tcp: dont drop MTU reduction indications

[linux-2.6/btrfs-unstable.git] / net / ipv4 / arp.c

/* linux/net/ipv4/arp.c
 *
 * Copyright (C) 1994 by Florian  La Roche
 *
 * This module implements the Address Resolution Protocol ARP (RFC 826),
 * which is used to convert IP addresses (or in the future maybe other
 * high-level addresses) into a low-level hardware address (like an Ethernet
 * address).
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version
 * 2 of the License, or (at your option) any later version.
 *
 * Fixes:
 *              Alan Cox        :       Removed the Ethernet assumptions in
 *                                      Florian's code
 *              Alan Cox        :       Fixed some small errors in the ARP
 *                                      logic
 *              Alan Cox        :       Allow >4K in /proc
 *              Alan Cox        :       Make ARP add its own protocol entry
 *              Ross Martin     :       Rewrote arp_rcv() and arp_get_info()
 *              Stephen Henson  :       Add AX25 support to arp_get_info()
 *              Alan Cox        :       Drop data when a device is downed.
 *              Alan Cox        :       Use init_timer().
 *              Alan Cox        :       Double lock fixes.
 *              Martin Seine    :       Move the arphdr structure
 *                                      to if_arp.h for compatibility.
 *                                      with BSD based programs.
 *              Andrew Tridgell :       Added ARP netmask code and
 *                                      re-arranged proxy handling.
 *              Alan Cox        :       Changed to use notifiers.
 *              Niibe Yutaka    :       Reply for this device or proxies only.
 *              Alan Cox        :       Don't proxy across hardware types!
 *              Jonathan Naylor :       Added support for NET/ROM.
 *              Mike Shaver     :       RFC1122 checks.
 *              Jonathan Naylor :       Only lookup the hardware address for
 *                                      the correct hardware type.
 *              Germano Caronni :       Assorted subtle races.
 *              Craig Schlenter :       Don't modify permanent entry
 *                                      during arp_rcv.
 *              Russ Nelson     :       Tidied up a few bits.
 *              Alexey Kuznetsov:       Major changes to caching and behaviour,
 *                                      eg intelligent arp probing and
 *                                      generation
 *                                      of host down events.
 *              Alan Cox        :       Missing unlock in device events.
 *              Eckes           :       ARP ioctl control errors.
 *              Alexey Kuznetsov:       Arp free fix.
 *              Manuel Rodriguez:       Gratuitous ARP.
 *              Jonathan Layes  :       Added arpd support through kerneld
 *                                      message queue (960314)
 *              Mike Shaver     :       /proc/sys/net/ipv4/arp_* support
 *              Mike McLagan    :       Routing by source
 *              Stuart Cheshire :       Metricom and grat arp fixes
 *                                      *** FOR 2.1 clean this up ***
 *              Lawrence V. Stefani: (08/12/96) Added FDDI support.
 *              Alan Cox        :       Took the AP1000 nasty FDDI hack and
 *                                      folded into the mainstream FDDI code.
 *                                      Ack spit, Linus how did you allow that
 *                                      one in...
 *              Jes Sorensen    :       Make FDDI work again in 2.1.x and
 *                                      clean up the APFDDI & gen. FDDI bits.
 *              Alexey Kuznetsov:       new arp state machine;
 *                                      now it is in net/core/neighbour.c.
 *              Krzysztof Halasa:       Added Frame Relay ARP support.
 *              Arnaldo C. Melo :       convert /proc/net/arp to seq_file
 *              Shmulik Hen:            Split arp_send to arp_create and
 *                                      arp_xmit so intermediate drivers like
 *                                      bonding can change the skb before
 *                                      sending (e.g. insert 8021q tag).
 *              Harald Welte    :       convert to make use of jenkins hash
 *              Jesper D. Brouer:       Proxy ARP PVLAN RFC 3069 support.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/module.h>
#include <linux/types.h>
#include <linux/string.h>
#include <linux/kernel.h>
#include <linux/capability.h>
#include <linux/socket.h>
#include <linux/sockios.h>
#include <linux/errno.h>
#include <linux/in.h>
#include <linux/mm.h>
#include <linux/inet.h>
#include <linux/inetdevice.h>
#include <linux/netdevice.h>
#include <linux/etherdevice.h>
#include <linux/fddidevice.h>
#include <linux/if_arp.h>
#include <linux/skbuff.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/stat.h>
#include <linux/init.h>
#include <linux/net.h>
#include <linux/rcupdate.h>
#include <linux/slab.h>
#ifdef CONFIG_SYSCTL
#include <linux/sysctl.h>
#endif

#include <net/net_namespace.h>
#include <net/ip.h>
#include <net/icmp.h>
#include <net/route.h>
#include <net/protocol.h>
#include <net/tcp.h>
#include <net/sock.h>
#include <net/arp.h>
#include <net/ax25.h>
#include <net/netrom.h>

#include <linux/uaccess.h>

#include <linux/netfilter_arp.h>

/*
 *      Interface to generic neighbour cache.
 */
static u32 arp_hash(const void *pkey, const struct net_device *dev, __u32 *hash_rnd);
static int arp_constructor(struct neighbour *neigh);
static void arp_solicit(struct neighbour *neigh, struct sk_buff *skb);
static void arp_error_report(struct neighbour *neigh, struct sk_buff *skb);
static void parp_redo(struct sk_buff *skb);

static const struct neigh_ops arp_generic_ops = {
        .family =               AF_INET,
        .solicit =              arp_solicit,
        .error_report =         arp_error_report,
        .output =               neigh_resolve_output,
        .connected_output =     neigh_connected_output,
};

static const struct neigh_ops arp_hh_ops = {
        .family =               AF_INET,
        .solicit =              arp_solicit,
        .error_report =         arp_error_report,
        .output =               neigh_resolve_output,
        .connected_output =     neigh_resolve_output,
};

static const struct neigh_ops arp_direct_ops = {
        .family =               AF_INET,
        .output =               neigh_direct_output,
        .connected_output =     neigh_direct_output,
};

