confd: add dhcp-server validation of address pool and/or static host

A valid DHCP server setup for a subnet is one of pool and/or at least one
static host entry/lease.  If pool is enabled the pool must have a start
and an end address.

To allow setting up a DHCP server with no pool and at least one static host
entry/lease, we make the pool a presence container, otherwise the pool will
always be set and trigger the below inference.

When an interactive CLI/Web user enables the address pool we infer a default
range .100-.250, but only for /24, C-class networks.  This is what most users
know and expect.

The YANG model now validates that:
- If an address pool is created, both start-address and end-address must be set
- Each subnet must have either a pool or at least one static host entry
- The pool container is now a presence container, so "no pool" fully deletes it

Fixes #1121

Signed-off-by: Joachim Wiberg <[email protected]>

Author: troglobit

doc/ChangeLog.md

@@ -3,7 +3,7 @@ Change Log
 
 All notable changes to the project are documented in this file.
 
-[v25.10.0][UNRELEASED] - 
+[v25.10.0][] - 2025-10-31
 -------------------------
 
 ### Changes
@@ -31,6 +31,10 @@ All notable changes to the project are documented in this file.
 
 - Fix #981: copying any file, including `running-config`, to the persistent
   back-end store for `startup-config`, does not take
+- Fix #1121: Ensure DHCP server does not crash if no address pool is set.  This
+  change infers a pool range (only) for /24 networks, and only when a pool is
+  enabled.  YANG validation for this and other use-cases is also included.  As
+  an unforeseen bonus, Infix now also support non-pool (static lease) setups
 - Fix #1146: Possible to set longer containers names than the system supports.
   Root cause, a limit of 15 characters implicitly imposed by the service mgmt
   daemon, Finit.  The length has not been increased to 64 characters (min: 2)

src/confd/src/infix-dhcp-server.c

@@ -223,10 +223,12 @@ static void add(const char *subnet, struct lyd_node *cfg)
 		start = lydx_get_cattr(node, "start-address");
 		end   = lydx_get_cattr(node, "end-address");
 
-		fprintf(fp, "\n# Subnet pool %s - %s\n", start, end);
-		fprintf(fp, "dhcp-range=%s%sset:%s,%s,%s,%s\n",
-			ifname ?: "", ifname ? "," : "", tag,
-			start, end, lydx_get_cattr(node, "lease-time"));
+		if (start && end) {
+			fprintf(fp, "\n# Subnet pool %s - %s\n", start, end);
+			fprintf(fp, "dhcp-range=%s%sset:%s,%s,%s,%s\n",
+				ifname ?: "", ifname ? "," : "", tag,
+				start, end, lydx_get_cattr(node, "lease-time"));
+		}
 	}
 
 err:
@@ -400,6 +402,7 @@ static int cand(sr_session_ctx_t *session, uint32_t sub_id, const char *module,
 		"router",
 		"dns-server",
 	};
+	sr_val_t *subnets = NULL;
 	size_t i, cnt = 0;
 
 	if (event != SR_EV_UPDATE && event != SR_EV_CHANGE)
@@ -413,6 +416,37 @@ static int cand(sr_session_ctx_t *session, uint32_t sub_id, const char *module,
 		srx_set_item(session, &inferred, 0, fmt, opt[i]);
 	}
 
+	/* Infer pool: .100 to .250 for /24 networks */
+	if (sr_get_items(session, CFG_XPATH "/subnet/subnet", 0, 0, &subnets, &cnt) == 0) {
+		for (i = 0; i < cnt; i++) {
+			const char *pool_xpathfmt = CFG_XPATH "/subnet[subnet='%s']/pool";
+			const char *host_xpathfmt = CFG_XPATH "/subnet[subnet='%s']/host";
+			const char *subnet = subnets[i].data.string_val;
+			sr_val_t pool_val = { .type = SR_STRING_T };
+			char start_addr[16], end_addr[16];
+			unsigned int a, b, c, d, len;
+			size_t pool_cnt = 0, host_cnt = 0;
+
+			if (sscanf(subnet, "%u.%u.%u.%u/%u", &a, &b, &c, &d, &len) != 5 || len != 24)
+				continue;
+
+			/* Don't auto-infer if pool or static hosts already exist */
+			if (!srx_nitems(session, &pool_cnt, pool_xpathfmt, subnet) && pool_cnt)
+				continue;
+			if (!srx_nitems(session, &host_cnt, host_xpathfmt, subnet) && host_cnt)
+				continue;
+
+			snprintf(start_addr, sizeof(start_addr), "%u.%u.%u.100", a, b, c);
+			snprintf(end_addr, sizeof(end_addr), "%u.%u.%u.250", a, b, c);
+
+			pool_val.data.string_val = start_addr;
+			srx_set_item(session, &pool_val, 0, CFG_XPATH "/subnet[subnet='%s']/pool/start-address", subnet);
+			pool_val.data.string_val = end_addr;
+			srx_set_item(session, &pool_val, 0, CFG_XPATH "/subnet[subnet='%s']/pool/end-address", subnet);
+		}
+		sr_free_values(subnets, cnt);
+	}
+
 	return 0;
 }
 

src/confd/yang/confd.inc

@@ -27,7 +27,7 @@ MODULES=(
 	"[email protected]"
 	"[email protected]"
 	"[email protected]"
-	"infix-dhcp-server@2025-01-29.yang"
+	"infix-dhcp-server@2025-10-28.yang"
 	"[email protected]"
 	"[email protected]"
 	"[email protected]"

src/confd/yang/confd/infix-dhcp-server.yang

@@ -20,6 +20,13 @@ module infix-dhcp-server {
   contact      "[email protected]";
   description  "This module implements a DHCPv4 server";
 
+  revision 2025-10-28 {
+    description "Make pool a presence container and add pool validation.
+                 Also, require each subnet to have either a pool or
+                 at least one static host entry.";
+    reference   "internal";
+  }
+
   revision 2025-01-29 {
     description "Initial revision adapted for Infix from DHCPv6 model.";
     reference   "internal";
@@ -132,6 +139,10 @@ module infix-dhcp-server {
       description "Subnet specific settings, including static host entries.";
       key "subnet";
 
+      must "pool or host" {
+        error-message "Subnet must have either a pool or at least one static host entry.";
+      }
+
       leaf subnet {
         description "Subnet to serve DHCP leases from.";
         type inet:ipv4-prefix;
@@ -160,8 +171,13 @@ module infix-dhcp-server {
       }
 
       container pool {
+        presence "Enable dynamic DHCP address pool for this subnet.";
         description "IP address pool for this subnet.";
 
+        must "start-address and end-address" {
+          error-message "Both start-address and end-address must be set if pool is configured.";
+        }
+
         leaf start-address {
           description "The start address of the DHCP address pool.";
           type inet:ipv4-address;