Hello Brian,

quoted

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?

Done, no change.

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\0\3252\0\0010T\275H\274\314\7\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=0}, 0) = 64

vs. 

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\0\3162\0\1+T\275H\255K\16\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=0}, 0) = -1 EINVAL (Invalid argument)

don't push too hard on ping6, I just included it to show that all
processes are affected sending to this particular group.

Bernhard