协议栈--net namespace

        在Linux 协议咱中引入了net namespace 后既已支持了多个实例，包含在net namespace 中元素有：进程、套接字、网络设备、proc文件
        We create a mechanism that from the users perspective allows creation of separate instances of the network stack.
        在linux 中命名空间的结构体为struct net，存在于linux/include/net/net_namespace.h
        

struct net {
    atomic_t        passive;    /* To decided when the network
                         * namespace should be freed.
                         */
    atomic_t        count;        /* To decided when the network
                         * namespace should be shut down.
                         */
#ifdef NETNS_REFCNT_DEBUG
    atomic_t        use_count;    /* To track references we
                         * destroy on demand
                         */
#endif
    spinlock_t        rules_mod_lock;

    struct list_head    list;        /* list of network namespaces */
    struct list_head    cleanup_list;    /* namespaces on death row */
    struct list_head    exit_list;    /* Use only net_mutex */

    struct user_namespace *user_ns;    /* Owning user namespace */

    unsigned int        proc_inum;

    struct proc_dir_entry     *proc_net;
    struct proc_dir_entry     *proc_net_stat;

#ifdef CONFIG_SYSCTL
    struct ctl_table_set    sysctls;
#endif

    struct sock         *rtnl;            /* rtnetlink socket */
    struct sock        *genl_sock;

    struct list_head     dev_base_head;
    struct hlist_head     *dev_name_head;
    struct hlist_head    *dev_index_head;
    unsigned int        dev_base_seq;    /* protected by rtnl_mutex */
    int            ifindex;
    unsigned int        dev_unreg_count;

    /* core fib_rules */
    struct list_head    rules_ops;


    struct net_device *loopback_dev; /* The loopback */
    struct netns_core    core;
    struct netns_mib    mib;
    struct netns_packet    packet;
    struct netns_unix    unx;
    struct netns_ipv4    ipv4;
#if IS_ENABLED(CONFIG_IPV6)
    struct netns_ipv6    ipv6;
#endif
#if IS_ENABLED(CONFIG_IEEE802154_6LOWPAN)
    struct netns_ieee802154_lowpan    ieee802154_lowpan;
#endif
#if defined(CONFIG_IP_SCTP) || defined(CONFIG_IP_SCTP_MODULE)
    struct netns_sctp    sctp;
#endif
#if defined(CONFIG_IP_DCCP) || defined(CONFIG_IP_DCCP_MODULE)
    struct netns_dccp    dccp;
#endif
#ifdef CONFIG_NETFILTER
    struct netns_nf        nf;
    struct netns_xt        xt;
#if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
    struct netns_ct        ct;
#endif
#if defined(CONFIG_NF_TABLES) || defined(CONFIG_NF_TABLES_MODULE)
    struct netns_nftables    nft;
#endif
#if IS_ENABLED(CONFIG_NF_DEFRAG_IPV6)
    struct netns_nf_frag    nf_frag;
#endif
    struct sock        *nfnl;
    struct sock        *nfnl_stash;
#endif
#ifdef CONFIG_WEXT_CORE
    struct sk_buff_head    wext_nlevents;
#endif
    struct net_generic __rcu    *gen;

    /* Note : following structs are cache line aligned */
#ifdef CONFIG_XFRM
    struct netns_xfrm    xfrm;
#endif
#if IS_ENABLED(CONFIG_IP_VS)
    struct netns_ipvs    *ipvs;
#endif
    struct sock        *diag_nlsk;
    atomic_t        fnhe_genid;
};

        对于设备：在list_netdevice 函数中将设备分别挂到了三条链表上：
        

list_add_tail_rcu(&dev->dev_list, &net->dev_base_head);
hlist_add_head_rcu(&dev->name_hlist, dev_name_hash(net, dev->name));
hlist_add_head_rcu(&dev->index_hlist,dev_index_hash(net, dev->ifindex));

        同样struct net_device 也有回朔的变量 net.
        对于 net 本身而言，也被连接到了net_namespace_list 上，系统初始化的时候自动生成一个命名空间：init_net

        在用户态可以通过ip netns  对network namespace进行查看，新建，删除等操作。

Ref:
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