On FreeBSD there is a setsockopt option SO_USER_COOKIE which allows

setting a 32 bit value on each socket. This can be used by applications
and DTrace as a rendezvous point so that an applicaton's data can
more easily be captured at run time. Expose the user cookie via
DTrace by updating the translator in tcp.d and add a quick test
program, a TCP server, that sets the cookie on each connection
accepted.

Reviewed by:	hiren
MFC after:	1 week
Sponsored by:	Limelight Networks
Differential Revision:	https://reviews.freebsd.org/D7152

cddl/lib/libdtrace/tcp.d

@@ -126,6 +126,7 @@ typedef struct tcpsinfo {
 	int tcps_retransmit;		/* retransmit send event, boolean */
 	int tcps_srtt;			/* smoothed RTT in units of (TCP_RTT_SCALE*hz) */
 	int tcps_debug;		/* socket has SO_DEBUG set */
+	int tcps_cookie;	/* expose the socket's SO_USER_COOKIE */
 	int32_t tcps_dupacks;	/* consecutive dup acks received */
 	uint32_t tcps_rtttime;	/* RTT measurement start time */
 	uint32_t tcps_rtseq;	/* sequence # being timed */
@@ -224,6 +225,8 @@ translator tcpsinfo_t < struct tcpcb *p > {
 	tcps_srtt =             p == NULL ? -1  : p->t_srtt;   /* smoothed RTT in units of (TCP_RTT_SCALE*hz) */
 	tcps_debug =		p == NULL ? 0 :
 	    p->t_inpcb->inp_socket->so_options & 1;
+	tcps_cookie =		p == NULL ? -1 :
+	    p->t_inpcb->inp_socket->so_user_cookie;
 	tcps_dupacks =		p == NULL ? -1  : p->t_dupacks;
 	tcps_rtttime =		p == NULL ? -1  : p->t_rtttime;
 	tcps_rtseq =		p == NULL ? -1  : p->t_rtseq;

tests/sys/netinet/Makefile

@@ -5,7 +5,7 @@ BINDIR=		${TESTSDIR}
 
 ATF_TESTS_SH=	fibs_test
 
-PROGS=	udp_dontroute
+PROGS=	udp_dontroute tcp_user_cookie
 
 MAN=
 

tests/sys/netinet/tcp_user_cookie.c

@@ -0,0 +1,111 @@
+/*
+ *  Copyright (c) 2016 Limelight Networks
+ *  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,
+ *     without modification.
+ *  2. Redistributions in binary form must reproduce at minimum a disclaimer
+ *     substantially similar to the "NO WARRANTY" disclaimer below
+ *     ("Disclaimer") and any redistribution must be conditioned upon
+ *     including a substantially similar Disclaimer requirement for further
+ *     binary redistribution.
+ *
+ *  NO WARRANTY
+ *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ *  "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ *  LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR
+ *  A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ *  HOLDERS OR CONTRIBUTORS BE LIABLE FOR 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 DAMAGES.
+ *
+ *  Authors: George Neville-Neil
+ *
+ * $FreeBSD$
+ */
+
+#include <arpa/inet.h>
+#include <netinet/in.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+
+#include <err.h>
+#include <sysexits.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#define buflen 80
+
+/*
+ * Setup a TCP server listening on a port for connections, all of
+ * which subseuqently have their user cookie set.
+ */
+int
+main(int argc, char **argv)
+{
+	struct sockaddr_in srv;
+	int sock, accepted, port, cookie;
+	int ret;
+	char recvbuf[buflen];
+
+	if (argc != 3) {
+		fprintf(stderr, "Usage: %s port cookie\n", argv[0]);
+		exit(2);
+	}
+
+	port = atoi(argv[1]);
+	cookie = atoi(argv[2]);
+
+	sock = socket(PF_INET, SOCK_STREAM, 0);
+	if (sock < 0)
+		err(EXIT_FAILURE, "socket");
+
+	srv.sin_len = sizeof(srv);
+	srv.sin_family = AF_INET;
+	srv.sin_port = htons(port);
+	srv.sin_addr.s_addr = INADDR_ANY;
+
+	if (bind(sock, (struct sockaddr *)&srv, srv.sin_len) < 0)
+		err(EX_OSERR, "failed to bind to port %d", port);
+
+	if (listen(sock, 5) < 0)
+		err(EX_OSERR, "failed to listen on socket");
+
+	ret = setsockopt(sock, SOL_SOCKET, SO_USER_COOKIE, &cookie, sizeof(cookie));
+	if (ret < 0)
+		err(EX_OSERR, "setsockopt(SO_USER_COOKIE)");
+
+	while (1) {
+
+		accepted = accept(sock, NULL, 0);
+
+		if (accepted < 0)
+			err(EX_OSERR, "accept failed");
+
+		ret = setsockopt(accepted, SOL_SOCKET, SO_USER_COOKIE,
+				 &cookie, sizeof(cookie));
+		if (ret < 0)
+			err(EX_OSERR, "setsockopt(SO_USER_COOKIE)");
+
+		ret = read(accepted, &recvbuf, buflen);
+
+		if (ret < 0)
+			warn("failed read");
+
+		close(accepted);
+	}
+
+	return (0);
+}