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Add in TCP MSS and TV values to get through some firewalls #14

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Add in TCP MSS and TV values to get through some firewalls #14

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58 changes: 55 additions & 3 deletions probe.c
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Original file line number Diff line number Diff line change
Expand Up @@ -128,6 +128,39 @@ u_short allocateid(void)
return ids[n++];
}

static int set_options_payload(uint8_t *payload,uint32_t timeval,uint16_t mss)
{
int len = 0;

//set mss value
payload[len++] = 0x02;
payload[len++] = 0x04;
payload[len++] = (uint8_t)((mss >> 8) & 0xFF);
payload[len++] = (uint8_t)(mss & 0xFF);

//set SACK
payload[len++] = 0x04;
payload[len++] = 0x02;

//set timeval
payload[len++] = 0x08;
payload[len++] = 0x0a;
payload[len++] = (uint8_t)((timeval >> 24) & 0xFF);
payload[len++] = (uint8_t)((timeval >> 16) & 0xFF);
payload[len++] = (uint8_t)((timeval >> 8) & 0xFF);
payload[len++] = (uint8_t)(timeval & 0xFF);
len+=4;

//set NOP
payload[len++] = 0x01;

//set Window scale
payload[len++] = 0x03;
payload[len++] = 0x03;
payload[len] = 0x07;
return 0;
}

/*
* Sends out a TCP SYN packet with the specified TTL, and returns a
* proberecord structure describing the packet sent, so we know what
Expand All @@ -139,11 +172,12 @@ void probe(proberecord *record, int ttl, int q)
{
static u_char *payload;
int i, size, ret;
uint8_t opt_payload[20] = {0x00};

#if (LIBNET_API_VERSION < 110)
static u_char *buf;
#else
static libnet_ptag_t ip_tag, tcp_tag, data_tag;
static libnet_ptag_t ip_tag, tcp_tag, data_tag, tcp_op_tag;
#endif

size = LIBNET_IPV4_H + LIBNET_TCP_H + o_pktlen;
Expand Down Expand Up @@ -205,6 +239,13 @@ void probe(proberecord *record, int ttl, int q)
NULL, /* data */
0, /* datasize? */
buf); /* buffer */

set_options_payload(opt_payload,1450435047,1460);

libnet_build_tcp_options(
opt_payload,
20,
buf);

libnet_build_tcp(
record->src_prt, /* source port */
Expand All @@ -216,7 +257,7 @@ void probe(proberecord *record, int ttl, int q)
(o_ack ? TH_ACK : 0) |
(o_ecn ? TH_CWR|TH_ECN : 0), /* control */

0, /* window */
5840, /* window */
0, /* urgent? */
payload, /* data */
o_pktlen, /* datasize */
Expand All @@ -235,6 +276,17 @@ void probe(proberecord *record, int ttl, int q)

if (data_tag < 0)
fatal("Can't add payload: %s\n", libnet_geterror(libnet_context));

set_options_payload(opt_payload,1450435047,1460);

tcp_op_tag = libnet_build_tcp_options(
opt_payload,
20,
libnet_context,
0);

if (tcp_op_tag < 0)
fatal("Can't build TCP options header: %s\n", libnet_geterror(libnet_context));

/* Add the TCP header */
tcp_tag = libnet_build_tcp(
Expand All @@ -247,7 +299,7 @@ void probe(proberecord *record, int ttl, int q)
(o_ack ? TH_ACK : 0) |
(o_ecn ? TH_CWR|TH_ECN : 0), /* control */

0, /* window */
5840, /* window */
0, /* checksum TBD */
0, /* urgent? */
LIBNET_TCP_H + o_pktlen, /* TCP PDU size */
Expand Down