sgsnemu: Fix build/run against linux < 4.11 (no sysctl addr_gen_mode support)

On older systems (like debian 8), the enum is not present in the header
file and build will fail (as saw in osmocom's OBS instance).
Furthermore, the sysctl to change the value was added at a later point
in time, which means compiling can go fine but running may fail due to
the sysctl not being available.

This is a fix-up to Change-Id I1d51f3ca91edbb3b788939982ab63264182ec2ce

Change-Id: I208970d5b16ea7148444d414b0a6f68c8d9a086c

configure.ac

@@ -131,6 +131,15 @@ AC_EGREP_HEADER(struct iphdr, netinet/ip.h,
  AC_DEFINE([HAVE_IPHDR])],
  AC_MSG_RESULT(no))
 
+# Address generation modes (enum) implemented in linux 3.17 (bc91b0f07ada5535427373a4e2050877bcc12218)
+# /proc/sys/net/ipv6/conf/${iface}/addr_gen_mode was added in linux 4.11 (d35a00b8e33dab7385f724e713ae71c8be0a49f4)
+AC_MSG_CHECKING(whether enum in6_addr_gen_mode.IN6_ADDR_GEN_MODE_NONE exists)
+AH_TEMPLATE(HAVE_IN6_ADDR_GEN_MODE_NONE)
+AC_EGREP_HEADER(IN6_ADDR_GEN_MODE_NONE, linux/if_link.h,
+[AC_MSG_RESULT(yes)
+ AC_DEFINE([HAVE_IN6_ADDR_GEN_MODE_NONE])],
+ AC_MSG_RESULT(no))
+
 # Checks for library functions.
 AC_PROG_GCC_TRADITIONAL
 # AC_FUNC_MALLOC

sgsnemu/sgsnemu.c

@@ -15,6 +15,8 @@
  *
  */
 
+#include "config.h"
+
 #ifdef __linux__
 #define _GNU_SOURCE 1		/* strdup() prototype, broken arpa/inet.h */
 #endif
@@ -47,10 +49,11 @@
 #include <time.h>
 
 #if defined(__linux__)
+#if defined(HAVE_IN6_ADDR_GEN_MODE_NONE)
 #include <linux/if_link.h>
+#endif // HAVE_IN6_ADDR_GEN_MODE_NONE
 #endif
 
-#include "config.h"
 #include "../lib/tun.h"
 #include "../lib/ippool.h"
 #include "../lib/syserr.h"
@@ -1702,13 +1705,16 @@ int main(int argc, char **argv)
 			exit(1);
 		}
 
-#if defined(__linux__)
-		/* Avoid tunnel setting its own link-local addr automatically, we don't need it. */
+#if defined(__linux__) && defined(HAVE_IN6_ADDR_GEN_MODE_NONE)
+		/* Avoid tunnel setting its own link-local addr automatically,
+		   we don't need it. Don't exit on error since this sysctl is
+		   only available starting with linux 4.11. */
 		snprintf(buf, sizeof(buf), "%u", IN6_ADDR_GEN_MODE_NONE);
 		if (proc_ipv6_conf_write(options.tun_dev_name, "addr_gen_mode", buf) < 0) {
 			SYS_ERR(DSGSN, LOGL_ERROR, errno,
-				"Failed to disable addr_gen_mode on %s\n", options.tun_dev_name);
-			exit(1);
+				"Failed to disable addr_gen_mode on %s, an extra link-local "
+				"ip address will appear on the tun device.\n",
+				options.tun_dev_name);
 		}
 #endif