Network layer

int ip_local_deliver(struct sk_buff *skb)
{
        /*
         *      Reassemble IP fragments.
         */

        if (ip_hdr(skb)->frag_off & htons(IP_MF | IP_OFFSET)) {
                if (ip_defrag(skb, IP_DEFRAG_LOCAL_DELIVER))
                        return 0;
        }

        return NF_HOOK(PF_INET, NF_INET_LOCAL_IN, skb, skb->dev, NULL,
                       ip_local_deliver_finish);
}

int ip_defrag(struct sk_buff *skb, u32 user)
{
        struct ipq *qp;
        struct net *net;

        net = skb->dev ? dev_net(skb->dev) : dev_net(skb_dst(skb)->dev);
        IP_INC_STATS_BH(net, IPSTATS_MIB_REASMREQDS);

        /* Start by cleaning up the memory. */
        if (atomic_read(&net->ipv4.frags.mem) > net->ipv4.frags.high_thresh)
                ip_evictor(net);

        /* Lookup (or create) queue header */
        if ((qp = ip_find(net, ip_hdr(skb), user)) != NULL) {
                int ret;

                spin_lock(&qp->q.lock);

                ret = ip_frag_queue(qp, skb);

                spin_unlock(&qp->q.lock);
                ipq_put(qp);
                return ret;
        }

        IP_INC_STATS_BH(net, IPSTATS_MIB_REASMFAILS);
        kfree_skb(skb);
        return -ENOMEM;
}

int inet_add_protocol(const struct net_protocol *prot, unsigned char protocol)
{
        int hash, ret;

        hash = protocol & (MAX_INET_PROTOS - 1);

        spin_lock_bh(&inet_proto_lock);
        if (inet_protos[hash]) {
                ret = -1;
        } else {
                inet_protos[hash] = prot;
                ret = 0;
        }
        spin_unlock_bh(&inet_proto_lock);

        return ret;
}

int ip_route_input(struct sk_buff *skb, __be32 daddr, __be32 saddr,
                   u8 tos, struct net_device *dev)
{
        struct rtable * rth;
        unsigned        hash;
        int iif = dev->ifindex;
        struct net *net;

        net = dev_net(dev);

        if (!rt_caching(net))
                goto skip_cache;

        tos &= IPTOS_RT_MASK;
        hash = rt_hash(daddr, saddr, iif, rt_genid(net));

        rcu_read_lock();
        for (rth = rcu_dereference(rt_hash_table[hash].chain); rth;
             rth = rcu_dereference(rth->u.dst.rt_next)) {
                if (((rth->fl.fl4_dst ^ daddr) |
                     (rth->fl.fl4_src ^ saddr) |
                     (rth->fl.iif ^ iif) |
                     rth->fl.oif |
                     (rth->fl.fl4_tos ^ tos)) == 0 &&
                    rth->fl.mark == skb->mark &&
                    net_eq(dev_net(rth->u.dst.dev), net) &&
                    !rt_is_expired(rth)) {
                        dst_use(&rth->u.dst, jiffies);
                        RT_CACHE_STAT_INC(in_hit);
                        rcu_read_unlock();
                        skb_dst_set(skb, &rth->u.dst);
                        return 0;
                }
                RT_CACHE_STAT_INC(in_hlist_search);
        }
        rcu_read_unlock();

skip_cache:
        if (ipv4_is_multicast(daddr)) {
                struct in_device *in_dev;

                rcu_read_lock();
                if ((in_dev = __in_dev_get_rcu(dev)) != NULL) {
                        int our = ip_check_mc(in_dev, daddr, saddr,
                                ip_hdr(skb)->protocol);
                        if (our
#ifdef CONFIG_IP_MROUTE
                            || (!ipv4_is_local_multicast(daddr) &&
                                IN_DEV_MFORWARD(in_dev))
#endif
                            ) {
                                rcu_read_unlock();
                                return ip_route_input_mc(skb, daddr, saddr,
                                                         tos, dev, our);
                        }
                }
                rcu_read_unlock();
                return -EINVAL;
        }
        return ip_route_input_slow(skb, daddr, saddr, tos, dev);
}

int ip_queue_xmit(struct sk_buff *skb, int ipfragok)
{
        struct sock *sk = skb->sk;
        struct inet_sock *inet = inet_sk(sk);
        struct ip_options *opt = inet->opt;
        struct rtable *rt;
        struct iphdr *iph;
        /* Skip all of this if the packet is already routed,
         * f.e. by something like SCTP.
         */
        rt = skb_rtable(skb);
        if (rt != NULL)
                goto packet_routed;

