hide-eid/pass-1/packet.c

#include <string.h>
#include <assert.h>

#include "util.h"
#include "rlocs.h"
#include "packet.h"

// shamelessly copied from:
// http://www.roman10.net/how-to-calculate-iptcpudp-checksumpart-2-implementation/
unsigned short compute_checksum(unsigned short *addr, unsigned int count) {

  unsigned long sum = 0;

  while (count > 1) {
    sum += * addr++;
    count -= 2;
  }

  //if any bytes left, pad the bytes and add
  if(count > 0) {
    sum += ((*addr)&htons(0xFF00));
  }

  //Fold sum to 16 bits: add carrier to result
  while (sum>>16) {
      sum = (sum & 0xffff) + (sum >> 16);
  }

  //one's complement
  sum = ~sum;

  return ((unsigned short)sum);

}

void compute_ip_checksum(struct iphdr* pkt)
{
  pkt->check = 0x0000;
  pkt->check = compute_checksum( (unsigned short*) pkt, pkt->ihl * 4 );
}


int wrap_ipv4_packet(struct rlocs* reg, struct recv_pkt* pkt, struct rsp_data* out)
{
    out->count = 3;
    assert( out->count < MAX_IOVS );
    
    unsigned char *scratch = &out->scratch[0];
    
    // iovec 0: wrapping header
    struct iphdr* wrap_hdr = (struct iphdr*) scratch;
    unsigned int wrap_hdr_size = sizeof( struct iphdr );
    scratch += wrap_hdr_size;
    
    memset( wrap_hdr, 0, wrap_hdr_size );
    
    wrap_hdr->version  = 0x04;
    wrap_hdr->ihl      = wrap_hdr_size / 4;
    wrap_hdr->ttl      = IPDEFTTL;
    wrap_hdr->protocol = IPPROTO_HIDE_EID;
    
    out->iovs[0].iov_base = wrap_hdr;
    out->iovs[0].iov_len = wrap_hdr_size;
    
    // TODO: id, still needs filling now.    
    
    // We need to know source and destination rlocs to construct the packet
    struct rloc* s_rloc;
    struct rloc* d_rloc;
    struct in_addr tmp;
    
    // TODO: check endianness of saddr/daddr
    tmp.s_addr = pkt->hdr.ip.saddr;
    if ( ( s_rloc = rloc_find_for_ipv4( reg, &tmp ) ) == NULL ) {
        warn( "Couldn't find source rloc, dropping packet" );
        // TODO: fallback behaviour here?
        return 0;
    }
    
    tmp.s_addr = pkt->hdr.ip.daddr;    
    if ( ( d_rloc = rloc_find_for_ipv4( reg, &tmp ) ) == NULL ) {
        warn( "Couldn't find destination rloc, dropping packet" );
        // TODO: fallback behaviour here?
        return 0;
    }
    
    wrap_hdr->saddr = s_rloc->addr.ip4.s_addr;
    wrap_hdr->daddr = d_rloc->addr.ip4.s_addr;
   
    // iovec 1: encrypted part. 
    // FIXME: Need to inspect the protocol field and gobble up the TCP/UDP/etc
    // header as well, for decent anonymity. TCP/UDP ports are an obvious way
    // to perform a correlation attack.
    // RSA pubkey encryption with 4096-bit keys gobbles up at least 512 bytes
    // of space, so we make sure to use it.
    ssize_t enc_size;
    size_t orig_data_size = ntohs( pkt->hdr.ip.tot_len );
    size_t bytes_to_encrypt;
    
    if ( orig_data_size > 512 ) {
        bytes_to_encrypt = pkt->hdr.ip.ihl * 4;
    } else {
        bytes_to_encrypt = orig_data_size;
    }

    off_t enc_max_len = IP_MAXPACKET - wrap_hdr_size - orig_data_size - bytes_to_encrypt;    
    
    // We use two bytes to store the size of the encrypted blob
    unsigned short *pkt_enc_size = (unsigned short *) scratch;
    scratch += 2;

    enc_size = rloc_encrypt( d_rloc, (unsigned char *)&pkt->hdr, bytes_to_encrypt, scratch, enc_max_len - 2 );
    if ( enc_size  < 0 ) { 
        warn( "failed to encrypt, dropping packet" );
        return 0;
    }
    
    *pkt_enc_size = htons( enc_size );
    enc_size += 2;
    scratch = (unsigned char*) pkt_enc_size;
    

    
    warn( "Encrypted size: 2 + %zu", enc_size - 2);
    
    out->iovs[1].iov_base = scratch;
    out->iovs[1].iov_len = enc_size;
    scratch += enc_size;
    
    // iovec 2: unencrypted remains
    if ( bytes_to_encrypt == orig_data_size ) {
        out->count = 2;
        out->iovs[2].iov_base = NULL;
        out->iovs[2].iov_len = 0;
    } else {    
        out->iovs[2].iov_base = (char *) pkt + bytes_to_encrypt;
        out->iovs[2].iov_len  = ntohs( pkt->hdr.ip.tot_len ) - bytes_to_encrypt;
    }
        
    wrap_hdr->tot_len = htons( wrap_hdr_size + enc_size + out->iovs[2].iov_len );
    compute_ip_checksum( wrap_hdr );
    
    info( "Finished building return packet" );

    return 1;
}

int wrap_ipv6_packet(struct rlocs *reg, struct recv_pkt* pkt, struct rsp_data* out)
{
    warn( "STUB: wrap_ipv6_packet" );
    return 0;   
}



int unwrap_ipv4_packet(struct rlocs* reg, struct recv_pkt* pkt, struct rsp_data* out)
{
    warn( "STUB: unwrap_ipv4_packet" );
    return 0;
}

int unwrap_ipv6_packet(struct rlocs *reg, struct recv_pkt* pkt, struct rsp_data* out)
{
    warn( "STUB: unwrap_ipv6_packet" );

    return 0;   
}