slitheen/relay_station/flow.c

/* Name: flow.c
 * Author: Cecylia Bocovich <cbocovic@uwaterloo.ca>
 *
 * This file contains functions for manipulating tagged flows. 
 *
 * The check_flow function checks to see whether the packet that is currently
 * being processed belongs to a tagged flow.
 *
 * The add_packet function attempts to reconstruct a TLS record containing
 * handshake data from the contents of previously received packets
 *
 * The update_flow function processes TLS handshake messages and calls the
 * necessary functions from crypto.c when a message has been received.
 *
 * There are several functions associated with the resumption of previously
 * tagged TLS sessions.
 *
 */

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <pthread.h>
#include <errno.h>
#include <semaphore.h>

#include "flow.h"
#include "crypto.h"
#include "slitheen.h"
#include "relay.h"
#include "util.h"

static flow_table *table;
static session_cache *sessions;
data_queue *downstream_queue;
client_table *clients;

sem_t flow_table_lock;

/* Initialize the table of tagged flows */
int init_tables(void) {

	table = emalloc(sizeof(flow_table));
	table->first_entry = NULL;
	table->len = 0;

	sem_init(&flow_table_lock, 0, 1);

	clients = emalloc(sizeof(client_table));
	clients->first = NULL;
	printf("initialized downstream queue\n");

	return 0;
}


/* Add a new flow to the tagged flow table */
flow *add_flow(struct packet_info *info) {
	flow_entry *entry = emalloc(sizeof(flow_entry));

	flow *new_flow = emalloc(sizeof(flow));

	entry->f = new_flow;
	entry->next = NULL;

	new_flow->src_ip = info->ip_hdr->src;
	new_flow->dst_ip = info->ip_hdr->dst;
	new_flow->src_port = info->tcp_hdr->src_port;
	new_flow->dst_port = info->tcp_hdr->dst_port;

	new_flow->upstream_app_data = emalloc(sizeof(app_data_queue));
	new_flow->upstream_app_data->first_packet = NULL;
	new_flow->downstream_app_data = emalloc(sizeof(app_data_queue));
	new_flow->downstream_app_data->first_packet = NULL;

	new_flow->upstream_seq_num = ntohl(info->tcp_hdr->sequence_num);
	new_flow->downstream_seq_num = ntohl(info->tcp_hdr->ack_num);

	new_flow->streams=NULL;
	new_flow->downstream_queue=NULL;

	sem_init(&(new_flow->flow_lock), 0, 1);
	new_flow->state = TLS_CLNT_HELLO;
	new_flow->in_encrypted = 0;
	new_flow->out_encrypted = 0;
	new_flow->application = 0;
	new_flow->resume_session = 0;
	new_flow->current_session = NULL;

	new_flow->us_packet_chain = emalloc(sizeof(packet_chain));
	
	new_flow->us_packet_chain->expected_seq_num = ntohl(info->tcp_hdr->sequence_num);
	new_flow->us_packet_chain->record_len = 0;
	new_flow->us_packet_chain->remaining_record_len = 0;
	new_flow->us_packet_chain->first_packet = NULL;
	new_flow->ds_packet_chain = emalloc(sizeof(packet_chain));

	new_flow->ds_packet_chain->expected_seq_num = ntohl(info->tcp_hdr->ack_num);
	new_flow->ds_packet_chain->record_len = 0;
	new_flow->ds_packet_chain->remaining_record_len = 0;
	new_flow->ds_packet_chain->first_packet = NULL;
	sem_init(&(new_flow->packet_chain_lock), 0, 1);
	
	new_flow->upstream_queue = NULL;
	new_flow->upstream_remaining = 0;
	sem_init(&(new_flow->upstream_queue_lock), 0, 1);
	new_flow->outbox = NULL;
	new_flow->outbox_len = 0;
	new_flow->outbox_offset = 0;
	new_flow->partial_record_header = NULL;
	new_flow->partial_record_header_len = 0;
	new_flow->remaining_record_len = 0;
	new_flow->remaining_response_len = 0;
	new_flow->httpstate = PARSE_HEADER;
	new_flow->replace_response = 0;

	new_flow->ecdh = NULL;


	new_flow->finish_md_ctx = EVP_MD_CTX_create();
	const EVP_MD *md = EVP_sha384();
	EVP_DigestInit_ex(new_flow->finish_md_ctx, md, NULL);