        /* Make sure we can route this packet. */
        rt = (struct rtable *)__sk_dst_check(sk, 0);
        if (rt == NULL) {
                __be32 daddr;

                /* Use correct destination address if we have options. */
                daddr = inet->daddr;
                if(opt && opt->srr)
                        daddr = opt->faddr;

                {
                        struct flowi fl = { .oif = sk->sk_bound_dev_if,
                                            .mark = sk->sk_mark,
                                            .nl_u = { .ip4_u =
                                                      { .daddr = daddr,
                                                        .saddr = inet->saddr,
                                                        .tos = RT_CONN_FLAGS(sk) } },
                                            .proto = sk->sk_protocol,
                                            .flags = inet_sk_flowi_flags(sk),
                                            .uli_u = { .ports =
                                                       { .sport = inet->sport,
                                                         .dport = inet->dport } } };

                        /* If this fails, retransmit mechanism of transport layer will
                         * keep trying until route appears or the connection times
                         * itself out.
                         */
                        security_sk_classify_flow(sk, &fl);
                        if (ip_route_output_flow(sock_net(sk), &rt, &fl, sk, 0))
                                goto no_route;
                }
                sk_setup_caps(sk, &rt->u.dst);
        }
        skb_dst_set(skb, dst_clone(&rt->u.dst));

packet_routed:
        if (opt && opt->is_strictroute && rt->rt_dst != rt->rt_gateway)
                goto no_route;

        /* OK, we know where to send it, allocate and build IP header. */
        skb_push(skb, sizeof(struct iphdr) + (opt ? opt->optlen : 0));
        skb_reset_network_header(skb);
        iph = ip_hdr(skb);
        *((__be16 *)iph) = htons((4 << 12) | (5 << 8) | (inet->tos & 0xff));
        if (ip_dont_fragment(sk, &rt->u.dst) && !ipfragok)
                iph->frag_off = htons(IP_DF);
        else
                iph->frag_off = 0;
        iph->ttl      = ip_select_ttl(inet, &rt->u.dst);
        iph->protocol = sk->sk_protocol;
        iph->saddr    = rt->rt_src;
        iph->daddr    = rt->rt_dst;
        /* Transport layer set skb->h.foo itself. */

        if (opt && opt->optlen) {
                iph->ihl += opt->optlen >> 2;
                ip_options_build(skb, opt, inet->daddr, rt, 0);
        }

        ip_select_ident_more(iph, &rt->u.dst, sk,
                             (skb_shinfo(skb)->gso_segs ?: 1) - 1);

        skb->priority = sk->sk_priority;
        skb->mark = sk->sk_mark;

        return ip_local_out(skb);

no_route:
        IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTNOROUTES);
        kfree_skb(skb);
        return -EHOSTUNREACH;
}

int nf_hook_slow(u_int8_t pf, unsigned int hook, struct sk_buff *skb,
                 struct net_device *indev,
                 struct net_device *outdev,
                 int (*okfn)(struct sk_buff *),
                 int hook_thresh)
{
        struct list_head *elem;
        unsigned int verdict;
        int ret = 0;

        /* We may already have this, but read-locks nest anyway */
        rcu_read_lock();

        elem = &nf_hooks[pf][hook];
next_hook:
        verdict = nf_iterate(&nf_hooks[pf][hook], skb, hook, indev,
                             outdev, &elem, okfn, hook_thresh);
        if (verdict == NF_ACCEPT || verdict == NF_STOP) {
                ret = 1;
        } else if (verdict == NF_DROP) {
                kfree_skb(skb);
                ret = -EPERM;
        } else if ((verdict & NF_VERDICT_MASK) == NF_QUEUE) {
                if (!nf_queue(skb, elem, pf, hook, indev, outdev, okfn,
                              verdict >> NF_VERDICT_BITS))
                        goto next_hook;
        }
        rcu_read_unlock();
        return ret;
}