diff --git a/contrib/addons/tcp_md5/README b/contrib/addons/tcp_md5/README
new file mode 100644
index 0000000..a6408ca
--- /dev/null
+++ b/contrib/addons/tcp_md5/README
@@ -0,0 +1,27 @@
+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.
diff --git a/contrib/addons/tcp_md5/tcp_md5.c b/contrib/addons/tcp_md5/tcp_md5.c
new file mode 100644
index 0000000..e7815d8
--- /dev/null
+++ b/contrib/addons/tcp_md5/tcp_md5.c
@@ -0,0 +1,534 @@
+/**
+ * @file: An implementation of TCP MD5 signatures by using various hooks in
+ * lwIP to implement custom tcp options and custom socket options.
+ */
+
+/*
+ * Copyright (c) 2018 Simon Goldschmidt
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without modification,
+ * are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
+ * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
+ * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
+ * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
+ * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
+ * OF SUCH DAMAGE.
+ *
+ * Author: Simon Goldschmidt <goldsimon@gmx.de>
+ */
+
+#include "tcp_md5.h"
+#include "lwip/ip_addr.h"
+#include "lwip/sys.h"
+#include "lwip/prot/tcp.h"
+#include "lwip/priv/tcp_priv.h"
+#include "lwip/sockets.h"
+#include "lwip/priv/sockets_priv.h"
+#include "lwip/api.h"
+#include <string.h>
+
+/* pull in md5 of ppp? */
+#include "netif/ppp/ppp_opts.h"
+#if !PPP_SUPPORT || (!LWIP_USE_EXTERNAL_POLARSSL && !LWIP_USE_EXTERNAL_MBEDTLS)
+#undef  LWIP_INCLUDED_POLARSSL_MD5
+#define LWIP_INCLUDED_POLARSSL_MD5 1
+#include "netif/ppp/polarssl/md5.h"
+#endif
+
+#if !LWIP_TCP_PCB_NUM_EXT_ARGS
+#error tcp_md5 needs LWIP_TCP_PCB_NUM_EXT_ARGS
+#endif
+
+#define LWIP_TCP_OPT_MD5          19 /* number of the md5 option */
+#define LWIP_TCP_OPT_LEN_MD5      18 /* length of the md5 option */
+#define LWIP_TCP_OPT_LEN_MD5_OUT  20 /* 18 + alignment */
+
+#define LWIP_TCP_MD5_DIGEST_LEN   16
+
+/* This keeps the md5 state internally */
+struct tcp_md5_conn_info {
+  struct tcp_md5_conn_info *next;
+  ip_addr_t remote_addr;
+  u16_t remote_port;
+  u8_t key[TCP_MD5SIG_MAXKEYLEN];
+  u16_t key_len;
+};
+
+/* Callback function prototypes: */
+static void tcp_md5_extarg_destroy(u8_t id, void *data);
+static err_t tcp_md5_extarg_passive_open(u8_t id, struct tcp_pcb_listen *lpcb, struct tcp_pcb *cpcb);
+/* Define our tcp ext arg callback structure: */
+const struct tcp_ext_arg_callbacks tcp_md5_ext_arg_callbacks = {
+  tcp_md5_extarg_destroy,
+  tcp_md5_extarg_passive_open
+};
+
+static u8_t tcp_md5_extarg_id = LWIP_TCP_PCB_NUM_EXT_ARG_ID_INVALID;
+static u8_t tcp_md5_opts_buf[40];
+
+/** Initialize this module (allocates a tcp ext arg id) */
+void
+tcp_md5_init(void)
+{
+  tcp_md5_extarg_id = tcp_ext_arg_alloc_id();
+}
+
+/* Create a conn-info structure that holds the md5 state per connection */
+static struct tcp_md5_conn_info *
+tcp_md5_conn_info_alloc(void)
+{
+  return (struct tcp_md5_conn_info *)mem_malloc(sizeof(struct tcp_md5_conn_info));
+}
+
+/* Frees a conn-info structure that holds the md5 state per connection */
