dhcpv4: support IPv6-only preferred (RFC8925)

This adds support for RFC8925/IPv6-only preferred to the DHCPv4 server.

Closes: https://github.com/openwrt/odhcpd/pull/235

Signed-off-by: David Härdeman <david@hardeman.nu>
Link: https://github.com/openwrt/odhcpd/pull/327
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

5 files changed, 44 insertions, 5 deletions

diff --git a/README.md b/README.md
index 663986a..3907089 100644
--- a/README.md
+++ b/README.md
@@ -122,6 +122,7 @@ and may also receive information from ubus
 | ntp			|list	|`<local address>`| NTP servers to announce accepts IPv4 and IPv6 |
 | upstream		|list	| -	| A list of interfaces which can be used as a source of configuration information (e.g. for NTP servers, if not set explicitly). |
 | captive_portal_uri |string | no  | The API URI to be sent in RFC8910 captive portal options, via DHCPv4, DHCPv6, and ICMPv6 RA. |
+| ipv6_only_preferred   |integer| 0 | Indicate that IPv6-only mode is preferred (RFC8925) [V6ONLY_WAIT time in seconds] |
 
 [//]: # "dhcpv6_raw - string - not documented, may change when generic DHCPv4/DHCPv6 options are added"
 
diff --git a/src/config.c b/src/config.c
index b7f32ee..ec110b5 100644
--- a/src/config.c
+++ b/src/config.c
@@ -22,6 +22,7 @@
 #include "odhcpd.h"
 #include "router.h"
 #include "dhcpv6-pxe.h"
+#include "dhcpv4.h"
 
 static struct blob_buf b;
 
@@ -139,6 +140,7 @@ enum {
 	IFACE_ATTR_MAX_VALID_LIFETIME,
 	IFACE_ATTR_NTP,
 	IFACE_ATTR_CAPTIVE_PORTAL_URI,
+	IFACE_ATTR_IPV6_ONLY_PREFERRED,
 	IFACE_ATTR_MAX
 };
 
@@ -192,6 +194,7 @@ static const struct blobmsg_policy iface_attrs[IFACE_ATTR_MAX] = {
 	[IFACE_ATTR_MAX_VALID_LIFETIME] = { .name = "max_valid_lifetime", .type = BLOBMSG_TYPE_STRING },
 	[IFACE_ATTR_NTP] = { .name = "ntp", .type = BLOBMSG_TYPE_ARRAY },
 	[IFACE_ATTR_CAPTIVE_PORTAL_URI] = { .name = "captive_portal_uri", .type = BLOBMSG_TYPE_STRING },
+	[IFACE_ATTR_IPV6_ONLY_PREFERRED] = { .name = "ipv6_only_preferred", .type = BLOBMSG_TYPE_INT32 },
 };
 
 const struct uci_blob_param_list interface_attr_list = {
@@ -1652,6 +1655,19 @@ int config_parse_interface(void *data, size_t len, const char *name, bool overwr
 		}
 	}
 
+	if ((c = tb[IFACE_ATTR_IPV6_ONLY_PREFERRED])) {
+		uint32_t v6only_wait = blobmsg_get_u32(c);
+
+		if (v6only_wait > 0 && v6only_wait < DHCPV4_MIN_V6ONLY_WAIT) {
+			warn("Invalid %s value configured for interface '%s', clamped to %d",
+			     iface_attrs[IFACE_ATTR_IPV6_ONLY_PREFERRED].name,
+			     iface->name, DHCPV4_MIN_V6ONLY_WAIT);
+			v6only_wait = DHCPV4_MIN_V6ONLY_WAIT;
+		}
+
+		iface->dhcpv4_v6only_wait = v6only_wait;
+	}
+
 	if ((c = tb[IFACE_ATTR_RA_PREFERENCE])) {
 		const char *prio = blobmsg_get_string(c);
 
diff --git a/src/dhcpv4.c b/src/dhcpv4.c
index 97c1ec2..17c34ba 100644
--- a/src/dhcpv4.c
+++ b/src/dhcpv4.c
@@ -789,6 +789,7 @@ enum {
 	IOV_DNR,
 	IOV_DNR_BODY,
 	IOV_CAPTIVE_PORTAL,
+	IOV_IPV6_ONLY_PREF,
 	IOV_END,
 	IOV_PADDING,
 	IOV_TOTAL
@@ -810,6 +811,7 @@ void dhcpv4_handle_msg(void *src_addr, void *data, size_t len,
 	uint8_t *req_clientid = NULL;
 	size_t req_clientid_len = 0;
 	bool req_accept_fr = false;
+	bool ipv6_only = false;
 