	new_flow->clnt_read_ctx = NULL;
	new_flow->clnt_write_ctx = NULL;
	new_flow->srvr_read_ctx = NULL;
	new_flow->srvr_write_ctx = NULL;

	memset(new_flow->read_seq, 0, 8);
	memset(new_flow->write_seq, 0, 8);


	sem_wait(&flow_table_lock);
	flow_entry *last = table->first_entry;
	if(last == NULL){
		table->first_entry = entry;
	} else {
		for(int i=0; i< table->len-1; i++){
			last = last->next;
		}
		last->next = entry;
	}
	table->len ++;
	sem_post(&flow_table_lock);

	return new_flow;

}

/** Observes TLS handshake messages and updates the state of
 *  the flow
 *
 *  Inputs:
 *  	f: the tagged flow
 *  	record: a complete TLS record
 *
 *  Output:
 *  	0 on success, 1 on failure
 */
int update_flow(flow *f, uint8_t *record, uint8_t incoming) {
	const struct record_header *record_hdr;
	const struct handshake_header *handshake_hdr;
	uint8_t *p;

	record_hdr = (struct record_header*) record;
	int record_len;

	record_len = RECORD_LEN(record_hdr)+RECORD_HEADER_LEN;

	switch(record_hdr->type){
		case HS:
			p = record;
			p += RECORD_HEADER_LEN;

			if((incoming && f->in_encrypted) || (!incoming && f->out_encrypted)){
#ifdef DEBUG_HS
				printf("Decrypting finished (%d bytes) (%x:%d -> %x:%d)\n", record_len - RECORD_HEADER_LEN, f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
				printf("Finished ciphertext:\n");
				for(int i=0; i< record_len; i++){
					printf("%02x ", record[i]);
				}
				printf("\n");
#endif
				int32_t n = encrypt(f, p, p, record_len - RECORD_HEADER_LEN, incoming, 0x16, 0);
				if(n<=0){
					printf("Error decrypting finished  (%x:%d -> %x:%d)\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
				}
#ifdef DEBUG_HS
				printf("Finished decrypted: (%x:%d -> %x:%d)\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
#endif
				p += EVP_GCM_TLS_EXPLICIT_IV_LEN;
				
#ifdef DEBUG_HS
				printf("record:\n");
				for(int i=0; i< n; i++){
					printf("%02x ", p[i]);
				}
				printf("\n");
#endif
				if(p[0] != 0x14){
					p[0] = 0x20; //trigger error
				}

				if(incoming){
					f->in_encrypted = 2;
				} else {
					f->out_encrypted = 2;
					update_context(f, p, n, incoming, 0x16, 0);
				}

			}
			handshake_hdr = (struct handshake_header*) p;
			f->state = handshake_hdr->type;

			switch(f->state){
				case TLS_CLNT_HELLO: 
#ifdef DEBUG_HS
					printf("Received tagged client hello (%x:%d -> %x:%d)\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
#endif
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish has with CLNT_HELLO msg\n");
						remove_flow(f);
						goto err;
					}
					if(check_session(f, p, HANDSHAKE_MESSAGE_LEN(handshake_hdr))){
						fprintf(stderr, "Error checking session, might cause problems\n");
					}

					break;
				case TLS_SERV_HELLO:
#ifdef DEBUG_HS
					printf("Received server hello (%x:%d -> %x:%d)\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
#endif
					if(f->resume_session){
						if(verify_session_id(f,p)){
							fprintf(stderr, "Failed to verify session id\n");
						}
					} else {
						if(save_session_id(f,p)){
							fprintf(stderr, "Failed to save session id\n");
						}
					}
					if(extract_server_random(f, p)){
						fprintf(stderr, "Failed to extract server random nonce\n");
						remove_flow(f);
						goto err;
					}

					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with SRVR_HELLO msg\n");
						remove_flow(f);
						goto err;
					}
					break;
				case TLS_NEW_SESS:
#ifdef DEBUG_HS
					printf("Received new session\n");
#endif
					if(save_session_ticket(f, p, HANDSHAKE_MESSAGE_LEN(handshake_hdr))){
						fprintf(stderr, "Failed to save session ticket\n");
					}
					
