update ssh config

hosts/candlekeep/configuration.nix

@@ -115,10 +115,91 @@
   services.openssh = {
     enable = true;
     settings = {
-      # Forbid root login through SSH.
-      PermitRootLogin = "no";
+      ########## Features ##########
+
+      # disallow ssh-agent forwarding to prevent lateral movement
+      AllowAgentForwarding = false;
+
+      # prevent TCP ports from being forwarded over SSH tunnels
+      # **please be aware that disabling TCP forwarding does not prevent port forwarding**
+      # any user with an interactive login shell can spin up his/her own instance of sshd
+      AllowTcpForwarding = false;
+
+      # prevent StreamLocal (Unix-domain socket) forwarding
+      AllowStreamLocalForwarding = false;
+
+      # disables all forwarding features
+      # overrides all other forwarding switches
+      DisableForwarding = true;
+
+      # disallow remote hosts from connecting to forwarded ports
+      # i.e. forwarded ports are forced to bind to 127.0.0.1 instad of 0.0.0.0
+      GatewayPorts = "no";
+
+      # prevent tun device forwarding
+      PermitTunnel = false;
+
+      # suppress MOTD
+      PrintMotd = false;
+
+      # disable X11 forwarding since it is not necessary
+      X11Forwarding = false;
+
+      ########## Authentication ##########
+
+      AllowUsers = ["${user}"];
+      AllowGroups = ["${user}"];
+
       # Use keys only. Remove if you want to SSH using password (not recommended)
       PasswordAuthentication = false;
+      HostbasedAuthentication = false;
+
+      # enable pubkey authentication
+      PubkeyAuthentication = true;
+
+      # Forbid root login through SSH.
+      PermitRootLogin = "no";
+
+      # nix enables pam by default
+      #UsePam = true;
+
+      ########## Cryptography ##########
+
+      # explicitly define cryptography algorithms to avoid the use of weak algorithms
+      # AES CTR modes have been removed to mitigate the Terrapin attack
+      #   https://terrapin-attack.com/
+
+      Ciphers = ["aes256-gcm@openssh.com" "aes128-gcm@openssh.com"];
+      Macs = ["hmac-sha2-256-etm@openssh.com" "hmac-sha2-512-etm@openssh.com" "umac-128-etm@openssh.com"];
+      KexAlgorithms = ["curve25519-sha256" "curve25519-sha256@libssh.org" "diffie-hellman-group16-sha512" "diffie-hellman-group18-sha512"];
+
+      ########## Connection Preferences ##########
+
+      # enforce SSH server to only use SSH protocol version 2
+      # SSHv1 contains security issues and should be avoided at all costs
+      # SSHv1 is disabled by default after OpenSSH 7.0, but this option is
+      #   specified anyways to ensure this configuration file's compatibility
+      #   with older versions of OpenSSH server
+      Protocol = 2;
+
+      # number of client alive messages sent without client responding
+      ClientAliveCountMax = 2;
+
+      # send a keepalive message to the client when the session has been idle for 300 seconds
+      # this prevents/detects connection timeouts
+      ClientAliveInterval = 300;
+
+      # compression before encryption might cause security issues
+      Compression = false;
+
+      # prevent SSH trust relationships from allowing lateral movements
+      IgnoreRhosts = true;
+
+      # log verbosely for addtional information
+      LogLevel = "VERBOSE";
+
+      # allow a maximum of two multiplexed sessions over a single TCP connection
+      MaxSessions = 2;
     };
   };