path: root/lib/libalias/alias_irc.c

/* Alias_irc.c intercepts packages contain IRC CTCP commands, and
	changes DCC commands to export a port on the aliasing host instead
	of an aliased host.

    For this routine to work, the DCC command must fit entirely into a
    single TCP packet.  This will usually happen, but is not
    guaranteed.

	 The interception is likely to change the length of the packet.
	 The handling of this is copied more-or-less verbatim from
	 ftp_alias.c

    This software is placed into the public domain with no restrictions
    on its distribution.

	 Initial version: Eivind Eklund <perhaps@yes.no> (ee) 97-01-29

         Version 2.1:  May, 1997 (cjm)
             Very minor changes to conform with
             local/global/function naming conventions
             withing the packet alising module.
*/

/* 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>
#include <limits.h>

#include "alias_local.h"

/* Local defines */
#define DBprintf(a)


void
AliasHandleIrcOut(struct ip *pip, /* IP packet to examine */
				 struct alias_link *link,		  /* Which link are we on? */
				 int maxsize		  /* Maximum size of IP packet including headers */
				 )
{       
    int hlen, tlen, dlen;
    struct in_addr true_addr;
    u_short true_port;
    char *sptr;
    struct tcphdr *tc;
	 int i;							  /* Iterator through the source */
        
/* 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 if data length is too short - assume an entire PRIVMSG in each packet. */
    if (dlen<sizeof(":A!a@n.n PRIVMSG A :aDCC 1 1a")-1)
        return;

/* Place string pointer at beginning of data */
    sptr = (char *) pip;  
    sptr += hlen;
	 maxsize -= hlen;				  /* We're interested in maximum size of data, not packet */

	 /* Search for a CTCP command [Note 1] */
	 for(	i=0; i<dlen; i++ ) {
		 if(sptr[i]=='\001')
			 goto lFOUND_CTCP;
	 }
	 return;					  /* No CTCP commands in  */
	 /* Handle CTCP commands - the buffer may have to be copied */
lFOUND_CTCP:
	 {
		 char newpacket[65536];	  /* Estimate of maximum packet size :) */
		 int  copyat = i;			  /* Same */
		 int  iCopy = 0;			  /* How much data have we written to copy-back string? */
		 unsigned long org_addr;  /* Original IP address */
		 unsigned short org_port; /* Original source port address */
	 lCTCP_START:
		 if( i >= dlen || iCopy >= sizeof(newpacket) )
			 goto lPACKET_DONE;
		 newpacket[iCopy++] = sptr[i++];	/* Copy the CTCP start character */
		 /* Start of a CTCP */
		 if( i+4 >= dlen )		  /* Too short for DCC */
			 goto lBAD_CTCP;
		 if( sptr[i+0] != 'D' )
			 goto lBAD_CTCP;
		 if( sptr[i+1] != 'C' )
			 goto lBAD_CTCP;
		 if( sptr[i+2] != 'C' )
			 goto lBAD_CTCP;
		 if( sptr[i+3] != ' ' )
			 goto lBAD_CTCP;
		 /* We have a DCC command - handle it! */
		 i+= 4;						  /* Skip "DCC " */
		 if( iCopy+4 > sizeof(newpacket) )
			 goto lPACKET_DONE;
		 newpacket[iCopy++] = 'D';
		 newpacket[iCopy++] = 'C';
		 newpacket[iCopy++] = 'C';
		 newpacket[iCopy++] = ' ';

		 DBprintf(("Found DCC\n"));
		 /* Skip any extra spaces (should not occur according to
          protocol, but DCC breaks CTCP protocol anyway */
		 while(sptr[i] == ' ') {
			 if( ++i >= dlen) {
				 DBprintf(("DCC packet terminated in just spaces\n"));
				 goto lPACKET_DONE;
			 }
		 }