static const struct neigh_ops arp_broken_ops = {
        .family =               AF_INET,
        .solicit =              arp_solicit,
        .error_report =         arp_error_report,
        .output =               neigh_compat_output,
        .connected_output =     neigh_compat_output,
};

struct neigh_table arp_tbl = {
        .family         = AF_INET,
        .key_len        = 4,
        .hash           = arp_hash,
        .constructor    = arp_constructor,
        .proxy_redo     = parp_redo,
        .id             = "arp_cache",
        .parms          = {
                .tbl                    = &arp_tbl,
                .base_reachable_time    = 30 * HZ,
                .retrans_time           = 1 * HZ,
                .gc_staletime           = 60 * HZ,
                .reachable_time         = 30 * HZ,
                .delay_probe_time       = 5 * HZ,
                .queue_len_bytes        = 64*1024,
                .ucast_probes           = 3,
                .mcast_probes           = 3,
                .anycast_delay          = 1 * HZ,
                .proxy_delay            = (8 * HZ) / 10,
                .proxy_qlen             = 64,
                .locktime               = 1 * HZ,
        },
        .gc_interval    = 30 * HZ,
        .gc_thresh1     = 128,
        .gc_thresh2     = 512,
        .gc_thresh3     = 1024,
};
EXPORT_SYMBOL(arp_tbl);

int arp_mc_map(__be32 addr, u8 *haddr, struct net_device *dev, int dir)
{
        switch (dev->type) {
        case ARPHRD_ETHER:
        case ARPHRD_FDDI:
        case ARPHRD_IEEE802:
                ip_eth_mc_map(addr, haddr);
                return 0;
        case ARPHRD_INFINIBAND:
                ip_ib_mc_map(addr, dev->broadcast, haddr);
                return 0;
        case ARPHRD_IPGRE:
                ip_ipgre_mc_map(addr, dev->broadcast, haddr);
                return 0;
        default:
                if (dir) {
                        memcpy(haddr, dev->broadcast, dev->addr_len);
                        return 0;
                }
        }
        return -EINVAL;
}


static u32 arp_hash(const void *pkey,
                    const struct net_device *dev,
                    __u32 *hash_rnd)
{
        return arp_hashfn(*(u32 *)pkey, dev, *hash_rnd);
}

static int arp_constructor(struct neighbour *neigh)
{
        __be32 addr = *(__be32 *)neigh->primary_key;
        struct net_device *dev = neigh->dev;
        struct in_device *in_dev;
        struct neigh_parms *parms;

        rcu_read_lock();
        in_dev = __in_dev_get_rcu(dev);
        if (in_dev == NULL) {
                rcu_read_unlock();
                return -EINVAL;
        }

        neigh->type = inet_addr_type(dev_net(dev), addr);

        parms = in_dev->arp_parms;
        __neigh_parms_put(neigh->parms);
        neigh->parms = neigh_parms_clone(parms);
        rcu_read_unlock();

        if (!dev->header_ops) {
                neigh->nud_state = NUD_NOARP;
                neigh->ops = &arp_direct_ops;
                neigh->output = neigh_direct_output;
        } else {
                /* Good devices (checked by reading texts, but only Ethernet is
                   tested)

                   ARPHRD_ETHER: (ethernet, apfddi)
                   ARPHRD_FDDI: (fddi)
                   ARPHRD_IEEE802: (tr)
                   ARPHRD_METRICOM: (strip)
                   ARPHRD_ARCNET:
                   etc. etc. etc.

                   ARPHRD_IPDDP will also work, if author repairs it.
                   I did not it, because this driver does not work even
                   in old paradigm.
                 */

#if 1
                /* So... these "amateur" devices are hopeless.
                   The only thing, that I can say now:
                   It is very sad that we need to keep ugly obsolete
                   code to make them happy.

                   They should be moved to more reasonable state, now
                   they use rebuild_header INSTEAD OF hard_start_xmit!!!
                   Besides that, they are sort of out of date
                   (a lot of redundant clones/copies, useless in 2.1),
                   I wonder why people believe that they work.
                 */
                switch (dev->type) {
                default:
                        break;
                case ARPHRD_ROSE:
#if IS_ENABLED(CONFIG_AX25)
                case ARPHRD_AX25:
#if IS_ENABLED(CONFIG_NETROM)
                case ARPHRD_NETROM:
#endif
                        neigh->ops = &arp_broken_ops;
                        neigh->output = neigh->ops->output;
                        return 0;
#else
                        break;
#endif
                }
#endif
                if (neigh->type == RTN_MULTICAST) {
                        neigh->nud_state = NUD_NOARP;
                        arp_mc_map(addr, neigh->ha, dev, 1);
                } else if (dev->flags & (IFF_NOARP | IFF_LOOPBACK)) {
                        neigh->nud_state = NUD_NOARP;
                        memcpy(neigh->ha, dev->dev_addr, dev->addr_len);
                } else if (neigh->type == RTN_BROADCAST ||
                           (dev->flags & IFF_POINTOPOINT)) {
                        neigh->nud_state = NUD_NOARP;
                        memcpy(neigh->ha, dev->broadcast, dev->addr_len);
                }

                if (dev->header_ops->cache)
                        neigh->ops = &arp_hh_ops;
                else
                        neigh->ops = &arp_generic_ops;

                if (neigh->nud_state & NUD_VALID)
                        neigh->output = neigh->ops->connected_output;
                else
                        neigh->output = neigh->ops->output;
        }
        return 0;
}

static void arp_error_report(struct neighbour *neigh, struct sk_buff *skb)
{
        dst_link_failure(skb);
        kfree_skb(skb);
}

static void arp_solicit(struct neighbour *neigh, struct sk_buff *skb)
{
        __be32 saddr = 0;
        u8  *dst_ha = NULL;
        struct net_device *dev = neigh->dev;
        __be32 target = *(__be32 *)neigh->primary_key;
        int probes = atomic_read(&neigh->probes);
        struct in_device *in_dev;

