ssh

SSH Hardening security

The tool ssh-audit can be used to audit a server for out of date ssh versions and making sure it does not support insecure encryption / key exchange algorithims. Pair the results of the audit tool with the ssh hardening guide.

Config File

~/.ssh/config Can be used for setting up aliases, forwarding ports, or assuring agent forwarding is done. e.g:

Host home
  HostName blah.blah.com
  ForwardAgent yes
  LocalForward 8080 192.168.1.150:443

This would enable typing ssh home and that be equivalent to ssh -A -L 8080:192.168.1.150:443 blah.blah.com . Or, in plainer english, set up a tunnel to blah.blah.com where the local port 8080 is forwarded to the port 443 on host 192.168.1.150, with agent forwading.

Caveat: this could create duplicate forwards if connecting multiple times

Reuse connections

Host example.org
  ControlMaster    auto
  ControlPath      ~/.ssh/%r@%h:%p.sock
  ControlPersist   yes

Remote commands

One useful idea is to start a tmux session straight off(Becker 2022):

Host example.org
  RemoteCommand tmux new -A -s default

Create tunnel in background

ssh -f -N -L 3000:server1:5029 me@server2

Use GPG Agent to authenticate ssh

source

Create authentication key

 gpg2 --expert --edit-key <KEY ID>
gpg> addkey
Please select what kind of key you want:
   (3) DSA (sign only)
   (4) RSA (sign only)
   (5) Elgamal (encrypt only)
   (6) RSA (encrypt only)
   (7) DSA (set your own capabilities)
   (8) RSA (set your own capabilities)
  (10) ECC (sign only)
  (11) ECC (set your own capabilities)
  (12) ECC (encrypt only)
  (13) Existing key
Your selection? 8

Possible actions for a RSA key: Sign Encrypt Authenticate
Current allowed actions: Sign Encrypt

   (S) Toggle the sign capability
   (E) Toggle the encrypt capability
   (A) Toggle the authenticate capability
   (Q) Finished

Your selection? s
Your selection? e
Your selection? a

Possible actions for a RSA key: Sign Encrypt Authenticate
Current allowed actions: Authenticate

   (S) Toggle the sign capability
   (E) Toggle the encrypt capability
   (A) Toggle the authenticate capability
   (Q) Finished

Your selection? q
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048)
Requested keysize is 2048 bits
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0)
Key does not expire at all
Is this correct? (y/N) y
Really create? (y/N) y

sec  rsa2048/8715AF32191DB135
     created: 2019-03-21  expires: 2021-03-20  usage: SC
     trust: ultimate      validity: ultimate
ssb  rsa2048/150F16909B9AA603
     created: 2019-03-21  expires: 2021-03-20  usage: E
ssb  rsa2048/17E7403F18CB1123
     created: 2019-03-21  expires: never       usage: A
[ultimate] (1). Brian Exelbierd

gpg> quit
Save changes? (y/N) y

Add key to ssh

$ gpg -K --with-keygrip
/home/bexelbie/.gnupg/pubring.kbx
------------------------------
sec   rsa2048 2019-03-21 [SC] [expires: 2021-03-20]
      96F33EA7F4E0F7051D75FC208715AF32191DB135
      Keygrip = 90E08830BC1AAD225E657AD4FBE638B3D8E50C9E
uid           [ultimate] Brian Exelbierd
ssb   rsa2048 2019-03-21 [E] [expires: 2021-03-20]
      Keygrip = 5FA04ABEBFBC5089E50EDEB43198B4895BCA2136
ssb   rsa2048 2019-03-21 [A]
      Keygrip = 7710BA0643CC022B92544181FF2EAC2A290CDC0E

$ echo 7710BA0643CC022B92544181FF2EAC2A290CDC0E >> ~/.gnupg/sshcontrol

Enable ssh-agent-support

echo "enable-ssh-support" >> ~/.gnupg/.gpg-agent-conf

Tell ssh about gpg

export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket)

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