i am writing an ipv6 enabled version of ping, and i want to dump informations
collected from ipv6 packets returned by icmpv6 error messages.
the matter is: before i can collect informations from the tcp or udp header
i have to skip the ipv6 extension headers.
i have found some code which does exactly this in the linux kernel, with a
different purpose. here is a note from alexei kuznetsov (linux kernel
developer):
/*
* Skip any extension headers. This is used by the ICMP module.
*
* Note that strictly speaking this conflicts with RFC1883 4.0:
* ...The contents and semantics of each extension header determine whether
* or not to proceed to the next header. Therefore, extension headers must
* be processed strictly in the order they appear in the packet; a
* receiver must not, for example, scan through a packet looking for a
* particular kind of extension header and process that header prior to
* processing all preceding ones.
*
* We do exactly this. This is a protocol bug. We can't decide after a
* seeing an unknown discard-with-error flavour TLV option if it's a
* ICMP error message or not (errors should never be send in reply to
* ICMP error messages).
*
* But I see no other way to do this. This might need to be reexamined
* when Linux implements ESP (and maybe AUTH) headers.
* --AK
*/
and this is what rfc2460 (section 4.0) says:
... to proceed to the next header. Therefore, extension headers must
be processed strictly in the order they appear in the packet; a
receiver must not, for example, scan through a packet looking for a
particular kind of extension header and process that header prior to
processing all preceding ones.
it seems to me that there is at least one situation in which ipv6
extension headers should be skipped - before a packet is sent back
within an icpmv6 error message, the kernel must be sure that it is not
an icmp error message.
i think there are many other situations in which low-level network programs
need to access payload data without processing extension headers. the
problems are two:
1) ipv6 standards do not seem to permit this
2) there is not a standard way to skip ext. headers
imho this wg should consider the following solutions:
1) permit to programs the skip of the ext. headers IF AND ONLY IF
they are only interested in collecting data for error reporting
purposes.
2) provide a standard macro that returns protocol id of the upper
layer protocol and the offset of the payload from the beginning of
the packet, to be included in the next version of
draft-ietf-ipngwg-rfc2292bis.
--
Aequam memento rebus in arduis servare mentem...
Mauro Tortonesi [EMAIL PROTECTED]
Ferrara Linux User Group http://www.ferrara.linux.it
Project6 - IPv6 for Linux http://project6.ferrara.linux.it
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