For route MTU comparison allow zero MTU to match interface MTU

Because some OS put the interface MTU into the route MTU if the
route does not define its own.

Author: rsmarples

src/dhcpcd.h

@@ -80,6 +80,7 @@ struct interface {
 	uint16_t hwtype; /* ARPHRD_ETHER for example */
 	unsigned char hwaddr[HWADDR_LEN];
 	uint8_t hwlen;
+	unsigned int mtu;
 	unsigned short vlanid;
 	unsigned int metric;
 	int carrier;

src/if-bsd.c

@@ -1252,6 +1252,7 @@ if_ifinfo(struct dhcpcd_ctx *ctx, const struct if_msghdr *ifm)
 	if ((ifp = if_findindex(ctx->ifaces, ifm->ifm_index)) == NULL)
 		return 0;
 
+	ifp->mtu = if_mtu(ifp);
 	link_state = if_carrier(ifp, &ifm->ifm_data);
 	dhcpcd_handlecarrier(ifp, link_state, (unsigned int)ifm->ifm_flags);
 	return 0;

src/if-linux.c

@@ -1053,7 +1053,7 @@ link_netlink(struct dhcpcd_ctx *ctx, void *arg, struct nlmsghdr *nlm)
 	struct interface *ifp = arg;
 	int r;
 	size_t len;
-	struct rtattr *rta, *hwaddr;
+	struct rtattr *rta, *hwaddr, *mtu;
 	struct ifinfomsg *ifi;
 	char ifn[IF_NAMESIZE + 1];
 
@@ -1082,7 +1082,7 @@ link_netlink(struct dhcpcd_ctx *ctx, void *arg, struct nlmsghdr *nlm)
 	rta = (void *)((char *)ifi + NLMSG_ALIGN(sizeof(*ifi)));
 	len = NLMSG_PAYLOAD(nlm, sizeof(*ifi));
 	*ifn = '\0';
-	hwaddr = NULL;
+	hwaddr = mtu = NULL;
 
 	for (; RTA_OK(rta, len); rta = RTA_NEXT(rta, len)) {
 		switch (rta->rta_type) {
@@ -1098,6 +1098,9 @@ link_netlink(struct dhcpcd_ctx *ctx, void *arg, struct nlmsghdr *nlm)
 		case IFLA_ADDRESS:
 			hwaddr = rta;
 			break;
+		case IFLA_MTU:
+			mtu = rta;
+			break;
 		}
 	}
 
@@ -1139,6 +1142,9 @@ link_netlink(struct dhcpcd_ctx *ctx, void *arg, struct nlmsghdr *nlm)
 		return 0;
 	}
 
+	if (mtu != NULL)
+		ifp->mtu = *(unsigned int *)RTA_DATA(mtu);
+
 	/* Re-read hardware address and friends */
 	if (!(ifi->ifi_flags & IFF_UP)) {
 		void *hwa = hwaddr != NULL ? RTA_DATA(hwaddr) : NULL;

src/if-sun.c

@@ -1059,6 +1059,7 @@ if_ifinfo(struct dhcpcd_ctx *ctx, const struct if_msghdr *ifm)
 		state = LINK_UP;
 		flags |= IFF_UP;
 	}
+	ifp->mtu = if_mtu(ifp);
 	dhcpcd_handlecarrier(ifp, state, flags);
 	return 0;
 }

src/if.c

@@ -182,6 +182,18 @@ if_setflag(struct interface *ifp, short setflag, short unsetflag)
 	return 0;
 }
 
+unsigned int
+if_mtu(struct interface *ifp)
+{
+	struct ifreq ifr = { .ifr_mtu = 0 };
+
+	strlcpy(ifr.ifr_name, ifp->name, sizeof(ifr.ifr_name));
+	if (ioctl(ifp->ctx->pf_inet_fd, SIOCGIFMTU, &ifr) == -1)
+		return 0;
+
+	return (unsigned int)ifr.ifr_mtu;
+}
+
 bool
 if_is_link_up(const struct interface *ifp)
 {
@@ -685,6 +697,7 @@ if_discover(struct dhcpcd_ctx *ctx, struct ifaddrs **ifaddrs,
 			}
 		}
 
+		ifp->mtu = if_mtu(ifp);
 		ifp->vlanid = if_vlanid(ifp);
 
 #ifdef SIOCGIFPRIORITY

src/if.h

@@ -163,6 +163,7 @@ int if_ioctl(struct dhcpcd_ctx *, ioctl_request_t, void *, size_t);
 #define	pioctl(ctx, req, data, len) ioctl((ctx)->pf_inet_fd, (req),(data),(len))
 #endif
 int if_setflag(struct interface *, short, short);
+unsigned int if_mtu(struct interface *);
 #define if_up(ifp) if_setflag((ifp), (IFF_UP | IFF_RUNNING), 0)
 #define if_down(ifp) if_setflag((ifp), 0, IFF_UP);
 bool if_is_link_up(const struct interface *);

src/route.c

@@ -506,9 +506,19 @@ rt_recvrt(int cmd, const struct rt *rt, pid_t pid)
 static bool
 rt_cmp_misc(struct rt *nrt, struct rt *ort)
 {
-	/* MTU changed */
+#if defined(__FreeBSD__) || defined(__DragonFly__)
+	/* FreeBSD puts the interface MTU into the route MTU
+	 * if the route does not define it's own. */
+	unsigned int nmtu, omtu;
+
+	nmtu = nrt->rt_mtu ? nrt->rt_mtu : nrt->rt_ifp->mtu;
+	omtu = ort->rt_mtu ? ort->rt_mtu : ort->rt_ifp->mtu;
+	if (omtu != nmtu)
+		return false;
+#else
 	if (ort->rt_mtu != nrt->rt_mtu)
 		return false;
+#endif
 
 #ifdef HAVE_ROUTE_LIFETIME
 	uint32_t deviation;