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/*
Alias_ftp.c performs special processing for FTP sessions under
TCP. Specifically, when a PORT command from the client side
is sent, it is intercepted and modified. The address is changed
to the gateway machine and an aliasing port is used.
For this routine to work, the PORT command must fit entirely
into a single TCP packet. This is typically the case, but exceptions
can easily be envisioned under the actual specifications.
Probably the most troubling aspect of the approach taken here is
that the new PORT command will typically be a different length, and
this causes a certain amount of bookkeeping to keep track of the
changes of sequence and acknowledgment numbers, since the client
machine is totally unaware of the modification to the TCP stream.
This software is placed into the public domain with no restrictions
on its distribution.
Initial version: August, 1996 (cjm)
*/
/* Includes */
#include <ctype.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <netinet/in_systm.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
/* Constants */
#define FTP_DATA_PORT_NUMBER 20
/* Prototypes */
#include "alias.p"
void
HandleFtpOut(pip, link)
struct ip *pip;
char *link;
{
int hlen, tlen, dlen;
struct in_addr true_addr;
u_short true_port;
char *sptr;
struct tcphdr *tc;
/* Calculate data length of TCP packet */
tc = (struct tcphdr *) ((char *) pip + (pip->ip_hl << 2));
hlen = (pip->ip_hl + tc->th_off) << 2;
tlen = ntohs(pip->ip_len);
dlen = tlen - hlen;
/* Return is data length is too long or too short */
if (dlen<10 || dlen>80)
return;
/* Place string pointer and beginning of data */
sptr = (char *) pip;
sptr += hlen;
/* Parse through string using state diagram method */
{
char ch, zero;
int i, state;
u_long a1, a2, a3, a4;
u_short p1, p2;
a1=0; a2=0; a3=0; a4=0; p1=0; p2=0;
zero = '0';
state=-4;
for (i=0; i<dlen; i++)
{
ch = sptr[i];
switch (state)
{
case -4: if (ch == 'P') state=-3; else return; break;
case -3: if (ch == 'O') state=-2; else return; break;
case -2: if (ch == 'R') state=-1; else return; break;
case -1: if (ch == 'T') state= 0; else return; break;
case 0 :
if (isdigit(ch)) {a1=ch-zero; state=1 ;} break;
case 1 :
if (isdigit(ch)) a1=10*a1+ch-zero; else state=2 ; break;
case 2 :
if (isdigit(ch)) {a2=ch-zero; state=3 ;} break;
case 3 :
if (isdigit(ch)) a2=10*a2+ch-zero; else state=4 ; break;
case 4 :
if (isdigit(ch)) {a3=ch-zero; state=5 ;} break;
case 5 :
if (isdigit(ch)) a3=10*a3+ch-zero; else state=6 ; break;
case 6 :
if (isdigit(ch)) {a4=ch-zero; state=7 ;} break;
case 7 :
if (isdigit(ch)) a4=10*a4+ch-zero; else state=8 ; break;
case 8 :
if (isdigit(ch)) {p1=ch-zero; state=9 ;} break;
case 9 :
if (isdigit(ch)) p1=10*p1+ch-zero; else state=10; break;
case 10:
if (isdigit(ch)) {p2=ch-zero; state=11;} break;
case 11:
if (isdigit(ch)) p2=10*p2+ch-zero; break;
}
}
if (state == 11)
{
true_port = htons((p1<<8) + p2);
true_addr.s_addr = htonl((a1<<24) + (a2<<16) +(a3<<8) + a4);
NewFtpPortCommand(pip, link, true_addr, true_port);
}
}
}
void
NewFtpPortCommand(pip, link, true_addr, true_port)
struct ip *pip;
char *link;
struct in_addr true_addr;
u_short true_port;
{
char *ftp_link;
/* Establish link to address and port found in PORT command */
ftp_link = FindTcpOut (true_addr,
GetDestAddress(link),
true_port,
htons(FTP_DATA_PORT_NUMBER));
if (ftp_link != NULL_PTR)
{
int slen, hlen, tlen, dlen;
struct tcphdr *tc;
/* Calculate data length of TCP packet */
tc = (struct tcphdr *) ((char *) pip + (pip->ip_hl << 2));
hlen = (pip->ip_hl + tc->th_off) << 2;
tlen = ntohs(pip->ip_len);
dlen = tlen - hlen;
/* Create new PORT command */
{
char stemp[80];
char *sptr;
u_short alias_port;
u_char *ptr;
int a1, a2, a3, a4, p1, p2;
struct in_addr aliasAddress;
/* Decompose alias address into quad format */
aliasAddress = GetAliasAddress();
ptr = (char *) &aliasAddress;
a1 = *ptr++; a2=*ptr++; a3=*ptr++; a4=*ptr;
/* Decompose alias port into pair format */
alias_port = GetAliasPort(ftp_link);
ptr = (char *) &alias_port;
p1 = *ptr++; p2=*ptr;
/* Generate command string */
sprintf(stemp, "PORT %d,%d,%d,%d,%d,%d\r\n",
a1,a2,a3,a4,p1,p2);
/* Save string length for IP header modification */
slen = strlen(stemp);
/* Copy into IP packet */
sptr = (char *) pip; sptr += hlen;
strcpy(sptr, stemp);
}
/* Save information regarding modified seq and ack numbers */
{
int delta;
SetAckModified(link);
delta = GetDeltaSeqOut(pip, link);
AddSeq(pip, link, delta+slen-dlen);
pip->ip_len = ntohs(hlen + slen);
}
/* Compute TCP checksum for revised packet */
tc->th_sum = 0;
tc->th_sum = TcpChecksum(pip);
}
else
{
fprintf(stderr,
"PacketAlias/HandleFtpOut: Cannot allocate FTP data port\n");
}
}
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