llmnrd: Poll hostname

Poll "/proc/sys/kernel/hostname", and update the hostname when it was 
changed. systemd has a similar mechanism.

Note that the hostname is not polled if it has been provided on the 
command line, or if "/proc/sys/kernel/hostname" cannot be opened.

diff -Naur llmnrd-0.4-orig/llmnr.c llmnrd-0.4/llmnr.c
--- llmnrd-0.4-orig/llmnr.c	2017-02-10 08:33:42.000000000 +0000
+++ llmnrd-0.4/llmnr.c	2017-02-27 14:08:15.330531281 +0000
@@ -51,6 +51,13 @@
 	llmnr_ipv6 = ipv6;
 }
 
+void llmnr_set_hostname(const char *hostname)
+{
+	llmnr_hostname[0] = strlen(hostname);
+	strncpy(&llmnr_hostname[1], hostname, LLMNR_LABEL_MAX_SIZE);
+	llmnr_hostname[LLMNR_LABEL_MAX_SIZE + 1] = '\0';
+}
+
 static bool llmnr_name_matches(const uint8_t *query)
 {
 	uint8_t n = llmnr_hostname[0];
diff -Naur llmnrd-0.4-orig/llmnr.h llmnrd-0.4/llmnr.h
--- llmnrd-0.4-orig/llmnr.h	2017-02-10 08:33:42.000000000 +0000
+++ llmnrd-0.4/llmnr.h	2017-02-27 14:06:22.125065973 +0000
@@ -22,6 +22,7 @@
 #include <stdbool.h>
 
 void llmnr_init(const char *hostname, bool ipv6);
+void llmnr_set_hostname(const char *hostname);
 void llmnr_recv(int sock);
 
 #endif /* LLMNR_H */
diff -Naur llmnrd-0.4-orig/llmnrd.c llmnrd-0.4/llmnrd.c
--- llmnrd-0.4-orig/llmnrd.c	2017-02-10 08:33:42.000000000 +0000
+++ llmnrd-0.4/llmnrd.c	2017-02-27 14:57:25.968718661 +0000
@@ -28,6 +28,10 @@
 #include <string.h>
 #include <unistd.h>
 
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
 #include <sys/ioctl.h>
 #include <sys/param.h>
 
@@ -43,6 +47,7 @@
 static bool llmnrd_running = true;
 static int llmnrd_sock_ipv4 = -1;
 static int llmnrd_sock_ipv6 = -1;
+static int llmnrd_fd_hostname = -1;
 
 static const char *short_opts = "H:i:p:6dhV";
 static const struct option long_opts[] = {
@@ -129,6 +134,22 @@
 	}
 }
 
+static void hostname_change_handle(char *hostname, int maxlen)
+{
+	char *newname;
+
+	newname = xzalloc(maxlen);
+	if (gethostname(newname, maxlen) == 0) {
+		newname[maxlen - 1] = '\0';
+		if (strcmp(hostname, newname) != 0) {
+			log_info("Hostname changed to %s\n", newname);
+			strcpy(hostname, newname);
+			llmnr_set_hostname(hostname);
+		}
+	}
+	free(newname);
+}
+
 int main(int argc, char **argv)
 {
 	int c, ret = -1;
@@ -183,6 +204,8 @@
 			return EXIT_FAILURE;
 		}
 		hostname[MAXHOSTNAMELEN - 1] = '\0';
+
+		llmnrd_fd_hostname = open("/proc/sys/kernel/hostname", O_RDONLY|O_CLOEXEC|O_NDELAY);
 	}
 
 	if (daemonize) {
@@ -216,10 +239,12 @@
 	nfds = max(llmnrd_sock_ipv4, llmnrd_sock_rtnl);
 	if (llmnrd_sock_ipv6 >= 0)
 		nfds = max(nfds, llmnrd_sock_ipv6);
+	if (llmnrd_fd_hostname >= 0)
+		nfds = max(nfds, llmnrd_fd_hostname);
 	nfds += 1;
 
 	while (llmnrd_running) {
-		fd_set rfds;
+		fd_set rfds, efds;
 
 		FD_ZERO(&rfds);
 		FD_SET(llmnrd_sock_ipv4, &rfds);
@@ -227,7 +252,11 @@
 		if (llmnrd_sock_ipv6 >= 0)
 			FD_SET(llmnrd_sock_ipv6, &rfds);
 
-		ret = select(nfds, &rfds, NULL, NULL, NULL);
+		FD_ZERO(&efds);
+		if (llmnrd_fd_hostname >= 0)
+			FD_SET(llmnrd_fd_hostname, &efds);
+
+		ret = select(nfds, &rfds, NULL, &efds, NULL);
 		if (ret < 0) {
 			if (errno != EINTR)
 				log_err("Failed to select() on socket: %s\n", strerror(errno));
@@ -240,6 +269,8 @@
 				iface_recv(llmnrd_sock_rtnl);
 			if (FD_ISSET(llmnrd_sock_ipv4, &rfds))
 				llmnr_recv(llmnrd_sock_ipv4);
+			if (llmnrd_fd_hostname >= 0 && FD_ISSET(llmnrd_fd_hostname, &efds))
+				hostname_change_handle(hostname, MAXHOSTNAMELEN);
 			if (llmnrd_sock_ipv6 >= 0 && FD_ISSET(llmnrd_sock_ipv6, &rfds))
 				llmnr_recv(llmnrd_sock_ipv6);
 		}