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with NEW_SESS msg\n");
						remove_flow(f);
						goto err;
					}
					
					break;
				case TLS_CERT:
#ifdef DEBUG_HS
					printf("Received cert\n");
#endif
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with CERT msg\n");
						remove_flow(f);
						goto err;
					}

					break;
				case TLS_SRVR_KEYEX:
#ifdef DEBUG_HS
					printf("Received server keyex\n");
#endif
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with SRVR_KEYEX msg\n");
						remove_flow(f);
						goto err;
					}

					if(extract_parameters(f, p)){
						printf("Error extracting params\n");
						remove_flow(f);
						goto err;
					}

					if(compute_master_secret(f)){
						printf("Error computing master secret\n");
						remove_flow(f);
						goto err;

					}

					break;

				case TLS_CERT_REQ:

					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with CERT_REQ msg\n");
						remove_flow(f);
						goto err;
					}

					break;
				case TLS_SRVR_HELLO_DONE:
#ifdef DEBUG_HS
					printf("Received server hello done\n");
#endif
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with HELLO_DONE msg\n");
						remove_flow(f);
						goto err;
					}

					break;
				case TLS_CERT_VERIFY:
#ifdef DEBUG_HS
					printf("received cert verify\n");
#endif
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with CERT_VERIFY msg\n");
						remove_flow(f);
						goto err;
					}

					break;

				case TLS_CLNT_KEYEX:
#ifdef DEBUG_HS
					printf("Received client key exchange\n");
#endif
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with CLNT_KEYEX msg\n");
						remove_flow(f);
						goto err;
					}

					break;
				case TLS_FINISHED:
#ifdef DEBUG_HS
					printf("Received finished (%d) (%x:%d -> %x:%d)\n", incoming, f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
#endif
					verify_finish_hash(f,p, incoming);
					
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with FINISHED msg\n");
						remove_flow(f);
						goto err;
					}

					//re-encrypt finished message
					if(incoming){
						//revert the sequence number
						memset(f->read_seq, 0, 8);

						int32_t n =  encrypt(f, record+RECORD_HEADER_LEN, record+RECORD_HEADER_LEN, record_len - (RECORD_HEADER_LEN+16), incoming, 0x16, 1);

#ifdef HS_DEBUG
						printf("New finished ciphertext:\n");
						for(int i=0; i< record_len; i++){
							printf("%02x ", record[i]);
						}
						printf("\n");
#endif

						if(n<=0){
							printf("Error re-encrypting finished  (%x:%d -> %x:%d)\n", f->src_ip.s_addr, ntohs(f->src_port),
									f->dst_ip.s_addr, ntohs(f->dst_port));
						}
					}

					if((f->in_encrypted == 2) && (f->out_encrypted == 2)){
						printf("Handshake complete!\n");
						f->application = 1;
					}

					break;
				default:
					printf("Error? (%x:%d -> %x:%d)...\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
					remove_flow(f);
					goto err;
					break;
			}
			break;
		case APP:
			printf("Application Data (%x:%d -> %x:%d)...\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
			break;
		case CCS:
#ifdef DEBUG_HS
			printf("CCS (%x:%d -> %x:%d) \n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
#endif
			/*Initialize ciphers */
			if ((!f->in_encrypted) && (!f->out_encrypted)){
				if(init_ciphers(f)){
					fprintf(stderr, "Failed to initialize ciphers\n");
					remove_flow(f);
					goto err;
				}
			}

			if(incoming){
				f->in_encrypted = 1;
			} else {
				f->out_encrypted = 1;
			}
			
			break;
		case ALERT:
			p = record;
			p += RECORD_HEADER_LEN;
			if(((incoming) && (f->in_encrypted > 0)) || ((!incoming) && (f->out_encrypted > 0))){
				encrypt(f, p, p, record_len - RECORD_HEADER_LEN, incoming, 0x16, 0);
				p += EVP_GCM_TLS_EXPLICIT_IV_LEN;
			}
			printf("Alert (%x:%d -> %x:%d) %02x %02x \n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port), p[0], p[1]);
			fflush(stdout);
			break;
		case HB:
			printf("Heartbeat\n");
			break;
		default:
			printf("Error: Not a Record (%x:%d -> %x:%d)\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
			fflush(stdout);
			goto err;
	}
	return 0;

err:
	return 1;
}

/** Removes the tagged flow from the flow table: happens when
 *  the station receives a TCP RST or FIN packet
 *
 *  Input:
 *  	index: the index into the flow table of the tagged flow
 *
 *  Output:
 *  	0 on success, 1 on failure
 */
int remove_flow(flow *f) {

