ipv6: assign DNS servers per session from RADIUS

The ipv6_nd and ipv6_dhcp modules could only advertise the DNS servers
configured in [ipv6-dns], the same set for every subscriber. RFC 6911
defines DNS-Server-IPv6-Address (attribute 169) for exactly this, and
the attribute was already in the shipped dictionary and in attr_defs.h;
nothing read it.

Give struct ap_session an ipv6_dns list, filled by the radius module
from that attribute, and have both modules advertise it when the session
has one: in the RDNSS option of the router advertisements, and in the
DNS_SERVERS option of DHCPv6 replies. Sessions without a list of their
own keep getting the configured servers, so nothing changes for anyone
not sending the attribute.

The selection is a single ipv6_dns_get() shared by both modules rather
than a copy in each: they already duplicate the whole [ipv6-dns] parser,
and two copies of a precedence rule are two chances to drift. It caps
what it returns, so neither the RDNSS option length (one byte, in units
of 8) nor the router advertisement buffer can be pushed around by what a
RADIUS server sends. The radius module caps at the same 3 servers as
[ipv6-dns] accepts and warns once when a reply carries more.

An Access-Accept which carries the attribute replaces the whole
previously assigned list rather than appending to it, so a re-authorized
session ends up with the servers of the latest reply and not with a
concatenation. One which does not carry it leaves the current list
alone, which is how the IPv4 MS-Primary-DNS-Server attribute already
behaves.

ipv6_dns_test.c covers the selection: assigned wins over configured,
empty list means "nothing assigned" rather than "no DNS", the cap holds
for both sources, and a caller with no room gets nothing rather than a
stomped buffer. Wired into the ASAN/UBSAN workflow.

Inspired by the per-session IPv6 DNS support in accel-ppp-ng (commit
2df6eb99), reimplemented against mainline's ap_session and ipdb types.

3 files changed, 175 insertions, 10 deletions

diff --git a/accel-pppd/ipv6/dhcpv6.c b/accel-pppd/ipv6/dhcpv6.c
index b284df5f..4a80bdb9 100644
--- a/accel-pppd/ipv6/dhcpv6.c
+++ b/accel-pppd/ipv6/dhcpv6.c
@@ -20,6 +20,7 @@
 #include "log.h"
 #include "ppp.h"
 #include "ipdb.h"
+#include "ipv6_dns.h"
 #include "events.h"
 #include "iputils.h"
 
@@ -220,16 +221,19 @@ static void insert_status(struct dhcpv6_packet *pkt, struct dhcpv6_option *opt,
 static void insert_oro(struct dhcpv6_packet *reply, struct dhcpv6_option *opt)
 {
 	struct dhcpv6_option *opt1;
-	int i, j;
+	int i, j, dns_count;
 	uint16_t *ptr;
 	struct in6_addr addr, *addr_ptr;
+	struct in6_addr dns[MAX_DNS_COUNT];
 
