7652512976
It does mean that it is now possible to run passive-mode FTP
server behind NAT.
- SECURITY: FTP aliasing engine now ensures that:
o the segment preceding a PORT/227 segment terminates with a \r\n;
o the IP address in the PORT/227 matches the source IP address of
the packet;
o the port number in the PORT command or 277 reply is greater than
or equal to 1024.
Submitted by: Erik Salander <erik@whistle.com>
Reviewed by: ru
530 lines
15 KiB
C
530 lines
15 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 command from the client side
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or PASV/227 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 PORT/227 must fit entirely
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into a 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 PORT/227 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 version also supports the EPRT command, which is functionally
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equivalent to the PORT command, but was designed to support both
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IPv4 and IPv6 addresses. See RFC 2428 for specifications.
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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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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 MIN_227_REPLY 16
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#define MAX_227_REPLY 128
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static int ParseFtpPortCommand(char *, int, struct ip *, struct alias_link *, int);
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static void ParseFtpEprtCommand(char *, int, struct ip *, struct alias_link *, int);
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static void NewFtpPortCommand(struct ip *, struct alias_link *, struct in_addr, u_short, int, int);
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static int ParseFtp227Reply(char *, int, struct ip *, struct alias_link *, int);
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static void NewFtp227Reply(struct ip *, struct alias_link *, struct in_addr, u_short, int);
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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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/* 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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/* When aliasing a client, check for the PORT/EPRT command */
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if (ntohs(tc->th_dport) == FTP_CONTROL_PORT_NUMBER) {
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/* Parse through string using state diagram method */
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if (!ParseFtpPortCommand(sptr, dlen, pip, link, maxpacketsize))
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ParseFtpEprtCommand(sptr, dlen, pip, link, maxpacketsize);
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} else {
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ParseFtp227Reply(sptr, dlen, pip, link, maxpacketsize);
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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(
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char *sptr,
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int dlen,
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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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struct in_addr true_addr;
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u_short true_port;
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char ch;
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int i, state;
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u_long a1, a2, a3, a4;
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u_short p1, p2;
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/* Return if data length is too long or too short */
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if (dlen<10 || dlen>80)
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return 0;
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a1=0; a2=0; a3=0; a4=0; p1=0; p2=0;
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state=-4;
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for (i=0; i<dlen; i++)
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{
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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 == '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 (isdigit(ch)) {a1=ch-'0'; state++;} break;
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case 1 :
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if (isdigit(ch)) a1=10*a1+ch-'0'; else state++; break;
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case 2 :
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if (isdigit(ch)) {a2=ch-'0'; state++;} break;
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case 3 :
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if (isdigit(ch)) a2=10*a2+ch-'0'; else state++; break;
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case 4 :
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if (isdigit(ch)) {a3=ch-'0'; state++;} break;
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case 5 :
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if (isdigit(ch)) a3=10*a3+ch-'0'; else state++; break;
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case 6 :
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if (isdigit(ch)) {a4=ch-'0'; state++;} break;
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case 7 :
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if (isdigit(ch)) a4=10*a4+ch-'0'; else state++; break;
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case 8 :
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if (isdigit(ch)) {p1=ch-'0'; state++;} break;
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case 9 :
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if (isdigit(ch)) p1=10*p1+ch-'0'; else state++; break;
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case 10:
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if (isdigit(ch)) {p2=ch-'0'; state++;} break;
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case 11:
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if (isdigit(ch)) p2=10*p2+ch-'0'; break;
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}
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}
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if (state == 11 && GetLastLineCrlfTermed(link))
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{
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true_port = htons((p1<<8) + p2);
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true_addr.s_addr = htonl((a1<<24) + (a2<<16) +(a3<<8) + a4);
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if ((pip->ip_src.s_addr == true_addr.s_addr) &&
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((p1<<8) >= IPPORT_RESERVED))
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NewFtpPortCommand(pip, link, true_addr, true_port, maxpacketsize, 0);
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return 1;
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}
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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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ParseFtpEprtCommand(
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char *sptr,
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int dlen,
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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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struct in_addr true_addr;
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u_short true_port;
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char ch, delim;
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int i, state;
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u_long a1, a2, a3, a4;
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u_short pt;
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/* Return if data length is too long or too short */
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if (dlen<10 || dlen>80)
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return;
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a1=0; a2=0; a3=0; a4=0; pt=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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{
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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; break;
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case -3: if (ch == 'P') state++; else return; break;
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case -2: if (ch == 'R') state++; else return; break;
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case -1: if (ch == 'T') state++; else return; break;
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case 0 :
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if (!isspace(ch)) {delim=ch; state++;} break;
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case 1 :
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if (ch=='1') /* IPv4 address */ state++; else return; break;
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case 2 :
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if (ch==delim) state++; else return; break;
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case 3 :
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if (isdigit(ch)) {a1=ch-'0'; state++;} else return; break;
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case 4 :
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if (isdigit(ch)) a1=10*a1+ch-'0';
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else if (ch=='.') state++;
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else return;
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break;
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case 5 :
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if (isdigit(ch)) {a2=ch-'0'; state++;} else return; break;
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case 6 :
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if (isdigit(ch)) a2=10*a2+ch-'0';
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else if (ch=='.') state++;
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else return;
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break;
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case 7:
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if (isdigit(ch)) {a3=ch-'0'; state++;} else return; break;
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case 8 :
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if (isdigit(ch)) a3=10*a3+ch-'0';
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else if (ch=='.') state++;
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else return;
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break;
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case 9 :
