Fwknop

"FireWall KNock OPerator" implements an authorization scheme called Single Packet Authorization (SPA) (encrypted, non-replayed, with an HMAC SHA-256)

• fwknop --key-gen
• Config files: /etc/fwknop/access.conf

See also

• Port knocking, fail2ban fwknop, DenyHosts
• OpenSSH (changelog): /etc/ssh/sshd_config | /etc/ssh/ssh_config | ~/.ssh/ | openSSL | sshd logs | sftp | scp | authorized_keys | ssh-keygen | ssh-keyscan | ssh-add | ssh-agent | ssh | Ssh -O stop | ssh-copy-id | CheckHostIP | UseKeychain, OpenSSF
• iptables ufw firewalld nftables firewall-cmd ipfw (FreeBSD) PF (OpenBSD), netsh advfirewall
• Security tools: Vulnerability scanner, port scan, Host sweep, nmap, nping, ncat, nc, psad, Gordon Lyon
• Port Knocking

https://serverfault.com/a/608976