555 lines
13 KiB
C
555 lines
13 KiB
C
/*
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Alias_ftp.c performs special processing for FTP sessions under
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TCP. Specifically, when a PORT/EPRT command from the client
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side or 227/229 reply from the server is sent, it is intercepted
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and modified. The address is changed to the gateway machine
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and an aliasing port is used.
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For this routine to work, the message must fit entirely into a
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single TCP packet. This is typically the case, but exceptions
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can easily be envisioned under the actual specifications.
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Probably the most troubling aspect of the approach taken here is
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that the new message will typically be a different length, and
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this causes a certain amount of bookkeeping to keep track of the
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changes of sequence and acknowledgment numbers, since the client
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machine is totally unaware of the modification to the TCP stream.
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This software is placed into the public domain with no restrictions
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on its distribution.
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References: RFC 959, RFC 2428.
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Initial version: August, 1996 (cjm)
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Version 1.6
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Brian Somers and Martin Renters identified an IP checksum
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error for modified IP packets.
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Version 1.7: January 9, 1996 (cjm)
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Differential checksum computation for change
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in IP packet length.
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Version 2.1: May, 1997 (cjm)
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Very minor changes to conform with
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local/global/function naming conventions
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within the packet aliasing module.
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Version 3.1: May, 2000 (eds)
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Add support for passive mode, alias the 227 replies.
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See HISTORY file for record of revisions.
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$FreeBSD$
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*/
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/* Includes */
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#include <ctype.h>
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#include <stdio.h>
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#include <string.h>
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#include <sys/types.h>
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#include <netinet/in_systm.h>
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#include <netinet/in.h>
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#include <netinet/ip.h>
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#include <netinet/tcp.h>
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#include "alias_local.h"
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#define FTP_CONTROL_PORT_NUMBER 21
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#define MAX_MESSAGE_SIZE 128
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enum ftp_message_type {
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FTP_PORT_COMMAND,
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FTP_EPRT_COMMAND,
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FTP_227_REPLY,
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FTP_229_REPLY,
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FTP_UNKNOWN_MESSAGE
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};
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static int ParseFtpPortCommand(char *, int);
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static int ParseFtpEprtCommand(char *, int);
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static int ParseFtp227Reply(char *, int);
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static int ParseFtp229Reply(char *, int);
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static void NewFtpMessage(struct ip *, struct alias_link *, int, int);
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static struct in_addr true_addr; /* in network byte order. */
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static u_short true_port; /* in host byte order. */
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void
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AliasHandleFtpOut(
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struct ip *pip, /* IP packet to examine/patch */
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struct alias_link *link, /* The link to go through (aliased port) */
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int maxpacketsize /* The maximum size this packet can grow to (including headers) */)
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{
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int hlen, tlen, dlen;
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char *sptr;
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struct tcphdr *tc;
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int ftp_message_type;
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/* Calculate data length of TCP packet */
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tc = (struct tcphdr *) ((char *) pip + (pip->ip_hl << 2));
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hlen = (pip->ip_hl + tc->th_off) << 2;
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tlen = ntohs(pip->ip_len);
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dlen = tlen - hlen;
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/* Place string pointer and beginning of data */
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sptr = (char *) pip;
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sptr += hlen;
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/*
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* Check that data length is not too long and previous message was
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* properly terminated with CRLF.
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*/
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if (dlen <= MAX_MESSAGE_SIZE && GetLastLineCrlfTermed(link)) {
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ftp_message_type = FTP_UNKNOWN_MESSAGE;
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if (ntohs(tc->th_dport) == FTP_CONTROL_PORT_NUMBER) {
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/*
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* When aliasing a client, check for the PORT/EPRT command.
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*/
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if (ParseFtpPortCommand(sptr, dlen))
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ftp_message_type = FTP_PORT_COMMAND;
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else if (ParseFtpEprtCommand(sptr, dlen))
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ftp_message_type = FTP_EPRT_COMMAND;
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} else {
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/*
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* When aliasing a server, check for the 227/229 reply.
