alias_irc.c revision 36711 
1/* Alias_irc.c intercepts packages contain IRC CTCP commands, and
2	changes DCC commands to export a port on the aliasing host instead
3	of an aliased host.
4
5    For this routine to work, the DCC command must fit entirely into a
6    single TCP packet.  This will usually happen, but is not
7    guaranteed.
8
9	 The interception is likely to change the length of the packet.
10	 The handling of this is copied more-or-less verbatim from
11	 ftp_alias.c
12
13    This software is placed into the public domain with no restrictions
14    on its distribution.
15
16	 Initial version: Eivind Eklund <perhaps@yes.no> (ee) 97-01-29
17
18         Version 2.1:  May, 1997 (cjm)
19             Very minor changes to conform with
20             local/global/function naming conventions
21             withing the packet alising module.
22*/
23
24/* Includes */
25#include <ctype.h>
26#include <stdio.h>
27#include <string.h>
28#include <sys/types.h>
29#include <netinet/in_systm.h>
30#include <netinet/in.h>
31#include <netinet/ip.h>
32#include <netinet/tcp.h>
33#include <limits.h>
34
35#include "alias_local.h"
36
37/* Local defines */
38#define DBprintf(a)
39
40
41void
42AliasHandleIrcOut(struct ip *pip, /* IP packet to examine */
43				 struct alias_link *link,		  /* Which link are we on? */
44				 int maxsize		  /* Maximum size of IP packet including headers */
45				 )
46{
47    int hlen, tlen, dlen;
48    struct in_addr true_addr;
49    u_short true_port;
50    char *sptr;
51    struct tcphdr *tc;
52	 int i;							  /* Iterator through the source */
53
54/* Calculate data length of TCP packet */
55    tc = (struct tcphdr *) ((char *) pip + (pip->ip_hl << 2));
56    hlen = (pip->ip_hl + tc->th_off) << 2;
57    tlen = ntohs(pip->ip_len);
58    dlen = tlen - hlen;
59
60	 /* Return if data length is too short - assume an entire PRIVMSG in each packet. */
61    if (dlen<sizeof(":A!a@n.n PRIVMSG A :aDCC 1 1a")-1)
62        return;
63
64/* Place string pointer at beginning of data */
65    sptr = (char *) pip;
66    sptr += hlen;
67	 maxsize -= hlen;				  /* We're interested in maximum size of data, not packet */
68
69	 /* Search for a CTCP command [Note 1] */
70	 for(	i=0; i<dlen; i++ ) {
71		 if(sptr[i]=='\001')
72			 goto lFOUND_CTCP;
73	 }
74	 return;					  /* No CTCP commands in  */
75	 /* Handle CTCP commands - the buffer may have to be copied */
76lFOUND_CTCP:
77	 {
78		 char newpacket[65536];	  /* Estimate of maximum packet size :) */
79		 int  copyat = i;			  /* Same */
80		 int  iCopy = 0;			  /* How much data have we written to copy-back string? */
81		 unsigned long org_addr;  /* Original IP address */
82		 unsigned short org_port; /* Original source port address */
83	 lCTCP_START:
84		 if( i >= dlen || iCopy >= sizeof(newpacket) )
85			 goto lPACKET_DONE;
86		 newpacket[iCopy++] = sptr[i++];	/* Copy the CTCP start character */
87		 /* Start of a CTCP */
88		 if( i+4 >= dlen )		  /* Too short for DCC */
89			 goto lBAD_CTCP;
90		 if( sptr[i+0] != 'D' )
91			 goto lBAD_CTCP;
92		 if( sptr[i+1] != 'C' )
93			 goto lBAD_CTCP;
94		 if( sptr[i+2] != 'C' )
95			 goto lBAD_CTCP;
96		 if( sptr[i+3] != ' ' )
97			 goto lBAD_CTCP;
98		 /* We have a DCC command - handle it! */
99		 i+= 4;						  /* Skip "DCC " */
100		 if( iCopy+4 > sizeof(newpacket) )
101			 goto lPACKET_DONE;
102		 newpacket[iCopy++] = 'D';
103		 newpacket[iCopy++] = 'C';
104		 newpacket[iCopy++] = 'C';
105		 newpacket[iCopy++] = ' ';
106
107		 DBprintf(("Found DCC\n"));
108		 /* Skip any extra spaces (should not occur according to
109          protocol, but DCC breaks CTCP protocol anyway */
