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   1 /*
   2  * This is a module which is used for rejecting packets.
   3  */
   4
   5 /* (C) 1999-2001 Paul `Rusty' Russell
   6  * (C) 2002-2004 Netfilter Core Team <coreteam@netfilter.org>
   7  *
   8  * This program is free software; you can redistribute it and/or modify
   9  * it under the terms of the GNU General Public License version 2 as
  10  * published by the Free Software Foundation.
  11  */
  12
  13 #include <linux/module.h>
  14 #include <linux/skbuff.h>
  15 #include <linux/ip.h>
  16 #include <linux/udp.h>
  17 #include <linux/icmp.h>
  18 #include <net/icmp.h>
  19 #include <net/ip.h>
  20 #include <net/tcp.h>
  21 #include <net/route.h>
  22 #include <net/dst.h>
  23 #include <linux/netfilter/x_tables.h>
  24 #include <linux/netfilter_ipv4/ip_tables.h>
  25 #include <linux/netfilter_ipv4/ipt_REJECT.h>
  26 #ifdef CONFIG_BRIDGE_NETFILTER
  27 #include <linux/netfilter_bridge.h>
  28 #endif
  29
  30 MODULE_LICENSE("GPL");
  31 MODULE_AUTHOR("Netfilter Core Team <coreteam@netfilter.org>");
  32 MODULE_DESCRIPTION("iptables REJECT target module");
  33
  34 #if 0
  35 #define DEBUGP printk
  36 #else
  37 #define DEBUGP(format, args...)
  38 #endif
  39
  40 /* Send RST reply */
  41 static void send_reset(struct sk_buff *oldskb, int hook)
  42 {
  43         struct sk_buff *nskb;
  44         struct iphdr *niph;
  45         struct tcphdr _otcph, *oth, *tcph;
  46         __be16 tmp_port;
  47         __be32 tmp_addr;
  48         int needs_ack;
  49         unsigned int addr_type;
  50
  51         /* IP header checks: fragment. */
  52         if (ip_hdr(oldskb)->frag_off & htons(IP_OFFSET))
  53                 return;
  54
  55         oth = skb_header_pointer(oldskb, ip_hdrlen(oldskb),
  56                                  sizeof(_otcph), &_otcph);
  57         if (oth == NULL)
  58                 return;
  59
  60         /* No RST for RST. */
  61         if (oth->rst)
  62                 return;
  63
  64         /* Check checksum */
  65         if (nf_ip_checksum(oldskb, hook, ip_hdrlen(oldskb), IPPROTO_TCP))
  66                 return;
  67
  68         /* We need a linear, writeable skb.  We also need to expand
  69            headroom in case hh_len of incoming interface < hh_len of
  70            outgoing interface */
  71         nskb = skb_copy_expand(oldskb, LL_MAX_HEADER, skb_tailroom(oldskb),
  72                                GFP_ATOMIC);
  73         if (!nskb)
  74                 return;
  75
  76         /* This packet will not be the same as the other: clear nf fields */
  77         nf_reset(nskb);
  78         nskb->mark = 0;
  79         skb_init_secmark(nskb);
  80
  81         skb_shinfo(nskb)->gso_size = 0;
  82         skb_shinfo(nskb)->gso_segs = 0;
  83         skb_shinfo(nskb)->gso_type = 0;
  84
  85         tcph = (struct tcphdr *)(skb_network_header(nskb) + ip_hdrlen(nskb));
  86
  87         /* Swap source and dest */
  88         niph = ip_hdr(nskb);
  89         tmp_addr = niph->saddr;
  90         niph->saddr = niph->daddr;
  91         niph->daddr = tmp_addr;
  92         tmp_port = tcph->source;
  93         tcph->source = tcph->dest;
  94         tcph->dest = tmp_port;
  95
  96         /* Truncate to length (no data) */
  97         tcph->doff = sizeof(struct tcphdr)/4;
  98         skb_trim(nskb, ip_hdrlen(nskb) + sizeof(struct tcphdr));
  99         niph->tot_len = htons(nskb->len);
 100
 101         if (tcph->ack) {
 102                 needs_ack = 0;
 103                 tcph->seq = oth->ack_seq;
 104                 tcph->ack_seq = 0;
 105         } else {
 106                 needs_ack = 1;
 107                 tcph->ack_seq = htonl(ntohl(oth->seq) + oth->syn + oth->fin +
 108                                       oldskb->len - ip_hdrlen(oldskb) -
 109                                       (oth->doff << 2));
 110                 tcph->seq = 0;
 111         }
 112
 113         /* Reset flags */
 114         ((u_int8_t *)tcph)[13] = 0;
 115         tcph->rst = 1;
 116         tcph->ack = needs_ack;
 117
 118         tcph->window = 0;
 119         tcph->urg_ptr = 0;
 120
 121         /* Adjust TCP checksum */
 122         tcph->check = 0;
 123         tcph->check = tcp_v4_check(sizeof(struct tcphdr),
 124                                    niph->saddr, niph->daddr,
 125                                    csum_partial(tcph,
 126                                                 sizeof(struct tcphdr), 0));
 127
 128         /* Set DF, id = 0 */
 129         niph->frag_off = htons(IP_DF);
 130         niph->id = 0;
 131
 132         addr_type = RTN_UNSPEC;
 133         if (hook != NF_IP_FORWARD
 134 #ifdef CONFIG_BRIDGE_NETFILTER
 135             || (nskb->nf_bridge && nskb->nf_bridge->mask & BRNF_BRIDGED)
 136 #endif
