all: implement RFC8910 captive portal (CP) option for DHCPv4

https://www.rfc-editor.org/rfc/rfc8910.html

Signed-off-by: Paul Donald <newtwen+github@gmail.com>
Link: https://github.com/openwrt/odhcpd/pull/315
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

4 files changed, 26 insertions, 1 deletions

diff --git a/README.md b/README.md
index 1ac7742..2d8c3f2 100644
--- a/README.md
+++ b/README.md
@@ -121,7 +121,7 @@ and may also receive information from ubus
 | prefix_filter		|string	|`::/0`	| Only advertise on-link prefixes within the provided IPv6 prefix; others are filtered out. [IPv6 prefix] |
 | 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 DHCPv6 and ICMPv6 RA. |
+| captive_portal_uri |string | no  | The API URI to be sent in RFC8910 captive portal options, via DHCPv4, DHCPv6, and ICMPv6 RA. |
 
 [//]: # "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 b2991ef..865dadf 100644
--- a/src/config.c
+++ b/src/config.c
@@ -1310,6 +1310,11 @@ int config_parse_interface(void *data, size_t len, const char *name, bool overwr
 	if ((c = tb[IFACE_ATTR_CAPTIVE_PORTAL_URI])) {
 		iface->captive_portal_uri = strdup(blobmsg_get_string(c));
 		iface->captive_portal_uri_len = strlen(iface->captive_portal_uri);
+		if (iface->captive_portal_uri_len > UINT8_MAX) {
+			warn("RFC8910 captive portal URI > %d characters for interface '%s': option via DHCPv4 not possible",
+				UINT8_MAX,
+				iface->name);
+		}
 		debug("Set RFC8910 captive portal URI: '%s' for interface '%s'",
 			iface->captive_portal_uri, iface->name);
 	}
diff --git a/src/dhcpv4.c b/src/dhcpv4.c
index cf5abf5..9ab7869 100644
--- a/src/dhcpv4.c
+++ b/src/dhcpv4.c
@@ -779,6 +779,7 @@ enum {
 	IOV_FR_NONCE_CAP,
 	IOV_DNR,
 	IOV_DNR_BODY,
+	IOV_CAPTIVE_PORTAL,
 	IOV_END,
 	IOV_PADDING,
 	IOV_TOTAL
@@ -920,6 +921,7 @@ void dhcpv4_handle_msg(void *src_addr, void *data, size_t len,
 		[IOV_FR_NONCE_CAP]	= { &reply_fr_nonce_cap, 0 },
 		[IOV_DNR]		= { &reply_dnr, 0 },
 		[IOV_DNR_BODY]		= { NULL, 0 },
+		[IOV_CAPTIVE_PORTAL]	= { NULL, 0 },
 		[IOV_END]		= { &reply_end, sizeof(reply_end) },
 		[IOV_PADDING]		= { NULL, 0 },
 	};
@@ -938,6 +940,7 @@ void dhcpv4_handle_msg(void *src_addr, void *data, size_t len,
 		DHCPV4_OPT_CLIENTID, // Must be in reply if present in req, RFC6842, §3
 		DHCPV4_OPT_AUTHENTICATION,
 		DHCPV4_OPT_SEARCH_DOMAIN,
+		DHCPV4_OPT_CAPTIVE_PORTAL,
 		DHCPV4_OPT_FORCERENEW_NONCE_CAPABLE,
 	};
 
@@ -1296,6 +1299,22 @@ void dhcpv4_handle_msg(void *src_addr, void *data, size_t len,
 			iov[IOV_DNR_BODY].iov_base = dnrs;
 			iov[IOV_DNR_BODY].iov_len = dnrs_len;
 			break;
+
+		case DHCPV4_OPT_CAPTIVE_PORTAL:
+			size_t uri_len = iface->captive_portal_uri_len;
+			if (uri_len == 0 || uri_len > UINT8_MAX)
+				break;
+
+			uint8_t *buf = alloca(2 + uri_len);
+			struct dhcpv4_option *opt = (struct dhcpv4_option *)buf;
+
+			opt->code = DHCPV4_OPT_CAPTIVE_PORTAL;
+			opt->len  = uri_len;
+			memcpy(opt->data, iface->captive_portal_uri, uri_len);
+
+			iov[IOV_CAPTIVE_PORTAL].iov_base = opt;
+			iov[IOV_CAPTIVE_PORTAL].iov_len  = 2 + uri_len;
+			break;
 		}
 	}
 
diff --git a/src/dhcpv4.h b/src/dhcpv4.h
index d2b31a3..f428022 100644
--- a/src/dhcpv4.h
+++ b/src/dhcpv4.h
@@ -78,6 +78,7 @@ enum dhcpv4_opt {
 	DHCPV4_OPT_CLIENTID = 61,
 	DHCPV4_OPT_USER_CLASS = 77,
 	DHCPV4_OPT_AUTHENTICATION = 90,
+	DHCPV4_OPT_CAPTIVE_PORTAL = 114, // RFC8910
 	DHCPV4_OPT_SEARCH_DOMAIN = 119,
 	DHCPV4_OPT_FORCERENEW_NONCE_CAPABLE = 145,
 	DHCPV4_OPT_DNR = 162,