MINOR: tcp: add new bind option "tcp-ss" to instruct the kernel to save the SYN

This option enables TCP_SAVE_SYN on the listening socket, which will
cause the kernel to try to save a copy of the SYN packet header (L2,
IP and TCP are supported). This can permit to check the source MAC
address of a client, or find certain TCP options such as a source
address encapsulated using RFC7974. It could also be used as an
alternate approach to retrieving the source and destination addresses
and ports. For now setting the option is enabled, but sample fetch
functions and converters will be needed to extract info.

doc/configuration.txt

@@ -17512,6 +17512,18 @@ tcp-md5sig <password>
   introduction of spoofed TCP segments into the connection stream. But it can
   be useful for any very long-lived TCP connections.
 
+tcp-ss <mode>
+  Sets the TCP Save SYN option for all incoming connections instantiated from
+  this listening socket. This option is available on Linux since version 4.3.
+  It instructs the kernel to try to keep a copy of the incoming IP packet
+  containing the TCP SYN flag, for later inspection. The option knows 3 modes:
+    - 0    SYN packet saving is disabled, this is the default
+    - 1    SYN packet saving is enabled, and contains IP and TCP headers
+    - 2    SYN packet saving is enabled, and contains ETH, IP and TCP headers
+
+  This only works for regular TCP connections, and is ignored for other
+  protocols (e.g. UNIX sockets). See also "fc.saved_syn".
+
 tcp-ut <delay>
   Sets the TCP User Timeout for all incoming connections instantiated from this
   listening socket. This option is available on Linux since version 2.6.37. It

include/haproxy/listener-t.h

@@ -204,6 +204,7 @@ struct bind_conf {
 	unsigned int backlog;      /* if set, listen backlog */
 	int maxconn;               /* maximum connections allowed on this listener */
 	int (*accept)(struct connection *conn); /* upper layer's accept() */
+	int tcp_ss;                /* for TCP, Save SYN */
 	int level;                 /* stats access level (ACCESS_LVL_*) */
 	int severity_output;       /* default severity output format in cli feedback messages */
 	short int nice;            /* nice value to assign to the instantiated tasks */

src/cfgparse-tcp.c

@@ -146,6 +146,25 @@ static int bind_parse_tcp_md5sig(char **args, int cur_arg, struct proxy *px, str
 }
 #endif
 
+#ifdef TCP_SAVE_SYN
+/* parse the "tcp-ss" bind keyword */
+static int bind_parse_tcp_ss(char **args, int cur_arg, struct proxy *px, struct bind_conf *conf, char **err)
+{
+	int tcp_ss = -1;
+
+	if (isdigit((unsigned char)*args[cur_arg + 1]))
+		tcp_ss = atoi(args[cur_arg + 1]);
+
+	if (tcp_ss < 0 || tcp_ss > 2) {
+		memprintf(err, "'%s' : TCP Save SYN option expects an integer argument from 0 to 2", args[cur_arg]);
+		return ERR_ALERT | ERR_FATAL;
+	}
+
+	conf->tcp_ss = tcp_ss;
+	return 0;
+}
+#endif
+
 #ifdef TCP_USER_TIMEOUT
 /* parse the "tcp-ut" bind keyword */
 static int bind_parse_tcp_ut(char **args, int cur_arg, struct proxy *px, struct bind_conf *conf, char **err)
@@ -332,6 +351,9 @@ static struct bind_kw_list bind_kws = { "TCP", { }, {
 #if defined(__linux__) && defined(TCP_MD5SIG)
 	{ "tcp-md5sig",    bind_parse_tcp_md5sig,   1 }, /* set TCP MD5 signature password */
 #endif
+#ifdef TCP_SAVE_SYN
+	{ "tcp-ss",        bind_parse_tcp_ss,       1 }, /* set TCP Save SYN option (0=no, 1=yes, 2=with MAC hdr) */
+#endif
 #ifdef TCP_USER_TIMEOUT
 	{ "tcp-ut",        bind_parse_tcp_ut,       1 }, /* set User Timeout on listening socket */
 #endif

src/proto_tcp.c

@@ -778,6 +778,17 @@ int tcp_bind_listener(struct listener *listener, char *errmsg, int errlen)
 		}
 	}
 #endif
+#if defined(TCP_SAVE_SYN)
+	if (listener->bind_conf->tcp_ss) {
+		if (setsockopt(fd, IPPROTO_TCP, TCP_SAVE_SYN,
+			       &listener->bind_conf->tcp_ss, sizeof(listener->bind_conf->tcp_ss)) == -1) {
+			chunk_appendf(msg, "%scannot set TCP Save SYN, (%s)", msg->data ? ", " : "",
+				      strerror(errno));
+			err |= ERR_WARN;
+		}
+	} else
+		setsockopt(fd, IPPROTO_TCP, TCP_SAVE_SYN, &zero, sizeof(zero));
+#endif
 #if defined(TCP_USER_TIMEOUT)
 	if (listener->bind_conf->tcp_ut) {
 		if (setsockopt(fd, IPPROTO_TCP, TCP_USER_TIMEOUT,