje Nov 11 '15 at 9:47
  • damm you are right about that. I only checked on an outdated VM ... thanks – Henrik Pingel Nov 11 '15 at 9:49
  • sshd -T will only offer information about the ciphers configured in the sshd_config file, not what can indeed be added to it as being supported by the binary – Daniel J. Jan 9 '19 at 10:21

How can I determine the supported MACs, Ciphers, Key length and KexAlogrithms supported by my ssh servers?

It looks like the answer on https://superuser.com/a/1219759/173408 is also an answer to your question. It fits in one line:

nmap --script ssh2-enum-algos -sV -p 22

Here is the output on a plain Debian 9.4 machine with current SSH version:

Starting Nmap 7.01 ( https://nmap.org ) at 2018-05-22 13:40 CEST
Nmap scan report for
Host is up (0.00024s latency).
22/tcp open  ssh     OpenSSH 7.4p1 Debian 10+deb9u3 (protocol 2.0)
| ssh2-enum-algos: 
|   kex_algorithms: (10)
|       curve25519-sha256
|       curve25519-sha256@libssh.org
|       ecdh-sha2-nistp256
|       ecdh-sha2-nistp384
|       ecdh-sha2-nistp521
|       diffie-hellman-group-exchange-sha256
|       diffie-hellman-group16-sha512
|       diffie-hellman-group18-sha512
|       diffie-hellman-group14-sha256
|       diffie-hellman-group14-sha1
|   server_host_key_algorithms: (5)
|       ssh-rsa
|       rsa-sha2-512
|       rsa-sha2-256
|       ecdsa-sha2-nistp256
|       ssh-ed25519
|   encryption_algorithms: (6)
|       chacha20-poly1305@openssh.com
|       aes128-ctr
|       aes192-ctr
|       aes256-ctr
|       aes128-gcm@openssh.com
|       aes256-gcm@openssh.com
|   mac_algorithms: (10)
|       umac-64-etm@openssh.com
|       umac-128-etm@openssh.com
|       hmac-sha2-256-etm@openssh.com
|       hmac-sha2-512-etm@openssh.com
|       hmac-sha1-etm@openssh.com
|       umac-64@openssh.com
|       umac-128@openssh.com
|       hmac-sha2-256
|       hmac-sha2-512
|       hmac-sha1
|   compression_algorithms: (2)
|       none
|_      zlib@openssh.com
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 0.52 seconds
  • 1
    I get a PORT STATE SERVICE VERSION 22/tcp filtered ssh with this command - although I can login to that same server via ssh. – hey Jul 4 '19 at 22:22
  • Did you literally use the command, or did you replace with the IP of your server? – Stéphane Gourichon Oct 14 '19 at 13:27
  • I used the IP of my server. – hey Oct 15 '19 at 21:34

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