dump rtable and dst_entry

ping www.baidu.com, ip_route_output_flow returns ffff880c834f6900crash> rtable ffff880c834f6900struct rtable {  dst = {    callback_head = {      next = 0xffff880c834f7b00,      func = 0x0    },    child = 0x0,    dev = 0xffff880c7ec18000,    ops = 0xffffffff81cd8640 <ipv4_dst_ops>,    _metrics = 0xffffffff818c2301,    expires = 0x0,    path = 0xffff880c834f6900,    from = 0x0,    xfrm = 0x0,    input = 0xffffffff816117b0 <dst_discard>,    output = 0xffffffff81655330 <ip_output>,    flags = 0x0,    error = 0x0,    obsolete = 0xffff,    header_len = 0x0,    trailer_len = 0x0,    __pad3 = 0x0,    tclassid = 0x0,    __pad_to_align_refcnt = {0x0, 0x0},    __refcnt = {      counter = 0x0    },    __use = 0x0,    lastuse = 0x103203a31,    lwtstate = 0x0,    {      next = 0x0,      rt_next = 0x0,      rt6_next = 0x0,      dn_next = 0x0    }  },  rt_genid = 0x57,  rt_flags = 0x0,  rt_type = 0x1,  rt_is_input = 0x0,  rt_uses_gateway = 0x1,  rt_iif = 0x0,  rt_gateway = 0xa00c80a,       # 10.200.0.10  rt_pmtu = 0x0,  rt_table_id = 0xfe,  rt_uncached = {    next = 0xffff880c834f69c0,    prev = 0xffff880c834f69c0  },  rt_uncached_list = 0x0}$ ip routedefault via 10.200.0.10 dev em1crash> net | grep ffff880c7ec18000ffff880c7ec18000  em1    10.200.0.168ping 192.168.1.19, ip_route_output_flow returns ffff880c58ed7300crash> rtable ffff880c58ed7300struct rtable {  dst = {    callback_head = {      next = 0xffff880c58ed6500,      func = 0xffffffff81219f40 <file_free_rcu>    },    child = 0x0,    dev = 0xffff880c693a0000,    ops = 0xffffffff81cd8640 <ipv4_dst_ops>,    _metrics = 0xffffffff818c2301,    expires = 0x0,    path = 0xffff880c58ed7300,    from = 0x0,    xfrm = 0x0,    input = 0xffffffff816117b0 <dst_discard>,    output = 0xffffffff81655330 <ip_output>,    flags = 0x0,    error = 0x0,    obsolete = 0xffff,    header_len = 0x0,    trailer_len = 0x0,    __pad3 = 0x0,    tclassid = 0x0,    __pad_to_align_refcnt = {0x0, 0x0},    __refcnt = {      counter = 0x0    },    __use = 0x0,    lastuse = 0x1030a4b07,    lwtstate = 0x0,    {      next = 0x0,      rt_next = 0x0,      rt6_next = 0x0,      dn_next = 0x0    }  },  rt_genid = 0x57,  rt_flags = 0x0,  rt_type = 0x1,  rt_is_input = 0x0,  rt_uses_gateway = 0x0,  rt_iif = 0x0,  rt_gateway = 0x0,     # directly connected  rt_pmtu = 0x0,  rt_table_id = 0xfe,  rt_uncached = {    next = 0xffff880c58ed73c0,    prev = 0xffff880c58ed73c0  },  rt_uncached_list = 0xffff880c58ed73d0}$ ip route192.168.1.0/24 dev p4p1 proto kernel scope link src 192.168.1.18crash> net | grep ffff880c693a0000ffff880c693a0000  p4p1   192.168.1.18

If rtable->rt_gateway is not NULL, find the mac address of the gateway using neighbouring subsystem.

If rtable->rt_gateway is NULL, the source and destination must be connected directly.

find the mac address of the destination ip address using neighbouring subsystem.

static struct neighbour *ipv4_neigh_lookup(const struct dst_entry *dst,                                           struct sk_buff *skb,                                           const void *daddr){        struct net_device *dev = dst->dev;        const __be32 *pkey = daddr;        const struct rtable *rt;        struct neighbour *n;        rt = (const struct rtable *) dst;        if (rt->rt_gateway)                pkey = (const __be32 *) &rt->rt_gateway;        else if (skb)                pkey = &ip_hdr(skb)->daddr;        n = __ipv4_neigh_lookup(dev, *(__force u32 *)pkey);        if (n)                return n;        return neigh_create(&arp_tbl, pkey, dev);}