        rcu_read_lock();
        in_dev = __in_dev_get_rcu(dev);
        if (!in_dev) {
                rcu_read_unlock();
                return;
        }
        switch (IN_DEV_ARP_ANNOUNCE(in_dev)) {
        default:
        case 0:         /* By default announce any local IP */
                if (skb && inet_addr_type(dev_net(dev),
                                          ip_hdr(skb)->saddr) == RTN_LOCAL)
                        saddr = ip_hdr(skb)->saddr;
                break;
        case 1:         /* Restrict announcements of saddr in same subnet */
                if (!skb)
                        break;
                saddr = ip_hdr(skb)->saddr;
                if (inet_addr_type(dev_net(dev), saddr) == RTN_LOCAL) {
                        /* saddr should be known to target */
                        if (inet_addr_onlink(in_dev, target, saddr))
                                break;
                }
                saddr = 0;
                break;
        case 2:         /* Avoid secondary IPs, get a primary/preferred one */
                break;
        }
        rcu_read_unlock();

        if (!saddr)
                saddr = inet_select_addr(dev, target, RT_SCOPE_LINK);

        probes -= neigh->parms->ucast_probes;
        if (probes < 0) {
                if (!(neigh->nud_state & NUD_VALID))
                        pr_debug("trying to ucast probe in NUD_INVALID\n");
                dst_ha = neigh->ha;
                read_lock_bh(&neigh->lock);
        } else {
                probes -= neigh->parms->app_probes;
                if (probes < 0) {
#ifdef CONFIG_ARPD
                        neigh_app_ns(neigh);
#endif
                        return;
                }
        }

        arp_send(ARPOP_REQUEST, ETH_P_ARP, target, dev, saddr,
                 dst_ha, dev->dev_addr, NULL);
        if (dst_ha)
                read_unlock_bh(&neigh->lock);
}

static int arp_ignore(struct in_device *in_dev, __be32 sip, __be32 tip)
{
        int scope;

        switch (IN_DEV_ARP_IGNORE(in_dev)) {
        case 0: /* Reply, the tip is already validated */
                return 0;
        case 1: /* Reply only if tip is configured on the incoming interface */
                sip = 0;
                scope = RT_SCOPE_HOST;
                break;
        case 2: /*
                 * Reply only if tip is configured on the incoming interface
                 * and is in same subnet as sip
                 */
                scope = RT_SCOPE_HOST;
                break;
        case 3: /* Do not reply for scope host addresses */
                sip = 0;
                scope = RT_SCOPE_LINK;
                break;
        case 4: /* Reserved */
        case 5:
        case 6:
        case 7:
                return 0;
        case 8: /* Do not reply */
                return 1;
        default:
                return 0;
        }
        return !inet_confirm_addr(in_dev, sip, tip, scope);
}

static int arp_filter(__be32 sip, __be32 tip, struct net_device *dev)
{
        struct rtable *rt;
        int flag = 0;
        /*unsigned long now; */
        struct net *net = dev_net(dev);

        rt = ip_route_output(net, sip, tip, 0, 0);
        if (IS_ERR(rt))
                return 1;
        if (rt->dst.dev != dev) {
                NET_INC_STATS_BH(net, LINUX_MIB_ARPFILTER);
                flag = 1;
        }
        ip_rt_put(rt);
        return flag;
}

/* OBSOLETE FUNCTIONS */

/*
 *      Find an arp mapping in the cache. If not found, post a request.
 *
 *      It is very UGLY routine: it DOES NOT use skb->dst->neighbour,
 *      even if it exists. It is supposed that skb->dev was mangled
 *      by a virtual device (eql, shaper). Nobody but broken devices
 *      is allowed to use this function, it is scheduled to be removed. --ANK
 */

static int arp_set_predefined(int addr_hint, unsigned char *haddr,
                              __be32 paddr, struct net_device *dev)
{
        switch (addr_hint) {
        case RTN_LOCAL:
                pr_debug("arp called for own IP address\n");
                memcpy(haddr, dev->dev_addr, dev->addr_len);
                return 1;
        case RTN_MULTICAST:
                arp_mc_map(paddr, haddr, dev, 1);
                return 1;
        case RTN_BROADCAST:
                memcpy(haddr, dev->broadcast, dev->addr_len);
                return 1;
        }
        return 0;
}


int arp_find(unsigned char *haddr, struct sk_buff *skb)
{
        struct net_device *dev = skb->dev;
        __be32 paddr;
        struct neighbour *n;

        if (!skb_dst(skb)) {
                pr_debug("arp_find is called with dst==NULL\n");
                kfree_skb(skb);
                return 1;
        }

        paddr = rt_nexthop(skb_rtable(skb), ip_hdr(skb)->daddr);
        if (arp_set_predefined(inet_addr_type(dev_net(dev), paddr), haddr,
                               paddr, dev))
                return 0;

        n = __neigh_lookup(&arp_tbl, &paddr, dev, 1);

        if (n) {
                n->used = jiffies;
                if (n->nud_state & NUD_VALID || neigh_event_send(n, skb) == 0) {
                        neigh_ha_snapshot(haddr, n, dev);
                        neigh_release(n);
                        return 0;
                }
                neigh_release(n);
        } else
                kfree_skb(skb);
        return 1;
}
EXPORT_SYMBOL(arp_find);

/* END OF OBSOLETE FUNCTIONS */

/*
 * Check if we can use proxy ARP for this path
 */
static inline int arp_fwd_proxy(struct in_device *in_dev,
                                struct net_device *dev, struct rtable *rt)
{
        struct in_device *out_dev;
        int imi, omi = -1;

        if (rt->dst.dev == dev)
                return 0;

        if (!IN_DEV_PROXY_ARP(in_dev))
                return 0;
        imi = IN_DEV_MEDIUM_ID(in_dev);
        if (imi == 0)
                return 1;
        if (imi == -1)
                return 0;

