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/*
* Copyright 2013 Google Inc.
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version 2
* of the License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
* 02110-1301, USA.
*/
/*
* Author: ncardwell@google.com (Neal Cardwell)
*
* Implementation for module for formatting TCP packets.
*/
#include "tcp_packet.h"
#include "ip_packet.h"
#include "tcp.h"
/* The full list of valid TCP bit flag characters */
static const char valid_tcp_flags[] = "FSRP.EWCU";
/* Are all the TCP flags in the given string valid? */
static bool is_tcp_flags_spec_valid(const char *flags, char **error)
{
const char *s;
for (s = flags; *s != '\0'; ++s) {
if (!strchr(valid_tcp_flags, *s)) {
asprintf(error, "Invalid TCP flag: '%c'", *s);
return false;
}
}
return true;
}
/* Parse tcpdump-style ASCII representation of flags to look for a flag */
static inline int is_tcp_flag_set(char flag, const char *flags)
{
return (strchr(flags, flag) != NULL) ? 1 : 0;
}
struct packet *new_tcp_packet(int address_family,
enum direction_t direction,
struct ip_info ip_info,
u16 src_port,
u16 dst_port,
const char *flags,
u32 start_sequence,
u16 tcp_payload_bytes,
u32 ack_sequence,
s32 window,
u16 urg_ptr,
const struct tcp_options *tcp_options,
char **error)
{
struct packet *packet = NULL; /* the newly-allocated result packet */
struct header *tcp_header = NULL; /* the TCP header info */
/* Calculate lengths in bytes of all sections of the packet */
const int ip_option_bytes = 0;
const int tcp_option_bytes = tcp_options ? tcp_options->length : 0;
const int ip_header_bytes = (ip_header_min_len(address_family) +
ip_option_bytes);
const int tcp_header_bytes = sizeof(struct tcp) + tcp_option_bytes;
const int ip_bytes =
ip_header_bytes + tcp_header_bytes + tcp_payload_bytes;
/* Sanity-check all the various lengths */
if (ip_option_bytes & 0x3) {
asprintf(error, "IP options are not padded correctly "
"to ensure IP header is a multiple of 4 bytes: "
"%d excess bytes", ip_option_bytes & 0x3);
return NULL;
}
if (tcp_option_bytes & 0x3) {
asprintf(error,
"TCP options are not padded correctly "
"to ensure TCP header is a multiple of 4 bytes: "
"%d excess bytes", tcp_option_bytes & 0x3);
return NULL;
}
assert((tcp_header_bytes & 0x3) == 0);
assert((ip_header_bytes & 0x3) == 0);
if (tcp_header_bytes > MAX_TCP_HEADER_BYTES) {
asprintf(error, "TCP header too large");
return NULL;
}
if (ip_bytes > MAX_TCP_DATAGRAM_BYTES) {
asprintf(error, "TCP segment too large");
return NULL;
}
if (!is_tcp_flags_spec_valid(flags, error))
return NULL;
/* Allocate and zero out a packet object of the desired size */
packet = packet_new(ip_bytes);
memset(packet->buffer, 0, ip_bytes);
packet->direction = direction;
packet->flags = 0;
packet->tos_chk = ip_info.tos.check;
/* Set IP header fields */
set_packet_ip_header(packet, address_family, ip_bytes,
ip_info.tos.value, ip_info.flow_label,
ip_info.ttl, IPPROTO_TCP);
tcp_header = packet_append_header(packet, HEADER_TCP, tcp_header_bytes);
tcp_header->total_bytes = tcp_header_bytes + tcp_payload_bytes;
/* Find the start of TCP sections of the packet */
packet->tcp = (struct tcp *) (ip_start(packet) + ip_header_bytes);
u8 *tcp_option_start = (u8 *) (packet->tcp + 1);
/* Set TCP header fields */
packet->tcp->src_port = htons(src_port);
packet->tcp->dst_port = htons(dst_port);
packet->tcp->seq = htonl(start_sequence);
packet->tcp->ack_seq = htonl(ack_sequence);
packet->tcp->doff = tcp_header_bytes / 4;
if (window == -1) {
if (direction == DIRECTION_INBOUND) {
asprintf(error, "window must be specified"
" for inbound packets");
return NULL;
}
packet->tcp->window = 0;
packet->flags |= FLAG_WIN_NOCHECK;
} else {
packet->tcp->window = htons(window);
}
packet->tcp->check = 0;
packet->tcp->urg_ptr = htons(urg_ptr);
packet->tcp->fin = is_tcp_flag_set('F', flags);
packet->tcp->syn = is_tcp_flag_set('S', flags);
packet->tcp->rst = is_tcp_flag_set('R', flags);
packet->tcp->psh = is_tcp_flag_set('P', flags);
packet->tcp->ack = is_tcp_flag_set('.', flags);
packet->tcp->urg = is_tcp_flag_set('U', flags);
packet->tcp->ece = is_tcp_flag_set('E', flags);
packet->tcp->cwr = is_tcp_flag_set('W', flags);
if (tcp_options == NULL) {
packet->flags |= FLAG_OPTIONS_NOCHECK;
} else if (tcp_options->length > 0) {
/* Copy TCP options into packet */
memcpy(tcp_option_start, tcp_options->data,
tcp_options->length);
}
packet->ip_bytes = ip_bytes;
return packet;
}
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