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if (isdigit(ch)) {a4=ch-'0'; state++;} else return; break;
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case 10:
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if (isdigit(ch)) a4=10*a4+ch-'0';
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else if (ch==delim) state++;
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else return;
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break;
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case 11:
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if (isdigit(ch)) {pt=ch-'0'; state++;} else return; break;
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case 12:
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if (isdigit(ch)) pt=10*pt+ch-'0';
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else if (ch==delim) state++;
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else return;
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break;
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}
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}
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if (state == 13 && GetLastLineCrlfTermed(link))
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{
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true_port = htons(pt);
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true_addr.s_addr = htonl((a1<<24) + (a2<<16) +(a3<<8) + a4);
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if ((pip->ip_src.s_addr == true_addr.s_addr) &&
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(pt >= IPPORT_RESERVED))
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NewFtpPortCommand(pip, link, true_addr, true_port, maxpacketsize, 1);
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}
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}
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static int
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ParseFtp227Reply(char *sptr,
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int dlen,
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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 */
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/* to (including headers) */
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{
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struct in_addr true_addr;
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u_short true_port;
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char ch;
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int i, state;
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u_long a1, a2, a3, a4;
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u_short p1, p2;
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/* Return if wrong size, in case more packet types added later */
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if (dlen<MIN_227_REPLY || dlen>MAX_227_REPLY)
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return 0;
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a1=0; a2=0; a3=0; a4=0; p1=0; p2=0;
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state=-3;
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for (i=0; i<dlen; i++)
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{
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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 (isdigit(ch)) {a1=ch-'0'; state++;} break;
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case 1 :
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if (isdigit(ch)) a1=10*a1+ch-'0'; else state++; break;
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case 2 :
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if (isdigit(ch)) {a2=ch-'0'; state++;} break;
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case 3 :
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if (isdigit(ch)) a2=10*a2+ch-'0'; else state++; break;
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case 4 :
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if (isdigit(ch)) {a3=ch-'0'; state++;} break;
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case 5 :
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if (isdigit(ch)) a3=10*a3+ch-'0'; else state++; break;
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case 6 :
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if (isdigit(ch)) {a4=ch-'0'; state++;} break;
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case 7 :
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if (isdigit(ch)) a4=10*a4+ch-'0'; else state++; break;
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case 8 :
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if (isdigit(ch)) {p1=ch-'0'; state++;} break;
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case 9 :
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if (isdigit(ch)) p1=10*p1+ch-'0'; else state++; break;
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case 10:
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if (isdigit(ch)) {p2=ch-'0'; state++;} break;
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case 11:
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if (isdigit(ch)) p2=10*p2+ch-'0'; break;
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}
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}
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if (state == 11 && GetLastLineCrlfTermed(link))
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{
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true_port = htons((p1<<8) + p2);
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true_addr.s_addr = htonl((a1<<24) + (a2<<16) +(a3<<8) + a4);
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if ((pip->ip_src.s_addr == true_addr.s_addr) &&
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((p1<<8) >= IPPORT_RESERVED))
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NewFtp227Reply(pip, link, true_addr, true_port, maxpacketsize);
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return 1;
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}
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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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NewFtpPortCommand(struct ip *pip,
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struct alias_link *link,
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struct in_addr true_addr,
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u_short true_port,
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int maxpacketsize,
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int is_eprt)
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{
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struct alias_link *ftp_link;
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/* Establish link to address and port found in PORT command */
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ftp_link = FindUdpTcpOut(true_addr, GetDestAddress(link),
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true_port, 0, IPPROTO_TCP);
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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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/* Punch hole in firewall */
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PunchFWHole(ftp_link);
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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 PORT command */
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{
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char stemp[80];
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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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if (is_eprt) {
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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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} else {
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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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/* 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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}
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/* Save string length for IP header modification */
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slen = strlen(stemp);
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/* Copy 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, PORT: Cannot allocate FTP data port\n");
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#endif
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}
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}
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|
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static void
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NewFtp227Reply(struct ip *pip,
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struct alias_link *link,
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struct in_addr true_addr,
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u_short true_port,
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int maxpacketsize)
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{
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struct alias_link *ftp_link;
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/* Establish link to address and port found in 227 reply */
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ftp_link = FindUdpTcpOut(true_addr, GetDestAddress(link),
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true_port, 0, IPPROTO_TCP);
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|
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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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/* 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 227 reply */
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{
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char stemp[MAX_227_REPLY+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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/* Decompose alias port into pair format */
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|
alias_port = GetAliasPort(ftp_link);
|
|
ptr = (char *) &alias_port;
|
|
p1 = *ptr++; p2=*ptr;
|
|
|
|
/* Generate command string */
|
|
sprintf(stemp, "227 Entering Passive Mode (%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 modified buffer into IP packet */
|
|
sptr = (char *) pip; sptr += hlen;
|
|
strncpy(sptr, stemp, maxpacketsize-hlen);
|
|
}
|
|
|
|
/* Save information regarding modified seq and ack numbers */
|
|
{
|
|
int delta;
|
|
|
|
SetAckModified(link);
|
|
delta = GetDeltaSeqOut(pip, link);
|
|
AddSeq(pip, link, delta+slen-dlen);
|
|
}
|
|
|
|
/* Revise IP header */
|
|
{
|
|
u_short new_len;
|
|
|
|
new_len = htons(hlen + slen);
|
|
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);
|
|
}
|
|
else
|
|
{
|
|
#ifdef DEBUG
|
|
fprintf(stderr,
|
|
"PacketAlias/HandleFtpOut, 227: Cannot allocate FTP data port\n");
|
|
#endif
|
|
}
|
|
}
|