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ip/dhcp6d: set source address to the receiving interface ip when multicast

to make sure the reply goes out the same interface
that it came in when it was multicast desination,
put the source ip of the receiving interface.

--- a/sys/src/cmd/ip/dhcp6d.c

+++ b/sys/src/cmd/ip/dhcp6d.c

@@ -315,6 +315,7 @@

 main(int argc, char *argv[])

 {

 	uchar ibuf[4096], obuf[4096];

+	Iplifc *lifc;

 	Req r[1];

 	int fd, n, i;

@@ -363,7 +364,8 @@

 			continue;

 		if((r->ifc = findifc(netmtpt, r->udp->ifcaddr)) == nil)

 			continue;

-		if(localonifc(r->ifc, r->udp->raddr) == nil)

+		lifc = localonifc(r->ifc, r->udp->raddr);

+		if(lifc == nil)

 			continue;

 		memmove(obuf, ibuf, Udphdrsize);

@@ -376,8 +378,8 @@

 		r->req.t = r->req.p[0];

 		if(debug)

-		fprint(2, "%I->%I(%s) typ=%d tra=%x\n",

-			r->udp->raddr, r->udp->laddr, r->ifc->dev,

+		fprint(2, "%I->%I(%I %s) typ=%d tra=%x\n",

+			r->udp->raddr, r->udp->laddr, lifc->ip, r->ifc->dev,

 			r->req.t, r->tra);

 		switch(r->req.t){

@@ -421,6 +423,9 @@

 		addoption(r, 3);

 		addoption(r, 6);

+

+		if(ISIPV6MCAST(((Udphdr*)obuf)->laddr))

+			ipmove(((Udphdr*)obuf)->laddr, lifc->ip);

 		write(fd, obuf, r->resp.p-obuf);

 		if(debug) fprint(2, "\n");