In file inet/inet6_opt.c:
inet6_opt_init (void *extbuf, socklen_t extlen)

What the method should do is written in RFC 3542. 
I tried to set up a hop-by-hop options header
but got errors back from sendmsg(). My findings are
as follows:
RFC 2460 says for HbH Opts and Dest Opts:
Hdr Ext Len          8-bit unsigned integer.  Length of the Hop-by
                     Hop Options header in 8-octet units, not
                     including the first 8 octets.

The problem is now that inet6_opt_init(extbuf, extlen)
with extlen == 8 results in a wrong setting of the
extension header length, because the glibc code sets
extp->ip6h_len = extlen / 8;
which yields 1, but it should be actually 0!
Passing an extlen of 0 is not allowed by the sanity
checks of the method and the RFC requires a _positive_ 
multiple of 8. So constructing a valid IPv6 HbH Opt Extension
header is not possible using the current implementation
of inet6_opt_init().

Fix: extp->ip6h_len = extlen / 8 - 1;

------- Additional Comment #1 From Roland Bless 2008-02-12 23:42 -------

Created an attachment (id=2255)
Proposed fix for the bug.

Fixing the bug is easy

------- Additional Comment #2 From Ulrich Drepper 2008-03-08 19:17 -------

Applied to the trunk.