 137            )
 138                 addr_type = RTN_LOCAL;
 139
 140         if (ip_route_me_harder(&nskb, addr_type))
 141                 goto free_nskb;
 142
 143         nskb->ip_summed = CHECKSUM_NONE;
 144
 145         /* Adjust IP TTL */
 146         niph->ttl = dst_metric(nskb->dst, RTAX_HOPLIMIT);
 147
 148         /* Adjust IP checksum */
 149         niph->check = 0;
 150         niph->check = ip_fast_csum(skb_network_header(nskb), niph->ihl);
 151
 152         /* "Never happens" */
 153         if (nskb->len > dst_mtu(nskb->dst))
 154                 goto free_nskb;
 155
 156         nf_ct_attach(nskb, oldskb);
 157
 158         NF_HOOK(PF_INET, NF_IP_LOCAL_OUT, nskb, NULL, nskb->dst->dev,
 159                 dst_output);
 160         return;
 161
 162  free_nskb:
 163         kfree_skb(nskb);
 164 }
 165
 166 static inline void send_unreach(struct sk_buff *skb_in, int code)
 167 {
 168         icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
 169 }
 170
 171 static unsigned int reject(struct sk_buff **pskb,
 172                            const struct net_device *in,
 173                            const struct net_device *out,
 174                            unsigned int hooknum,
 175                            const struct xt_target *target,
 176                            const void *targinfo)
 177 {
 178         const struct ipt_reject_info *reject = targinfo;
 179
 180         /* Our naive response construction doesn't deal with IP
 181            options, and probably shouldn't try. */
 182         if (ip_hdrlen(*pskb) != sizeof(struct iphdr))
 183                 return NF_DROP;
 184
 185         /* WARNING: This code causes reentry within iptables.
 186            This means that the iptables jump stack is now crap.  We
 187            must return an absolute verdict. --RR */
 188         switch (reject->with) {
 189         case IPT_ICMP_NET_UNREACHABLE:
 190                 send_unreach(*pskb, ICMP_NET_UNREACH);
 191                 break;
 192         case IPT_ICMP_HOST_UNREACHABLE:
 193                 send_unreach(*pskb, ICMP_HOST_UNREACH);
 194                 break;
 195         case IPT_ICMP_PROT_UNREACHABLE:
 196                 send_unreach(*pskb, ICMP_PROT_UNREACH);
 197                 break;
 198         case IPT_ICMP_PORT_UNREACHABLE:
 199                 send_unreach(*pskb, ICMP_PORT_UNREACH);
 200                 break;
 201         case IPT_ICMP_NET_PROHIBITED:
 202                 send_unreach(*pskb, ICMP_NET_ANO);
 203                 break;
 204         case IPT_ICMP_HOST_PROHIBITED:
 205                 send_unreach(*pskb, ICMP_HOST_ANO);
 206                 break;
 207         case IPT_ICMP_ADMIN_PROHIBITED:
 208                 send_unreach(*pskb, ICMP_PKT_FILTERED);
 209                 break;
 210         case IPT_TCP_RESET:
 211                 send_reset(*pskb, hooknum);
 212         case IPT_ICMP_ECHOREPLY:
 213                 /* Doesn't happen. */
 214                 break;
 215         }
 216
 217         return NF_DROP;
 218 }
 219
 220 static bool check(const char *tablename,
 221                   const void *e_void,
 222                   const struct xt_target *target,
 223                   void *targinfo,
 224                   unsigned int hook_mask)
 225 {
 226         const struct ipt_reject_info *rejinfo = targinfo;
 227         const struct ipt_entry *e = e_void;
 228
 229         if (rejinfo->with == IPT_ICMP_ECHOREPLY) {
 230                 printk("REJECT: ECHOREPLY no longer supported.\n");
 231                 return false;
 232         } else if (rejinfo->with == IPT_TCP_RESET) {
 233                 /* Must specify that it's a TCP packet */
 234                 if (e->ip.proto != IPPROTO_TCP
 235                     || (e->ip.invflags & XT_INV_PROTO)) {
 236                         DEBUGP("REJECT: TCP_RESET invalid for non-tcp\n");
 237                         return false;
 238                 }
 239         }
 240         return true;
 241 }
 242
 243 static struct xt_target ipt_reject_reg __read_mostly = {
 244         .name           = "REJECT",
 245         .family         = AF_INET,
 246         .target         = reject,
 247         .targetsize     = sizeof(struct ipt_reject_info),
 248         .table          = "filter",
 249         .hooks          = (1 << NF_IP_LOCAL_IN) | (1 << NF_IP_FORWARD) |
 250                           (1 << NF_IP_LOCAL_OUT),
 251         .checkentry     = check,
 252         .me             = THIS_MODULE,
 253 };
 254
 255 static int __init ipt_reject_init(void)
 256 {
 257         return xt_register_target(&ipt_reject_reg);
 258 }
 259
 260 static void __exit ipt_reject_fini(void)
 261 {
 262         xt_unregister_target(&ipt_reject_reg);
 263 }
 264
 265 module_init(ipt_reject_init);
 266 module_exit(ipt_reject_fini);