IP6_EXTHDR_CHECK宏的作用

IP6_EXTHDR_CHECK(m, off, hlen, ret)

因为一个数据包可以放在多个mbuf里，（mbuf link），而KAME的普遍实现，是要保证后面所操作的header或者option必须在一段连续的内存里，IP6_EXTHDR_CHECK是一个宏，用来检查从ipv6 header起始开始，到距离ipv6 header的偏移off，以及后面hlen长度都在同一个mbuf里面。

实现如下：

#define IP6_EXTHDR_CHECK(m, off, hlen, ret)                                 \

do {                                                                             \

    if ((m)->m_next != NULL) {                                                    \

         if ((m)->m_len < (off) + (hlen)) {                               \

                   ip6stat.ip6s_exthdrtoolong++;                                 \

                   m_freem(m);                                                      \

                   return ret;                                                 \

         }                                                                         \

    } else {                                                                           \

         if ((m)->m_len < (off) + (hlen)) {                               \

                   ip6stat.ip6s_tooshort++;                                 \

                   in6_ifstat_inc(m->m_pkthdr.rcvif, ifs6_in_truncated);     \

                   m_freem(m);                                                      \

                   return ret;                                                 \

         }                                                                         \

    }                                                                            \

} while (/*CONSTCOND*/ 0)

 

正常应用是，m是mbuf，当前mbuf的data距离ipv6 header的偏移的off，检查接下来的hlen是否和ipv6 header在连续内存中。

这个宏并不改变mbuf的内容，如果不满足条件，会直接return。所以KAME里某些地方在调用IP6_EXTHDR_CHECK后，重置ipv6 header或其他option指针，是没有必要的。