1 /* 2 Alias_ftp.c performs special processing for FTP sessions under 3 TCP. Specifically, when a PORT command from the client side 4 is sent, it is intercepted and modified. The address is changed 5 to the gateway machine and an aliasing port is used. 6 7 For this routine to work, the PORT command must fit entirely 8 into a single TCP packet. This is typically the case, but exceptions 9 can easily be envisioned under the actual specifications. 10 11 Probably the most troubling aspect of the approach taken here is 12 that the new PORT command will typically be a different length, and 13 this causes a certain amount of bookkeeping to keep track of the 14 changes of sequence and acknowledgment numbers, since the client 15 machine is totally unaware of the modification to the TCP stream. 16 17 18 This software is placed into the public domain with no restrictions 19 on its distribution. 20 21 Initial version: August, 1996 (cjm) 22 23 Version 1.6 24 Brian Somers and Martin Renters identified an IP checksum 25 error for modified IP packets. 26 27 Version 1.7: January 9, 1996 (cjm) 28 Differental checksum computation for change 29 in IP packet length. 30 31 Version 2.1: May, 1997 (cjm) 32 Very minor changes to conform with 33 local/global/function naming conventions 34 withing the packet alising module. 35 36 See HISTORY file for record of revisions. 37 38 $FreeBSD$ 39 */ 40 41 /* Includes */ 42 #include <ctype.h> 43 #include <stdio.h> 44 #include <string.h> 45 #include <sys/types.h> 46 #include <netinet/in_systm.h> 47 #include <netinet/in.h> 48 #include <netinet/ip.h> 49 #include <netinet/tcp.h> 50 51 #include "alias_local.h" 52 53 static void NewFtpPortCommand(struct ip *, struct alias_link *, struct in_addr, u_short, int); 54 55 56 57 void 58 AliasHandleFtpOut( 59 struct ip *pip, /* IP packet to examine/patch */ 60 struct alias_link *link, /* The link to go through (aliased port) */ 61 int maxpacketsize /* The maximum size this packet can grow to (including headers) */) 62 { 63 int hlen, tlen, dlen; 64 struct in_addr true_addr; 65 u_short true_port; 66 char *sptr; 67 struct tcphdr *tc; 68 69 /* Calculate data length of TCP packet */ 70 tc = (struct tcphdr *) ((char *) pip + (pip->ip_hl << 2)); 71 hlen = (pip->ip_hl + tc->th_off) << 2; 72 tlen = ntohs(pip->ip_len); 73 dlen = tlen - hlen; 74 75 /* Return is data length is too long or too short */ 76 if (dlen<10 || dlen>80) 77 return; 78 79 /* Place string pointer and beginning of data */ 80 sptr = (char *) pip; 81 sptr += hlen; 82 83 /* Parse through string using state diagram method */ 84 { 85 char ch, zero; 86 int i, state; 87 u_long a1, a2, a3, a4; 88 u_short p1, p2; 89 90 a1=0; a2=0; a3=0; a4=0; p1=0; p2=0; 91 zero = '0'; 92 state=-4; 93 for (i=0; i<dlen; i++) 94 { 95 ch = sptr[i]; 96 switch (state) 97 { 98 case -4: if (ch == 'P') state=-3; else return; break; 99 case -3: if (ch == 'O') state=-2; else return; break; 100 case -2: if (ch == 'R') state=-1; else return; break; 101 case -1: if (ch == 'T') state= 0; else return; break; 102 103 case 0 : 104 if (isdigit(ch)) {a1=ch-zero; state=1 ;} break; 105 case 1 : 106 if (isdigit(ch)) a1=10*a1+ch-zero; else state=2 ; break; 107 case 2 : 