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Linux/net/ipv4/ip_forward.c

  1 /*
  2  * INET         An implementation of the TCP/IP protocol suite for the LINUX
  3  *              operating system.  INET is implemented using the  BSD Socket
  4  *              interface as the means of communication with the user level.
  5  *
  6  *              The IP forwarding functionality.
  7  *
  8  * Authors:     see ip.c
  9  *
 10  * Fixes:
 11  *              Many            :       Split from ip.c , see ip_input.c for
 12  *                                      history.
 13  *              Dave Gregorich  :       NULL ip_rt_put fix for multicast
 14  *                                      routing.
 15  *              Jos Vos         :       Add call_out_firewall before sending,
 16  *                                      use output device for accounting.
 17  *              Jos Vos         :       Call forward firewall after routing
 18  *                                      (always use output device).
 19  *              Mike McLagan    :       Routing by source
 20  */
 21 
 22 #include <linux/types.h>
 23 #include <linux/mm.h>
 24 #include <linux/skbuff.h>
 25 #include <linux/ip.h>
 26 #include <linux/icmp.h>
 27 #include <linux/netdevice.h>
 28 #include <linux/slab.h>
 29 #include <net/sock.h>
 30 #include <net/ip.h>
 31 #include <net/tcp.h>
 32 #include <net/udp.h>
 33 #include <net/icmp.h>
 34 #include <linux/tcp.h>
 35 #include <linux/udp.h>
 36 #include <linux/netfilter_ipv4.h>
 37 #include <net/checksum.h>
 38 #include <linux/route.h>
 39 #include <net/route.h>
 40 #include <net/xfrm.h>
 41 
 42 static bool ip_may_fragment(const struct sk_buff *skb)
 43 {
 44         return unlikely((ip_hdr(skb)->frag_off & htons(IP_DF)) == 0) ||
 45                !skb->local_df;
 46 }
 47 
 48 static bool ip_exceeds_mtu(const struct sk_buff *skb, unsigned int mtu)
 49 {
 50         if (skb->len <= mtu || skb->local_df)
 51                 return false;
 52 
 53         if (skb_is_gso(skb) && skb_gso_network_seglen(skb) <= mtu)
 54                 return false;
 55 
 56         return true;
 57 }
 58 
 59 static bool ip_gso_exceeds_dst_mtu(const struct sk_buff *skb)
 60 {
 61         unsigned int mtu;
 62 
 63         if (skb->local_df || !skb_is_gso(skb))
 64                 return false;
 65 
 66         mtu = ip_dst_mtu_maybe_forward(skb_dst(skb), true);
 67 
 68         /* if seglen > mtu, do software segmentation for IP fragmentation on
 69          * output.  DF bit cannot be set since ip_forward would have sent
 70          * icmp error.
 71          */
 72         return skb_gso_network_seglen(skb) > mtu;
 73 }
 74 
 75 /* called if GSO skb needs to be fragmented on forward */
 76 static int ip_forward_finish_gso(struct sk_buff *skb)
 77 {
 78         struct dst_entry *dst = skb_dst(skb);
 79         netdev_features_t features;
 80         struct sk_buff *segs;
 81         int ret = 0;
 82 
 83         features = netif_skb_dev_features(skb, dst->dev);
 84         segs = skb_gso_segment(skb, features & ~NETIF_F_GSO_MASK);
 85         if (IS_ERR(segs)) {
 86                 kfree_skb(skb);
 87                 return -ENOMEM;
 88         }
 89 
 90         consume_skb(skb);
 91 
 92         do {
 93                 struct sk_buff *nskb = segs->next;
 94                 int err;
 95 
 96                 segs->next = NULL;
 97                 err = dst_output(segs);
 98 
 99                 if (err && ret == 0)
100                         ret = err;
101                 segs = nskb;
102         } while (segs);
103 
104         return ret;
105 }
106 
107 static int ip_forward_finish(struct sk_buff *skb)
108 {
109         struct ip_options *opt  = &(IPCB(skb)->opt);
110 
111         IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTFORWDATAGRAMS);
112         IP_ADD_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTOCTETS, skb->len);
113 
114         if (unlikely(opt->optlen))
115                 ip_forward_options(skb);
116 
117         if (ip_gso_exceeds_dst_mtu(skb))
118                 return ip_forward_finish_gso(skb);
119 
120         return dst_output(skb);
121 }
122 
123 int ip_forward(struct sk_buff *skb)
124 {
125         u32 mtu;
126         struct iphdr *iph;      /* Our header */
127         struct rtable *rt;      /* Route we use */
128         struct ip_options *opt  = &(IPCB(skb)->opt);
129 
130         if (skb_warn_if_lro(skb))
131                 goto drop;
132 
133         if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))
134                 goto drop;
135 
136         if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
137                 return NET_RX_SUCCESS;
138 
139         if (skb->pkt_type != PACKET_HOST)
140                 goto drop;
141 
142         skb_forward_csum(skb);
143 
144         /*
145          *      According to the RFC, we must first decrease the TTL field. If
146          *      that reaches zero, we must reply an ICMP control message telling
147          *      that the packet's lifetime expired.
148          */
149         if (ip_hdr(skb)->ttl <= 1)
150                 goto too_many_hops;
151 
152         if (!xfrm4_route_forward(skb))
153                 goto drop;
154 
155         rt = skb_rtable(skb);
156 
157         if (opt->is_strictroute && rt->rt_uses_gateway)
158                 goto sr_failed;
159 
160         IPCB(skb)->flags |= IPSKB_FORWARDED;
161         mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
162         if (!ip_may_fragment(skb) && ip_exceeds_mtu(skb, mtu)) {
163                 IP_INC_STATS(dev_net(rt->dst.dev), IPSTATS_MIB_FRAGFAILS);
164                 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
165                           htonl(mtu));
166                 goto drop;
167         }
168 
169         /* We are about to mangle packet. Copy it! */
170         if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))
171                 goto drop;
172         iph = ip_hdr(skb);
173 
174         /* Decrease ttl after skb cow done */
175         ip_decrease_ttl(iph);
176 
177         /*
178          *      We now generate an ICMP HOST REDIRECT giving the route
179          *      we calculated.
180          */
181         if (rt->rt_flags&RTCF_DOREDIRECT && !opt->srr && !skb_sec_path(skb))
182                 ip_rt_send_redirect(skb);
183 
184         skb->priority = rt_tos2priority(iph->tos);
185 
186         return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD, skb, skb->dev,
187                        rt->dst.dev, ip_forward_finish);
188 
189 sr_failed:
190         /*
191          *      Strict routing permits no gatewaying
192          */
193          icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
194          goto drop;
195 
196 too_many_hops:
197         /* Tell the sender its packet died... */
198         IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_INHDRERRORS);
199         icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
200 drop:
201         kfree_skb(skb);
202         return NET_RX_DROP;
203 }

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