	//Empty application data queues
	packet *tmp = f->upstream_app_data->first_packet;
	while(tmp != NULL){
		f->upstream_app_data->first_packet = tmp->next;
		free(tmp->data);
		free(tmp);
		tmp = f->upstream_app_data->first_packet;
	}

	tmp = f->downstream_app_data->first_packet;
	while(tmp != NULL){
		f->downstream_app_data->first_packet = tmp->next;
		free(tmp->data);
		free(tmp);
		tmp = f->downstream_app_data->first_packet;
	}

	//Clean up cipher ctxs
	EVP_MD_CTX_cleanup(f->finish_md_ctx);
	if(f->finish_md_ctx != NULL){
		EVP_MD_CTX_destroy(f->finish_md_ctx);
	}
	if(f->clnt_read_ctx != NULL){
		EVP_CIPHER_CTX_cleanup(f->clnt_read_ctx);
		OPENSSL_free(f->clnt_read_ctx);
		f->clnt_read_ctx = NULL;
	}
	if(f->clnt_write_ctx != NULL){
		EVP_CIPHER_CTX_cleanup(f->clnt_write_ctx);
		OPENSSL_free(f->clnt_write_ctx);
		f->clnt_write_ctx = NULL;
	}
	if(f->srvr_read_ctx != NULL){
		EVP_CIPHER_CTX_free(f->srvr_read_ctx);
	}
	if(f->srvr_write_ctx != NULL){
		EVP_CIPHER_CTX_free(f->srvr_write_ctx);
	}

	if(f->ecdh != NULL){
		EC_KEY_free(f->ecdh);
	}

	if(f->resume_session == 1){
		if(f->current_session->session_ticket != NULL){
			free(f->current_session->session_ticket);
		}
		free(f->current_session);
	}

	if(f->ds_packet_chain != NULL){
		packet *tmp = f->ds_packet_chain->first_packet;
		while(tmp != NULL){
			f->ds_packet_chain->first_packet = tmp->next;
			printf("Freed data %p\n", tmp->data);
			printf("Freed packet %p\n", tmp);
			free(tmp->data);
			free(tmp);
			tmp = f->ds_packet_chain->first_packet;
		}
	}
	free(f->ds_packet_chain);

	if(f->us_packet_chain != NULL){
		packet *tmp = f->us_packet_chain->first_packet;
		while(tmp != NULL){
			f->us_packet_chain->first_packet = tmp->next;
			printf("Freed data %p\n", tmp->data);
			printf("Freed packet %p\n", tmp);
			free(tmp->data);
			free(tmp);
			tmp = f->us_packet_chain->first_packet;
		}
	}
	free(f->us_packet_chain);
		
	if(f->upstream_queue != NULL){
		queue_block *tmp = f->upstream_queue;
		while(tmp != NULL){
			f->upstream_queue = tmp->next;
			printf("Freed data %p\n", tmp->data);
			printf("Freed packet %p\n", tmp);
			free(tmp->data);
			free(tmp);
			tmp = f->upstream_queue;
		}
	}

	sem_wait(&flow_table_lock);
	flow_entry *entry = table->first_entry;
	if(entry->f == f){
		table->first_entry = entry->next;
		free(entry->f);
		free(entry);
		printf("flow removed!\n");
		fflush(stdout);
		table->len --;
	} else {

		flow_entry *next;
		for(int i=0; i< table->len; i++){
			if(entry->next != NULL){
				next = entry->next;
			} else {
				printf("Flow not in table\n");
				break;
			}

			if(next->f == f){
				entry->next = next->next;
				free(next->f);
				free(next);
				printf("flow removed!\n");
				table->len --;
				break;
			}

			entry = next;
		}
	}
	sem_post(&flow_table_lock);

	return 1;
}

/** Returns the index of a flow in the flow table if
 *  it exists, returns 0 if it is not present.
 *
 *  Inputs:
 *  	observed: details for the observed flow
 *
 *  Output:
 *  	index of flow in table or -1 if it doesn't exist
	new_flow->src_ip = info->ip_hdr->src;
	new_flow->dst_ip = info->ip_hdr->dst;
	new_flow->src_port = info->tcp_hdr->src_port;
	new_flow->dst_port = info->tcp_hdr->dst_port;
 */
flow *check_flow(struct packet_info *info){
	/* Loop through flows in table and see if it exists */
	int i;
	flow_entry *entry = table->first_entry;
	flow *candidate;
	flow *found = NULL;
	if(entry == NULL)
		return NULL;

