Allow for setting the Next server IP in the DHCP header

In order to PXE boot a baremetal server using the OVN DHCP server we
need to allow users to set the "next-server" (siaddr) [0] field in the
DHCP header.

While investigating this issue by comparing the DHCPOFFER and DHCPACK
packets sent my dnsmasq and OVN we saw that the "next-server" field
was the problem for OVN, without it PXE booting was timing out while
fetching the iPXE image from the TFTP server (see the bugzilla ticket
below for reference).

To confirm this problem we created a bogus patch hardcoding the TFTP
address in the siaddr of the DHCP header (see the discussion in the
maillist below) and with this in place we were able to deploy a
baremetal node using the OVN DHCP end-to-end.

This patch is a proper implementation that creates a new DHCP
configuration option called "next_server" to allow users to set this
field dynamically. This patch uses the DHCP code 253 which is a unsed
code for DHCP specification as this is not a normal DHCP option but a
special use case in OVN.

[0]
https://github.com/openvswitch/ovs/blob/9dd3031d2e0e9597449e95428320ccaaff7d8b3d/lib/dhcp.h#L42

Reported-at: https://bugzilla.redhat.com/show_bug.cgi?id=2083629
Reported-at:
https://mail.openvswitch.org/pipermail/ovs-discuss/2022-May/051821.html
Signed-off-by: Lucas Alvares Gomes <lucasagomes@gmail.com>
Signed-off-by: Numan Siddique <numans@ovn.org>
(cherry picked from commit 0057cde)
Signed-off-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Dumitru Ceara <dceara@redhat.com>

NEWS

@@ -3,6 +3,8 @@ OVN v22.03.2 - xx xxx xxxx
   - Support (LSP.options:qos_min_rate) to guarantee minimal bandwidth available
     for a logical port.
   - Bump python version required for building OVN to 3.6.
+  - Added support for setting the Next server IP in the DHCP header
+    using the private DHCP option - 253 in native OVN DHCPv4 responder.
 
 OVN v22.03.1 - 03 Jun 2022
 --------------------------

controller/pinctrl.c

@@ -2207,30 +2207,56 @@ pinctrl_handle_put_dhcp_opts(
      *| 4 Bytes padding | 1 Byte (option end 0xFF ) | 4 Bytes padding|
      * --------------------------------------------------------------
      */
-    struct dhcp_opt_header *in_dhcp_opt =
-        (struct dhcp_opt_header *)reply_dhcp_opts_ptr->data;
-    if (in_dhcp_opt->code == DHCP_OPT_BOOTFILE_CODE) {
-        unsigned char *ptr = (unsigned char *)in_dhcp_opt;
-        int len = sizeof *in_dhcp_opt + in_dhcp_opt->len;
-        struct dhcp_opt_header *next_dhcp_opt =
-            (struct dhcp_opt_header *)(ptr + len);
-
-        if (next_dhcp_opt->code == DHCP_OPT_BOOTFILE_ALT_CODE) {
-            if (!ipxe_req) {
-                ofpbuf_pull(reply_dhcp_opts_ptr, len);
-                next_dhcp_opt->code = DHCP_OPT_BOOTFILE_CODE;
-            } else {
-                char *buf = xmalloc(len);
+    ovs_be32 next_server = in_dhcp_data->siaddr;
+    bool bootfile_name_set = false;
+    in_dhcp_ptr = reply_dhcp_opts_ptr->data;
+    end = (const char *)reply_dhcp_opts_ptr->data + reply_dhcp_opts_ptr->size;
+
+    while (in_dhcp_ptr < end) {
+        struct dhcp_opt_header *in_dhcp_opt =
+            (struct dhcp_opt_header *)in_dhcp_ptr;
+
+        switch (in_dhcp_opt->code) {
+        case DHCP_OPT_NEXT_SERVER_CODE:
+            next_server = get_unaligned_be32(DHCP_OPT_PAYLOAD(in_dhcp_opt));
+            break;
+        case DHCP_OPT_BOOTFILE_CODE: ;
+            unsigned char *ptr = (unsigned char *)in_dhcp_opt;
+            int len = sizeof *in_dhcp_opt + in_dhcp_opt->len;
+            struct dhcp_opt_header *next_dhcp_opt =
+                (struct dhcp_opt_header *)(ptr + len);
+
+            if (next_dhcp_opt->code == DHCP_OPT_BOOTFILE_ALT_CODE) {
+                if (!ipxe_req) {
+                    ofpbuf_pull(reply_dhcp_opts_ptr, len);
+                    next_dhcp_opt->code = DHCP_OPT_BOOTFILE_CODE;
+                } else {
+                    char *buf = xmalloc(len);
 
