net/ipv4/netfilter/nf_reject_ipv4.c

   1 /* (C) 1999-2001 Paul `Rusty' Russell
   2  * (C) 2002-2004 Netfilter Core Team <coreteam@netfilter.org>
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License version 2 as
   6  * published by the Free Software Foundation.
   7  */
   8
   9 #include <net/ip.h>
  10 #include <net/tcp.h>
  11 #include <net/route.h>
  12 #include <net/dst.h>
  13 #include <linux/netfilter_ipv4.h>
  14
  15 /* Send RST reply */
  16 void nf_send_reset(struct sk_buff *oldskb, int hook)
  17 {
  18         struct sk_buff *nskb;
  19         const struct iphdr *oiph;
  20         struct iphdr *niph;
  21         const struct tcphdr *oth;
  22         struct tcphdr _otcph, *tcph;
  23
  24         /* IP header checks: fragment. */
  25         if (ip_hdr(oldskb)->frag_off & htons(IP_OFFSET))
  26                 return;
  27
  28         oth = skb_header_pointer(oldskb, ip_hdrlen(oldskb),
  29                                  sizeof(_otcph), &_otcph);
  30         if (oth == NULL)
  31                 return;
  32
  33         /* No RST for RST. */
  34         if (oth->rst)
  35                 return;
  36
  37         if (skb_rtable(oldskb)->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST))
  38                 return;
  39
  40         /* Check checksum */
  41         if (nf_ip_checksum(oldskb, hook, ip_hdrlen(oldskb), IPPROTO_TCP))
  42                 return;
  43         oiph = ip_hdr(oldskb);
  44
  45         nskb = alloc_skb(sizeof(struct iphdr) + sizeof(struct tcphdr) +
  46                          LL_MAX_HEADER, GFP_ATOMIC);
  47         if (!nskb)
  48                 return;
  49
  50         skb_reserve(nskb, LL_MAX_HEADER);
  51
  52         skb_reset_network_header(nskb);
  53         niph = (struct iphdr *)skb_put(nskb, sizeof(struct iphdr));
  54         niph->version   = 4;
  55         niph->ihl       = sizeof(struct iphdr) / 4;
  56         niph->tos       = 0;
  57         niph->id        = 0;
  58         niph->frag_off  = htons(IP_DF);
  59         niph->protocol  = IPPROTO_TCP;
  60         niph->check     = 0;
  61         niph->saddr     = oiph->daddr;
  62         niph->daddr     = oiph->saddr;
  63
  64         skb_reset_transport_header(nskb);
  65         tcph = (struct tcphdr *)skb_put(nskb, sizeof(struct tcphdr));
  66         memset(tcph, 0, sizeof(*tcph));
  67         tcph->source    = oth->dest;
  68         tcph->dest      = oth->source;
  69         tcph->doff      = sizeof(struct tcphdr) / 4;
  70
  71         if (oth->ack)
  72                 tcph->seq = oth->ack_seq;
  73         else {
  74                 tcph->ack_seq = htonl(ntohl(oth->seq) + oth->syn + oth->fin +
  75                                       oldskb->len - ip_hdrlen(oldskb) -
  76                                       (oth->doff << 2));
  77                 tcph->ack = 1;
  78         }
  79
  80         tcph->rst       = 1;
  81         tcph->check = ~tcp_v4_check(sizeof(struct tcphdr), niph->saddr,
  82                                     niph->daddr, 0);
  83         nskb->ip_summed = CHECKSUM_PARTIAL;
  84         nskb->csum_start = (unsigned char *)tcph - nskb->head;
  85         nskb->csum_offset = offsetof(struct tcphdr, check);
  86
  87         /* ip_route_me_harder expects skb->dst to be set */
  88         skb_dst_set_noref(nskb, skb_dst(oldskb));
  89
  90         nskb->protocol = htons(ETH_P_IP);
  91         if (ip_route_me_harder(nskb, RTN_UNSPEC))
  92                 goto free_nskb;
  93
  94         niph->ttl       = ip4_dst_hoplimit(skb_dst(nskb));
  95
  96         /* "Never happens" */
  97         if (nskb->len > dst_mtu(skb_dst(nskb)))
  98                 goto free_nskb;
  99
 100         nf_ct_attach(nskb, oldskb);
 101
 102 #if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
 103         /* If we use ip_local_out for bridged traffic, the MAC source on
 104          * the RST will be ours, instead of the destination's.  This confuses
 105          * some routers/firewalls, and they drop the packet.  So we need to
 106          * build the eth header using the original destination's MAC as the
 107          * source, and send the RST packet directly.
 108          */
 109         if (oldskb->nf_bridge) {
 110                 struct ethhdr *oeth = eth_hdr(oldskb);
 111                 nskb->dev = oldskb->nf_bridge->physindev;
 112                 niph->tot_len = htons(nskb->len);
 113                 ip_send_check(niph);
 114                 if (dev_hard_header(nskb, nskb->dev, ntohs(nskb->protocol),
 115                                     oeth->h_source, oeth->h_dest, nskb->len) < 0)
 116                         goto free_nskb;
 117                 dev_queue_xmit(nskb);
 118         } else
 119 #endif
 120                 ip_local_out(nskb);
 121
 122         return;
 123
 124  free_nskb:
 125         kfree_skb(nskb);
 126 }
 127 EXPORT_SYMBOL_GPL(nf_send_reset);