		 DBprintf(("Transferring command...\n"));
		 while(sptr[i] != ' ') {
			 newpacket[iCopy++] = sptr[i];
			 if( ++i >= dlen || iCopy >= sizeof(newpacket) ) {
				 DBprintf(("DCC packet terminated during command\n"));
				 goto lPACKET_DONE;
			 }
		 }
		 /* Copy _one_ space */
		 if( i+1 < dlen && iCopy < sizeof(newpacket) )
			 newpacket[iCopy++] = sptr[i++];

		 DBprintf(("Done command - removing spaces\n"));
		 /* Skip any extra spaces (should not occur according to
          protocol, but DCC breaks CTCP protocol anyway */
		 while(sptr[i] == ' ') {
			 if( ++i >= dlen ) {
				 DBprintf(("DCC packet terminated in just spaces (post-command)\n"));
				 goto lPACKET_DONE;
			 }
		 }

		 DBprintf(("Transferring filename...\n"));
		 while(sptr[i] != ' ') {
			 newpacket[iCopy++] = sptr[i];
			 if( ++i >= dlen || iCopy >= sizeof(newpacket) ) {
				 DBprintf(("DCC packet terminated during filename\n"));
				 goto lPACKET_DONE;
			 }
		 }
		 /* Copy _one_ space */
		 if( i+1 < dlen && iCopy < sizeof(newpacket) )
			 newpacket[iCopy++] = sptr[i++];

		 DBprintf(("Done filename - removing spaces\n"));
		 /* Skip any extra spaces (should not occur according to
          protocol, but DCC breaks CTCP protocol anyway */
		 while(sptr[i] == ' ') {
			 if( ++i >= dlen ) {
				 DBprintf(("DCC packet terminated in just spaces (post-filename)\n"));
				 goto lPACKET_DONE;
			 }
		 }

		 DBprintf(("Fetching IP address\n"));
		 /* Fetch IP address */
		 org_addr = 0;
		 while(i<dlen && isdigit(sptr[i])) {
			 if( org_addr > ULONG_MAX/10UL )	{ /* Terminate on overflow */
				 DBprintf(("DCC Address overflow (org_addr == 0x%08lx, next char %c\n", org_addr, sptr[i]));
				 goto lBAD_CTCP;
			 }
			 org_addr *= 10;
			 org_addr += sptr[i++]-'0';
		 }
		 DBprintf(("Skipping space\n"));
		 if( i+1 >= dlen || sptr[i] != ' ' ) {
			 DBprintf(("Overflow (%d >= %d) or bad character (%02x) terminating IP address\n", i+1, dlen, sptr[i]));
			 goto lBAD_CTCP;
		 }
		 /* Skip any extra spaces (should not occur according to
          protocol, but DCC breaks CTCP protocol anyway, so we might
          as well play it safe */
		 while(sptr[i] == ' ') {
			 if( ++i >= dlen ) {
				 DBprintf(("Packet failure - space overflow.\n"));
				 goto lPACKET_DONE;
			 }
		 }
		 DBprintf(("Fetching port number\n"));
		 /* Fetch source port */
		 org_port = 0;
		 while(i<dlen && isdigit(sptr[i])) {
			 if( org_port > 6554 )	{ /* Terminate on overflow (65536/10 rounded up*/
				 DBprintf(("DCC: port number overflow\n"));
				 goto lBAD_CTCP;
			 }
			 org_port *= 10;
			 org_port += sptr[i++]-'0';
		 }
		 /* Skip illegal addresses (or early termination) */
		 if( i >= dlen || (sptr[i] != '\001' && sptr[i] != ' ') ) {
			 DBprintf(("Bad port termination\n"));
			 goto lBAD_CTCP;
		 }
		 DBprintf(("Got IP %lu and port %u\n", org_addr, (unsigned)org_port));

		 /* We've got the address and port - now alias it */
		 {
			 struct alias_link *dcc_link;
			 struct in_addr destaddr;
			 

			 true_port = htons(org_port);
			 true_addr.s_addr = htonl(org_addr);
			 destaddr.s_addr = 0;