110		 while(sptr[i] == ' ') {
111			 if( ++i >= dlen) {
112				 DBprintf(("DCC packet terminated in just spaces\n"));
113				 goto lPACKET_DONE;
114			 }
115		 }
116
117		 DBprintf(("Transferring command...\n"));
118		 while(sptr[i] != ' ') {
119			 newpacket[iCopy++] = sptr[i];
120			 if( ++i >= dlen || iCopy >= sizeof(newpacket) ) {
121				 DBprintf(("DCC packet terminated during command\n"));
122				 goto lPACKET_DONE;
123			 }
124		 }
125		 /* Copy _one_ space */
126		 if( i+1 < dlen && iCopy < sizeof(newpacket) )
127			 newpacket[iCopy++] = sptr[i++];
128
129		 DBprintf(("Done command - removing spaces\n"));
130		 /* Skip any extra spaces (should not occur according to
131          protocol, but DCC breaks CTCP protocol anyway */
132		 while(sptr[i] == ' ') {
133			 if( ++i >= dlen ) {
134				 DBprintf(("DCC packet terminated in just spaces (post-command)\n"));
135				 goto lPACKET_DONE;
136			 }
137		 }
138
139		 DBprintf(("Transferring filename...\n"));
140		 while(sptr[i] != ' ') {
141			 newpacket[iCopy++] = sptr[i];
142			 if( ++i >= dlen || iCopy >= sizeof(newpacket) ) {
143				 DBprintf(("DCC packet terminated during filename\n"));
144				 goto lPACKET_DONE;
145			 }
146		 }
147		 /* Copy _one_ space */
148		 if( i+1 < dlen && iCopy < sizeof(newpacket) )
149			 newpacket[iCopy++] = sptr[i++];
150
151		 DBprintf(("Done filename - removing spaces\n"));
152		 /* Skip any extra spaces (should not occur according to
153          protocol, but DCC breaks CTCP protocol anyway */
154		 while(sptr[i] == ' ') {
155			 if( ++i >= dlen ) {
156				 DBprintf(("DCC packet terminated in just spaces (post-filename)\n"));
157				 goto lPACKET_DONE;
158			 }
159		 }
160
161		 DBprintf(("Fetching IP address\n"));
162		 /* Fetch IP address */
163		 org_addr = 0;
164		 while(i<dlen && isdigit(sptr[i])) {
165			 if( org_addr > ULONG_MAX/10UL )	{ /* Terminate on overflow */
166				 DBprintf(("DCC Address overflow (org_addr == 0x%08lx, next char %c\n", org_addr, sptr[i]));
167				 goto lBAD_CTCP;
168			 }
169			 org_addr *= 10;
170			 org_addr += sptr[i++]-'0';
171		 }
172		 DBprintf(("Skipping space\n"));
173		 if( i+1 >= dlen || sptr[i] != ' ' ) {
174			 DBprintf(("Overflow (%d >= %d) or bad character (%02x) terminating IP address\n", i+1, dlen, sptr[i]));
175			 goto lBAD_CTCP;
176		 }
177		 /* Skip any extra spaces (should not occur according to
178          protocol, but DCC breaks CTCP protocol anyway, so we might
179          as well play it safe */
180		 while(sptr[i] == ' ') {
181			 if( ++i >= dlen ) {
182				 DBprintf(("Packet failure - space overflow.\n"));
183				 goto lPACKET_DONE;
184			 }
185		 }
186		 DBprintf(("Fetching port number\n"));
187		 /* Fetch source port */
188		 org_port = 0;
189		 while(i<dlen && isdigit(sptr[i])) {
190			 if( org_port > 6554 )	{ /* Terminate on overflow (65536/10 rounded up*/
191				 DBprintf(("DCC: port number overflow\n"));
192				 goto lBAD_CTCP;
193			 }
194			 org_port *= 10;
195			 org_port += sptr[i++]-'0';
196		 }
197		 /* Skip illegal addresses (or early termination) */
198		 if( i >= dlen || (sptr[i] != '\001' && sptr[i] != ' ') ) {
199			 DBprintf(("Bad port termination\n"));
200			 goto lBAD_CTCP;
201		 }
202		 DBprintf(("Got IP %lu and port %u\n", org_addr, (unsigned)org_port));
203
204		 /* We've got the address and port - now alias it */
205		 {
206			 struct alias_link *dcc_link;
207			 struct in_addr destaddr;
208
209
210			 true_port = htons(org_port);
211			 true_addr.s_addr = htonl(org_addr);
212			 destaddr.s_addr = 0;
213
214			 /* Steal the FTP_DATA_PORT - it doesn't really matter, and this