        /* place to check for proxy_arp for routes */

        out_dev = __in_dev_get_rcu(rt->dst.dev);
        if (out_dev)
                omi = IN_DEV_MEDIUM_ID(out_dev);

        return omi != imi && omi != -1;
}

/*
 * Check for RFC3069 proxy arp private VLAN (allow to send back to same dev)
 *
 * RFC3069 supports proxy arp replies back to the same interface.  This
 * is done to support (ethernet) switch features, like RFC 3069, where
 * the individual ports are not allowed to communicate with each
 * other, BUT they are allowed to talk to the upstream router.  As
 * described in RFC 3069, it is possible to allow these hosts to
 * communicate through the upstream router, by proxy_arp'ing.
 *
 * RFC 3069: "VLAN Aggregation for Efficient IP Address Allocation"
 *
 *  This technology is known by different names:
 *    In RFC 3069 it is called VLAN Aggregation.
 *    Cisco and Allied Telesyn call it Private VLAN.
 *    Hewlett-Packard call it Source-Port filtering or port-isolation.
 *    Ericsson call it MAC-Forced Forwarding (RFC Draft).
 *
 */
static inline int arp_fwd_pvlan(struct in_device *in_dev,
                                struct net_device *dev, struct rtable *rt,
                                __be32 sip, __be32 tip)
{
        /* Private VLAN is only concerned about the same ethernet segment */
        if (rt->dst.dev != dev)
                return 0;

        /* Don't reply on self probes (often done by windowz boxes)*/
        if (sip == tip)
                return 0;

        if (IN_DEV_PROXY_ARP_PVLAN(in_dev))
                return 1;
        else
                return 0;
}

/*
 *      Interface to link layer: send routine and receive handler.
 */

/*
 *      Create an arp packet. If (dest_hw == NULL), we create a broadcast
 *      message.
 */
struct sk_buff *arp_create(int type, int ptype, __be32 dest_ip,
                           struct net_device *dev, __be32 src_ip,
                           const unsigned char *dest_hw,
                           const unsigned char *src_hw,
                           const unsigned char *target_hw)
{
        struct sk_buff *skb;
        struct arphdr *arp;
        unsigned char *arp_ptr;
        int hlen = LL_RESERVED_SPACE(dev);
        int tlen = dev->needed_tailroom;

        /*
         *      Allocate a buffer
         */

        skb = alloc_skb(arp_hdr_len(dev) + hlen + tlen, GFP_ATOMIC);
        if (skb == NULL)
                return NULL;

        skb_reserve(skb, hlen);
        skb_reset_network_header(skb);
        arp = (struct arphdr *) skb_put(skb, arp_hdr_len(dev));
        skb->dev = dev;
        skb->protocol = htons(ETH_P_ARP);
        if (src_hw == NULL)
                src_hw = dev->dev_addr;
        if (dest_hw == NULL)
                dest_hw = dev->broadcast;

        /*
         *      Fill the device header for the ARP frame
         */
        if (dev_hard_header(skb, dev, ptype, dest_hw, src_hw, skb->len) < 0)
                goto out;

        /*
         * Fill out the arp protocol part.
         *
         * The arp hardware type should match the device type, except for FDDI,
         * which (according to RFC 1390) should always equal 1 (Ethernet).
         */
        /*
         *      Exceptions everywhere. AX.25 uses the AX.25 PID value not the
         *      DIX code for the protocol. Make these device structure fields.
         */
        switch (dev->type) {
        default:
                arp->ar_hrd = htons(dev->type);
                arp->ar_pro = htons(ETH_P_IP);
                break;

#if IS_ENABLED(CONFIG_AX25)
        case ARPHRD_AX25:
                arp->ar_hrd = htons(ARPHRD_AX25);
                arp->ar_pro = htons(AX25_P_IP);
                break;

#if IS_ENABLED(CONFIG_NETROM)
        case ARPHRD_NETROM:
                arp->ar_hrd = htons(ARPHRD_NETROM);
                arp->ar_pro = htons(AX25_P_IP);
                break;
#endif
#endif

#if IS_ENABLED(CONFIG_FDDI)
        case ARPHRD_FDDI:
                arp->ar_hrd = htons(ARPHRD_ETHER);
                arp->ar_pro = htons(ETH_P_IP);
                break;
#endif
        }

        arp->ar_hln = dev->addr_len;
        arp->ar_pln = 4;
        arp->ar_op = htons(type);

        arp_ptr = (unsigned char *)(arp + 1);

        memcpy(arp_ptr, src_hw, dev->addr_len);
        arp_ptr += dev->addr_len;
        memcpy(arp_ptr, &src_ip, 4);
        arp_ptr += 4;
        if (target_hw != NULL)
                memcpy(arp_ptr, target_hw, dev->addr_len);
        else
                memset(arp_ptr, 0, dev->addr_len);
        arp_ptr += dev->addr_len;
        memcpy(arp_ptr, &dest_ip, 4);

        return skb;

out:
        kfree_skb(skb);
        return NULL;
}
EXPORT_SYMBOL(arp_create);

/*
 *      Send an arp packet.
 */
void arp_xmit(struct sk_buff *skb)
{
        /* Send it off, maybe filter it using firewalling first.  */
        NF_HOOK(NFPROTO_ARP, NF_ARP_OUT, skb, NULL, skb->dev, dev_queue_xmit);
}
EXPORT_SYMBOL(arp_xmit);

/*
 *      Create and send an arp packet.
 */
void arp_send(int type, int ptype, __be32 dest_ip,
              struct net_device *dev, __be32 src_ip,
              const unsigned char *dest_hw, const unsigned char *src_hw,
              const unsigned char *target_hw)
{
        struct sk_buff *skb;

        /*
         *      No arp on this interface.
         */

        if (dev->flags&IFF_NOARP)
                return;

        skb = arp_create(type, ptype, dest_ip, dev, src_ip,
                         dest_hw, src_hw, target_hw);
        if (skb == NULL)
                return;

        arp_xmit(skb);
}
EXPORT_SYMBOL(arp_send);