+static void
+tcp_md5_conn_info_free(struct tcp_md5_conn_info *info)
+{
+  mem_free(info);
+}
+
+/* A pcb is about to be destroyed. Free its extdata */
+static void
+tcp_md5_extarg_destroy(u8_t id, void *data)
+{
+  struct tcp_md5_conn_info *iter;
+
+  LWIP_ASSERT("tcp_md5_extarg_id != LWIP_TCP_PCB_NUM_EXT_ARG_ID_INVALID",
+    tcp_md5_extarg_id != LWIP_TCP_PCB_NUM_EXT_ARG_ID_INVALID);
+  LWIP_ASSERT("id == tcp_md5_extarg_id", id == tcp_md5_extarg_id);
+  LWIP_UNUSED_ARG(id);
+
+  iter = (struct tcp_md5_conn_info *)data;
+  while (iter != NULL) {
+    struct tcp_md5_conn_info *info = iter;
+    iter = iter->next;
+    tcp_md5_conn_info_free(info);
+  }
+}
+
+/* Try to find an md5 connection info for the specified remote connection */
+static struct tcp_md5_conn_info *
+tcp_md5_get_info(const struct tcp_pcb *pcb, const ip_addr_t *remote_ip, u16_t remote_port)
+{
+  if (pcb != NULL) {
+    struct tcp_md5_conn_info *info = (struct tcp_md5_conn_info *)tcp_ext_arg_get(pcb, tcp_md5_extarg_id);
+    while (info != NULL) {
+      if (ip_addr_eq(&info->remote_addr, remote_ip)) {
+        if (info->remote_port == remote_port) {
+          return info;
+        }
+      }
+      info = info->next;
+    }
+  }
+  return NULL;
+}
+
+/* Passive open: copy md5 connection info from listen pcb to connection pcb
+ * or return error (connection will be closed)
+ */
+static err_t
+tcp_md5_extarg_passive_open(u8_t id, struct tcp_pcb_listen *lpcb, struct tcp_pcb *cpcb)
+{
+  struct tcp_md5_conn_info *iter;
+
+  LWIP_ASSERT("lpcb != NULL", lpcb != NULL);
+  LWIP_ASSERT("cpcb != NULL", cpcb != NULL);
+  LWIP_ASSERT("tcp_md5_extarg_id != LWIP_TCP_PCB_NUM_EXT_ARG_ID_INVALID",
+    tcp_md5_extarg_id != LWIP_TCP_PCB_NUM_EXT_ARG_ID_INVALID);
+  LWIP_ASSERT("id == tcp_md5_extarg_id", id == tcp_md5_extarg_id);
+  LWIP_UNUSED_ARG(id);
+
+  iter = (struct tcp_md5_conn_info *)tcp_ext_arg_get((struct tcp_pcb *)lpcb, id);
+  while (iter != NULL) {
+    if (iter->remote_port == cpcb->remote_port) {
+      if (ip_addr_eq(&iter->remote_addr, &cpcb->remote_ip)) {
+        struct tcp_md5_conn_info *info = tcp_md5_conn_info_alloc();
+        if (info != NULL) {
+          memcpy(info, iter, sizeof(struct tcp_md5_conn_info));
+          tcp_ext_arg_set(cpcb, id, info);
+          tcp_ext_arg_set_callbacks(cpcb, id, &tcp_md5_ext_arg_callbacks);
+          return ERR_OK;
+        } else {
+          return ERR_MEM;
+        }
+      }
+    }
+    iter = iter->next;
+  }
+  /* remote connection not found */
+  return ERR_VAL;
+}
+
+/* Parse tcp header options and return 1 if an md5 signature option was found */
+static int
+tcp_md5_parseopt(const u8_t *opts, u16_t optlen, u8_t *md5_digest_out)
+{
+  u8_t data;
+  u16_t optidx;
+
+  /* Parse the TCP MSS option, if present. */
+  if (optlen != 0) {
+    for (optidx = 0; optidx < optlen; ) {
+      u8_t opt = opts[optidx++];
+      switch (opt) {
+        case LWIP_TCP_OPT_EOL:
+          /* End of options. */
+          LWIP_DEBUGF(TCP_INPUT_DEBUG, ("tcp_parseopt: EOL\n"));
+          return 0;
