devip: implement ipv4 arp timeout with icmp host unreachable notification

The IPv4 ARP cache used to indefinitely buffer packets in the Arpent hold list.
This is bad in case of a router, because it opens a 1 second
(retransmit time) window to leak all the to be forwarded packets.

This change makes the ipv4 arp code path similar to the IPv6 neighbour
solicitation path, using the retransmit process to time out old entries
(after 3 arp retransmits => 3 seconds).

A new function arpcontinue() has been added that unifies the point when
we schedule the (ipv6 sol retransmit) / (ipv4 arp timeout) and reduce
the hold queue to the last packet and unlock the cache.

As a bonus, we also now send a icmp host unreachable notification
for the dropped packets.

1 files changed, 12 insertions, 57 deletions

diff --git a/sys/src/9/ip/ethermedium.c b/sys/src/9/ip/ethermedium.c
index 2bb022dcf..0bcd0c165 100644
--- a/sys/src/9/ip/ethermedium.c
+++ b/sys/src/9/ip/ethermedium.c
@@ -17,15 +17,6 @@ struct Etherhdr
 	uchar	t[2];
 };
 
-static uchar ipbroadcast[IPaddrlen] = {
-	0xff,0xff,0xff,0xff,
-	0xff,0xff,0xff,0xff,
-	0xff,0xff,0xff,0xff,
-	0xff,0xff,0xff,0xff,
-};
-
-static uchar etherbroadcast[] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff };
-
 static void	etherread4(void *a);
 static void	etherread6(void *a);
 static void	etherbind(Ipifc *ifc, int argc, char **argv);
@@ -35,8 +26,7 @@ static void	etheraddmulti(Ipifc *ifc, uchar *a, uchar *ia);
 static void	etherremmulti(Ipifc *ifc, uchar *a, uchar *ia);
 static void	etherareg(Fs *f, Ipifc *ifc, Iplifc *lifc, uchar *ip);
 static Block*	multicastarp(Fs *f, Arpent *a, Medium*, uchar *mac);
-static void	sendarp(Ipifc *ifc, Arpent *a);
-static void	sendndp(Ipifc *ifc, Arpent *a);
+static void	sendarpreq(Ipifc *ifc, Arpent *a);
 static int	multicastea(uchar *ea, uchar *ip);
 static void	recvarpproc(void*);
 static void	etherpref2addr(uchar *pref, uchar *ea);
@@ -276,12 +266,17 @@ etherbwrite(Ipifc *ifc, Block *bp, int version, uchar *ip)
 		/* check for broadcast or multicast */
 		bp = multicastarp(er->f, a, ifc->m, mac);
 		if(bp == nil){
+			/* don't do anything if it's been less than a second since the last */
+			if(NOW - a->ctime < RETRANS_TIMER){
+				arprelease(er->f->arp, a);
+				return;
+			}
 			switch(version){
 			case V4:
-				sendarp(ifc, a);
+				sendarpreq(ifc, a);		/* unlocks arp */
 				break;
 			case V6:
-				sendndp(ifc, a);
+				ndpsendsol(er->f, ifc, a);	/* unlocks arp */
 				break;
 			default:
 				panic("etherbwrite: version %d", version);
@@ -442,7 +437,7 @@ etherremmulti(Ipifc *ifc, uchar *a, uchar *)
  *  (only v4, v6 uses the neighbor discovery, rfc1970)
  */
 static void
-sendarp(Ipifc *ifc, Arpent *a)
+sendarpreq(Ipifc *ifc, Arpent *a)
 {
 	int n;
 	Block *bp;
@@ -450,25 +445,8 @@ sendarp(Ipifc *ifc, Arpent *a)
 	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){
-		arprelease(er->f->arp, a);
-		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)
-			break;
-		a->hold = bp->list;
-		freeblist(bp);
-	}
-
 	memmove(targ, a->ip+IPv4off, IPv4addrlen);
-	arprelease(er->f->arp, a);
+	arpcontinue(er->f->arp, a);
 
 	if(!ipv4local(ifc, src, 0, targ))
 		return;
@@ -477,8 +455,8 @@ sendarp(Ipifc *ifc, Arpent *a)
 	if(n < ifc->m->mintu)
 		n = ifc->m->mintu;
 	bp = allocb(n);
-	memset(bp->rp, 0, n);
 	e = (Etherarp*)bp->rp;
+	memset(e, 0, n);
 	memmove(e->tpa, targ, sizeof(e->tpa));
 	memmove(e->spa, src, sizeof(e->spa));
 	memmove(e->sha, ifc->mac, sizeof(e->sha));
@@ -496,29 +474,6 @@ sendarp(Ipifc *ifc, Arpent *a)
 	devtab[er->achan->type]->bwrite(er->achan, bp, 0);
 }
 
-static void
-sendndp(Ipifc *ifc, Arpent *a)
-{
-	Block *bp;
-	Etherrock *er = ifc->arg;
-
-	/* don't do anything if it's been less than a second since the last */
-	if(NOW - a->ctime < ReTransTimer){
-		arprelease(er->f->arp, a);
-		return;
-	}
-
-	/* remove all but the last message */
-	while((bp = a->hold) != nil){
-		if(bp == a->last)
-			break;
-		a->hold = bp->list;
-		freeblist(bp);
-	}
-
-	ndpsendsol(er->f, ifc, a);	/* unlocks arp */
-}
-
 /*
  *  send a gratuitous arp to refresh arp caches
  */
@@ -534,8 +489,8 @@ sendgarp(Ipifc *ifc, uchar *ip)
 	if(n < ifc->m->mintu)
 		n = ifc->m->mintu;
 	bp = allocb(n);
-	memset(bp->rp, 0, n);
 	e = (Etherarp*)bp->rp;
+	memset(e, 0, n);
 	memmove(e->tpa, ip+IPv4off, sizeof(e->tpa));
 	memmove(e->spa, ip+IPv4off, sizeof(e->spa));
 	memmove(e->sha, ifc->mac, sizeof(e->sha));