108 if (isdigit(ch)) {a2=ch-zero; state=3 ;} break; 109 case 3 : 110 if (isdigit(ch)) a2=10*a2+ch-zero; else state=4 ; break; 111 case 4 : 112 if (isdigit(ch)) {a3=ch-zero; state=5 ;} break; 113 case 5 : 114 if (isdigit(ch)) a3=10*a3+ch-zero; else state=6 ; break; 115 case 6 : 116 if (isdigit(ch)) {a4=ch-zero; state=7 ;} break; 117 case 7 : 118 if (isdigit(ch)) a4=10*a4+ch-zero; else state=8 ; break; 119 case 8 : 120 if (isdigit(ch)) {p1=ch-zero; state=9 ;} break; 121 case 9 : 122 if (isdigit(ch)) p1=10*p1+ch-zero; else state=10; break; 123 case 10: 124 if (isdigit(ch)) {p2=ch-zero; state=11;} break; 125 case 11: 126 if (isdigit(ch)) p2=10*p2+ch-zero; break; 127 } 128 } 129 130 if (state == 11) 131 { 132 true_port = htons((p1<<8) + p2); 133 true_addr.s_addr = htonl((a1<<24) + (a2<<16) +(a3<<8) + a4); 134 NewFtpPortCommand(pip, link, true_addr, true_port, maxpacketsize); 135 } 136 } 137 } 138 139 static void 140 NewFtpPortCommand(struct ip *pip, 141 struct alias_link *link, 142 struct in_addr true_addr, 143 u_short true_port, 144 int maxpacketsize) 145 { 146 struct alias_link *ftp_link; 147 148 /* Establish link to address and port found in PORT command */ 149 ftp_link = FindUdpTcpOut(true_addr, GetDestAddress(link), 150 true_port, 0, IPPROTO_TCP); 151 152 if (ftp_link != NULL) 153 { 154 int slen, hlen, tlen, dlen; 155 struct tcphdr *tc; 156 157 #ifndef NO_FW_PUNCH 158 /* Punch hole in firewall */ 159 PunchFWHole(ftp_link); 160 #endif 161 162 /* Calculate data length of TCP packet */ 163 tc = (struct tcphdr *) ((char *) pip + (pip->ip_hl << 2)); 164 hlen = (pip->ip_hl + tc->th_off) << 2; 165 tlen = ntohs(pip->ip_len); 166 dlen = tlen - hlen; 167 168 /* Create new PORT command */ 169 { 170 char stemp[80]; 171 char *sptr; 172 u_short alias_port; 173 u_char *ptr; 174 int a1, a2, a3, a4, p1, p2; 175 struct in_addr alias_address; 176 177 /* Decompose alias address into quad format */ 178 alias_address = GetAliasAddress(link); 179 ptr = (u_char *) &alias_address.s_addr; 180 a1 = *ptr++; a2=*ptr++; a3=*ptr++; a4=*ptr; 181 182 /* Decompose alias port into pair format */ 183 alias_port = GetAliasPort(ftp_link); 184 ptr = (char *) &alias_port; 185 p1 = *ptr++; p2=*ptr; 186 187 /* Generate command string */ 188 sprintf(stemp, "PORT %d,%d,%d,%d,%d,%d\r\n", 189 a1,a2,a3,a4,p1,p2); 190 191 /* Save string length for IP header modification */ 192 slen = strlen(stemp); 193 194 /* Copy into IP packet */ 195 sptr = (char *) pip; sptr += hlen; 196 strncpy(sptr, stemp, maxpacketsize-hlen); 197 } 198 199 /* Save information regarding modified seq and ack numbers */ 200 { 201 int delta; 202 203 SetAckModified(link); 204 delta = GetDeltaSeqOut(pip, link); 205 AddSeq(pip, link, delta+slen-dlen); 206 } 207 208 /* Revise IP header */ 209 { 210 u_short new_len; 211 212 new_len = htons(hlen + slen); 213 DifferentialChecksum(&pip->ip_sum, 214 &new_len, 215 &pip->ip_len, 216 1); 217 pip->ip_len = new_len; 218 } 219 220 /* Compute TCP checksum for revised packet */ 221 tc->th_sum = 0; 222 tc->th_sum = TcpChecksum(pip); 223 } 224 else 225 { 226 #ifdef DEBUG 227 fprintf(stderr, 228 "PacketAlias/HandleFtpOut: Cannot allocate FTP data port\n"); 229 #endif 230 } 231 } 232