+        case LWIP_TCP_OPT_NOP:
+          /* NOP option. */
+          LWIP_DEBUGF(TCP_INPUT_DEBUG, ("tcp_parseopt: NOP\n"));
+          break;
+        case LWIP_TCP_OPT_MD5:
+          LWIP_DEBUGF(TCP_INPUT_DEBUG, ("tcp_parseopt: MD5\n"));
+          if (opts[optidx++] != LWIP_TCP_OPT_LEN_MD5 || (optidx - 2 + LWIP_TCP_OPT_LEN_MD5) > optlen) {
+            /* Bad length */
+            LWIP_DEBUGF(TCP_INPUT_DEBUG, ("tcp_parseopt: bad length\n"));
+            return 0;
+          }
+          /* An MD5 option with the right option length. */
+          memcpy(md5_digest_out, &opts[optidx], LWIP_TCP_MD5_DIGEST_LEN);
+          /* no need to process the options further */
+          return 1;
+          break;
+        default:
+          LWIP_DEBUGF(TCP_INPUT_DEBUG, ("tcp_parseopt: other\n"));
+          data = opts[optidx++];
+          if (data < 2) {
+            LWIP_DEBUGF(TCP_INPUT_DEBUG, ("tcp_parseopt: bad length\n"));
+            /* If the length field is zero, the options are malformed
+               and we don't process them further. */
+            return 0;
+          }
+          /* All other options have a length field, so that we easily
+             can skip past them. */
+          optidx += data - 2;
+      }
+    }
+  }
+  return 0;
+}
+
+/* Get tcp options into contiguous memory. May be required if input pbufs
+ * are chained.
+ */
+static const u8_t*
+tcp_md5_options_singlebuf(struct tcp_hdr *hdr, u16_t optlen, u16_t opt1len, u8_t *opt2)
+{
+  const u8_t *opts;
+  LWIP_ASSERT("hdr != NULL", hdr != NULL);
+  LWIP_ASSERT("optlen >= opt1len", optlen >= opt1len);
+  opts = (const u8_t *)hdr + TCP_HLEN;
+  if (optlen == opt1len) {
+    /* arleady in one piece */
+    return opts;
+  }
+  if (optlen > sizeof(tcp_md5_opts_buf)) {
+    /* options too long */
+    return NULL;
+  }
+  LWIP_ASSERT("opt2 != NULL", opt2 != NULL);
+  /* copy first part */
+  memcpy(tcp_md5_opts_buf, opts, opt1len);
+  /* copy second part */
+  memcpy(&tcp_md5_opts_buf[opt1len], opt2, optlen - opt1len);
+  return tcp_md5_opts_buf;
+}
+
+/* Create the md5 digest for a given segment */
+static int
+tcp_md5_create_digest(const ip_addr_t *ip_src, const ip_addr_t *ip_dst, const struct tcp_hdr *hdr,
+                      const u8_t *key, size_t key_len, u8_t *digest_out, struct pbuf *p)
+{
+  md5_context ctx;
+  u8_t tmp8;
+  u16_t tmp16;
+  const size_t addr_len = IP_ADDR_RAW_SIZE(*ip_src);
+
+  if (p != NULL) {
+    LWIP_ASSERT("pbuf must not point to tcp header here!", (const void *)hdr != p->payload);
+  }
+
+  /* Generate the hash, using MD5. */
+  md5_starts(&ctx);
+  /* 1. the TCP pseudo-header (in the order: source IP address,
+          destination IP address, zero-padded protocol number, and
+          segment length) */
+  md5_update(&ctx, (const unsigned char*)ip_src, addr_len);
+  md5_update(&ctx, (const unsigned char*)ip_dst, addr_len);
+  tmp8 = 0; /* zero-padded */
+  md5_update(&ctx, &tmp8, 1);
+  tmp8 = IP_PROTO_TCP;
+  md5_update(&ctx, &tmp8, 1);
+  tmp16 = lwip_htons(TCPH_HDRLEN_BYTES(hdr) + (p ? p->tot_len : 0));