215				 would probably allow it through at least _some_
216				 firewalls. */
217			 dcc_link = FindUdpTcpOut (true_addr,
218											destaddr,
219										   true_port,
220											0, IPPROTO_TCP);
221			 DBprintf(("Got a DCC link\n"));
222			 if ( dcc_link ) {
223				 struct in_addr alias_address;	/* Address from aliasing */
224				 u_short alias_port;	/* Port given by aliasing */
225
226#ifndef NO_FW_PUNCH
227				 /* Generate firewall hole as appropriate */
228				 PunchFWHole(dcc_link);
229#endif
230
231				 alias_address = GetAliasAddress(link);
232				 iCopy += snprintf(&newpacket[iCopy],
233										 sizeof(newpacket)-iCopy,
234										 "%lu ", (u_long)htonl(alias_address.s_addr));
235				 if( iCopy >= sizeof(newpacket) ) { /* Truncated/fit exactly - bad news */
236					 DBprintf(("DCC constructed packet overflow.\n"));
237					 goto lBAD_CTCP;
238				 }
239				 alias_port = GetAliasPort(dcc_link);
240				 iCopy += snprintf(&newpacket[iCopy],
241										 sizeof(newpacket)-iCopy,
242										 "%u", htons(alias_port) );
243				 /* Done - truncated cases will be taken care of by lBAD_CTCP */
244				 DBprintf(("Aliased IP %lu and port %u\n", alias_address.s_addr, (unsigned)alias_port));
245			 }
246		 }
247		 /* An uninteresting CTCP - state entered right after '\001' has
248          been pushed.  Also used to copy the rest of a DCC, after IP
249          address and port has been handled */
250	 lBAD_CTCP:
251		 for(; i<dlen && iCopy<sizeof(newpacket); i++,iCopy++) {
252			 newpacket[iCopy] = sptr[i]; /* Copy CTCP unchanged */
253			 if(sptr[i] == '\001') {
254				 goto lNORMAL_TEXT;
255			 }
256		 }
257		 goto lPACKET_DONE;
258		 /* Normal text */
259	 lNORMAL_TEXT:
260		 for(; i<dlen && iCopy<sizeof(newpacket); i++,iCopy++) {
261			 newpacket[iCopy] = sptr[i]; /* Copy CTCP unchanged */
262			 if(sptr[i] == '\001') {
263				 goto lCTCP_START;
264			 }
265		 }
266		 /* Handle the end of a packet */
267	 lPACKET_DONE:
268		 iCopy = iCopy > maxsize-copyat ? maxsize-copyat : iCopy;
269		 memcpy(sptr+copyat, newpacket, iCopy);
270
271/* Save information regarding modified seq and ack numbers */
272        {
273            int delta;
274
275            SetAckModified(link);
276            delta = GetDeltaSeqOut(pip, link);
277            AddSeq(pip, link, delta+copyat+iCopy-dlen);
278        }
279
280		  /* Revise IP header */
281        {
282			  u_short new_len;
283
284			  new_len = htons(hlen + iCopy + copyat);
285			  DifferentialChecksum(&pip->ip_sum,
286										  &new_len,
287										  &pip->ip_len,
288										  1);
289			  pip->ip_len = new_len;
290        }
291
292		  /* Compute TCP checksum for revised packet */
293        tc->th_sum = 0;
294        tc->th_sum = TcpChecksum(pip);
295		  return;
296	 }
297}
298
299/* Notes:
300	[Note 1]
301	The initial search will most often fail; it could be replaced with a 32-bit specific search.
302	Such a search would be done for 32-bit unsigned value V:
303	V ^= 0x01010101;				  (Search is for null bytes)
304	if( ((V-0x01010101)^V) & 0x80808080 ) {
305     (found a null bytes which was a 01 byte)
306	}
307   To assert that the processor is 32-bits, do
308   extern int ircdccar[32];        (32 bits)
309   extern int ircdccar[CHAR_BIT*sizeof(unsigned int)];
310   which will generate a type-error on all but 32-bit machines.
311
312	[Note 2] This routine really ought to be replaced with one that
313	creates a transparent proxy on the aliasing host, to allow arbitary
314	changes in the TCP stream.  This should not be too difficult given
315	this base;  I (ee) will try to do this some time later.
316	*/
317