	sem_wait(&flow_table_lock);
	/* Check first in this direction */
	for(i=0; i<table->len; i++){
		if(entry == NULL){
			printf("Error: entry is null\n");
			break;
		}
		candidate = entry->f;
		if(candidate->src_ip.s_addr == info->ip_hdr->src.s_addr){
			if(candidate->dst_ip.s_addr == info->ip_hdr->dst.s_addr){
				if(candidate->src_port == info->tcp_hdr->src_port){
					if(candidate->dst_port == info->tcp_hdr->dst_port){
						found = candidate;
					}
				}
			}
		}
		entry = entry->next;
	}


	entry = table->first_entry;
	/* Then in the other direction */
	for(i=0; i<table->len; i++){
		if(entry == NULL){
			printf("Error: entry is null\n");
			break;
		}
		candidate = entry->f;
		if(candidate->src_ip.s_addr == info->ip_hdr->dst.s_addr){
			if(candidate->dst_ip.s_addr == info->ip_hdr->src.s_addr){
				if(candidate->src_port == info->tcp_hdr->dst_port){
					if(candidate->dst_port == info->tcp_hdr->src_port){
						found = candidate;
					}
				}
			}
		}
		entry = entry->next;
	}
	sem_post(&flow_table_lock);

	return found;
}

int init_session_cache(void){
	sessions = emalloc(sizeof(session_cache));

	sessions->length = 0;
	sessions->first_session = NULL;

	return 0;
}

/** Called from ServerHello, verifies that the session id returned matches
 *  the session id requested from the client hello
 *
 *  Input:
 *  	f: the tagged flow
 *  	hs: a pointer to the ServerHello message
 *
 *  Output:
 *  	0 if success, 1 if failed
 */
int verify_session_id(flow *f, uint8_t *hs){
	
	//increment pointer to point to sessionid
	uint8_t *p = hs + HANDSHAKE_HEADER_LEN;
	p += 2; //skip version
	p += SSL3_RANDOM_SIZE; //skip random

	uint8_t id_len = (uint8_t) p[0];
	p ++;
	
	//check to see if it matches flow's session id set by ClientHello
	if(f->current_session != NULL && f->current_session->session_id_len > 0 && !memcmp(f->current_session->session_id, p, id_len)){
		//if it matched, update flow with master secret :D
#ifdef DEBUG_HS
		printf("Session id matched!\n");
		printf("First session id (%p->%p):", sessions, sessions->first_session);
#endif
		session *last = sessions->first_session;
		int found = 0;
		for(int i=0; ((i<sessions->length) && (!found)); i++){
#ifdef DEBUG_HS
			printf("Checking saved session id: ");
			for (int j=0; j< last->session_id_len; j++){
				printf("%02x ", last->session_id[j]);
			}
			printf("\n");
#endif
			if(!memcmp(last->session_id, f->current_session->session_id, id_len)){
				memcpy(f->master_secret, last->master_secret, SSL3_MASTER_SECRET_SIZE);
				found = 1;
			}
			last = last->next;
		}
		if((!found) && (f->current_session->session_ticket_len > 0)){
			last = sessions->first_session;
			for(int i=0; ((i<sessions->length) && (!found)); i++){
				if(last->session_ticket_len == f->current_session->session_ticket_len){
				if(!memcmp(last->session_ticket, f->current_session->session_ticket, f->current_session->session_ticket_len)){
					memcpy(f->master_secret, last->master_secret, SSL3_MASTER_SECRET_SIZE);
					found = 1;
#ifdef DEBUG_HS
					printf("Found new session ticket (%x:%d -> %x:%d)\n", f->src_ip.s_addr, f->src_port, f->dst_ip.s_addr, f->dst_port);
					for(int i=0; i< last->session_ticket_len; i++){
						printf("%02x ", last->session_ticket[i]);
					}
					printf("\n");
#endif
				}
				}
				last = last->next;
			}
		}

