не работает knock по tcp, да и в апстриме есть версия поновее

$ strace -fF knock 127.0.0.1 123
....
open("/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=153, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd33480e000
read(3, "# Generated by resolvconf\n# Do n"..., 4096) = 153
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0x7fd33480e000, 4096)            = 0
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 3
fcntl(3, F_GETFL)                       = 0x2 (flags O_RDWR)
fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK)    = 0
connect(3, {sa_family=AF_INET, sin_port=htons(123), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 EINPROGRESS (Operation now in progress)
close(3)                                = 0