 	/* Reply variables */
 	struct dhcpv4_message reply = {
@@ -901,6 +903,11 @@ void dhcpv4_handle_msg(void *src_addr, void *data, size_t len,
 	struct dhcpv4_option reply_dnr = {
 		.code = DHCPV4_OPT_DNR,
 	};
+	struct dhcpv4_option_u32 reply_ipv6_only = {
+		.code = DHCPV4_OPT_IPV6_ONLY_PREFERRED,
+		.len = sizeof(uint32_t),
+		.data = htonl(iface->dhcpv4_v6only_wait),
+	};
 	uint8_t reply_end = DHCPV4_OPT_END;
 
 	struct iovec iov[IOV_TOTAL] = {
@@ -931,6 +938,7 @@ void dhcpv4_handle_msg(void *src_addr, void *data, size_t len,
 		[IOV_DNR]		= { &reply_dnr, 0 },
 		[IOV_DNR_BODY]		= { NULL, 0 },
 		[IOV_CAPTIVE_PORTAL]	= { NULL, 0 },
+		[IOV_IPV6_ONLY_PREF]	= { &reply_ipv6_only, 0 },
 		[IOV_END]		= { &reply_end, sizeof(reply_end) },
 		[IOV_PADDING]		= { NULL, 0 },
 	};
@@ -986,10 +994,12 @@ void dhcpv4_handle_msg(void *src_addr, void *data, size_t len,
 				return;
 			break;
 		case DHCPV4_OPT_REQOPTS:
-			if (opt->len > 0) {
-				req_opts = opt->data;
-				req_opts_len = opt->len;
-			}
+			req_opts = opt->data;
+			req_opts_len = opt->len;
+			if (iface->dhcpv4_v6only_wait)
+				for (uint8_t i = 0; i < opt->len; i++)
+					if (opt->data[i] == DHCPV4_OPT_IPV6_ONLY_PREFERRED)
+						ipv6_only = true;
 			break;
 		case DHCPV4_OPT_CLIENTID:
 			if (opt->len >= 2) {
@@ -1028,6 +1038,9 @@ void dhcpv4_handle_msg(void *src_addr, void *data, size_t len,
 			     &reply_incl_fr, &fr_serverid);
 		return;
 	case DHCPV4_MSG_DISCOVER:
+		if (ipv6_only)
+			break;
+		_o_fallthrough;
 	case DHCPV4_MSG_REQUEST:
 		lease = dhcpv4_lease(iface, req_msg, req->chaddr, req_clientid,
 				     req_clientid_len, req_addr, &req_leasetime,
@@ -1041,7 +1054,7 @@ void dhcpv4_handle_msg(void *src_addr, void *data, size_t len,
 	/* We are at the point where we know the client expects a reply */
 	switch (req_msg) {
 	case DHCPV4_MSG_DISCOVER:
-		if (!lease)
+		if (!lease && !ipv6_only)
 			return;
 		reply_msg.data = DHCPV4_MSG_OFFER;
 		break;
@@ -1296,6 +1309,10 @@ void dhcpv4_handle_msg(void *src_addr, void *data, size_t len,
 			iov[IOV_DNR_BODY].iov_len = dnrs_len;
 			break;
 
+		case DHCPV4_OPT_IPV6_ONLY_PREFERRED:
+			iov[IOV_IPV6_ONLY_PREF].iov_len = sizeof(reply_ipv6_only);
+			break;
+
 		case DHCPV4_OPT_CAPTIVE_PORTAL:
 			size_t uri_len = iface->captive_portal_uri_len;
 			if (uri_len == 0 || uri_len > UINT8_MAX)
diff --git a/src/dhcpv4.h b/src/dhcpv4.h
index 045f78b..e601da6 100644
--- a/src/dhcpv4.h
+++ b/src/dhcpv4.h
@@ -29,6 +29,9 @@
 #define DHCPV4_FR_MIN_DELAY	500
 #define DHCPV4_FR_MAX_FUZZ	500
 
+// RFC8925, §3.4
+#define DHCPV4_MIN_V6ONLY_WAIT	300
+
 // RFC4361, §6.1
 #define DHCPV4_CLIENTID_TYPE_DUID_IAID 255
 
@@ -80,6 +83,7 @@ enum dhcpv4_opt {
 	DHCPV4_OPT_CLIENTID = 61,
 	DHCPV4_OPT_USER_CLASS = 77,
 	DHCPV4_OPT_AUTHENTICATION = 90,
+	DHCPV4_OPT_IPV6_ONLY_PREFERRED = 108, // RFC8925
 	DHCPV4_OPT_CAPTIVE_PORTAL = 114, // RFC8910
 	DHCPV4_OPT_DNS_DOMAIN_SEARCH = 119,
 	DHCPV4_OPT_FORCERENEW_NONCE_CAPABLE = 145,
diff --git a/src/odhcpd.h b/src/odhcpd.h
index a5ba496..8a6461c 100644
--- a/src/odhcpd.h
+++ b/src/odhcpd.h
@@ -450,6 +450,7 @@ struct interface {
 	struct in_addr *dhcpv4_routers;	// IPv4 addresses for routers on this subnet
 	size_t dhcpv4_routers_cnt;	// Count of router addresses
 	bool dhcpv4_forcereconf;
+	uint32_t dhcpv4_v6only_wait;	// V6ONLY_WAIT for the IPv6-only preferred option (RFC8925)
 
 	// DNS
 	struct in_addr *dns_addrs4;	// IPv4 DNS server addresses to announce