	} else if (f->current_session != NULL &&  f->current_session->session_id_len > 0){
		//check to see if server's hello extension matches the ticket
		save_session_id(f, p);
	}

	return 0;

}

/* Called from ClientHello. Checks to see if the session id len is > 0. If so,
 * saves sessionid for later verification. Also checks to see if a session
 * ticket is included as an extension.
 *
 *  Input:
 *  	f: the tagged flow
 *  	hs: a pointer to the ServerHello message
 *
 *  Output:
 *  	0 if success, 1 if failed
 */
int check_session(flow *f, uint8_t *hs, uint32_t len){

	uint8_t *p = hs + HANDSHAKE_HEADER_LEN;
	p += 2; //skip version
	p += SSL3_RANDOM_SIZE; //skip random

	session *new_session = emalloc(sizeof(session));
	new_session->session_id_len = (uint8_t) p[0];
	new_session->session_ticket_len = 0;
	p  ++;

	if(new_session->session_id_len > 0){
		f->resume_session = 1;
		memcpy(new_session->session_id, p, new_session->session_id_len);
		new_session->next = NULL;
#ifdef DEBUG_HS
		printf("Requested new session (%x:%d -> %x:%d)\n", f->src_ip.s_addr, f->src_port, f->dst_ip.s_addr, f->dst_port);
		printf("session id: \n");
		for(int i=0; i< new_session->session_id_len; i++){
			printf("%02x ", p[i]);
		}
		printf("\n");
#endif

		f->current_session = new_session;
	}

	p += new_session->session_id_len;
	
	//check to see if there is a session ticket included

	//skip to extensions
	uint16_t ciphersuite_len = (p[0] << 8) + p[1];
	p += 2 + ciphersuite_len;
	uint8_t compress_meth_len = p[0];
	p += 1 + compress_meth_len;
	
	//search for SessionTicket TLS extension
	if(2 + SSL3_RANDOM_SIZE + new_session->session_id_len + 1 + 2 + ciphersuite_len + 1 + compress_meth_len > len){
		//no extension
		if(f->current_session == NULL)
			free(new_session);
		return 0;
	}
	uint16_t extensions_len = (p[0] << 8) + p[1];
	p += 2;
	while(extensions_len > 0){
		uint16_t type = (p[0] << 8) + p[1];
		p += 2;
		uint16_t ext_len = (p[0] << 8) + p[1];
		p += 2;
		if(type == 0x23){
			if(ext_len > 0){
				f->resume_session = 1;
				new_session->session_ticket_len = ext_len;
				new_session->session_ticket = emalloc(ext_len);
				memcpy(new_session->session_ticket, p, ext_len);
				f->current_session = new_session;
			}
		}
		p += ext_len;
		extensions_len -= (4 + ext_len);
	}

	if(!f->resume_session){
		//see if a ticket is incuded
		free(new_session);
	}

	return 0;
}
	

/* Called from ServerHello during full handshake. Adds the session id to the
 * cache for later resumptions
 *
 *  Input:
 *  	f: the tagged flow
 *  	hs: a pointer to the ServerHello message
 *
 *  Output:
 *  	0 if success, 1 if failed
 */
int save_session_id(flow *f, uint8_t *hs){

	//increment pointer to point to sessionid
	uint8_t *p = hs + HANDSHAKE_HEADER_LEN;
	p += 2; //skip version
	p += SSL3_RANDOM_SIZE; //skip random
	
	session *new_session = emalloc(sizeof(session));

	new_session->session_id_len = (uint8_t) p[0];
	if(new_session->session_id_len <= 0){
		//if this value is zero, the session is non-resumable or the
		//server will issue a NewSessionTicket handshake message
		free(new_session);
		return 0;
	}
	p++;
	memcpy(new_session->session_id, p, new_session->session_id_len);
	new_session->next = NULL;

	if(f->current_session != NULL){
		free(f->current_session);
	}
	f->resume_session = 0;
	f->current_session = new_session;

	if(sessions->first_session == NULL){
		sessions->first_session = new_session;
		printf("First session id (%p->%p):", sessions, sessions->first_session);
		for(int i=0; i< new_session->session_id_len; i++){
			printf(" %02x", sessions->first_session->session_id[i]);
		}
		printf("\n");
	} else {
		session *last = sessions->first_session;

		for(int i=0; i< sessions->length -1; i++){
			if(last == NULL){
				printf("UH OH: last is null?\n");
				fflush(stdout);
			}
			last = last->next;
		}
		last->next = new_session;
	}

	sessions->length ++;

	printf("Saved session id:");
	for(int i=0; i< new_session->session_id_len; i++){
		printf(" %02x", new_session->session_id[i]);
	}
	printf("\n");

	printf("THERE ARE NOW %d saved sessions\n", sessions->length);

	return 0;