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*/
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if (ParseFtp227Reply(sptr, dlen))
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ftp_message_type = FTP_227_REPLY;
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else if (ParseFtp229Reply(sptr, dlen))
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ftp_message_type = FTP_229_REPLY;
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}
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if (ftp_message_type != FTP_UNKNOWN_MESSAGE)
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NewFtpMessage(pip, link, maxpacketsize, ftp_message_type);
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}
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/* Track the msgs which are CRLF term'd for PORT/PASV FW breach */
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if (dlen) { /* only if there's data */
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sptr = (char *) pip; /* start over at beginning */
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tlen = ntohs(pip->ip_len); /* recalc tlen, pkt may have grown */
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SetLastLineCrlfTermed(link,
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(sptr[tlen-2] == '\r') && (sptr[tlen-1] == '\n'));
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}
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}
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static int
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ParseFtpPortCommand(char *sptr, int dlen)
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{
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char ch;
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int i, state;
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u_int32_t addr;
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u_short port;
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u_int8_t octet;
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/* Format: "PORT A,D,D,R,PO,RT". */
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/* Return if data length is too short. */
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if (dlen < 18)
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return 0;
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addr = port = octet = 0;
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state = -4;
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for (i = 0; i < dlen; i++) {
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ch = sptr[i];
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switch (state) {
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case -4: if (ch == 'P') state++; else return 0; break;
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case -3: if (ch == 'O') state++; else return 0; break;
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case -2: if (ch == 'R') state++; else return 0; break;
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case -1: if (ch == 'T') state++; else return 0; break;
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case 0:
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if (isspace(ch))
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break;
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else
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state++;
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case 1: case 3: case 5: case 7: case 9: case 11:
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if (isdigit(ch)) {
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octet = ch - '0';
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state++;
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} else
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return 0;
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break;
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case 2: case 4: case 6: case 8:
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if (isdigit(ch))
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octet = 10 * octet + ch - '0';
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else if (ch == ',') {
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addr = (addr << 8) + octet;
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state++;
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} else
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return 0;
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break;
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case 10: case 12:
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if (isdigit(ch))
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octet = 10 * octet + ch - '0';
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else if (ch == ',' || state == 12) {
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port = (port << 8) + octet;
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state++;
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} else
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return 0;
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break;
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}
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}
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if (state == 13) {
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true_addr.s_addr = htonl(addr);
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true_port = port;
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return 1;
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} else
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return 0;
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}
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static int
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ParseFtpEprtCommand(char *sptr, int dlen)
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{
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char ch, delim;
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int i, state;
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u_int32_t addr;
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u_short port;
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u_int8_t octet;
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/* Format: "EPRT |1|A.D.D.R|PORT|". */
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/* Return if data length is too short. */
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if (dlen < 18)
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return 0;
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addr = port = octet = 0;
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delim = '|'; /* XXX gcc -Wuninitialized */
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state = -4;
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for (i = 0; i < dlen; i++) {
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ch = sptr[i];
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switch (state)
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{
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case -4: if (ch == 'E') state++; else return 0; break;
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case -3: if (ch == 'P') state++; else return 0; break;
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case -2: if (ch == 'R') state++; else return 0; break;
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case -1: if (ch == 'T') state++; else return 0; break;
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case 0:
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if (!isspace(ch)) {
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delim = ch;
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state++;
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}
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break;
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case 1:
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if (ch == '1') /* IPv4 address */
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state++;
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else
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return 0;
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break;
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case 2:
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if (ch == delim)
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state++;
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else
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return 0;
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break;
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case 3: case 5: case 7: case 9:
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if (isdigit(ch)) {
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octet = ch - '0';
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state++;
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} else
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return 0;
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break;
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case 4: case 6: case 8: case 10:
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if (isdigit(ch))
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octet = 10 * octet + ch - '0';
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else if (ch == '.' || state == 10) {
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addr = (addr << 8) + octet;
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state++;
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} else
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return 0;
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break;
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case 11:
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if (isdigit(ch)) {
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port = ch - '0';
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state++;
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} else
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return 0;
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break;
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case 12:
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if (isdigit(ch))
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port = 10 * port + ch - '0';
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else if (ch == delim)
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state++;
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else
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return 0;
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break;
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}
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}
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if (state == 13) {
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true_addr.s_addr = htonl(addr);
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true_port = port;
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return 1;
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} else
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return 0;
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}
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static int
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ParseFtp227Reply(char *sptr, int dlen)
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{
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char ch;
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int i, state;
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u_int32_t addr;
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u_short port;
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u_int8_t octet;
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/* Format: "227 Entering Passive Mode (A,D,D,R,PO,RT)" */
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/* Return if data length is too short. */
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if (dlen < 17)
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return 0;
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addr = port = octet = 0;
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state = -3;
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for (i = 0; i < dlen; i++) {
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ch = sptr[i];
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switch (state)
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{
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case -3: if (ch == '2') state++; else return 0; break;
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case -2: if (ch == '2') state++; else return 0; break;
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case -1: if (ch == '7') state++; else return 0; break;
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case 0:
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if (ch == '(')
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state++;
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break;
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case 1: case 3: case 5: case 7: case 9: case 11:
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if (isdigit(ch)) {
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octet = ch - '0';
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state++;
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} else
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return 0;
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break;
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case 2: case 4: case 6: case 8:
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if (isdigit(ch))
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octet = 10 * octet + ch - '0';
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else if (ch == ',') {
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addr = (addr << 8) + octet;
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state++;
