Also clamp MSS on IPv6

Some routers erratically drop "Packet too big" icmp messages, and PMTU
discovery then doesn't work. We can however easily clamp MSS on IPv6 too.

l2tpns.c

@@ -90,6 +90,7 @@ int guest_accounts_num = 0;     // Number of guest users
 // calculated from config->l2tp_mtu
 uint16_t MRU = 0;		// PPP MRU
 uint16_t MSS = 0;		// TCP MSS
+uint16_t MSS6 = 0;		// TCPv6 MSS
 
 struct cli_session_actions *cli_session_actions = NULL;	// Pending session changes requested by CLI
 struct cli_tunnel_actions *cli_tunnel_actions = NULL;	// Pending tunnel changes required by CLI
@@ -1322,7 +1323,7 @@ int tun_write(uint8_t * data, int size)
 }
 
 // adjust tcp mss to avoid fragmentation (called only for tcp packets with syn set)
-void adjust_tcp_mss(sessionidt s, tunnelidt t, uint8_t *buf, int len, uint8_t *tcp)
+static void adjust_tcp_mss(sessionidt s, tunnelidt t, uint8_t *buf, int len, uint8_t *tcp, in_addr_t src, in_addr_t dst, uint16_t mss_clamp)
 {
 	int d = (tcp[12] >> 4) * 4;
 	uint8_t *mss = 0;
@@ -1359,23 +1360,34 @@ void adjust_tcp_mss(sessionidt s, tunnelidt t, uint8_t *buf, int len, uint8_t *t
 	if (!mss) return; // not found
 	orig = ntohs(*(uint16_t *) mss);
 
-	if (orig <= MSS) return; // mss OK
+	if (orig <= mss_clamp) return; // mss OK
 
 	LOG(5, s, t, "TCP: %s:%u -> %s:%u SYN%s: adjusted mss from %u to %u\n",
-		fmtaddr(*(in_addr_t *) (buf + 12), 0), ntohs(*(uint16_t *) tcp),
+		fmtaddr(src, 0), ntohs(*(uint16_t *) tcp),
-		fmtaddr(*(in_addr_t *) (buf + 16), 1), ntohs(*(uint16_t *) (tcp + 2)),
+		fmtaddr(dst, 1), ntohs(*(uint16_t *) (tcp + 2)),
-		(tcp[13] & TCP_FLAG_ACK) ? ",ACK" : "", orig, MSS);
+		(tcp[13] & TCP_FLAG_ACK) ? ",ACK" : "", orig, mss_clamp);
 
 	// set mss
-	*(int16_t *) mss = htons(MSS);
+	*(int16_t *) mss = htons(mss_clamp);
 
 	// adjust checksum (see rfc1141)
-	sum = orig + (~MSS & 0xffff);
+	sum = orig + (~mss_clamp & 0xffff);
 	sum += ntohs(*(uint16_t *) (tcp + 16));
 	sum = (sum & 0xffff) + (sum >> 16);
 	*(uint16_t *) (tcp + 16) = htons(sum + (sum >> 16));
 }
 
+void adjust_tcp4_mss(sessionidt s, tunnelidt t, uint8_t *buf, int len, uint8_t *tcp)
+{
+	adjust_tcp_mss(s, t, buf, len, tcp, *(in_addr_t *) (buf + 12), *(in_addr_t *) (buf + 16), MSS);
+}
+
+void adjust_tcp6_mss(sessionidt s, tunnelidt t, uint8_t *buf, int len, uint8_t *tcp)
+{
+	/* Showing the lower part of IPv6 addresses */
+	adjust_tcp_mss(s, t, buf, len, tcp, *(in_addr_t *) (buf + 20), *(in_addr_t *) (buf + 36), MSS6);
+}
+
 void processmpframe(sessionidt s, tunnelidt t, uint8_t *p, uint16_t l, uint8_t extra)
 {
 	uint16_t proto;
@@ -1565,7 +1577,7 @@ void processipout(uint8_t *buf, int len)
 	{
 		int ihl = (buf[0] & 0xf) * 4; // length of IP header
 		if (len >= ihl + 20 && (buf[ihl + 13] & TCP_FLAG_SYN) && ((buf[ihl + 12] >> 4) > 5))
-			adjust_tcp_mss(s, t, buf, len, buf + ihl);
+			adjust_tcp4_mss(s, t, buf, len, buf + ihl);
 	}
 
 	if (sp->tbf_out)
@@ -1792,6 +1804,14 @@ static void processipv6out(uint8_t * buf, int len)
 
 	// FIXME: add DoS prevention/filters?
 
