Bernhard Schmidt wrote:
> The working group:
> 
> # strace -e recvmsg,sendmsg ping6 -c 1 -I eth0 ff02::2
> recvmsg(4, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"0\0\0\0\24\0\2\0\22\37\274H4x\0\0\2\10\200\376\1\0\0\0\10\0\1\0\177\0\0\1\10"..., 4096}], msg_controllen=0, msg_flags=0}, 0) = 108
> recvmsg(4, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"@\0\0\0\24\0\2\0\22\37\274H4x\0\0\n\200\200\376\1\0\0\0\24\0\1\0\0\0\0\0\0"..., 4096}], msg_controllen=0, msg_flags=0}, 0) = 256
> recvmsg(4, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"\24\0\0\0\3\0\2\0\22\37\274H4x\0\0\0\0\0\0\1\0\0\0\24\0\1\0\0\0\0\0\0"..., 4096}], msg_controllen=0, msg_flags=0}, 0) = 20
> sendmsg(3, {msg_name(28)={sa_family=AF_INET6, sin6_port=htons(58), inet_pton(AF_INET6, "ff02::2", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, msg_iov(1)=[{"\200\0\0\0004x\0\1\22\37\274H\366\v\5\0\10\t\n\v\f\r\16\17\20\21\22\23\24\25\26\27\30"..., 64}], msg_controllen=32, {cmsg_len=32, cmsg_level=SOL_IPV6, cmsg_type=, ...}, msg_flags=MSG_PEEK|MSG_PROXY|MSG_WAITALL|MSG_CONFIRM|MSG_FIN|MSG_SYN|MSG_RST|MSG_CMSG_CLOEXEC|0x8bc0000}, 0) = 64
> recvmsg(3, {msg_name(28)={sa_family=AF_INET6, sin6_port=htons(0), inet_pton(AF_INET6, "fe80::216:3eff:feb9:29f5", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=if_nametoindex("eth0")}, msg_iov(1)=[{"\201\0\305\n4x\0\1\22\37\274H\366\v\5\0\10\t\n\v\f\r\16\17\20\21\22\23\24\25\26\27\30"..., 4208}], msg_controllen=36, {cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=0x1d /* SCM_??? */, ...}, msg_flags=0}, 0) = 64
> 
> 
> The non-working RIPng group:
> # strace -e recvmsg,sendmsg ping6 -c 1 -I eth0 ff02::9
> recvmsg(4, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"0\0\0\0\24\0\2\0/\37\274HSx\0\0\2\10\200\376\1\0\0\0\10\0\1\0\177\0\0\1\10"..., 4096}], msg_controllen=0, msg_flags=0}, 0) = 108
> recvmsg(4, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"@\0\0\0\24\0\2\0/\37\274HSx\0\0\n\200\200\376\1\0\0\0\24\0\1\0\0\0\0\0\0"..., 4096}], msg_controllen=0, msg_flags=0}, 0) = 256
> recvmsg(4, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"\24\0\0\0\3\0\2\0/\37\274HSx\0\0\0\0\0\0\1\0\0\0\24\0\1\0\0\0\0\0\0"..., 4096}], msg_controllen=0, msg_flags=0}, 0) = 20
> sendmsg(3, {msg_name(28)={sa_family=AF_INET6, sin6_port=htons(58), inet_pton(AF_INET6, "ff02::9", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, msg_iov(1)=[{"\200\0\0\0Sx\0\1/\37\274H}\202\n\0\10\t\n\v\f\r\16\17\20\21\22\23\24\25\26\27\30"..., 64}], msg_controllen=32, {cmsg_len=32, cmsg_level=SOL_IPV6, cmsg_type=, ...}, msg_flags=MSG_OOB|MSG_DONTROUTE|MSG_PEEK|MSG_CTRUNC|MSG_WAITALL|MSG_TRUNC|MSG_CONFIRM|MSG_FIN|MSG_SYN|MSG_RST|MSG_CMSG_CLOEXEC|0x8bc0000}, 0) = -1 EINVAL (Invalid argument)
> recvmsg(3, 0xbf8c8350, MSG_ERRQUEUE|MSG_DONTWAIT) = -1 EAGAIN (Resource temporarily unavailable)
> sendmsg(3, {msg_name(28)={sa_family=AF_INET6, sin6_port=htons(58), inet_pton(AF_INET6, "ff02::9", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, msg_iov(1)=[{"\200\0\0\0Sx\0\1/\37\274H\354\212\n\0\10\t\n\v\f\r\16\17\20\21\22\23\24\25\26\27\30"..., 64}], msg_controllen=32, {cmsg_len=32, cmsg_level=SOL_IPV6, cmsg_type=, ...}, msg_flags=MSG_CTRUNC}, 0) = -1 EINVAL (Invalid argument)
> recvmsg(3, 0xbf8c8350, MSG_ERRQUEUE|MSG_DONTWAIT) = -1 EAGAIN (Resource temporarily unavailable)
> ping: sendmsg: Invalid argument
> recvmsg(3, {msg_name(28)={sa_family=AF_INET6, sin6_port=htons(0), inet_pton(AF_INET6, "fe80::216:3eff:feb9:29f5", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=if_nametoindex("eth0")}, msg_iov(1)=[{"\201\0\374\223Sx\0\1/\37\274H}\202\n\0\10\t\n\v\f\r\16\17\20\21\22\23\24\25\26\27\30"..., 4208}], msg_controllen=36, {cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=0x1d /* SCM_??? */, ...}, msg_flags=0}, 0) = 64
> 
> 
> So the flags look different, but why?

Well, at least in the ping6 sources I have, msg_flags is never 
initialized before the sendmsg() call, and since it's allocated on the 
stack it can have random bits set.  Can you rebuild your ping6 with the 
attached patch and retry?

-Brian