Detect kernel support for v4-over-v6.

Detection is based on the kernel version, and can be overridden by
the configuration option `v4-over-v6 {true|false}`. If v4-over-v6 is not
supported, ignores v4-over-v6 routes early.

babeld.c

@@ -64,6 +64,7 @@ int debug = 0;
 int link_detect = 0;
 int all_wireless = 0;
 int has_ipv6_subtrees = 0;
+int has_v4ov6 = 0;
 int default_wireless_hello_interval = -1;
 int default_wired_hello_interval = -1;
 int resend_delay = -1;
@@ -169,6 +170,7 @@ main(int argc, char **argv)
     protocol_port = 6696;
     change_smoothing_half_life(4);
     has_ipv6_subtrees = kernel_has_ipv6_subtrees();
+    has_v4ov6 = kernel_has_v4ov6();
 
     while(1) {
         opt = getopt(argc, argv,

babeld.h

@@ -92,6 +92,7 @@ extern const char *logfile, *pidfile, *state_file;
 extern int link_detect;
 extern int all_wireless;
 extern int has_ipv6_subtrees;
+extern int has_v4ov6;
 
 extern unsigned char myid[8];
 extern int have_id;

babeld.man

@@ -200,6 +200,11 @@ This specifies whether to use native source-specific IPv6 forwarding
 rather than multiple routing tables.  The default is chosen automatically
 depending on the kernel version.
 .TP
+.BR v4-over-v6 " {" true | false }
+This specifies whether to use v4-over-v6 routes (IPv4 routes with an IPv6
+next-hop).  This should not be enabled unless your kernel supports it.  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
 option

configuration.c

@@ -850,6 +850,7 @@ parse_option(int c, gnc_t gnc, void *closure, char *token)
               strcmp(token, "daemonise") == 0 ||
               strcmp(token, "skip-kernel-setup") == 0 ||
               strcmp(token, "ipv6-subtrees") == 0 ||
+              strcmp(token, "v4-over-v6") == 0 ||
               strcmp(token, "reflect-kernel-metric") == 0) {
         int b;
         c = getbool(c, &b, gnc, closure);
@@ -866,6 +867,8 @@ parse_option(int c, gnc_t gnc, void *closure, char *token)
             skip_kernel_setup = b;
         else if(strcmp(token, "ipv6-subtrees") == 0)
             has_ipv6_subtrees = b;
+        else if(strcmp(token, "v4-over-v6") == 0)
+            has_v4ov6 = b;
         else if(strcmp(token, "reflect-kernel-metric") == 0)
             reflect_kernel_metric = b;
         else

kernel.h

@@ -105,6 +105,7 @@ int gettime(struct timeval *tv);
 int read_random_bytes(void *buf, int len);
 int kernel_older_than(const char *sysname, int version, int sub_version);
 int kernel_has_ipv6_subtrees(void);
+int kernel_has_v4ov6(void);
 int add_rule(int prio, const unsigned char *src_prefix, int src_plen,
              int table);
 int flush_rule(int prio, int family);

kernel_netlink.c

@@ -940,6 +940,14 @@ kernel_has_ipv6_subtrees(void)
     return (kernel_older_than("Linux", 3, 11) == 0);
 }
 
+int
+kernel_has_v4ov6(void)
+{
+    /* v4-over-v6 was introduced in Linux by commit
+       d15662682db232da77136cd348f4c9df312ca6f9 first released as 5.2 */
+    return (kernel_older_than("Linux", 5, 2) == 0);
+}
+
 int
 kernel_route(int operation, int table,
              const unsigned char *dest, unsigned short plen,

kernel_socket.c

@@ -401,6 +401,12 @@ kernel_has_ipv6_subtrees(void)
     return 0;
 }
 
+int
+kernel_has_v4ov6(void)
+{
+    return 0;
+}
+
 int
 kernel_route(int operation, int table,
              const unsigned char *dest, unsigned short plen,

message.c

@@ -686,6 +686,13 @@ parse_packet(const unsigned char *from, struct interface *ifp,
                        message[2]);
                 goto done;
             }
+            if(message[2] == AE_V4OV6 && !has_v4ov6) {
+                /* We can safely ignore the prefix update that might come
+                   alongside with this TLV, since we ignore every v4-over-v6
+                   TLVs */
+                debugf("Ignoring v4-over-v6 route (unsupported).\n");
+                goto done;
+            }
             DO_NTOHS(interval, message + 6);
             DO_NTOHS(seqno, message + 8);
             DO_NTOHS(metric, message + 10);
@@ -1215,6 +1222,8 @@ really_buffer_update(struct buffered *buf, struct interface *ifp,
 
     if(v4) {
         if(!ifp->ipv4) {
+            if(!has_v4ov6)
+                return;
             ae = AE_V4OV6;
         } else {
             ae = AE_IPV4;