Determine default value of has_ipv6_subtrees dynamically.

Based on the kernel version, can be overridden explicitly.

babeld.c

@@ -123,6 +123,7 @@ main(int argc, char **argv)
     parse_address("ff02:0:0:0:0:0:1:6", protocol_group, NULL);
     protocol_port = 6696;
     change_smoothing_half_life(4);
+    has_ipv6_subtrees = kernel_has_ipv6_subtrees();
 
     while(1) {
         opt = getopt(argc, argv, "m:p:h:H:i:k:A:sruS:d:g:lwz:M:t:T:c:C:DL:I:");

babeld.man

@@ -190,13 +190,8 @@ equivalent to the command-line option
 .TP
 .BR ipv6-subtrees " {" true | false }
 This specifies whether to use native source-specific IPv6 forwarding
-rather than multiple routing tables.  The default is
-.BR false ,
-setting this to
-.B true
-is recommended on sufficiently recent
-.I Linux
-kernels.
+rather than multiple routing tables.  The default is chosen automatically
+depending on the kernel version.
 .TP
 .BI debug " level"
 This specifies the debugging level, and is equivalent to the command-line

kernel.c

@@ -20,6 +20,7 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 */
 
+#include <sys/utsname.h>
 #include <sys/time.h>
 #include <sys/param.h>
 #include <time.h>
@@ -109,3 +110,21 @@ add_import_table(int table)
     import_tables[import_table_count++] = table;
     return 0;
 }
+
+int
+kernel_older_than(const char *sysname, int version, int sub_version)
+{
+    struct utsname un;
+    int rc;
+    int v = 0;
+    int sub_v = 0;
+    rc = uname(&un);
+    if (rc < 0)
+        return -1;
+    if (strcmp(sysname, un.sysname) != 0)
+        return -1;
+    rc = sscanf(un.release, "%d.%d", &v, &sub_v);
+    if (rc < 2)
+        return -1;
+    return (v < version || (v == version && sub_v < sub_version));
+}

kernel.h

@@ -78,3 +78,5 @@ int kernel_addresses(char *ifname, int ifindex, int ll,
 int if_eui64(char *ifname, int ifindex, unsigned char *eui);
 int gettime(struct timeval *tv);
 int read_random_bytes(void *buf, size_t len);
+int kernel_older_than(const char *sysname, int version, int sub_version);
+int kernel_has_ipv6_subtrees(void);

kernel_netlink.c

@@ -925,6 +925,12 @@ kernel_disambiguate(int v4)
     return !v4 && has_ipv6_subtrees;
 }
 
+int
+kernel_has_ipv6_subtrees(void)
+{
+    return (kernel_older_than("Linux", 3, 11) == 0);
+}
+
 int
 kernel_route(int operation, const unsigned char *dest, unsigned short plen,
              const unsigned char *src, unsigned short src_plen,

kernel_socket.c

@@ -388,6 +388,12 @@ kernel_disambiguate(int v4)
     return 0;
 }
 
+int
+kernel_has_ipv6_subtrees(void)
+{
+    return 0;
+}
+
 int
 kernel_route(int operation, const unsigned char *dest, unsigned short plen,
              const unsigned char *src, unsigned short src_plen,