 	for (i = ntohs(opt->hdr->len) / 2, ptr = (uint16_t *)opt->hdr->data; i; i--, ptr++) {
 		if (ntohs(*ptr) == D6_OPTION_DNS_SERVERS) {
-			if (conf_dns_count) {
-				opt1 = dhcpv6_option_alloc(reply, D6_OPTION_DNS_SERVERS, conf_dns_count * sizeof(addr));
-				for (j = 0, addr_ptr = (struct in6_addr *)opt1->hdr->data; j < conf_dns_count; j++, addr_ptr++)
-					memcpy(addr_ptr, conf_dns + j, sizeof(addr));
+			dns_count = ipv6_dns_get(reply->ses, conf_dns, conf_dns_count,
+						 dns, MAX_DNS_COUNT);
+			if (dns_count) {
+				opt1 = dhcpv6_option_alloc(reply, D6_OPTION_DNS_SERVERS, dns_count * sizeof(addr));
+				for (j = 0, addr_ptr = (struct in6_addr *)opt1->hdr->data; j < dns_count; j++, addr_ptr++)
+					memcpy(addr_ptr, dns + j, sizeof(addr));
 			}
 		} else if (ntohs(*ptr) == D6_OPTION_DOMAIN_LIST) {
 			if (conf_dnssl_size) {
diff --git a/accel-pppd/ipv6/ipv6_dns_test.c b/accel-pppd/ipv6/ipv6_dns_test.c
new file mode 100644
index 00000000..72b4751c
--- /dev/null
+++ b/accel-pppd/ipv6/ipv6_dns_test.c
@@ -0,0 +1,157 @@
+/*
+ * Standalone test for the IPv6 DNS server selection shared by the ipv6_nd and
+ * ipv6_dhcp modules.
+ *
+ * Not part of the cmake build. Compile and run from the top of the tree, with
+ * a configured build directory around for config.h:
+ *   gcc -O2 -Wall -D_GNU_SOURCE -DAP_SESSIONID_LEN=16 \
+ *       -I accel-pppd/include -I accel-pppd/triton -I build \
+ *       -o /tmp/ipv6_dns_test accel-pppd/ipv6/ipv6_dns_test.c && /tmp/ipv6_dns_test
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <arpa/inet.h>
+
+#include "ipv6_dns.h"
+
+static int failures;
+#define CHECK(cond) do { if (!(cond)) { \
+	fprintf(stderr, "FAIL %s:%d: %s\n", __FILE__, __LINE__, #cond); failures++; } } while (0)
+
+#define MAX_DNS_COUNT 3		/* as in nd.c and dhcpv6.c */
+
+static struct in6_addr a6(const char *str)
+{
+	struct in6_addr addr;
+
+	if (inet_pton(AF_INET6, str, &addr) != 1) {
+		fprintf(stderr, "bad address %s\n", str);
+		exit(1);
+	}
+
+	return addr;
+}
+
+static int is(const struct in6_addr *addr, const char *str)
+{
+	struct in6_addr expect = a6(str);
+
+	return memcmp(addr, &expect, sizeof(expect)) == 0;
+}
+
+/* A session carrying 'count' DNS servers taken from 'str' */
+static struct ap_session *session_with(const char **str, int count)
+{
+	struct ap_session *ses = calloc(1, sizeof(*ses));
+	struct ipv6db_item_t *item = calloc(1, sizeof(*item));
+	int i;
+
+	INIT_LIST_HEAD(&item->addr_list);
+	for (i = 0; i < count; i++) {
+		struct ipv6db_addr_t *a = calloc(1, sizeof(*a));
+
+		a->addr = a6(str[i]);
+		a->prefix_len = 128;
+		list_add_tail(&a->entry, &item->addr_list);
+	}
+
+	ses->ipv6_dns = item;
+
+	return ses;
+}
+
+static void session_free(struct ap_session *ses)
+{
+	if (ses->ipv6_dns) {
+		while (!list_empty(&ses->ipv6_dns->addr_list)) {
+			struct ipv6db_addr_t *a = list_entry(ses->ipv6_dns->addr_list.next,
+							     typeof(*a), entry);
+
+			list_del(&a->entry);
+			free(a);
+		}
+		free(ses->ipv6_dns);
+	}
+
+	free(ses);
+}
+
+int main(void)
+{
+	static const char *four[] = { "2001:db8::1", "2001:db8::2",
+				      "2001:db8::3", "2001:db8::4" };
+	struct in6_addr conf_dns[MAX_DNS_COUNT];
+	struct in6_addr dns[MAX_DNS_COUNT];
+	struct ap_session *ses;
+	int n;
+
+	conf_dns[0] = a6("fc00::53");
+	conf_dns[1] = a6("fc00::54");
+
+	/* No session at all: the configured servers, as before the feature */
+	n = ipv6_dns_get(NULL, conf_dns, 2, dns, MAX_DNS_COUNT);
+	CHECK(n == 2);
+	CHECK(is(&dns[0], "fc00::53"));
+	CHECK(is(&dns[1], "fc00::54"));
+
+	/* Session without assigned servers: same fallback */
+	ses = calloc(1, sizeof(*ses));
+	n = ipv6_dns_get(ses, conf_dns, 2, dns, MAX_DNS_COUNT);
+	CHECK(n == 2);
+	CHECK(is(&dns[0], "fc00::53"));
+	free(ses);
+
+	/* Nothing configured and nothing assigned: advertise nothing */
+	n = ipv6_dns_get(NULL, conf_dns, 0, dns, MAX_DNS_COUNT);
+	CHECK(n == 0);
+
+	/* Assigned servers win over the configured ones */
+	ses = session_with(four, 2);
+	n = ipv6_dns_get(ses, conf_dns, 2, dns, MAX_DNS_COUNT);
+	CHECK(n == 2);
+	CHECK(is(&dns[0], "2001:db8::1"));
+	CHECK(is(&dns[1], "2001:db8::2"));
+
+	/* ... and win even when nothing is configured */
+	n = ipv6_dns_get(ses, conf_dns, 0, dns, MAX_DNS_COUNT);
+	CHECK(n == 2);
+	CHECK(is(&dns[0], "2001:db8::1"));
+
+	session_free(ses);
+
+	/* An empty assigned list is "nothing assigned", not "no DNS" */
+	ses = session_with(four, 0);
+	n = ipv6_dns_get(ses, conf_dns, 2, dns, MAX_DNS_COUNT);
+	CHECK(n == 2);
+	CHECK(is(&dns[0], "fc00::53"));
+	session_free(ses);
+
+	/* More assigned than fit: keep the first max, never overrun */
+	ses = session_with(four, 4);
+	memset(dns, 0, sizeof(dns));
+	n = ipv6_dns_get(ses, conf_dns, 2, dns, MAX_DNS_COUNT);
+	CHECK(n == MAX_DNS_COUNT);
+	CHECK(is(&dns[0], "2001:db8::1"));
+	CHECK(is(&dns[1], "2001:db8::2"));
+	CHECK(is(&dns[2], "2001:db8::3"));
+
+	/* Same for the configured ones, should they ever exceed max */
+	n = ipv6_dns_get(NULL, conf_dns, 99, dns, MAX_DNS_COUNT);
+	CHECK(n == MAX_DNS_COUNT);
+
+	/* A caller with no room gets nothing rather than a stomped buffer */
+	n = ipv6_dns_get(ses, conf_dns, 2, dns, 0);
+	CHECK(n == 0);
+
+	session_free(ses);
+
+	if (failures) {
+		fprintf(stderr, "%d failure(s)\n", failures);
+		return 1;
+	}
+
+	printf("all tests passed\n");
+
+	return 0;
+}
diff --git a/accel-pppd/ipv6/nd.c b/accel-pppd/ipv6/nd.c
index 6dd00ee2..8b22da14 100644
--- a/accel-pppd/ipv6/nd.c
+++ b/accel-pppd/ipv6/nd.c
@@ -17,6 +17,7 @@
 #include "events.h"
 #include "mempool.h"
 #include "ipdb.h"
+#include "ipv6_dns.h"
 #include "iputils.h"
 
