c - 为什么要在 libpcap 程序中重新声明 TCP 和 IP header ？

Question

在libpcap的教程程序中，我看到以下结构:

/* IP header */
struct sniff_ip {
    u_char  ip_vhl;                 
    u_char  ip_tos;                 
    u_short ip_len;                 
    u_short ip_id;                  
    u_short ip_off;                 
    #define IP_RF 0x8000            
    #define IP_DF 0x4000            
    #define IP_MF 0x2000            
    #define IP_OFFMASK 0x1fff       
    u_char  ip_ttl;                 
    u_char  ip_p;                   
    u_short ip_sum;                 
    struct  in_addr ip_src,ip_dst;  
};
#define IP_HL(ip)               (((ip)->ip_vhl) & 0x0f)
#define IP_V(ip)                (((ip)->ip_vhl) >> 4)

/* TCP header */
typedef u_int tcp_seq;

struct sniff_tcp {
    u_short th_sport;               
    u_short th_dport;               
    tcp_seq th_seq;                 
    tcp_seq th_ack;                 
    u_char  th_offx2;               
    #define TH_OFF(th)      (((th)->th_offx2 & 0xf0) >> 4)
    u_char  th_flags;
    #define TH_FIN  0x01
    #define TH_SYN  0x02
    #define TH_RST  0x04
    #define TH_PUSH 0x08
    #define TH_ACK  0x10
    #define TH_URG  0x20
    #define TH_ECE  0x40
    #define TH_CWR  0x80
    #define TH_FLAGS        (TH_FIN|TH_SYN|TH_RST|TH_ACK|TH_URG|TH_ECE|TH_CWR)
    u_short th_win;                 
    u_short th_sum;                 
    u_short th_urp;                 
};

为什么上面的 header 结构已被重新声明，并且没有使用 netinet/tcp.h 中的 TCP 和 IP 结构，netinet/ip.h 没有使用？ 使用这个自定义结构有什么优势吗？

Answer 1

简短的回答:这只是一个例子。

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正如本教程作者对本文的评论中所指出的，存在可移植性问题，因为并非所有操作系统都提供此类网络结构，另一方面，RFC 明确定义了格式，因此可以使用自己的结构使用过。

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教程演变

原始文档于 2005 年进行了修改(有注释“Guy Harris 进一步编辑和开发”)。原始文档可以在 cvs 中找到 1.1 版本。 :

First, we must have the actual structures define before we can typecast to them. The following is the structure definitions that I use to describe a TCP/IP packet over Ethernet. All three definitions that I use are taken directly out of the POSIX libraries. Normally I would have simply just used the definitions in those libraries, but it has been my experience that the libraries vary slightly from platform to platform, making it complicated to implement them quickly. So for demonstration purposes we will just avoid that mess and simply copy the relevant structures. All of these, incidentally, can be found in include/netinet on your local Unix system. Here are the structures:

有 Linux 结构的原始副本(现在它们在示例中进行了修改)。解释:

Note: On my Slackware Linux 8 box (stock kernel 2.2.19) I found that code using the above structures would not compile. The problem, as it turns out, was in include/features.h, which implements a POSIX interface unless _BSD_SOURCE is defined. If it was not defined, then I had to use a different structure definition for the TCP header. The more universal solution, that does not prevent the code from working on FreeBSD or OpenBSD (where it had previously worked fine), is simply to do the following:
#define _BSD_SOURCE 1
prior to including any of your header files. This will ensure that a BSD style API is being used. Again, if you don't wish to do this, then you can simply use the alternative TCP header structure, which I've linked to here, along with some quick notes about using it.

因此，在文档版本之后，在 v1.2 中修改了结构，并在 v1.6 中更改了文本，并添加了以下提交注释:

Don't talk about any of this coming from POSIX (it doesn't) and don't note that they might be available on a UN*X system (don't take your packet layouts from the OS; it might not have them, or they might not have all the protocol features you want).