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.
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