-                memcpy(buf, in_dhcp_opt, len);
-                ofpbuf_pull(reply_dhcp_opts_ptr,
-                            sizeof *in_dhcp_opt + next_dhcp_opt->len);
-                memcpy(reply_dhcp_opts_ptr->data, buf, len);
-                free(buf);
+                    memcpy(buf, in_dhcp_opt, len);
+                    ofpbuf_pull(reply_dhcp_opts_ptr,
+                                sizeof *in_dhcp_opt + next_dhcp_opt->len);
+                    memcpy(reply_dhcp_opts_ptr->data, buf, len);
+                    free(buf);
+                }
+            }
+            bootfile_name_set = true;
+            break;
+        case DHCP_OPT_BOOTFILE_ALT_CODE:
+            if (!bootfile_name_set) {
+                in_dhcp_opt->code = DHCP_OPT_BOOTFILE_CODE;
             }
+            break;
+        }
+
+        in_dhcp_ptr += sizeof *in_dhcp_opt;
+        if (in_dhcp_ptr > end) {
+            break;
+        }
+        in_dhcp_ptr += in_dhcp_opt->len;
+        if (in_dhcp_ptr > end) {
+            break;
         }
-    } else if (in_dhcp_opt->code == DHCP_OPT_BOOTFILE_ALT_CODE) {
-        in_dhcp_opt->code = DHCP_OPT_BOOTFILE_CODE;
     }
 
     uint16_t new_l4_size = UDP_HEADER_LEN + DHCP_HEADER_LEN + 16;
@@ -2263,6 +2289,7 @@ pinctrl_handle_put_dhcp_opts(
 
     if (*in_dhcp_msg_type != OVN_DHCP_MSG_INFORM) {
         dhcp_data->yiaddr = (msg_type == DHCP_MSG_NAK) ? 0 : *offer_ip;
+        dhcp_data->siaddr = (msg_type == DHCP_MSG_NAK) ? 0 : next_server;
     } else {
         dhcp_data->yiaddr = 0;
     }

lib/actions.c

@@ -2862,10 +2862,21 @@ encode_PUT_DHCPV4_OPTS(const struct ovnact_put_opts *pdo,
         opt_header[1] = strlen(c->string);
         ofpbuf_put(ofpacts, c->string, opt_header[1]);
     }
+    /* Encode next_server opt (253) */
+    const struct ovnact_gen_option *next_server_opt = find_opt(
+        pdo->options, pdo->n_options, DHCP_OPT_NEXT_SERVER_CODE);
+    if (next_server_opt) {
+        uint8_t *opt_header = ofpbuf_put_zeros(ofpacts, 2);
+        const union expr_constant *c = next_server_opt->value.values;
+        opt_header[0] = next_server_opt->option->code;
+        opt_header[1] = sizeof(ovs_be32);
+        ofpbuf_put(ofpacts, &c->value.ipv4, sizeof(ovs_be32));
+    }
 
     for (size_t i = 0; i < pdo->n_options; i++) {
         const struct ovnact_gen_option *o = &pdo->options[i];
-        if (o != offerip_opt && o != boot_opt && o != boot_alt_opt) {
+        if (o != offerip_opt && o != boot_opt && o != boot_alt_opt && \
+            o != next_server_opt) {
             encode_put_dhcpv4_option(o, ofpacts);
         }
     }

lib/ovn-l7.h

@@ -145,6 +145,14 @@ struct gen_opts_map {
 #define DHCP_OPT_TCP_KEEPALIVE_INTERVAL \
     DHCP_OPTION("tcp_keepalive_interval", 38, "uint32")
 
+/* Use unused 253 option for DHCP next-server DHCP option.
+ * This option is used for setting the "Next server IP address"
+ * field in the DHCP header.
+ */
+#define DHCP_OPT_NEXT_SERVER_CODE 253
+#define DHCP_OPT_NEXT_SERVER \
+    DHCP_OPTION("next_server", DHCP_OPT_NEXT_SERVER_CODE, "ipv4")
+
 static inline uint32_t
 gen_opt_hash(char *opt_name)
 {

northd/ovn-northd.c

@@ -246,6 +246,7 @@ static struct gen_opts_map supported_dhcp_opts[] = {
     DHCP_OPT_BROADCAST_ADDRESS,
     DHCP_OPT_NETBIOS_NAME_SERVER,
     DHCP_OPT_NETBIOS_NODE_TYPE,
+    DHCP_OPT_NEXT_SERVER,
 };
 
 static struct gen_opts_map supported_dhcpv6_opts[] = {

ovn-nb.xml

@@ -3549,6 +3549,13 @@
           </p>
         </column>
 
+        <column name="options" key="next_server">
+          <p>
+            The DHCPv4 option code for setting the "Next server IP
+            address" field in the DHCP header.
+          </p>
+        </column>
+
       </group>
 
