From: Paul Donald <newtwen+github@gmail.com>
Date: Fri, 10 Oct 2025 11:42:11 +0000 (+0200)
Subject: config: clamp ra_mtu into 1280-65535 range
X-Git-Url: http://git.openwrt.org/?a=commitdiff_plain;h=465f19c9c2e37023ddf9604a12b91b7252a20561;p=project%2Fodhcpd.git

config: clamp ra_mtu into 1280-65535 range

(ipv6 packets can be up to 4GB in size...)

The old logic had a logic error of || instead of &&

Signed-off-by: Paul Donald <newtwen+github@gmail.com>
Link: https://github.com/openwrt/odhcpd/pull/225
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
---

diff --git a/src/config.c b/src/config.c
index 05daf5d..3cd9b52 100644
--- a/src/config.c
+++ b/src/config.c
@@ -1389,13 +1389,18 @@ int config_parse_interface(void *data, size_t len, const char *name, bool overwr
 	}
 
 	if ((c = tb[IFACE_ATTR_RA_MTU])) {
-		uint32_t ra_mtu = blobmsg_get_u32(c);
-
-		if (ra_mtu >= 1280 || ra_mtu <= 65535)
-			iface->ra_mtu = ra_mtu;
-		else
-			syslog(LOG_ERR, "Invalid %s value configured for interface '%s'",
-					iface_attrs[IFACE_ATTR_RA_MTU].name, iface->name);
+		uint32_t original_ra_mtu, ra_mtu;
+		original_ra_mtu = ra_mtu = blobmsg_get_u32(c);
+		if (ra_mtu < RA_MTU_MIN)
+			ra_mtu = RA_MTU_MIN;
+		else if (ra_mtu > RA_MTU_MAX)
+			ra_mtu = RA_MTU_MAX;
+		iface->ra_mtu = ra_mtu;
+
+		if (original_ra_mtu != ra_mtu) {
+			syslog(LOG_WARNING, "Clamped invalid %s value configured for interface '%s' to %d",
+				iface_attrs[IFACE_ATTR_RA_MTU].name, iface->name, iface->ra_mtu);
+		}
 	}
 
 	if ((c = tb[IFACE_ATTR_RA_SLAAC]))
diff --git a/src/router.h b/src/router.h
index 6d82666..afcf3df 100644
--- a/src/router.h
+++ b/src/router.h
@@ -83,6 +83,12 @@ struct icmpv6_opt {
 		RETRANS_TIMER                 1,000 milliseconds
 */
 #define RETRANS_TIMER_MAX				60000
+/* RFC2460 §5
+   IPv6 requires that every link in the internet have an MTU of 1280
+   octets or greater. 
+*/
+#define RA_MTU_MIN						1280
+#define RA_MTU_MAX						65535
 
 #define ND_RA_FLAG_PROXY		0x4
 #define ND_RA_PREF_HIGH			(1 << 3)