+  md5_update(&ctx, (const unsigned char*)&tmp16, 2);
+  /* 2. the TCP header, excluding options, and assuming a checksum of
+          zero */
+  md5_update(&ctx, (const unsigned char*)hdr, sizeof(struct tcp_hdr));
+  /* 3. the TCP segment data (if any) */
+  if ((p != NULL) && (p->tot_len != 0)) {
+    struct pbuf *q;
+    for (q = p; q != NULL; q = q->next) {
+      md5_update(&ctx, (const unsigned char*)q->payload, q->len);
+    }
+  }
+  /* 4. an independently-specified key or password, known to both TCPs
+          and presumably connection-specific */
+  md5_update(&ctx, key, key_len);
+
+  md5_finish(&ctx, digest_out);
+  return 1;
+}
+
+/* Duplicate a tcp header and make sure the fields are in network byte order */
+static void
+tcp_md5_dup_tcphdr(struct tcp_hdr *tcphdr_copy, const struct tcp_hdr *tcphdr_in, int tcphdr_in_is_host_order)
+{
+  memcpy(tcphdr_copy, tcphdr_in, sizeof(struct tcp_hdr));
+  tcphdr_copy->chksum = 0; /* checksum is zero for the pseudo header */
+  if (tcphdr_in_is_host_order) {
+    /* lwIP writes the TCP header values back to the buffer, we need to invert that here: */
+    tcphdr_copy->src = lwip_htons(tcphdr_copy->src);
+    tcphdr_copy->dest = lwip_htons(tcphdr_copy->dest);
+    tcphdr_copy->seqno = lwip_htonl(tcphdr_copy->seqno);
+    tcphdr_copy->ackno = lwip_htonl(tcphdr_copy->ackno);
+    tcphdr_copy->wnd = lwip_htons(tcphdr_copy->wnd);
+    tcphdr_copy->urgp = lwip_htons(tcphdr_copy->urgp);
+  }
+}
+
+/* Check if md5 is enabled on a given pcb */
+static int
+tcp_md5_is_enabled_on_pcb(const struct tcp_pcb *pcb)
+{
+  if (tcp_md5_extarg_id != LWIP_TCP_PCB_NUM_EXT_ARG_ID_INVALID) {
+    struct tcp_md5_conn_info *info = (struct tcp_md5_conn_info *)tcp_ext_arg_get(pcb, tcp_md5_extarg_id);
+    if (info != NULL) {
+      return 1;
+    }
+  }
+  return 0;
+}
+
+/* Check if md5 is enabled on a given listen pcb */
+static int
+tcp_md5_is_enabled_on_lpcb(const struct tcp_pcb_listen *lpcb)
+{
+  /* same as for connection pcbs */
+  return tcp_md5_is_enabled_on_pcb((const struct tcp_pcb *)lpcb);
+}
+
+/* Hook implementation for LWIP_HOOK_TCP_OPT_LENGTH_SEGMENT */
+u8_t
+tcp_md5_get_additional_option_length(const struct tcp_pcb *pcb, u8_t internal_option_length)
+{
+  if ((pcb != NULL) && tcp_md5_is_enabled_on_pcb(pcb)) {
+    u8_t new_option_length = internal_option_length + LWIP_TCP_OPT_LEN_MD5_OUT;
+    LWIP_ASSERT("overflow", new_option_length > internal_option_length);
+    LWIP_ASSERT("options too long", new_option_length <= TCP_MAX_OPTION_BYTES);
+    return new_option_length;
+  }
+  return internal_option_length;
+}
+
+/* Hook implementation for LWIP_HOOK_TCP_INPACKET_PCB when called for listen pcbs */
+static err_t
+tcp_md5_check_listen(struct tcp_pcb_listen* lpcb, struct tcp_hdr *hdr, u16_t optlen, u16_t opt1len, u8_t *opt2)
+{
+  LWIP_ASSERT("lpcb != NULL", lpcb != NULL);
+
+  if (tcp_md5_is_enabled_on_lpcb(lpcb)) {
+    const u8_t *opts;
+    u8_t digest_received[LWIP_TCP_MD5_DIGEST_LEN];
+    u8_t digest_calculated[LWIP_TCP_MD5_DIGEST_LEN];
