confd: add support for container hostname format specifiers

This patch makes it possible to use format specifiers to "uniqiefy" a
container's hostname in the same way as with ietf-system hostname.

The following specifiers are currently supported:

 - %h default hostname from /etc/os-release
 - %i value of ID from /etc/os-release
 - %m last three octets of base MAC, e.g., c0-ff-ee

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

doc/container.md

@@ -609,6 +609,17 @@ container ID (hash) is used, but this can be easily changed:
     admin@example:/> container shell system
     root@sys101:/#
 
+In fact, the container `hostname` setting supports the same format
+specifiers as the host's `hostname` setting:
+
+ - `%i`: OS ID, from `/etc/os-release`, from Menuconfig branding
+ - `%h`: Default hostname, from `/etc/os-release`, from branding
+ - `%m`: NIC specific part of base MAC, e.g., to `c0-ff-ee`
+ - `%%`: Literal %
+
+The most useful combination is probably `"container-name-%m"`, which in
+this example give the container hostname `container-name-c0-ff-ee`.
+
 [^1]: this does not apply to the admin-exec command `container run`.
     This command is intended to be used for testing and evaluating
 	container images.  Such containers are given a private network

src/confd/src/ietf-system.c

@@ -1625,7 +1625,7 @@ static char *get_mac(struct confd *confd, char *mac, size_t len)
  *
  * XXX: PLEASE REFACTOR THIS INTO A PYTHON HELPER FOR FUTURE EXTENSIONS, OR BUGS!
  */
-static int hostnamefmt(struct confd *confd, char **fmt)
+int hostnamefmt(struct confd *confd, char **fmt)
 {
 	size_t hostlen, fmt_len;
 	char *hostname;

src/confd/src/infix-containers.c

@@ -21,6 +21,9 @@
 #define  ACTIVE_QUEUE "/var/lib/containers/active"
 #define  LOGGER       "logger -t container -p local1.notice"
 
+int hostnamefmt(struct confd *confd, char **fmt);
+
+
 static int add(const char *name, struct lyd_node *cif)
 {
 	const char *image = lydx_get_cattr(cif, "image");
@@ -46,8 +49,16 @@ static int add(const char *name, struct lyd_node *cif)
 	LYX_LIST_FOR_EACH(lyd_child(cif), node, "search")
 		fprintf(fp, " --dns-search %s", lyd_get_value(node));
 
-	if ((string = lydx_get_cattr(cif, "hostname")))
-		fprintf(fp, " --hostname %s", string);
+	if ((string = lydx_get_cattr(cif, "hostname"))) {
+		char *fmt = (char *)string;
+
+		if (hostnamefmt(&confd, &fmt)) {
+			ERRNO("%s: failed setting custom hostname", name);
+		} else {
+			fprintf(fp, " --hostname %s", fmt);
+			free(fmt);
+		}
+	}
 
 	if (lydx_is_enabled(cif, "read-only"))
 		fprintf(fp, " --read-only");

src/confd/yang/infix-containers.yang

@@ -22,6 +22,17 @@ module infix-containers {
     prefix infix-if;
   }
 
+  import infix-system {
+    prefix infix-sys;
+  }
+
+  revision 2024-10-14  {
+    description "Two major changes:
+                  - Allow changing name of host interfaces inside container
+                  - Support hostname format specifiers, like ietf-system";
+    reference "internal";
+  }
+
   revision 2024-03-27  {
     description "Add support for capabilities.";
     reference "internal";
@@ -160,8 +171,11 @@ module infix-containers {
       }
 
       leaf hostname {
-	description "Sets the container host name that is available inside the container.";
-	type inet:domain-name;
+	description "Sets the container host name, available inside the container.
+
+                     Format specifiers are for, default hostname, ID, and the
+                     last three octets in base MAC, e.g., c0-ff-ee";
+	type infix-sys:hostname;
       }
 
       leaf privileged {