/*
 *      Process an arp request.
 */

static int arp_process(struct sk_buff *skb)
{
        struct net_device *dev = skb->dev;
        struct in_device *in_dev = __in_dev_get_rcu(dev);
        struct arphdr *arp;
        unsigned char *arp_ptr;
        struct rtable *rt;
        unsigned char *sha;
        __be32 sip, tip;
        u16 dev_type = dev->type;
        int addr_type;
        struct neighbour *n;
        struct net *net = dev_net(dev);

        /* arp_rcv below verifies the ARP header and verifies the device
         * is ARP'able.
         */

        if (in_dev == NULL)
                goto out;

        arp = arp_hdr(skb);

        switch (dev_type) {
        default:
                if (arp->ar_pro != htons(ETH_P_IP) ||
                    htons(dev_type) != arp->ar_hrd)
                        goto out;
                break;
        case ARPHRD_ETHER:
        case ARPHRD_FDDI:
        case ARPHRD_IEEE802:
                /*
                 * ETHERNET, and Fibre Channel (which are IEEE 802
                 * devices, according to RFC 2625) devices will accept ARP
                 * hardware types of either 1 (Ethernet) or 6 (IEEE 802.2).
                 * This is the case also of FDDI, where the RFC 1390 says that
                 * FDDI devices should accept ARP hardware of (1) Ethernet,
                 * however, to be more robust, we'll accept both 1 (Ethernet)
                 * or 6 (IEEE 802.2)
                 */
                if ((arp->ar_hrd != htons(ARPHRD_ETHER) &&
                     arp->ar_hrd != htons(ARPHRD_IEEE802)) ||
                    arp->ar_pro != htons(ETH_P_IP))
                        goto out;
                break;
        case ARPHRD_AX25:
                if (arp->ar_pro != htons(AX25_P_IP) ||
                    arp->ar_hrd != htons(ARPHRD_AX25))
                        goto out;
                break;
        case ARPHRD_NETROM:
                if (arp->ar_pro != htons(AX25_P_IP) ||
                    arp->ar_hrd != htons(ARPHRD_NETROM))
                        goto out;
                break;
        }

        /* Understand only these message types */

        if (arp->ar_op != htons(ARPOP_REPLY) &&
            arp->ar_op != htons(ARPOP_REQUEST))
                goto out;

/*
 *      Extract fields
 */
        arp_ptr = (unsigned char *)(arp + 1);
        sha     = arp_ptr;
        arp_ptr += dev->addr_len;
        memcpy(&sip, arp_ptr, 4);
        arp_ptr += 4;
        arp_ptr += dev->addr_len;
        memcpy(&tip, arp_ptr, 4);
/*
 *      Check for bad requests for 127.x.x.x and requests for multicast
 *      addresses.  If this is one such, delete it.
 */
        if (ipv4_is_multicast(tip) ||
            (!IN_DEV_ROUTE_LOCALNET(in_dev) && ipv4_is_loopback(tip)))
                goto out;

/*
 *     Special case: We must set Frame Relay source Q.922 address
 */
        if (dev_type == ARPHRD_DLCI)
                sha = dev->broadcast;

/*
 *  Process entry.  The idea here is we want to send a reply if it is a
 *  request for us or if it is a request for someone else that we hold
 *  a proxy for.  We want to add an entry to our cache if it is a reply
 *  to us or if it is a request for our address.
 *  (The assumption for this last is that if someone is requesting our
 *  address, they are probably intending to talk to us, so it saves time
 *  if we cache their address.  Their address is also probably not in
 *  our cache, since ours is not in their cache.)
 *
 *  Putting this another way, we only care about replies if they are to
 *  us, in which case we add them to the cache.  For requests, we care
 *  about those for us and those for our proxies.  We reply to both,
 *  and in the case of requests for us we add the requester to the arp
 *  cache.
 */

        /* Special case: IPv4 duplicate address detection packet (RFC2131) */
        if (sip == 0) {
                if (arp->ar_op == htons(ARPOP_REQUEST) &&
                    inet_addr_type(net, tip) == RTN_LOCAL &&
                    !arp_ignore(in_dev, sip, tip))
                        arp_send(ARPOP_REPLY, ETH_P_ARP, sip, dev, tip, sha,
                                 dev->dev_addr, sha);
                goto out;
        }

        if (arp->ar_op == htons(ARPOP_REQUEST) &&
            ip_route_input(skb, tip, sip, 0, dev) == 0) {

                rt = skb_rtable(skb);
                addr_type = rt->rt_type;

                if (addr_type == RTN_LOCAL) {
                        int dont_send;

                        dont_send = arp_ignore(in_dev, sip, tip);
                        if (!dont_send && IN_DEV_ARPFILTER(in_dev))
                                dont_send = arp_filter(sip, tip, dev);
                        if (!dont_send) {
                                n = neigh_event_ns(&arp_tbl, sha, &sip, dev);
                                if (n) {
                                        arp_send(ARPOP_REPLY, ETH_P_ARP, sip,
                                                 dev, tip, sha, dev->dev_addr,
                                                 sha);
                                        neigh_release(n);
                                }
                        }
                        goto out;
                } else if (IN_DEV_FORWARD(in_dev)) {
                        if (addr_type == RTN_UNICAST  &&
                            (arp_fwd_proxy(in_dev, dev, rt) ||
                             arp_fwd_pvlan(in_dev, dev, rt, sip, tip) ||
                             (rt->dst.dev != dev &&
                              pneigh_lookup(&arp_tbl, net, &tip, dev, 0)))) {
                                n = neigh_event_ns(&arp_tbl, sha, &sip, dev);
                                if (n)
                                        neigh_release(n);

