读了一下《Professional Linux Kernel Architecture》的Network这一章。由于本书讲得比较新，可以说是市面上目前讲Linux内核版本最新的著作了，涉及到了2.6.24版本。其中，有很多微妙的变化，由于struct sk_buff是内核网络机构的基础，因此我也比较关心这个结构的变化： struct sk_buff { /* These two members must be first. */ struct sk_buff *next; struct sk_buff *prev; struct sock *sk; ktime_t tstamp; struct net_device *dev; struct dst_entry *dst; struct sec_path *sp; /* * This is the control buffer. It is free to use for every * layer. Please put your private variables there. If you * want to keep them across layers you have to do a skb_clone() * first. This is owned by whoever has the skb queued ATM. */ char cb[48]; unsigned int len, data_len; __u16 mac_len, hdr_len; union { __wsum csum; struct { __u16 csum_start; __u16 csum_offset; }; }; __u32 priority; __u8 local_df:1, cloned:1, ip_summed:2, nohdr:1, nfctinfo:3; __u8 pkt_type:3, fclone:2, ipvs_property:1, peeked:1, nf_trace:1; __be16 protocol; void (*destructor)(struct sk_buff *skb); #if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE) struct nf_conntrack *nfct; struct sk_buff *nfct_reasm; #endif #ifdef CONFIG_BRIDGE_NETFILTER struct nf_bridge_info *nf_bridge; #endif int iif; #ifdef CONFIG_NETDEVICES_MULTIQUEUE __u16 queue_mapping; #endif #ifdef CONFIG_NET_SCHED __u16 tc_index; /* traffic control index */ #ifdef CONFIG_NET_CLS_ACT __u16 tc_verd; /* traffic control verdict */ #endif #endif /* 2 byte hole */ #ifdef CONFIG_NET_DMA dma_cookie_t dma_cookie; #endif #ifdef CONFIG_NETWORK_SECMARK __u32 secmark; #endif __u32 mark; sk_buff_data_t transport_header; sk_buff_data_t network_header; sk_buff_data_t mac_header; /* These elements must be at the end, see alloc_skb() for details. */ sk_buff_data_t tail; sk_buff_data_t end; unsigned char *head, *data; unsigned int truesize; atomic_t users; }; 用红色标记这三个成员，分别是传输头，网络头以及mac头相对于Sk_buff的head的偏移。有了这三个成员，可以说为内核编程人员提供了更便利的获取传输层、网络层和MAC层头的偏移。并且，内核也新增了几个函数，来提供获取这些偏移的接口： #ifdef NET_SKBUFF_DATA_USES_OFFSET 如果使用了offset来表示偏移的话，就是说是一个相对偏移的情况： static inline unsigned char *skb_transport_header(const struct sk_buff *skb) { return skb->head skb->transport_header; } static inline void skb_reset_transport_header(struct sk_buff *skb) { skb->transport_header = skb->data - skb->head; } static inline void skb_set_transport_header(struct sk_buff *skb, const int offset) { skb_reset_transport_header(skb); skb->transport_header = offset; } static inline unsigned char *skb_network_header(const struct sk_buff *skb) { return skb->head skb->network_header; } static inline void skb_reset_network_header(struct sk_buff *skb) { skb->network_header = skb->data - skb->head; } static inline void skb_set_network_header(struct sk_buff *skb, const int offset) { skb_reset_network_header(skb); skb->network_header = offset; } static inline unsigned char *skb_mac_header(const struct sk_buff *skb) { return skb->head skb->mac_header; } static inline int skb_mac_header_was_set(const struct sk_buff *skb) { return skb->mac_header != ~0U; } static inline void skb_reset_mac_header(struct sk_buff *skb) { skb->mac_header = skb->data - skb->head; } static inline void skb_set_mac_header(struct sk_buff *skb, const int offset) { skb_reset_mac_header(skb); skb->mac_header = offset; } #else /* NET_SKBUFF_DATA_USES_OFFSET */ 不使用相对偏移的情况 static inline unsigned char *skb_transport_header(const struct sk_buff *skb) { return skb->transport_header; } static inline void skb_reset_transport_header(struct sk_buff *skb) { skb->transport_header = skb->data; } static inline void skb_set_transport_header(struct sk_buff *skb, const int offset) { skb->transport_header = skb->data offset; } static inline unsigned char *skb_network_header(const struct sk_buff *skb) { return skb->network_header; } static inline void skb_reset_network_header(struct sk_buff *skb) { skb->network_header = skb->data; } static inline void skb_set_network_header(struct sk_buff *skb, const int offset) { skb->network_header = skb->data offset; } static inline unsigned char *skb_mac_header(const struct sk_buff *skb) { return skb->mac_header; } static inline int skb_mac_header_was_set(const struct sk_buff *skb) { return skb->mac_header != NULL; } static inline void skb_reset_mac_header(struct sk_buff *skb) { skb->mac_header = skb->data; } static inline void skb_set_mac_header(struct sk_buff *skb, const int offset) { skb->mac_header = skb->data offset; } #endif /* NET_SKBUFF_DATA_USES_OFFSET */ 1、TCP层获取相关偏移的函数 static inline struct tcphdr *tcp_hdr(const struct sk_buff *skb) { return (struct tcphdr *)skb_transport_header(skb); } 这个函数用来获得sk_buff结构中TCP头的指针 static inline unsigned int tcp_hdrlen(const struct sk_buff *skb) { return tcp_hdr(skb)->doff * 4; } 这个函数用来获得TCP头的长度 static inline unsigned int tcp_optlen(const struct sk_buff *skb) { return (tcp_hdr(skb)->doff - 5) * 4; } 获取tcp option的长度 2、IP相关的函数 static inline struct iphdr *ip_hdr(const struct sk_buff *skb) { return (struct iphdr *)skb_network_header(skb); } 该函数获得ip头 static inline struct iphdr *ipip_hdr(const struct sk_buff *skb) { return (struct iphdr *)skb_transport_header(skb); } 该函数获得ipip头，实际上偏移已经跑到了传输层的开始 3、MAC相关函数 static inline struct ebt_802_3_hdr *ebt_802_3_hdr(const struct sk_buff *skb) { return (struct ebt_802_3_hdr *)skb_mac_header(skb); } 获取802.3MAC头指针。 可以说，这些函数，为我们编写内核程序提供了极大的便捷，而不用再花更多的精力去考虑计算各层的指针偏移，可以把更多的精力用来考虑策略性的设计。 文章出处：http://www.diybl.com/course/6_system/linux/Linuxjs/20081216/153921.html |
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