Set MSS to default Linux MSS in synscan modules (zmap#673)

* Set MSS to default Linux MSS in synscan modules For both synscan and synackscan, set MSS to 1450. This is the default Linux MSS, and the option is required to be set for certain hosts to respond. Fixes zmap#601 * set_tcp_options to set_mss_option
tumi8 · Aug 2, 2021 · 6819c65 · 6819c65
1 parent 967c589
commit 6819c65
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Showing 4 changed files with 29 additions and 6 deletions.
diff --git a/src/probe_modules/module_tcp_synackscan.c b/src/probe_modules/module_tcp_synackscan.c
@@ -21,7 +21,8 @@
 #include "packet.h"
 #include "module_tcp_synscan.h"
 
-#define ZMAP_TCP_SYNACKSCAN_PACKET_LEN 54
+#define ZMAP_TCP_SYNACKSCAN_TCP_HEADER_LEN 24
+#define ZMAP_TCP_SYNACKSCAN_PACKET_LEN 58
 
 probe_module_t module_tcp_synackscan;
 static uint32_t num_ports;
@@ -40,10 +41,11 @@ static int synackscan_init_perthread(void *buf, macaddr_t *src, macaddr_t *gw,
 	struct ether_header *eth_header = (struct ether_header *)buf;
 	make_eth_header(eth_header, src, gw);
 	struct ip *ip_header = (struct ip *)(&eth_header[1]);
-	uint16_t len = htons(sizeof(struct ip) + sizeof(struct tcphdr));
+	uint16_t len = htons(sizeof(struct ip) + ZMAP_TCP_SYNACKSCAN_TCP_HEADER_LEN);
 	make_ip_header(ip_header, IPPROTO_TCP, len);
 	struct tcphdr *tcp_header = (struct tcphdr *)(&ip_header[1]);
 	make_tcp_header(tcp_header, dst_port, TH_SYN | TH_ACK);
+	set_mss_option(tcp_header);
 	return EXIT_SUCCESS;
 }
 
@@ -69,7 +71,7 @@ static int synackscan_make_packet(void *buf, UNUSED size_t *buf_len,
 	tcp_header->th_ack = tcp_ack;
 	tcp_header->th_sum = 0;
 	tcp_header->th_sum =
-	    tcp_checksum(sizeof(struct tcphdr), ip_header->ip_src.s_addr,
+	    tcp_checksum(ZMAP_TCP_SYNACKSCAN_TCP_HEADER_LEN, ip_header->ip_src.s_addr,
 			 ip_header->ip_dst.s_addr, tcp_header);
 
 	ip_header->ip_sum = 0;

diff --git a/src/probe_modules/module_tcp_synscan.c b/src/probe_modules/module_tcp_synscan.c
@@ -20,7 +20,8 @@
 #include "probe_modules.h"
 #include "packet.h"
 
-#define ZMAP_TCP_SYNSCAN_PACKET_LEN 54
+#define ZMAP_TCP_SYNSCAN_TCP_HEADER_LEN 24
+#define ZMAP_TCP_SYNSCAN_PACKET_LEN 58
 
 probe_module_t module_tcp_synscan;
 static uint32_t num_ports;
@@ -39,10 +40,11 @@ static int synscan_init_perthread(void *buf, macaddr_t *src, macaddr_t *gw,
 	struct ether_header *eth_header = (struct ether_header *)buf;
 	make_eth_header(eth_header, src, gw);
 	struct ip *ip_header = (struct ip *)(&eth_header[1]);
-	uint16_t len = htons(sizeof(struct ip) + sizeof(struct tcphdr));
+	uint16_t len = htons(sizeof(struct ip) + ZMAP_TCP_SYNSCAN_TCP_HEADER_LEN);
 	make_ip_header(ip_header, IPPROTO_TCP, len);
 	struct tcphdr *tcp_header = (struct tcphdr *)(&ip_header[1]);
 	make_tcp_header(tcp_header, dst_port, TH_SYN);
+	set_mss_option(tcp_header);
 	return EXIT_SUCCESS;
 }
 
@@ -65,7 +67,7 @@ static int synscan_make_packet(void *buf, size_t *buf_len,
 	tcp_header->th_seq = tcp_seq;
 	tcp_header->th_sum = 0;
 	tcp_header->th_sum =
-	    tcp_checksum(sizeof(struct tcphdr), ip_header->ip_src.s_addr,
+	    tcp_checksum(ZMAP_TCP_SYNSCAN_TCP_HEADER_LEN, ip_header->ip_src.s_addr,
 			 ip_header->ip_dst.s_addr, tcp_header);
 
 	ip_header->ip_sum = 0;

diff --git a/src/probe_modules/packet.c b/src/probe_modules/packet.c
@@ -118,6 +118,24 @@ void make_tcp_header(struct tcphdr *tcp_header, port_h_t dest_port,
 	tcp_header->th_dport = htons(dest_port);
 }
 
+size_t set_mss_option(struct tcphdr *tcp_header) {
+	// This only sets MSS, which is a single-word option.
+	size_t header_size = tcp_header->th_off * 4;
+	uint8_t *base = (uint8_t *) tcp_header;
+	uint8_t *last_opt = (uint8_t*) base + header_size;
+
+	// TCP Option "header"
+	last_opt[0] = 2; // MSS
+	last_opt[1] = 4; // MSS is 4 bytes long
+
+	// Default Linux MSS is 1460, which 0x05b4
+	last_opt[2] = 0x05;
+	last_opt[3] = 0xb4;
+
+	tcp_header->th_off += 1;
+	return tcp_header->th_off*4;
+}
+
 void make_udp_header(struct udphdr *udp_header, port_h_t dest_port,
 		     uint16_t len)
 {

diff --git a/src/probe_modules/packet.h b/src/probe_modules/packet.h
@@ -28,6 +28,7 @@ void make_eth_header(struct ether_header *ethh, macaddr_t *src, macaddr_t *dst);
 
 void make_ip_header(struct ip *iph, uint8_t, uint16_t);
 void make_tcp_header(struct tcphdr *, port_h_t, uint16_t);
+size_t set_mss_option(struct tcphdr *tcp_header);
 void make_icmp_header(struct icmp *);
 void make_udp_header(struct udphdr *udp_header, port_h_t dest_port,
 		     uint16_t len);