       <group title="Boolean DHCP Options">

tests/ovn.at

@@ -1404,9 +1404,9 @@ reg0[0] = lookup_arp_ip(inport, eth.dst);
 # put_dhcp_opts
 reg1[0] = put_dhcp_opts(offerip = 1.2.3.4, router = 10.0.0.1);
     encodes as controller(userdata=00.00.00.02.00.00.00.00.00.01.de.10.00.00.00.40.01.02.03.04.03.04.0a.00.00.01,pause)
-reg2[5] = put_dhcp_opts(offerip=10.0.0.4,router=10.0.0.1,netmask=255.255.254.0,mtu=1400,domain_name="ovn.org",wpad="https://example.org",bootfile_name="https://127.0.0.1/boot.ipxe",path_prefix="/tftpboot");
-    formats as reg2[5] = put_dhcp_opts(offerip = 10.0.0.4, router = 10.0.0.1, netmask = 255.255.254.0, mtu = 1400, domain_name = "ovn.org", wpad = "https://example.org", bootfile_name = "https://127.0.0.1/boot.ipxe", path_prefix = "/tftpboot");
-    encodes as controller(userdata=00.00.00.02.00.00.00.00.00.01.de.10.00.00.00.25.0a.00.00.04.43.1b.68.74.74.70.73.3a.2f.2f.31.32.37.2e.30.2e.30.2e.31.2f.62.6f.6f.74.2e.69.70.78.65.03.04.0a.00.00.01.01.04.ff.ff.fe.00.1a.02.05.78.0f.07.6f.76.6e.2e.6f.72.67.fc.13.68.74.74.70.73.3a.2f.2f.65.78.61.6d.70.6c.65.2e.6f.72.67.d2.09.2f.74.66.74.70.62.6f.6f.74,pause)
+reg2[5] = put_dhcp_opts(offerip=10.0.0.4,router=10.0.0.1,netmask=255.255.254.0,mtu=1400,domain_name="ovn.org",wpad="https://example.org",bootfile_name="https://127.0.0.1/boot.ipxe",path_prefix="/tftpboot", next_server=10.0.0.9);
+    formats as reg2[5] = put_dhcp_opts(offerip = 10.0.0.4, router = 10.0.0.1, netmask = 255.255.254.0, mtu = 1400, domain_name = "ovn.org", wpad = "https://example.org", bootfile_name = "https://127.0.0.1/boot.ipxe", path_prefix = "/tftpboot", next_server = 10.0.0.9);
+    encodes as controller(userdata=00.00.00.02.00.00.00.00.00.01.de.10.00.00.00.25.0a.00.00.04.43.1b.68.74.74.70.73.3a.2f.2f.31.32.37.2e.30.2e.30.2e.31.2f.62.6f.6f.74.2e.69.70.78.65.fd.04.0a.00.00.09.03.04.0a.00.00.01.01.04.ff.ff.fe.00.1a.02.05.78.0f.07.6f.76.6e.2e.6f.72.67.fc.13.68.74.74.70.73.3a.2f.2f.65.78.61.6d.70.6c.65.2e.6f.72.67.d2.09.2f.74.66.74.70.62.6f.6f.74,pause)
 reg0[15] = put_dhcp_opts(offerip=10.0.0.4,router=10.0.0.1,netmask=255.255.255.0,mtu=1400,ip_forward_enable=1,default_ttl=121,dns_server={8.8.8.8,7.7.7.7},classless_static_route={30.0.0.0/24,10.0.0.4,40.0.0.0/16,10.0.0.6,0.0.0.0/0,10.0.0.1},ethernet_encap=1,router_discovery=0,tftp_server_address={10.0.0.4,10.0.0.5},arp_cache_timeout=10,tcp_keepalive_interval=10);
     formats as reg0[15] = put_dhcp_opts(offerip = 10.0.0.4, router = 10.0.0.1, netmask = 255.255.255.0, mtu = 1400, ip_forward_enable = 1, default_ttl = 121, dns_server = {8.8.8.8, 7.7.7.7}, classless_static_route = {30.0.0.0/24, 10.0.0.4, 40.0.0.0/16, 10.0.0.6, 0.0.0.0/0, 10.0.0.1}, ethernet_encap = 1, router_discovery = 0, tftp_server_address = {10.0.0.4, 10.0.0.5}, arp_cache_timeout = 10, tcp_keepalive_interval = 10);
     encodes as controller(userdata=00.00.00.02.00.00.00.00.00.01.de.10.00.00.00.6f.0a.00.00.04.03.04.0a.00.00.01.01.04.ff.ff.ff.00.1a.02.05.78.13.01.01.17.01.79.06.08.08.08.08.08.07.07.07.07.79.14.18.1e.00.00.0a.00.00.04.10.28.00.0a.00.00.06.00.0a.00.00.01.24.01.01.1f.01.00.96.08.0a.00.00.04.0a.00.00.05.23.04.00.00.00.0a.26.04.00.00.00.0a,pause)

tests/test-ovn.c

@@ -196,6 +196,7 @@ create_gen_opts(struct hmap *dhcp_opts, struct hmap *dhcpv6_opts,
     dhcp_opt_add(dhcp_opts, "broadcast_address", 28, "ipv4");
     dhcp_opt_add(dhcp_opts, "netbios_name_server", 44, "ipv4");
     dhcp_opt_add(dhcp_opts, "netbios_node_type", 46, "uint8");
+    dhcp_opt_add(dhcp_opts, "next_server", 253, "ipv4");
 
     /* DHCPv6 options. */
     hmap_init(dhcpv6_opts);