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 39 /* Includes */ 40 #include <ctype.h> 41 #include <stdio.h> 42 #include <string.h> 43 #include <sys/types.h> 44 #include <netinet/in_systm.h> 45 #include <netinet/in.h> 46 #include <netinet/ip.h> 47 #include <netinet/tcp.h> 48 49 #include "alias_local.h" 50 51 static void NewFtpPortCommand(struct ip *, struct alias_link *, struct in_addr, u_short, int); 52 53 54 55 void 56 AliasHandleFtpOut( 57 struct ip *pip, /* IP packet to examine/patch */ 58 struct alias_link *link, /* The link to go through (aliased port) */ 59 int maxpacketsize /* The maximum size this packet can grow to (including headers) */) 60 { 61 int hlen, tlen, dlen; 62 struct in_addr true_addr; 63 u_short true_port; 64 char *sptr; 65 struct tcphdr *tc; 66 67 /* Calculate data length of TCP packet */ 68 tc = (struct tcphdr *) ((char *) pip + (pip->ip_hl << 2)); 69 hlen = (pip->ip_hl + tc->th_off) << 2; 70 tlen = ntohs(pip->ip_len); 71 dlen = tlen - hlen; 72 73 /* Return is data length is too long or too short */ 74 if (dlen<10 || dlen>80) 75 return; 76 77 /* Place string pointer and beginning of data */ 78 sptr = (char *) pip; 79 sptr += hlen; 80 81 /* Parse through string using state diagram method */ 82 { 83 char ch, zero; 84 int i, state; 85 u_long a1, a2, a3, a4; 86 u_short p1, p2; 87 88 a1=0; a2=0; a3=0; a4=0; p1=0; p2=0; 89 zero = '0'; 90 state=-4; 91 for (i=0; i<dlen; i++) 92 { 93 ch = sptr[i]; 94 switch (state) 95 { 96 case -4: if (ch == 'P') state=-3; else return; break; 97 case -3: if (ch == 'O') state=-2; else return; break; 98 case -2: if (ch == 'R') state=-1; else return; break; 99 case -1: if (ch == 'T') state= 0; else return; break; 100 101 case 0 : 102 if (isdigit(ch)) {a1=ch-zero; state=1 ;} break; 103 case 1 : 104 if (isdigit(ch)) a1=10*a1+ch-zero; else state=2 ; break; 105 case 2 : 106 if (isdigit(ch)) {a2=ch-zero; state=3 ;} break; 107 case 3 : 108 if (isdigit(ch)) a2=10*a2+ch-zero; else state=4 ; break; 109 case 4 : 110 if (isdigit(ch)) {a3=ch-zero; state=5 ;} break; 111 case 5 : 112 if (isdigit(ch)) a3=10*a3+ch-zero; else state=6 ; break; 113 case 6 : 114 if (isdigit(ch)) {a4=ch-zero; state=7 ;} break; 115 case 7 : 116 if (isdigit(ch)) a4=10*a4+ch-zero; else state=8 ; break; 117 case 8 : 118 if (isdigit(ch)) {p1=ch-zero; state=9 ;} break; 119 case 9 : 120 if (isdigit(ch)) p1=10*p1+ch-zero; else state=10; break; 121 case 10: 122 if (isdigit(ch)) {p2=ch-zero; state=11;} break; 123 case 11: 124 if (isdigit(ch)) p2=10*p2+ch-zero; break; 125 } 126 } 127 128 if (state == 11) 129 { 130 true_port = htons((p1<<8) + p2); 131 true_addr.s_addr = htonl((a1<<24) + (a2<<16) +(a3<<8) + a4); 132 NewFtpPortCommand(pip, link, true_addr, true_port, maxpacketsize); 133 } 134 } 135 } 136 137 static void 138 NewFtpPortCommand(struct ip *pip, 139 struct alias_link *link, 140 struct in_addr true_addr, 141 u_short true_port, 142 int maxpacketsize) 143 { 144 struct alias_link *ftp_link; 145 146 /* Establish link to address and port found in PORT command */ 147 ftp_link = FindUdpTcpOut(true_addr, GetDestAddress(link), 148 true_port, 0, IPPROTO_TCP); 149 150 if (ftp_link != NULL) 151 { 152 int slen, hlen, tlen, dlen; 153 struct tcphdr *tc; 154 155 #ifndef NO_FW_PUNCH 156 /* Punch hole in firewall */ 157 PunchFWHole(ftp_link); 158 #endif 159 160 /* Calculate data length of TCP packet */ 161 tc = (struct tcphdr *) ((char *) pip + (pip->ip_hl << 2)); 162 hlen = (pip->ip_hl + tc->th_off) << 2; 163 tlen = ntohs(pip->ip_len); 164 dlen = tlen - hlen; 165 166 /* Create new PORT command */ 167 { 168 char stemp[80]; 169 char *sptr; 170 u_short alias_port; 171 u_char *ptr; 172 int a1, a2, a3, a4, p1, p2; 173 struct in_addr alias_address; 174 175 /* Decompose alias address into quad format */ 176 alias_address = GetAliasAddress(link); 177 ptr = (u_char *) &alias_address.s_addr; 178 a1 = *ptr++; a2=*ptr++; a3=*ptr++; a4=*ptr; 179 180 /* Decompose alias port into pair format */ 181 alias_port = GetAliasPort(ftp_link); 182 ptr = (char *) &alias_port; 183 p1 = *ptr++; p2=*ptr; 184 185 /* Generate command string */ 186 sprintf(stemp, "PORT %d,%d,%d,%d,%d,%d\r\n", 187 a1,a2,a3,a4,p1,p2); 188 189 /* Save string length for IP header modification */ 190 slen = strlen(stemp); 191 192 /* Copy into IP packet */ 193 sptr = (char *) pip; sptr += hlen; 194 strncpy(sptr, stemp, maxpacketsize-hlen); 195 } 196 197 /* Save information regarding modified seq and ack numbers */ 198 { 199 int delta; 200 201 SetAckModified(link); 202 delta = GetDeltaSeqOut(pip, link); 203 AddSeq(pip, link, delta+slen-dlen); 204 } 205 206 /* Revise IP header */ 207 { 208 u_short new_len; 209 210 new_len = htons(hlen + slen); 211 DifferentialChecksum(&pip->ip_sum, 212 &new_len, 213 &pip->ip_len, 214 1); 215 pip->ip_len = new_len; 216 } 217 218 /* Compute TCP checksum for revised packet */ 219 tc->th_sum = 0; 220 tc->th_sum = TcpChecksum(pip); 221 } 222 else 223 { 224 fprintf(stderr, 225 "PacketAlias/HandleFtpOut: Cannot allocate FTP data port\n"); 226 } 227 } 228