+	// adjust MSS on SYN and SYN,ACK packets with options
+	if (buf[6] == IPPROTO_TCP)
+	{
+		int ihl = 40; // length of IPv6 header
+		if (len >= ihl + 20 && (buf[ihl + 13] & TCP_FLAG_SYN) && ((buf[ihl + 12] >> 4) > 5))
+			adjust_tcp6_mss(s, t, buf, len, buf + ihl);
+	}
+
 	if (sp->tbf_out)
 	{
 		// Are we throttling this session?
@@ -5391,6 +5411,7 @@ static void update_config()
 
 #define L2TP_HDRS		(20+8+6+4)	// L2TP data encaptulation: ip + udp + l2tp (data) + ppp (inc hdlc)
 #define TCP_HDRS		(20+20)		// TCP encapsulation: ip + tcp
+#define TCP6_HDRS		(40+20)		// TCP encapsulation: ipv6 + tcp
 
 	if (config->l2tp_mtu <= 0)		config->l2tp_mtu = 1500; // ethernet default
 	else if (config->l2tp_mtu < MINMTU)	config->l2tp_mtu = MINMTU;
@@ -5404,6 +5425,7 @@ static void update_config()
 		MRU = PPPoE_MRU;
 
 	MSS = MRU - TCP_HDRS;
+	MSS6 = MRU - TCP6_HDRS;
 
 	// Update radius
 	config->numradiusservers = 0;

l2tpns.h

@@ -969,7 +969,8 @@ void filter_session(sessionidt s, int filter_in, int filter_out);
 void send_garp(in_addr_t ip);
 void tunnelsend(uint8_t *buf, uint16_t l, tunnelidt t);
 int tun_write(uint8_t *data, int size);
-void adjust_tcp_mss(sessionidt s, tunnelidt t, uint8_t *buf, int len, uint8_t *tcp);
+void adjust_tcp4_mss(sessionidt s, tunnelidt t, uint8_t *buf, int len, uint8_t *tcp);
+void adjust_tcp6_mss(sessionidt s, tunnelidt t, uint8_t *buf, int len, uint8_t *tcp);
 void sendipcp(sessionidt s, tunnelidt t);
 void sendipv6cp(sessionidt s, tunnelidt t);
 void processudp(uint8_t *buf, int len, struct sockaddr_in *addr, uint16_t indexudpfd);
@@ -1054,6 +1055,7 @@ struct event_data {
 
 extern uint16_t MRU;
 extern uint16_t MSS;
+extern uint16_t MSS6;
 
 // macros for handling help in cli commands
 #define CLI_HELP_REQUESTED	(argc > 0 && argv[argc-1][strlen(argv[argc-1])-1] == '?')

ppp.c

@@ -1803,7 +1803,7 @@ void processipin(sessionidt s, tunnelidt t, uint8_t *p, uint16_t l)
 	{
 		int ihl = (p[0] & 0xf) * 4; // length of IP header
 		if (l >= ihl + 20 && (p[ihl + 13] & TCP_FLAG_SYN) && ((p[ihl + 12] >> 4) > 5))
-			adjust_tcp_mss(s, t, p, l, p + ihl);
+			adjust_tcp4_mss(s, t, p, l, p + ihl);
 	}
 
 	// Add on the tun header
@@ -2284,6 +2284,14 @@ void processipv6in(sessionidt s, tunnelidt t, uint8_t *p, uint16_t l)
 		return;
 	}
 
+	// adjust MSS on SYN and SYN,ACK packets with options
+	if (p[6] == IPPROTO_TCP)
+	{
+		int ihl = 40; // length of IPv6 header
+		if (l >= ihl + 20 && (p[ihl + 13] & TCP_FLAG_SYN) && ((p[ihl + 12] >> 4) > 5))
+			adjust_tcp6_mss(s, t, p, l, p + ihl);
+	}
+
 	// Check if it's a Router Solicition message.
 	if (*(p + 6) == 58 && *(p + 7) == 255 && *(p + 24) == 0xFF && *(p + 25) == 2 &&
 			*(uint32_t *)(p + 26) == 0 && *(uint32_t *)(p + 30) == 0 &&