+    const struct tcp_md5_conn_info *info = tcp_md5_get_info((struct tcp_pcb *)lpcb, ip_current_src_addr(), hdr->src);
+    if (info != NULL) {
+      opts = tcp_md5_options_singlebuf(hdr, optlen, opt1len, opt2);
+      if (opts != NULL) {
+        if (tcp_md5_parseopt(opts, optlen, digest_received)) {
+          struct tcp_hdr tcphdr_copy;
+          tcp_md5_dup_tcphdr(&tcphdr_copy, hdr, 1);
+          if (tcp_md5_create_digest(ip_current_src_addr(), ip_current_dest_addr(), &tcphdr_copy, info->key, info->key_len, digest_calculated, NULL)) {
+            /* everything set up, compare the digests */
+            if (!memcmp(digest_received, digest_calculated, LWIP_TCP_MD5_DIGEST_LEN)) {
+              /* equal */
+              return ERR_OK;
+            }
+            /* not equal */
+          }
+        }
+      }
+    }
+    /* md5 enabled on this pcb but no match or other error -> fail */
+    return ERR_VAL;
+  }
+  return ERR_OK;
+}
+
+/* Hook implementation for LWIP_HOOK_TCP_INPACKET_PCB */
+err_t
+tcp_md5_check_inpacket(struct tcp_pcb* pcb, struct tcp_hdr *hdr, u16_t optlen, u16_t opt1len, u8_t *opt2, struct pbuf *p)
+{
+  LWIP_ASSERT("pcb != NULL", pcb != NULL);
+
+  if (pcb->state == LISTEN) {
+    return tcp_md5_check_listen((struct tcp_pcb_listen *)pcb, hdr, optlen, opt1len, opt2);
+  }
+
+  if (tcp_md5_is_enabled_on_pcb(pcb)) {
+    const struct tcp_md5_conn_info *info = tcp_md5_get_info(pcb, ip_current_src_addr(), hdr->src);
+    if (info != NULL) {
+      const u8_t *opts;
+      u8_t digest_received[LWIP_TCP_MD5_DIGEST_LEN];
+      u8_t digest_calculated[LWIP_TCP_MD5_DIGEST_LEN];
+      opts = tcp_md5_options_singlebuf(hdr, optlen, opt1len, opt2);
+      if (opts != NULL) {
+        if (tcp_md5_parseopt(opts, optlen, digest_received)) {
+          struct tcp_hdr hdr_copy;
+          tcp_md5_dup_tcphdr(&hdr_copy, hdr, 1);
+          if (tcp_md5_create_digest(&pcb->remote_ip, &pcb->local_ip, &hdr_copy, info->key, info->key_len, digest_calculated, p)) {
+            /* everything set up, compare the digests */
+            if (!memcmp(digest_received, digest_calculated, LWIP_TCP_MD5_DIGEST_LEN)) {
+              /* equal */
+              return ERR_OK;
+            }
+            /* not equal */
+          }
+        }
+      }
+    }
+    /* md5 enabled on this pcb but no match or other error -> fail */
+    return ERR_VAL;
+  }
+  return ERR_OK;
+}
+
+/* Hook implementation for LWIP_HOOK_TCP_ADD_TX_OPTIONS */
+u32_t *
+tcp_md5_add_tx_options(struct pbuf *p, struct tcp_hdr *hdr, const struct tcp_pcb *pcb, u32_t *opts)
+{
+  LWIP_ASSERT("p != NULL", p != NULL);
+  LWIP_ASSERT("hdr != NULL", hdr != NULL);
+  LWIP_ASSERT("pcb != NULL", pcb != NULL);
+  LWIP_ASSERT("opts != NULL", opts != NULL);
+
+  if (tcp_md5_is_enabled_on_pcb(pcb)) {
+    u8_t digest_calculated[LWIP_TCP_MD5_DIGEST_LEN];
+    u32_t *opts_ret = opts + 5; /* we use 20 bytes: 2 bytes padding + 18 bytes for this option */
+    u8_t *ptr = (u8_t*)opts;
+
+    const struct tcp_md5_conn_info *info = tcp_md5_get_info(pcb, &pcb->remote_ip, pcb->remote_port);