                                if (NEIGH_CB(skb)->flags & LOCALLY_ENQUEUED ||
                                    skb->pkt_type == PACKET_HOST ||
                                    in_dev->arp_parms->proxy_delay == 0) {
                                        arp_send(ARPOP_REPLY, ETH_P_ARP, sip,
                                                 dev, tip, sha, dev->dev_addr,
                                                 sha);
                                } else {
                                        pneigh_enqueue(&arp_tbl,
                                                       in_dev->arp_parms, skb);
                                        return 0;
                                }
                                goto out;
                        }
                }
        }

        /* Update our ARP tables */

        n = __neigh_lookup(&arp_tbl, &sip, dev, 0);

        if (IN_DEV_ARP_ACCEPT(in_dev)) {
                /* Unsolicited ARP is not accepted by default.
                   It is possible, that this option should be enabled for some
                   devices (strip is candidate)
                 */
                if (n == NULL &&
                    (arp->ar_op == htons(ARPOP_REPLY) ||
                     (arp->ar_op == htons(ARPOP_REQUEST) && tip == sip)) &&
                    inet_addr_type(net, sip) == RTN_UNICAST)
                        n = __neigh_lookup(&arp_tbl, &sip, dev, 1);
        }

        if (n) {
                int state = NUD_REACHABLE;
                int override;

                /* If several different ARP replies follows back-to-back,
                   use the FIRST one. It is possible, if several proxy
                   agents are active. Taking the first reply prevents
                   arp trashing and chooses the fastest router.
                 */
                override = time_after(jiffies, n->updated + n->parms->locktime);

                /* Broadcast replies and request packets
                   do not assert neighbour reachability.
                 */
                if (arp->ar_op != htons(ARPOP_REPLY) ||
                    skb->pkt_type != PACKET_HOST)
                        state = NUD_STALE;
                neigh_update(n, sha, state,
                             override ? NEIGH_UPDATE_F_OVERRIDE : 0);
                neigh_release(n);
        }

out:
        consume_skb(skb);
        return 0;
}

static void parp_redo(struct sk_buff *skb)
{
        arp_process(skb);
}


/*
 *      Receive an arp request from the device layer.
 */

static int arp_rcv(struct sk_buff *skb, struct net_device *dev,
                   struct packet_type *pt, struct net_device *orig_dev)
{
        struct arphdr *arp;

        /* ARP header, plus 2 device addresses, plus 2 IP addresses.  */
        if (!pskb_may_pull(skb, arp_hdr_len(dev)))
                goto freeskb;

        arp = arp_hdr(skb);
        if (arp->ar_hln != dev->addr_len ||
            dev->flags & IFF_NOARP ||
            skb->pkt_type == PACKET_OTHERHOST ||
            skb->pkt_type == PACKET_LOOPBACK ||
            arp->ar_pln != 4)
                goto freeskb;

        skb = skb_share_check(skb, GFP_ATOMIC);
        if (skb == NULL)
                goto out_of_mem;

        memset(NEIGH_CB(skb), 0, sizeof(struct neighbour_cb));

        return NF_HOOK(NFPROTO_ARP, NF_ARP_IN, skb, dev, NULL, arp_process);

freeskb:
        kfree_skb(skb);
out_of_mem:
        return 0;
}

/*
 *      User level interface (ioctl)
 */

/*
 *      Set (create) an ARP cache entry.
 */

static int arp_req_set_proxy(struct net *net, struct net_device *dev, int on)
{
        if (dev == NULL) {
                IPV4_DEVCONF_ALL(net, PROXY_ARP) = on;
                return 0;
        }
        if (__in_dev_get_rtnl(dev)) {
                IN_DEV_CONF_SET(__in_dev_get_rtnl(dev), PROXY_ARP, on);
                return 0;
        }
        return -ENXIO;
}

static int arp_req_set_public(struct net *net, struct arpreq *r,
                struct net_device *dev)
{
        __be32 ip = ((struct sockaddr_in *)&r->arp_pa)->sin_addr.s_addr;
        __be32 mask = ((struct sockaddr_in *)&r->arp_netmask)->sin_addr.s_addr;

        if (mask && mask != htonl(0xFFFFFFFF))
                return -EINVAL;
        if (!dev && (r->arp_flags & ATF_COM)) {
                dev = dev_getbyhwaddr_rcu(net, r->arp_ha.sa_family,
                                      r->arp_ha.sa_data);
                if (!dev)
                        return -ENODEV;
        }
        if (mask) {
                if (pneigh_lookup(&arp_tbl, net, &ip, dev, 1) == NULL)
                        return -ENOBUFS;
                return 0;
        }

        return arp_req_set_proxy(net, dev, 1);
}

static int arp_req_set(struct net *net, struct arpreq *r,
                       struct net_device *dev)
{
        __be32 ip;
        struct neighbour *neigh;
        int err;

        if (r->arp_flags & ATF_PUBL)
                return arp_req_set_public(net, r, dev);

        ip = ((struct sockaddr_in *)&r->arp_pa)->sin_addr.s_addr;
        if (r->arp_flags & ATF_PERM)
                r->arp_flags |= ATF_COM;
        if (dev == NULL) {
                struct rtable *rt = ip_route_output(net, ip, 0, RTO_ONLINK, 0);

                if (IS_ERR(rt))
                        return PTR_ERR(rt);
                dev = rt->dst.dev;
                ip_rt_put(rt);
                if (!dev)
                        return -EINVAL;
        }
        switch (dev->type) {
#if IS_ENABLED(CONFIG_FDDI)
        case ARPHRD_FDDI:
                /*
                 * According to RFC 1390, FDDI devices should accept ARP
                 * hardware types of 1 (Ethernet).  However, to be more
                 * robust, we'll accept hardware types of either 1 (Ethernet)
                 * or 6 (IEEE 802.2).
                 */
                if (r->arp_ha.sa_family != ARPHRD_FDDI &&
                    r->arp_ha.sa_family != ARPHRD_ETHER &&
                    r->arp_ha.sa_family != ARPHRD_IEEE802)
                        return -EINVAL;
                break;
#endif
        default:
                if (r->arp_ha.sa_family != dev->type)
                        return -EINVAL;
                break;
        }

