source: azure_iot_hub_f767zi/trunk/asp_baseplatform/lwip/contrib-2.1.0/addons/tcp_md5/README@ 457

This folder provides an example implementation of how to add custom tcp header
options and custom socket options.

It does this by implementing the (seldom used) tcp md5 signature.

To enable it, add an LWIP_HOOK_FILENAME hook file, include tcp_md5.h in it and
define these hooks:

  #define LWIP_HOOK_TCP_INPACKET_PCB(pcb, hdr, optlen, opt1len, opt2, p) tcp_md5_check_inpacket(pcb, hdr, optlen, opt1len, opt2, p)
  #define LWIP_HOOK_TCP_OPT_LENGTH_SEGMENT(pcb, internal_len)            tcp_md5_get_additional_option_length(pcb, internal_len)
  #define LWIP_HOOK_TCP_ADD_TX_OPTIONS(p, hdr, pcb, opts)                tcp_md5_add_tx_options(p, hdr, pcb,  opts)
  #define LWIP_HOOK_SOCKETS_SETSOCKOPT(s, sock, level, optname, optval, optlen, err) tcp_md5_setsockopt_hook(sock, level, optname, optval, optlen, err)

Then, in your sockets application, enable md5 signature on a socket like this:

  struct tcp_md5sig md5;
  struct sockaddr_storage addr_remote; /* Initialize this to remote address and port */
  memcpy(&md5.tcpm_addr, &addr_remote, sizeof(addr_remote));
  strcpy(md5.tcpm_key, key); /* this is the md5 key per connection */
  md5.tcpm_keylen = strlen(key);
  if ((ret = setsockopt(sockfd, IPPROTO_TCP, TCP_MD5SIG, &md5, sizeof(md5))) < 0) {
    perror("setsockopt TCP_MD5SIG");
    return;
  }

After that, your connection (client) or all incoming connections (server) require
tcp md5 signatures.