+    if (info != NULL) {
+      struct tcp_hdr hdr_copy;
+      size_t hdrsize = TCPH_HDRLEN_BYTES(hdr);
+      tcp_md5_dup_tcphdr(&hdr_copy, hdr, 0);
+      /* p->payload points to the tcp header */
+      LWIP_ASSERT("p->payload == hdr", p->payload == hdr);
+      if (!pbuf_remove_header(p, hdrsize)) {
+        u8_t ret;
+        if (!tcp_md5_create_digest(&pcb->local_ip, &pcb->remote_ip, &hdr_copy, info->key, info->key_len, digest_calculated, p)) {
+          info = NULL;
+        }
+        ret = pbuf_add_header_force(p, hdrsize);
+        LWIP_ASSERT("tcp_md5_add_tx_options: pbuf_add_header_force failed", !ret);
+        LWIP_UNUSED_ARG(ret);
+      } else  {
+        LWIP_ASSERT("error", 0);
+      }
+    }
+    if (info == NULL) {
+      /* create an invalid signature by zeroing the digest */
+      memset(&digest_calculated, 0, sizeof(digest_calculated));
+    }
+
+    *ptr++ = LWIP_TCP_OPT_NOP;
+    *ptr++ = LWIP_TCP_OPT_NOP;
+    *ptr++ = LWIP_TCP_OPT_MD5;
+    *ptr++ = LWIP_TCP_OPT_LEN_MD5;
+    memcpy(ptr, digest_calculated, LWIP_TCP_MD5_DIGEST_LEN);
+    ptr += LWIP_TCP_MD5_DIGEST_LEN;
+    LWIP_ASSERT("ptr == opts_ret", ptr == (u8_t *)opts_ret);
+    return opts_ret;
+  }
+  return opts;
+}
+
+/* Hook implementation for LWIP_HOOK_SOCKETS_SETSOCKOPT */
+int
+tcp_md5_setsockopt_hook(struct lwip_sock *sock, int level, int optname, const void *optval, socklen_t optlen, int *err)
+{
+  LWIP_ASSERT("sock != NULL", sock != NULL);
+  LWIP_ASSERT("err != NULL", err != NULL);
+
+  if ((level == IPPROTO_TCP) && (optname == TCP_MD5SIG)) {
+    const struct tcp_md5sig *md5 = (const struct tcp_md5sig*)optval;
+    if ((optval == NULL) || (optlen < sizeof(struct tcp_md5sig))) {
+      *err = EINVAL;
+    } else {
+      if (sock->conn && (NETCONNTYPE_GROUP(netconn_type(sock->conn)) == NETCONN_TCP) && (sock->conn->pcb.tcp != NULL)) {
+        if (tcp_md5_extarg_id == LWIP_TCP_PCB_NUM_EXT_ARG_ID_INVALID) {
+          /* not initialized */
+          *err = EINVAL;
+        } else {
+          struct tcp_md5_conn_info *info = tcp_md5_conn_info_alloc();
+          if (info == NULL) {
+            *err = ENOMEM;
+          } else {
+            int addr_valid = 0;
+            /* OK, fill and link this request */
+            memcpy(info->key, md5->tcpm_key, TCP_MD5SIG_MAXKEYLEN);
+            info->key_len = md5->tcpm_keylen;
+            memset(&info->remote_addr, 0, sizeof(info->remote_addr));
+            if (md5->tcpm_addr.ss_family == AF_INET) {
+#if LWIP_IPV4
+              const struct sockaddr_in *sin = (const struct sockaddr_in *)&md5->tcpm_addr;
+              memcpy(&info->remote_addr, &sin->sin_addr, sizeof(sin->sin_addr));
+              IP_SET_TYPE_VAL(info->remote_addr, IPADDR_TYPE_V4);
+              info->remote_port = lwip_htons(sin->sin_port);
+              addr_valid = 1;
+#endif /* LWIP_IPV4 */
+            } else if (md5->tcpm_addr.ss_family == AF_INET6) {
+#if LWIP_IPV6
+              const struct sockaddr_in6 *sin6 = (const struct sockaddr_in6 *)&md5->tcpm_addr;