        neigh = __neigh_lookup_errno(&arp_tbl, &ip, dev);
        err = PTR_ERR(neigh);
        if (!IS_ERR(neigh)) {
                unsigned int state = NUD_STALE;
                if (r->arp_flags & ATF_PERM)
                        state = NUD_PERMANENT;
                err = neigh_update(neigh, (r->arp_flags & ATF_COM) ?
                                   r->arp_ha.sa_data : NULL, state,
                                   NEIGH_UPDATE_F_OVERRIDE |
                                   NEIGH_UPDATE_F_ADMIN);
                neigh_release(neigh);
        }
        return err;
}

static unsigned int arp_state_to_flags(struct neighbour *neigh)
{
        if (neigh->nud_state&NUD_PERMANENT)
                return ATF_PERM | ATF_COM;
        else if (neigh->nud_state&NUD_VALID)
                return ATF_COM;
        else
                return 0;
}

/*
 *      Get an ARP cache entry.
 */

static int arp_req_get(struct arpreq *r, struct net_device *dev)
{
        __be32 ip = ((struct sockaddr_in *) &r->arp_pa)->sin_addr.s_addr;
        struct neighbour *neigh;
        int err = -ENXIO;

        neigh = neigh_lookup(&arp_tbl, &ip, dev);
        if (neigh) {
                read_lock_bh(&neigh->lock);
                memcpy(r->arp_ha.sa_data, neigh->ha, dev->addr_len);
                r->arp_flags = arp_state_to_flags(neigh);
                read_unlock_bh(&neigh->lock);
                r->arp_ha.sa_family = dev->type;
                strlcpy(r->arp_dev, dev->name, sizeof(r->arp_dev));
                neigh_release(neigh);
                err = 0;
        }
        return err;
}

int arp_invalidate(struct net_device *dev, __be32 ip)
{
        struct neighbour *neigh = neigh_lookup(&arp_tbl, &ip, dev);
        int err = -ENXIO;

        if (neigh) {
                if (neigh->nud_state & ~NUD_NOARP)
                        err = neigh_update(neigh, NULL, NUD_FAILED,
                                           NEIGH_UPDATE_F_OVERRIDE|
                                           NEIGH_UPDATE_F_ADMIN);
                neigh_release(neigh);
        }

        return err;
}
EXPORT_SYMBOL(arp_invalidate);

static int arp_req_delete_public(struct net *net, struct arpreq *r,
                struct net_device *dev)
{
        __be32 ip = ((struct sockaddr_in *) &r->arp_pa)->sin_addr.s_addr;
        __be32 mask = ((struct sockaddr_in *)&r->arp_netmask)->sin_addr.s_addr;

        if (mask == htonl(0xFFFFFFFF))
                return pneigh_delete(&arp_tbl, net, &ip, dev);

        if (mask)
                return -EINVAL;

        return arp_req_set_proxy(net, dev, 0);
}

static int arp_req_delete(struct net *net, struct arpreq *r,
                          struct net_device *dev)
{
        __be32 ip;

        if (r->arp_flags & ATF_PUBL)
                return arp_req_delete_public(net, r, dev);

        ip = ((struct sockaddr_in *)&r->arp_pa)->sin_addr.s_addr;
        if (dev == NULL) {
                struct rtable *rt = ip_route_output(net, ip, 0, RTO_ONLINK, 0);
                if (IS_ERR(rt))
                        return PTR_ERR(rt);
                dev = rt->dst.dev;
                ip_rt_put(rt);
                if (!dev)
                        return -EINVAL;
        }
        return arp_invalidate(dev, ip);
}

/*
 *      Handle an ARP layer I/O control request.
 */

int arp_ioctl(struct net *net, unsigned int cmd, void __user *arg)
{
        int err;
        struct arpreq r;
        struct net_device *dev = NULL;

        switch (cmd) {
        case SIOCDARP:
        case SIOCSARP:
                if (!capable(CAP_NET_ADMIN))
                        return -EPERM;
        case SIOCGARP:
                err = copy_from_user(&r, arg, sizeof(struct arpreq));
                if (err)
                        return -EFAULT;
                break;
        default:
                return -EINVAL;
        }

        if (r.arp_pa.sa_family != AF_INET)
                return -EPFNOSUPPORT;

        if (!(r.arp_flags & ATF_PUBL) &&
            (r.arp_flags & (ATF_NETMASK | ATF_DONTPUB)))
                return -EINVAL;
        if (!(r.arp_flags & ATF_NETMASK))
                ((struct sockaddr_in *)&r.arp_netmask)->sin_addr.s_addr =
                                                           htonl(0xFFFFFFFFUL);
        rtnl_lock();
        if (r.arp_dev[0]) {
                err = -ENODEV;
                dev = __dev_get_by_name(net, r.arp_dev);
                if (dev == NULL)
                        goto out;

                /* Mmmm... It is wrong... ARPHRD_NETROM==0 */
                if (!r.arp_ha.sa_family)
                        r.arp_ha.sa_family = dev->type;
                err = -EINVAL;
                if ((r.arp_flags & ATF_COM) && r.arp_ha.sa_family != dev->type)
                        goto out;
        } else if (cmd == SIOCGARP) {
                err = -ENODEV;
                goto out;
        }

        switch (cmd) {
        case SIOCDARP:
                err = arp_req_delete(net, &r, dev);
                break;
        case SIOCSARP:
                err = arp_req_set(net, &r, dev);
                break;
        case SIOCGARP:
                err = arp_req_get(&r, dev);
                break;
        }
out:
        rtnl_unlock();
        if (cmd == SIOCGARP && !err && copy_to_user(arg, &r, sizeof(r)))
                err = -EFAULT;
        return err;
}

static int arp_netdev_event(struct notifier_block *this, unsigned long event,
                            void *ptr)
{
        struct net_device *dev = ptr;

        switch (event) {
        case NETDEV_CHANGEADDR:
                neigh_changeaddr(&arp_tbl, dev);
                rt_cache_flush(dev_net(dev), 0);
                break;
        default:
                break;
        }

        return NOTIFY_DONE;
}

static struct notifier_block arp_netdev_notifier = {
        .notifier_call = arp_netdev_event,
};

