Linux Kernel Network Programming – dst_entry data-structure – Introduction and code-walk

dst_entry() is one of the important Linux Kernel Networking subsystem data-structure which is used while navigating the packet flow within the IPv4 stack. It contains function pointers one for input and one for output.

For example a dst_entry instance (net/ipv4/ip_input.c) for a locally received packet would have an input function pointer to a local ip receiving function. A packet which was not intended for the local host, would have a dst_entry with an input function pointer to a packet forwarding function (net/ipv4/ip_forward.c).

For more details refer Linux Source:
Network_data_flow_through_kernel API Map ↗
http://lxr.free-electrons.com/source/include/net/dst.h ↗
http://lxr.free-electrons.com/source/net/core/dst.c ↗
http://lxr.free-electrons.com/source/net/ipv4/ip_input.c#L396 ↗
http://lxr.free-electrons.com/source/include/linux/skbuff.h ↗
http://lxr.free-electrons.com/source/include/linux/skbuff.h#L665 ↗
skb_dst() – http://lxr.free-electrons.com/source/include/linux/skbuff.h#L845 ↗
skb_dst_set() – http://lxr.free-electrons.com/source/include/linux/skbuff.h#L864 ↗

And here is the copy paste of struct dst_entry data-structure (/include/net/dst.h) from the Kernel-source version 4.13 for quick reference:

struct dst_entry {
	struct net_device       *dev;
	struct rcu_head		rcu_head;
	struct dst_entry	*child;
	struct  dst_ops	        *ops;
	unsigned long		_metrics;
	unsigned long           expires;
	struct dst_entry	*path;
	struct dst_entry	*from;
#ifdef CONFIG_XFRM
	struct xfrm_state	*xfrm;
#else
	void			*__pad1;
#endif
	int			(*input)(struct sk_buff *);
	int			(*output)(struct net *net, struct sock *sk, struct sk_buff *skb);

	unsigned short		flags;
#define DST_HOST		0x0001
#define DST_NOXFRM		0x0002
#define DST_NOPOLICY		0x0004
#define DST_NOCOUNT		0x0008
#define DST_FAKE_RTABLE		0x0010
#define DST_XFRM_TUNNEL		0x0020
#define DST_XFRM_QUEUE		0x0040
#define DST_METADATA		0x0080

	short			error;

	/* A non-zero value of dst->obsolete forces by-hand validation
	 * of the route entry.  Positive values are set by the generic
	 * dst layer to indicate that the entry has been forcefully
	 * destroyed.
	 *
	 * Negative values are used by the implementation layer code to
	 * force invocation of the dst_ops->check() method.
	 */
	short			obsolete;
#define DST_OBSOLETE_NONE	0
#define DST_OBSOLETE_DEAD	2
#define DST_OBSOLETE_FORCE_CHK	-1
#define DST_OBSOLETE_KILL	-2
	unsigned short		header_len;	/* more space at head required */
	unsigned short		trailer_len;	/* space to reserve at tail */
	unsigned short		__pad3;

#ifdef CONFIG_IP_ROUTE_CLASSID
	__u32			tclassid;
#else
	__u32			__pad2;
#endif

#ifdef CONFIG_64BIT
	/*
	 * Align __refcnt to a 64 bytes alignment
	 * (L1_CACHE_SIZE would be too much)
	 */
	long			__pad_to_align_refcnt[2];
#endif
	/*
	 * __refcnt wants to be on a different cache line from
	 * input/output/ops or performance tanks badly
	 */
	atomic_t		__refcnt;	/* client references	*/
	int			__use;
	unsigned long		lastuse;
	struct lwtunnel_state   *lwtstate;
	union {
		struct dst_entry	*next;
		struct rtable __rcu	*rt_next;
		struct rt6_info		*rt6_next;
		struct dn_route __rcu	*dn_next;
	};
};

And here is the copy paste of API skb_dst() which returns the dst_entry linked to the corresponding skbuff instance (include/linux/skbuff.h) from the Kernel-source version 4.13 for quick reference:

/**
 * skb_dst - returns skb dst_entry
 * @skb: buffer
 *
 * Returns skb dst_entry, regardless of reference taken or not.
 */
static inline struct dst_entry *skb_dst(const struct sk_buff *skb)
{
	/* If refdst was not refcounted, check we still are in a 
	 * rcu_read_lock section
	 */
	WARN_ON((skb->_skb_refdst & SKB_DST_NOREF) &&
		!rcu_read_lock_held() &&
		!rcu_read_lock_bh_held());
	return (struct dst_entry *)(skb->_skb_refdst & SKB_DST_PTRMASK);
}

So here is my detailed YouTube video on the same and overall various aspects of Kernel network subsystem development and architecture.

I also conduct sessions/classes on Systems and Network Software Programming, Linux Kernel Programming and Architecture. If you are interested, click HERE for more details.

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