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devip: don't send arp requests from null address

during dhcp, ipconfig assigns the null address :: which makes
ipforme() return Runi for any destination, which can trigger
arp resolution when we attempt to reply. so have v4local()
skip the null address and have sendarp() check the return
status of v4local(), avoing the spurious arp requests.

--- a/sys/src/9/ip/ethermedium.c

+++ b/sys/src/9/ip/ethermedium.c

@@ -449,6 +449,7 @@

 	Block *bp;

 	Etherarp *e;

 	Etherrock *er = ifc->arg;

+	uchar targ[IPv4addrlen], src[IPv4addrlen];

 	/* don't do anything if it's been less than a second since the last */

 	if(NOW - a->ctime < 1000){

@@ -456,6 +457,9 @@

 		return;

 	}

+	/* try to keep it around for a second more */

+	a->ctime = NOW;

+

 	/* remove all but the last message */

 	while((bp = a->hold) != nil){

 		if(bp == a->last)

@@ -464,10 +468,12 @@

 		freeblist(bp);

 	}

-	/* try to keep it around for a second more */

-	a->ctime = NOW;

+	memmove(targ, a->ip+IPv4off, IPv4addrlen);

 	arprelease(er->f->arp, a);

+	if(!ipv4local(ifc, src, targ))

+		return;

+

 	n = sizeof(Etherarp);

 	if(n < ifc->m->mintu)

 		n = ifc->m->mintu;

@@ -474,8 +480,8 @@

 	bp = allocb(n);

 	memset(bp->rp, 0, n);

 	e = (Etherarp*)bp->rp;

-	memmove(e->tpa, a->ip+IPv4off, sizeof(e->tpa));

-	ipv4local(ifc, e->spa, e->tpa);

+	memmove(e->tpa, targ, sizeof(e->tpa));

+	memmove(e->spa, src, sizeof(e->spa));

 	memmove(e->sha, ifc->mac, sizeof(e->sha));

 	memset(e->d, 0xff, sizeof(e->d));		/* ethernet broadcast */

 	memmove(e->s, ifc->mac, sizeof(e->s));

--- a/sys/src/9/ip/ipifc.c

+++ b/sys/src/9/ip/ipifc.c

@@ -1274,7 +1274,7 @@

 	b = -1;

 	for(lifc = ifc->lifc; lifc != nil; lifc = lifc->next){

-		if((lifc->type & Rv4) == 0)

+		if((lifc->type & Rv4) == 0 || ipcmp(lifc->local, IPnoaddr) == 0)

 			continue;

 		a = comprefixlen(lifc->local+IPv4off, remote, IPv4addrlen);

 		if(a > b){