/* Note, that it is not on notifier chain.
   It is necessary, that this routine was called after route cache will be
   flushed.
 */
void arp_ifdown(struct net_device *dev)
{
        neigh_ifdown(&arp_tbl, dev);
}


/*
 *      Called once on startup.
 */

static struct packet_type arp_packet_type __read_mostly = {
        .type = cpu_to_be16(ETH_P_ARP),
        .func = arp_rcv,
};

static int arp_proc_init(void);

void __init arp_init(void)
{
        neigh_table_init(&arp_tbl);

        dev_add_pack(&arp_packet_type);
        arp_proc_init();
#ifdef CONFIG_SYSCTL
        neigh_sysctl_register(NULL, &arp_tbl.parms, "ipv4", NULL);
#endif
        register_netdevice_notifier(&arp_netdev_notifier);
}

#ifdef CONFIG_PROC_FS
#if IS_ENABLED(CONFIG_AX25)

/* ------------------------------------------------------------------------ */
/*
 *      ax25 -> ASCII conversion
 */
static char *ax2asc2(ax25_address *a, char *buf)
{
        char c, *s;
        int n;

        for (n = 0, s = buf; n < 6; n++) {
                c = (a->ax25_call[n] >> 1) & 0x7F;

                if (c != ' ')
                        *s++ = c;
        }

        *s++ = '-';
        n = (a->ax25_call[6] >> 1) & 0x0F;
        if (n > 9) {
                *s++ = '1';
                n -= 10;
        }

        *s++ = n + '0';
        *s++ = '\0';

        if (*buf == '\0' || *buf == '-')
                return "*";

        return buf;
}
#endif /* CONFIG_AX25 */

#define HBUFFERLEN 30

static void arp_format_neigh_entry(struct seq_file *seq,
                                   struct neighbour *n)
{
        char hbuffer[HBUFFERLEN];
        int k, j;
        char tbuf[16];
        struct net_device *dev = n->dev;
        int hatype = dev->type;

        read_lock(&n->lock);
        /* Convert hardware address to XX:XX:XX:XX ... form. */
#if IS_ENABLED(CONFIG_AX25)
        if (hatype == ARPHRD_AX25 || hatype == ARPHRD_NETROM)
                ax2asc2((ax25_address *)n->ha, hbuffer);
        else {
#endif
        for (k = 0, j = 0; k < HBUFFERLEN - 3 && j < dev->addr_len; j++) {
                hbuffer[k++] = hex_asc_hi(n->ha[j]);
                hbuffer[k++] = hex_asc_lo(n->ha[j]);
                hbuffer[k++] = ':';
        }
        if (k != 0)
                --k;
        hbuffer[k] = 0;
#if IS_ENABLED(CONFIG_AX25)
        }
#endif
        sprintf(tbuf, "%pI4", n->primary_key);
        seq_printf(seq, "%-16s 0x%-10x0x%-10x%s     *        %s\n",
                   tbuf, hatype, arp_state_to_flags(n), hbuffer, dev->name);
        read_unlock(&n->lock);
}

static void arp_format_pneigh_entry(struct seq_file *seq,
                                    struct pneigh_entry *n)
{
        struct net_device *dev = n->dev;
        int hatype = dev ? dev->type : 0;
        char tbuf[16];

        sprintf(tbuf, "%pI4", n->key);
        seq_printf(seq, "%-16s 0x%-10x0x%-10x%s     *        %s\n",
                   tbuf, hatype, ATF_PUBL | ATF_PERM, "00:00:00:00:00:00",
                   dev ? dev->name : "*");
}

static int arp_seq_show(struct seq_file *seq, void *v)
{
        if (v == SEQ_START_TOKEN) {
                seq_puts(seq, "IP address       HW type     Flags       "
                              "HW address            Mask     Device\n");
        } else {
                struct neigh_seq_state *state = seq->private;

                if (state->flags & NEIGH_SEQ_IS_PNEIGH)
                        arp_format_pneigh_entry(seq, v);
                else
                        arp_format_neigh_entry(seq, v);
        }

        return 0;
}

static void *arp_seq_start(struct seq_file *seq, loff_t *pos)
{
        /* Don't want to confuse "arp -a" w/ magic entries,
         * so we tell the generic iterator to skip NUD_NOARP.
         */
        return neigh_seq_start(seq, pos, &arp_tbl, NEIGH_SEQ_SKIP_NOARP);
}

/* ------------------------------------------------------------------------ */

static const struct seq_operations arp_seq_ops = {
        .start  = arp_seq_start,
        .next   = neigh_seq_next,
        .stop   = neigh_seq_stop,
        .show   = arp_seq_show,
};

static int arp_seq_open(struct inode *inode, struct file *file)
{
        return seq_open_net(inode, file, &arp_seq_ops,
                            sizeof(struct neigh_seq_state));
}

static const struct file_operations arp_seq_fops = {
        .owner          = THIS_MODULE,
        .open           = arp_seq_open,
        .read           = seq_read,
        .llseek         = seq_lseek,
        .release        = seq_release_net,
};


static int __net_init arp_net_init(struct net *net)
{
        if (!proc_net_fops_create(net, "arp", S_IRUGO, &arp_seq_fops))
                return -ENOMEM;
        return 0;
}

static void __net_exit arp_net_exit(struct net *net)
{
        proc_net_remove(net, "arp");
}

static struct pernet_operations arp_net_ops = {
        .init = arp_net_init,
        .exit = arp_net_exit,
};

static int __init arp_proc_init(void)
{
        return register_pernet_subsys(&arp_net_ops);
}

#else /* CONFIG_PROC_FS */

static int __init arp_proc_init(void)
{
        return 0;
}

#endif /* CONFIG_PROC_FS */