confd: add configurable mDNS hostname (avahi host-name)

Add a `hostname` leaf to the `mdns` YANG container with default `"%h"`,
allowing operators to override the avahi host-name used for mDNS A/AAAA
records without reflashing. The default expands to DEFAULT_HOSTNAME from
os-release, preserving existing behaviour for unconfigured deployments.

Format specifiers %h/%i/%m are supported via the existing hostnamefmt()
infrastructure, which is also fixed to copy the const fmt argument to a
local buffer before modification (UB when called with a libyang-owned
string).

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/confd/src/services.c

@@ -340,12 +340,20 @@ static void fput_list(FILE *fp, struct lyd_node *cfg, const char *list, const ch
 
 #define AVAHI_CONF "/etc/avahi/avahi-daemon.conf"
 
-static void mdns_conf(struct lyd_node *cfg)
+static void mdns_conf(struct confd *confd, struct lyd_node *cfg)
 {
-	const char *hostname = fgetkey("/etc/os-release", "DEFAULT_HOSTNAME");
+	char hname[HOST_NAME_MAX + 1];
+	const char *hostname;
+	const char *fmt;
 	struct lyd_node *ctx;
 	FILE *fp;
 
+	fmt = lydx_get_cattr(cfg, "hostname");   /* "%h" when unset (YANG default) */
+	if (!hostnamefmt(confd, fmt, hname, sizeof(hname), NULL, 0))
+		hostname = hname;
+	else
+		hostname = fgetkey("/etc/os-release", "DEFAULT_HOSTNAME");
+
 	fp = fopen(AVAHI_CONF, "w");
 	if (!fp) {
 		ERRNO("failed creating %s", AVAHI_CONF);
@@ -424,7 +432,7 @@ static int mdns_change(sr_session_ctx_t *session, struct lyd_node *config, struc
 	ena = lydx_is_enabled(srv, "enabled");
 	if (ena) {
 		/* Generate/update avahi-daemon.conf */
-		mdns_conf(srv);
+		mdns_conf(confd, srv);
 
 		/* Generate/update basic mDNS service records */
 		mdns_records(MDNS_UPDATE, all);

src/confd/src/system.c

@@ -1540,27 +1540,33 @@ static char *get_mac(struct confd *confd, char *mac, size_t len)
 int hostnamefmt(struct confd *confd, const char *fmt, char *hostnm, size_t hostlen,
 		char *domain, size_t domlen)
 {
+	char buf[HOST_NAME_MAX + 1];
 	char mac[10];
-	char *ptr;
+	char *f, *ptr;
 	size_t i;
 
 	if (!fmt || !*fmt)
 		return -1;
 
+	strlcpy(buf, fmt, sizeof(buf));
+	f = buf;
+
 	memset(hostnm, 0, hostlen);
 	if (domain)
 		memset(domain, 0, domlen);
 
-	ptr = strchr(fmt, '.');
+	ptr = strchr(f, '.');
 	if (ptr) {
 		*ptr++ = 0;
 		if (domain)
 			strlcpy(domain, ptr, domlen);
 	}
 
-	for (i = 0; i < strlen(fmt); i++) {
-		if (fmt[i] == '%') {
-			switch (fmt[++i]) {
+	for (i = 0; i < strlen(f); i++) {
+		if (f[i] == '%') {
+			if (f[++i] == '\0')
+				break;
+			switch (f[i]) {
 			case 'i':
 				strlcat(hostnm, id, hostlen);
 				break;
@@ -1577,7 +1583,7 @@ int hostnamefmt(struct confd *confd, const char *fmt, char *hostnm, size_t hostl
 				break; /* Unknown, skip */
 			}
 		} else {
-			char ch[2] = { fmt[i], 0 };
+			char ch[2] = { f[i], 0 };
 			strlcat(hostnm, ch, hostlen);
 		}
 	}

src/confd/yang/confd.inc

@@ -43,7 +43,7 @@ MODULES=(
 	"infix-firewall-icmp-types@2025-04-26.yang"
 	"infix-meta@2025-12-10.yang"
 	"infix-system@2026-03-09.yang"
-	"infix-services@2025-12-10.yang"
+	"infix-services@2026-03-03.yang"
 	"ieee802-ethernet-interface@2019-06-21.yang"
 	"infix-ethernet-interface@2024-02-27.yang"
 	"infix-factory-default@2023-06-28.yang"

src/confd/yang/confd/infix-services.yang

@@ -16,6 +16,9 @@ module infix-services {
     reference
       "RFC 9640: YANG Data Types and Groupings for Cryptography";
   }
+  import infix-system {
+    prefix infix-sys;
+  }
 
   import ietf-keystore {
     prefix ks;
@@ -25,6 +28,10 @@ module infix-services {
   contact      "kernelkit@googlegroups.com";
   description  "Infix services, generic.";
 
+  revision 2026-03-03 {
+    description "Add hostname leaf to mdns container for avahi host-name override.";
+    reference "internal";
+  }
   revision 2025-12-10 {
     description "Adapt to changes in final version of ietf-keystore";
     reference "internal";
@@ -80,6 +87,18 @@ module infix-services {
       type inet:domain-name;
     }
 
+    leaf hostname {
+      description "Hostname for mDNS A/AAAA records (avahi host-name setting).
+
+                   When not set the default is the build-time branding hostname.
+                   Supports the same format specifiers as /system/hostname:
+                     %h  Default hostname (DEFAULT_HOSTNAME from os-release)
+                     %i  OS ID from os-release
+                     %m  Last three octets of base MAC (e.g. c0-ff-ee)";
+      type infix-sys:hostname;
+      default "%h";
+    }
+
     container interfaces {
       description "Filter interfaces to act on.";