+              memcpy(&info->remote_addr, &sin6->sin6_addr, sizeof(sin6->sin6_addr));
+              IP_SET_TYPE_VAL(info->remote_addr, IPADDR_TYPE_V6);
+              info->remote_port = lwip_htons(sin6->sin6_port);
+              addr_valid = 1;
+#endif /* LWIP_IPV6 */
+            }
+            if (addr_valid) {
+              /* store it */
+              tcp_ext_arg_set_callbacks(sock->conn->pcb.tcp, tcp_md5_extarg_id, &tcp_md5_ext_arg_callbacks);
+              info->next = (struct tcp_md5_conn_info *)tcp_ext_arg_get(sock->conn->pcb.tcp, tcp_md5_extarg_id);
+              tcp_ext_arg_set(sock->conn->pcb.tcp, tcp_md5_extarg_id, info);
+            } else {
+              *err = EINVAL;
+              tcp_md5_conn_info_free(info);
+            }
+          }
+        }
+      } else {
+        /* not a tcp netconn */
+        *err = EINVAL;
+      }
+    }
+    return 1;
+  }
+  return 0;
+}
diff --git a/contrib/addons/tcp_md5/tcp_md5.h b/contrib/addons/tcp_md5/tcp_md5.h
new file mode 100644
index 0000000..e55740a
--- /dev/null
+++ b/contrib/addons/tcp_md5/tcp_md5.h
@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) 2018 Simon Goldschmidt
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without modification,
+ * are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
+ * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
+ * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
+ * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
+ * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
+ * OF SUCH DAMAGE.
+ *
+ * Author: Simon Goldschmidt <goldsimon@gmx.de>
+ *
+ * To use the hooks in this file, make sure this file is included in LWIP_HOOK_FILENAME
+ * 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)
+ */
+
+#ifndef LWIP_HDR_CONTRIB_ADDONS_TCP_MD5_H
+#define LWIP_HDR_CONTRIB_ADDONS_TCP_MD5_H
+
+#include "lwip/opt.h"
+#include "lwip/ip_addr.h"
+#include "lwip/err.h"
+
+#include "lwip/priv/sockets_priv.h"
+#include "lwip/priv/tcp_priv.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* setsockopt definitions and structs: */
+
+/* This is the optname (for level = IPPROTO_TCP) */
+#ifndef TCP_MD5SIG
+#define TCP_MD5SIG 14
+#endif
+
+#define TCP_MD5SIG_MAXKEYLEN 80
+
+/* This is the optval type */
+struct tcp_md5sig {
+  struct  sockaddr_storage tcpm_addr;
+  u16_t   __tcpm_pad1;
+  u16_t   tcpm_keylen;
+  u32_t   __tcpm_pad2;
+  u8_t    tcpm_key[TCP_MD5SIG_MAXKEYLEN];
+};
+
+/* socket setsockopt hook: */
+int tcp_md5_setsockopt_hook(struct lwip_sock *sock, int level, int optname, const void *optval, u32_t optlen, int *err);
+
+/* Internal hook functions */
+void tcp_md5_init(void);
+err_t tcp_md5_check_inpacket(struct tcp_pcb* pcb, struct tcp_hdr *hdr, u16_t optlen, u16_t opt1len, u8_t *opt2, struct pbuf *p);
+u8_t tcp_md5_get_additional_option_length(const struct tcp_pcb *pcb, u8_t internal_option_length);
+u32_t *tcp_md5_add_tx_options(struct pbuf *p, struct tcp_hdr *hdr, const struct tcp_pcb *pcb, u32_t *opts);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* LWIP_HDR_CONTRIB_ADDONS_TCP_MD5_H */