			 /* Steal the FTP_DATA_PORT - it doesn't really matter, and this
				 would probably allow it through at least _some_
				 firewalls. */
			 dcc_link = FindUdpTcpOut (true_addr,
											destaddr,
										   true_port,
											0, IPPROTO_TCP);
			 DBprintf(("Got a DCC link\n"));
			 if ( dcc_link ) {
				 struct in_addr alias_address;	/* Address from aliasing */
				 u_short alias_port;	/* Port given by aliasing */

				 alias_address = GetAliasAddress(link);
				 iCopy += snprintf(&newpacket[iCopy],
										 sizeof(newpacket)-iCopy, 
										 "%lu ", htonl(alias_address.s_addr));
				 if( iCopy >= sizeof(newpacket) ) { /* Truncated/fit exactly - bad news */
					 DBprintf(("DCC constructed packet overflow.\n"));
					 goto lBAD_CTCP;
				 }
				 alias_port = GetAliasPort(dcc_link);
				 iCopy += snprintf(&newpacket[iCopy],
										 sizeof(newpacket)-iCopy, 
										 "%u", htons(alias_port) );
				 /* Done - truncated cases will be taken care of by lBAD_CTCP */
				 DBprintf(("Aliased IP %lu and port %u\n", alias_address.s_addr, (unsigned)alias_port));
			 }
		 }
		 /* An uninteresting CTCP - state entered right after '\001' has
          been pushed.  Also used to copy the rest of a DCC, after IP
          address and port has been handled */
	 lBAD_CTCP:
		 for(; i<dlen && iCopy<sizeof(newpacket); i++,iCopy++) {
			 newpacket[iCopy] = sptr[i]; /* Copy CTCP unchanged */
			 if(sptr[i] == '\001') {
				 goto lNORMAL_TEXT;
			 }
		 }
		 goto lPACKET_DONE;
		 /* Normal text */
	 lNORMAL_TEXT:
		 for(; i<dlen && iCopy<sizeof(newpacket); i++,iCopy++) {
			 newpacket[iCopy] = sptr[i]; /* Copy CTCP unchanged */
			 if(sptr[i] == '\001') {
				 goto lCTCP_START;
			 }
		 }
		 /* Handle the end of a packet */
	 lPACKET_DONE:
		 iCopy = iCopy > maxsize-copyat ? maxsize-copyat : iCopy;
		 memcpy(sptr+copyat, newpacket, iCopy);

/* Save information regarding modified seq and ack numbers */
        {
            int delta;

            SetAckModified(link);
            delta = GetDeltaSeqOut(pip, link);
            AddSeq(pip, link, delta+copyat+iCopy-dlen);
        }

		  /* Revise IP header */
        {
			  u_short new_len;
			  
			  new_len = htons(hlen + iCopy + copyat);
			  DifferentialChecksum(&pip->ip_sum,
										  &new_len,
										  &pip->ip_len,
										  1);
			  pip->ip_len = new_len;
        }

		  /* Compute TCP checksum for revised packet */
        tc->th_sum = 0;
        tc->th_sum = TcpChecksum(pip);
		  return;
	 }
}

/* Notes:
	[Note 1]
	The initial search will most often fail; it could be replaced with a 32-bit specific search.
	Such a search would be done for 32-bit unsigned value V:
	V ^= 0x01010101;				  (Search is for null bytes)
	if( ((V-0x01010101)^V) & 0x80808080 ) {
     (found a null bytes which was a 01 byte)
	}
   To assert that the processor is 32-bits, do
   extern int ircdccar[32];        (32 bits)
   extern int ircdccar[CHAR_BIT*sizeof(unsigned int)];
   which will generate a type-error on all but 32-bit machines.

	[Note 2] This routine really ought to be replaced with one that
	creates a transparent proxy on the aliasing host, to allow arbitary
	changes in the TCP stream.  This should not be too difficult given
	this base;  I (ee) will try to do this some time later.
	*/