The maximum length of an interface name is 16

This is described in POSIX:
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/net_if.h.html

	The <net/if.h> header shall define the following symbolic constant
	for the length of a buffer containing an interface name (including
	the terminating NULL character):

	IF_NAMESIZE
		Interface name length.

Note that proper macro name is IF_NAMESIZE, IFNAMSIZ is the legacy name.

The value of IF_NAMESIZE is usually 16 as far as I can tell, at least on
recent Linux and macOS machines.

src/config.h

@@ -19,6 +19,7 @@
 #define OPENFORTIVPN_CONFIG_H
 
 #include <netinet/in.h>
+#include <net/if.h>
 
 #include <errno.h>
 #include <stdint.h>
@@ -92,7 +93,7 @@ struct vpn_config {
 	unsigned int  otp_delay;
 	int         no_ftm_push;
 	char		*pinentry;
-	char		iface_name[FIELD_SIZE + 1];
+	char		iface_name[IF_NAMESIZE];
 	char		realm[REALM_SIZE + 1];
 
 	int	set_routes;

src/main.c

@@ -433,8 +433,8 @@ int main(int argc, char **argv)
 			}
 			if (strcmp(long_options[option_index].name,
 			           "ifname") == 0) {
-				strncpy(cli_cfg.iface_name, optarg, FIELD_SIZE);
-				cli_cfg.iface_name[FIELD_SIZE] = '\0';
+				strncpy(cli_cfg.iface_name, optarg, IF_NAMESIZE - 1);
+				cli_cfg.iface_name[IF_NAMESIZE - 1] = '\0';
 				break;
 			}
 			if (strcmp(long_options[option_index].name,

src/tunnel.c

@@ -585,7 +585,7 @@ static int tcp_connect(struct tunnel *tunnel)
 	int ret, handle;
 	struct sockaddr_in server;
 	char *env_proxy;
-	const int iface_len = strnlen(tunnel->config->iface_name, IFNAMSIZ);
+	const int iface_len = strnlen(tunnel->config->iface_name, IF_NAMESIZE);
 
 	handle = socket(AF_INET, SOCK_STREAM, 0);
 
@@ -647,7 +647,7 @@ static int tcp_connect(struct tunnel *tunnel)
 		log_debug("SO_RCVBUF: %d\n", ret);
 #endif
 
-	if (iface_len == IFNAMSIZ) {
+	if (iface_len == IF_NAMESIZE) {
 		log_error("socket: Too long iface name\n");
 		goto err_post_socket;
 	}
@@ -659,8 +659,8 @@ static int tcp_connect(struct tunnel *tunnel)
 		struct ifreq ifr;
 
 		memset(&ifr, 0, sizeof(ifr));
-		if (strlcpy(ifr.ifr_name, tunnel->config->iface_name, IFNAMSIZ)
-		    >= IFNAMSIZ) {
+		if (strlcpy(ifr.ifr_name, tunnel->config->iface_name, IF_NAMESIZE)
+		    >= IF_NAMESIZE) {
 			log_error("interface name too long\n");
 			goto err_post_socket;
 		}