diff --git a/lib/sockopt.c b/lib/sockopt.c
index f8fa946..a2038a5 100644
--- a/lib/sockopt.c
+++ b/lib/sockopt.c
@@ -22,6 +22,7 @@
 #include <zebra.h>
 #include "log.h"
 #include "sockopt.h"
+#include "sockunion.h"
 
 int
 setsockopt_so_recvbuf (int sock, int size)
@@ -480,3 +481,70 @@
 
   iph->ip_id = ntohs(iph->ip_id);
 }
+
+int
+sockopt_tcp_signature (int sock, union sockunion *su, const char *password)
+{
+#if HAVE_DECL_TCP_MD5SIG
+#ifndef GNU_LINUX
+  /*
+   * XXX Need to do PF_KEY operation here to add/remove an SA entry,
+   * and add/remove an SP entry for this peer's packet flows also.
+   */
+  int md5sig = password && *password ? 1 : 0;
+#else
+  int keylen = password ? strlen (password) : 0;
+  struct tcp_md5sig md5sig;
+  union sockunion *su2, *susock;
+  int ret;
+  
+  /* Figure out whether the socket and the sockunion are the same family..
+   * adding AF_INET to AF_INET6 needs to be v4 mapped, you'd think..
+   */
+  if (!(susock = sockunion_getsockname (sock)))
+    return -1;
+  
+  if (susock->sa.sa_family == su->sa.sa_family)
+    su2 = su;
+  else
+    {
+      /* oops.. */
+      su2 = susock;
+      
+      if (su2->sa.sa_family == AF_INET)
+        {
+          sockunion_free (susock);
+          return -1;
+        };
+      
+      /* If this does not work, then all users of this sockopt will need to
+       * differentiate between IPv4 and IPv6, and keep seperate sockets for
+       * each. 
+       *
+       * Sadly, it doesn't seem to work at present. It's unknown whether
+       * this is a bug or not.
+       */
+      if (su2->sa.sa_family == AF_INET6
+          && su->sa.sa_family == AF_INET)
+        {
+           su2->sin6.sin6_family = AF_INET6;
+           /* V4Map the address */
+           memset (&su2->sin6.sin6_addr, 0, sizeof (struct in6_addr));
+           su2->sin6.sin6_addr.s6_addr32[2] = htonl(0xffff);
+           memcpy (&su2->sin6.sin6_addr.s6_addr32[3], &su->sin.sin_addr, 4);
+        }
+    }
+  
+  memset (&md5sig, 0, sizeof (md5sig));
+  memcpy (&md5sig.tcpm_addr, su2, sizeof (*su2));
+  md5sig.tcpm_keylen = keylen;
+  if (keylen)
+    memcpy (md5sig.tcpm_key, password, keylen);
+#endif /* GNU_LINUX */
+  ret = setsockopt (sock, IPPROTO_TCP, TCP_MD5SIG, &md5sig, sizeof md5sig);
+  sockunion_free (susock);
+  return ret;
+#else /* HAVE_TCP_MD5SIG */
+  return -2;
+#endif /* HAVE_TCP_MD5SIG */
+}