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} else
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return 0;
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break;
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case 10: case 12:
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if (isdigit(ch))
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octet = 10 * octet + ch - '0';
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else if (ch == ',' || (state == 12 && ch == ')')) {
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port = (port << 8) + octet;
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state++;
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} else
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return 0;
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break;
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}
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}
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if (state == 13) {
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true_port = port;
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true_addr.s_addr = htonl(addr);
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return 1;
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} else
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return 0;
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}
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static int
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ParseFtp229Reply(char *sptr, int dlen)
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{
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char ch, delim;
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int i, state;
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u_short port;
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/* Format: "229 Entering Extended Passive Mode (|||PORT|)" */
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/* Return if data length is too short. */
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if (dlen < 11)
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return 0;
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port = 0;
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delim = '|'; /* XXX gcc -Wuninitialized */
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state = -3;
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for (i = 0; i < dlen; i++) {
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ch = sptr[i];
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switch (state)
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{
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case -3: if (ch == '2') state++; else return 0; break;
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case -2: if (ch == '2') state++; else return 0; break;
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case -1: if (ch == '9') state++; else return 0; break;
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case 0:
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if (ch == '(')
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state++;
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break;
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case 1:
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delim = ch;
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state++;
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break;
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case 2: case 3:
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if (ch == delim)
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state++;
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else
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return 0;
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break;
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case 4:
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if (isdigit(ch)) {
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port = ch - '0';
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state++;
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} else
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return 0;
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break;
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case 5:
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if (isdigit(ch))
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port = 10 * port + ch - '0';
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else if (ch == delim)
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state++;
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else
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return 0;
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break;
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case 6:
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if (ch == ')')
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state++;
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else
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return 0;
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break;
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}
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}
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if (state == 7) {
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true_port = port;
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return 1;
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} else
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return 0;
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}
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static void
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NewFtpMessage(struct ip *pip,
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struct alias_link *link,
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int maxpacketsize,
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int ftp_message_type)
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{
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struct alias_link *ftp_link;
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/* Security checks. */
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if (ftp_message_type != FTP_229_REPLY &&
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pip->ip_src.s_addr != true_addr.s_addr)
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return;
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if (true_port < IPPORT_RESERVED)
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return;
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/* Establish link to address and port found in FTP control message. */
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ftp_link = FindUdpTcpOut(true_addr, GetDestAddress(link),
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htons(true_port), 0, IPPROTO_TCP, 1);
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if (ftp_link != NULL)
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{
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int slen, hlen, tlen, dlen;
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struct tcphdr *tc;
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#ifndef NO_FW_PUNCH
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if (ftp_message_type == FTP_PORT_COMMAND ||
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ftp_message_type == FTP_EPRT_COMMAND) {
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/* Punch hole in firewall */
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PunchFWHole(ftp_link);
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}
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#endif
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/* Calculate data length of TCP packet */
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tc = (struct tcphdr *) ((char *) pip + (pip->ip_hl << 2));
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hlen = (pip->ip_hl + tc->th_off) << 2;
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tlen = ntohs(pip->ip_len);
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dlen = tlen - hlen;
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/* Create new FTP message. */
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{
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char stemp[MAX_MESSAGE_SIZE + 1];
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char *sptr;
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u_short alias_port;
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u_char *ptr;
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int a1, a2, a3, a4, p1, p2;
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struct in_addr alias_address;
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/* Decompose alias address into quad format */
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alias_address = GetAliasAddress(link);
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ptr = (u_char *) &alias_address.s_addr;
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a1 = *ptr++; a2=*ptr++; a3=*ptr++; a4=*ptr;
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alias_port = GetAliasPort(ftp_link);
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switch (ftp_message_type)
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{
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case FTP_PORT_COMMAND:
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case FTP_227_REPLY:
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/* Decompose alias port into pair format. */
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ptr = (char *) &alias_port;
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p1 = *ptr++; p2=*ptr;
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if (ftp_message_type == FTP_PORT_COMMAND) {
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/* Generate PORT command string. */
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sprintf(stemp, "PORT %d,%d,%d,%d,%d,%d\r\n",
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a1,a2,a3,a4,p1,p2);
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} else {
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/* Generate 227 reply string. */
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sprintf(stemp,
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"227 Entering Passive Mode (%d,%d,%d,%d,%d,%d)\r\n",
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a1,a2,a3,a4,p1,p2);
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}
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break;
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case FTP_EPRT_COMMAND:
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/* Generate EPRT command string. */
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sprintf(stemp, "EPRT |1|%d.%d.%d.%d|%d|\r\n",
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a1,a2,a3,a4,ntohs(alias_port));
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break;
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case FTP_229_REPLY:
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/* Generate 229 reply string. */
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sprintf(stemp, "229 Entering Extended Passive Mode (|||%d|)\r\n",
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ntohs(alias_port));
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break;
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}
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/* Save string length for IP header modification */
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slen = strlen(stemp);
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/* Copy modified buffer into IP packet. */
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sptr = (char *) pip; sptr += hlen;
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strncpy(sptr, stemp, maxpacketsize-hlen);
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}
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/* Save information regarding modified seq and ack numbers */
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{
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int delta;
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SetAckModified(link);
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delta = GetDeltaSeqOut(pip, link);
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AddSeq(pip, link, delta+slen-dlen);
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}
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/* Revise IP header */
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{
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u_short new_len;
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new_len = htons(hlen + slen);
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DifferentialChecksum(&pip->ip_sum,
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&new_len,
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&pip->ip_len,
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1);
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pip->ip_len = new_len;
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}
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/* Compute TCP checksum for revised packet */
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tc->th_sum = 0;
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tc->th_sum = TcpChecksum(pip);
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}
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else
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{
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#ifdef DEBUG
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fprintf(stderr,
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"PacketAlias/HandleFtpOut: Cannot allocate FTP data port\n");
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#endif
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}
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}
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