}

/* Called from NewSessionTicket. Adds the session ticket to the
 * cache for later resumptions
 *
 *  Input:
 *  	f: the tagged flow
 *  	hs: a pointer to the ServerHello message
 *
 *  Output:
 *  	0 if success, 1 if failed
 */
int save_session_ticket(flow *f, uint8_t *hs, uint32_t len){
#ifdef DEBUG_HS
	printf("TICKET HDR:");
	for(int i=0; i< HANDSHAKE_HEADER_LEN; i++){
		printf("%02x ", hs[i]);
	}
	printf("\n");
#endif
	uint8_t *p = hs + HANDSHAKE_HEADER_LEN;
	p += 4; //skip lifetime TODO: add to session struct
	session *new_session = ecalloc(1, sizeof(session));

	new_session->session_id_len = 0;
	
	new_session->session_ticket_len = (p[0] << 8) + p[1];
	p += 2;

	uint8_t *ticket = emalloc(new_session->session_ticket_len);

	memcpy(ticket, p, new_session->session_ticket_len);
	new_session->session_ticket = ticket;
	memcpy(new_session->master_secret, f->master_secret, SSL3_MASTER_SECRET_SIZE);

	if(sessions->first_session == NULL){
		sessions->first_session = new_session;
	} else {
		session *last = sessions->first_session;

		for(int i=0; i< (sessions->length-1); i++){
			if(last == NULL){
				printf("UH OH: last is null?\n");
				fflush(stdout);
			}
			last = last->next;
		}
		last->next = new_session;
	}

	sessions->length ++;

#ifdef DEBUG_HS
	printf("Saved session ticket:");
	for(int i=0; i< new_session->session_ticket_len; i++){
		printf(" %02x", p[i]);
	}
	printf("\n");
	fflush(stdout);

	printf("Saved session master secret:");
	for(int i=0; i< SSL3_MASTER_SECRET_SIZE; i++){
		printf(" %02x", new_session->master_secret[i]);
	}
	printf("\n");
	fflush(stdout);

	printf("THERE ARE NOW %d saved sessions\n", sessions->length);
	fflush(stdout);
#endif

	return 0;
}

/* Adds a packet the flow's packet chain. If it can complete a record, gives
 * this record to update_flow */
int add_packet(flow *f, struct packet_info *info){
	if (info->tcp_hdr == NULL || info->app_data_len <= 0){
		return 0;
	}

	packet *new_packet = emalloc(sizeof(packet));

	new_packet->seq_num = ntohl(info->tcp_hdr->sequence_num);
	new_packet->len = info->app_data_len;

	uint8_t *packet_data = emalloc(new_packet->len);
	memcpy(packet_data, info->app_data, new_packet->len);

	new_packet->data = packet_data;
	new_packet->next = NULL;
	uint8_t incoming = (info->ip_hdr->src.s_addr == f->src_ip.s_addr) ? 0 : 1;
	packet_chain *chain = 
		(info->ip_hdr->src.s_addr == f->src_ip.s_addr) ? f->us_packet_chain : f->ds_packet_chain;

	if(new_packet->seq_num < chain->expected_seq_num){
		//see if this packet contains any data we are missing
		printf("Received replayed packet O.o\n");
		free(new_packet->data);
		free(new_packet);

	} else {//new_packet->seq_num >= chain->expected_seq_num
	
		//Find appropriate place in chain
		packet *previous = NULL;
		packet *next = chain->first_packet;
		while(next != NULL && (next->seq_num <= new_packet->seq_num)){
			previous = next;
			next = next->next;
		}

		//place packet after current
		if(previous == NULL){
			//goes at the beginning of chain
			new_packet->next = chain->first_packet;
			chain->first_packet = new_packet;

			//if this is a new record, find lengths
			if(new_packet->seq_num == chain->expected_seq_num){
				const struct record_header *record_hdr = (struct record_header *) new_packet->data;
				chain->record_len = RECORD_LEN(record_hdr)+RECORD_HEADER_LEN;
				chain->remaining_record_len = chain->record_len;
			}
			