 #include "memdebug.h"
@@ -107,7 +108,8 @@ static void ipv6_nd_send_ra(struct ipv6_nd_handler_t *h, struct sockaddr_in6 *ds
 	//struct nd_opt_mtu *mtu;
 	struct ipv6db_addr_t *a;
 	struct in6_addr addr, peer_addr;
-	int i, prefix_len;
+	struct in6_addr dns[MAX_DNS_COUNT];
+	int i, prefix_len, dns_count;
 
 	if (!buf) {
 		log_emerg("out of memory\n");
@@ -174,15 +176,17 @@ static void ipv6_nd_send_ra(struct ipv6_nd_handler_t *h, struct sockaddr_in6 *ds
 		rinfo++;
 	}*/
 
-	if (conf_dns_count) {
+	dns_count = ipv6_dns_get(ses, conf_dns, conf_dns_count, dns, MAX_DNS_COUNT);
+
+	if (dns_count) {
 		rdnssinfo = (struct nd_opt_rdnss_info_local *)pinfo;
 		memset(rdnssinfo, 0, sizeof(*rdnssinfo));
 		rdnssinfo->nd_opt_rdnssi_type = ND_OPT_RDNSS_INFORMATION;
-		rdnssinfo->nd_opt_rdnssi_len = 1 + 2 * conf_dns_count;
+		rdnssinfo->nd_opt_rdnssi_len = 1 + 2 * dns_count;
 		rdnssinfo->nd_opt_rdnssi_lifetime = htonl(conf_rdnss_lifetime);
 		rdnss_addr = (struct in6_addr *)rdnssinfo->nd_opt_rdnssi;
-		for (i = 0; i < conf_dns_count; i++) {
-			memcpy(rdnss_addr, &conf_dns[i], sizeof(*rdnss_addr));
+		for (i = 0; i < dns_count; i++) {
+			memcpy(rdnss_addr, &dns[i], sizeof(*rdnss_addr));
 			rdnss_addr++;
 		}
 	} else