		} else {
			new_packet->next = next;
			previous->next = new_packet;
		}

		if(new_packet->seq_num == chain->expected_seq_num){
			chain->expected_seq_num += new_packet->len;

			uint32_t record_offset = 0; //offset into record for updating info with any changes
			uint32_t info_offset = 0; //offset into info for updating with changes
			uint32_t info_len = 0; //number of bytes that possibly changed

			//while there is still data left:
			uint32_t available_data = new_packet->len;

			while(available_data > 0){

				//if full record, give to update_flow
				if(chain->remaining_record_len <= new_packet->len){
					chain->remaining_record_len = 0;
					uint8_t *record = emalloc(chain->record_len);
					uint32_t record_len = chain->record_len;
					uint32_t tmp_len = chain->record_len;

					packet *next = chain->first_packet;
					while(tmp_len > 0){
						if(tmp_len >= next->len){
							memcpy(record+chain->record_len - tmp_len, next->data, next->len);
							if(next == new_packet){
								new_packet = NULL;
								record_offset = chain->record_len - tmp_len;
								info_len = next->len;
							}

							tmp_len -= next->len;
							chain->first_packet = next->next;
							free(next->data);
							free(next);
							next = chain->first_packet;
							available_data = 0;
						} else {
							memcpy(record+chain->record_len - tmp_len, next->data, tmp_len);
							if(next == new_packet){
								record_offset = chain->record_len - tmp_len;
								info_len = tmp_len;
							}

							memmove(next->data, next->data+tmp_len, next->len - tmp_len);
							next->len -= tmp_len;
							available_data -= tmp_len;
							tmp_len = 0;
							//this is going to be a new record
							const struct record_header *record_hdr = (struct record_header *) next->data;
							chain->record_len = RECORD_LEN(record_hdr)+RECORD_HEADER_LEN;
							chain->remaining_record_len = chain->record_len;
#ifdef DEBUG
							printf("Found record of type %d\n", record_hdr->type);
							fflush(stdout);
#endif

						}
					}
					//if handshake is complete, send to relay code
					//TODO: check to see if this code needs to replace info->data
					if(f->application == 1){
						//update packet info and send to replace_packet
						printf("Packet contains application data!\n");
						struct packet_info *copy_info = copy_packet_info(info);
						copy_info->app_data = record;
						copy_info->app_data_len = record_len;
						replace_packet(f, copy_info);
						free(copy_info->app_data);
						free(copy_info);
					} else {
						update_flow(f, record, incoming);

						//check to see if last finished message received
						if(f->application ==1){

#ifdef DEBUG
							printf("Replacing info->data with finished message (%d bytes).\n", info_len);

							printf("Previous bytes:\n");
							for(int i=0; i<info_len; i++){
								printf("%02x ", info->app_data[info_offset+i]);
							}
							printf("\n");
							printf("New bytes:\n");
							for(int i=0; i<info_len; i++){
								printf("%02x ", record[record_offset+i]);
							}
							printf("\n");
							printf("SLITHEEN: Previous packet contents:\n");
							for(int i=0; i< info->app_data_len; i++){
								printf("%02x ", info->app_data[i]);
							}
							printf("\n");
#endif
							memcpy(info->app_data+info_offset, record+record_offset, info_len);
#ifdef DEBUG
							printf("SLITHEEN: Current packet contents:\n");
							for(int i=0; i< info->app_data_len; i++){
								printf("%02x ", info->app_data[i]);
							}
							printf("\n");
#endif

							//update TCP checksum
							tcp_checksum(info);
						}
						free(record);

						if(new_packet != NULL){
							info_offset += info_len;
						}

					}
				} else {
					chain->remaining_record_len -= new_packet->len;
					//see if this packet filled a hole
					new_packet = new_packet->next;
					if(new_packet != NULL &&
							new_packet->seq_num == chain->expected_seq_num){
						available_data = new_packet->len;
						chain->expected_seq_num += new_packet->len;
					} else {
						available_data = 0;
					}
				}
			}
		
		} else {//
			//add to end of packet_chain
			printf("Missing packet (expected %d, received %d)\n", chain->expected_seq_num, new_packet->seq_num);
		}
	}
	return 0;

}