Rework v4-via-v6 configuration.

We now accept v4-via-v6 routes if the kernel supports installing
them, and announce them if the kernel is able to source ICMPv4
without an IPv4 address.  The latter behaviour is configurable,
the former is not.

babeld.c

@@ -63,7 +63,7 @@ int debug = 0;
 int link_detect = 0;
 int all_wireless = 0;
 int has_ipv6_subtrees = 0;
-int has_v4viav6 = 0;
+int has_v4viav6 = 0, safe_v4viav6 = 0;
 int default_wireless_hello_interval = -1;
 int default_wired_hello_interval = -1;
 int resend_delay = -1;
@@ -153,6 +153,7 @@ main(int argc, char **argv)
     change_smoothing_half_life(4);
     has_ipv6_subtrees = kernel_has_ipv6_subtrees();
     has_v4viav6 = kernel_has_v4viav6();
+    safe_v4viav6 = kernel_safe_v4viav6();
 
     while(1) {
         opt = getopt(argc, argv,

babeld.man

@@ -200,11 +200,6 @@ 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
@@ -470,6 +465,10 @@ and
 .BR 0
 otherwise.
 .TP
+.B$ v4-via-v6 " {" true | false }
+Whether to announce IPv4 routes through this interface even when there is
+no IPv4 address.  The default depends on the kernel version.
+.TP
 .BI key " id"
 Enable HMAC security on this interface, and use the key
 .IR id .

configuration.c

@@ -700,6 +700,12 @@ parse_anonymous_ifconf(int c, gnc_t gnc, void *closure,
             if(c < -1)
                 goto error;
             if_conf->accept_bad_signatures = v;
+        } else if(strcmp(token, "v4-via-v6") == 0) {
+            int v;
+            c = getbool(c, &v, gnc, closure);
+            if(c < -1)
+                goto error;
+            if_conf->v4viav6 = v;
         } else {
             goto error;
         }
@@ -917,6 +923,7 @@ merge_ifconf(struct interface_conf *dest,
     MERGE(rtt_min);
     MERGE(rtt_max);
     MERGE(max_rtt_penalty);
+    MERGE(v4viav6);
     MERGE(key);
 
 #undef MERGE
@@ -1029,7 +1036,6 @@ 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);
@@ -1046,8 +1052,6 @@ 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_v4viav6 = b;
         else if(strcmp(token, "reflect-kernel-metric") == 0)
             reflect_kernel_metric = b;
         else

interface.c

@@ -280,6 +280,7 @@ interface_updown(struct interface *ifp, int up)
 {
     int mtu, rc, type;
     struct ipv6_mreq mreq;
+    int v4viav6;
 
     if((!!up) == if_up(ifp))
         return 0;
@@ -458,6 +459,17 @@ interface_updown(struct interface *ifp, int up)
         else
             ifp->flags &= ~IF_RFC6126;
 
+        if(IF_CONF(ifp, v4viav6) == CONFIG_NO)
+            v4viav6 = 0;
+        else if(IF_CONF(ifp, v4viav6 == CONFIG_YES))
+            v4viav6 = 1;
+        else
+            v4viav6 = kernel_safe_v4viav6();
+        if(v4viav6)
+            ifp->flags |= IF_V4VIAV6;
+        else
+            ifp->flags &= ~IF_V4VIAV6;
+
         rc = check_link_local_addresses(ifp);
         if(rc < 0) {
             goto fail;

interface.h

@@ -56,6 +56,7 @@ struct interface_conf {
     char enable_timestamps;
     char rfc6126;
     char accept_bad_signatures;
+    char v4viav6;
     int channel;
     unsigned int rtt_decay;
     unsigned int rtt_min;
@@ -89,6 +90,8 @@ struct interface_conf {
 #define IF_ACCEPT_BAD_SIGNATURES (1 << 8)
 /* Use Babel over DTLS on this interface. */
 #define IF_DTLS (1 << 9)
+/* Announce v4-via-v6 routes through this interface. */
+#define IF_V4VIAV6 (1 << 10)
 
 /* Only INTERFERING can appear on the wire. */
 #define IF_CHANNEL_UNKNOWN 0

kernel.h

@@ -96,6 +96,7 @@ 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_v4viav6(void);
+int kernel_safe_v4viav6(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

@@ -951,6 +951,13 @@ kernel_has_v4viav6(void)
     return (kernel_older_than("Linux", 5, 2) == 0);
 }
 
+/* Whether the kernel is able to source ICMPv4 without an IPv4 address. */
+int
+kernel_safe_v4viav6(void)
+{
+    return (kernel_older_than("Linux", 5, 13) == 0);
+}
+
 int
 kernel_route(int operation, int table,
              const unsigned char *dest, unsigned short plen,

kernel_socket.c

@@ -401,6 +401,12 @@ kernel_has_v4viav6(void)
     return 0;
 }
 
+int
+kernel_safe_v4viav6(void)
+{
+    return 0;
+}
+
 int
 kernel_route(int operation, int table,
              const unsigned char *dest, unsigned short plen,

message.c

@@ -1461,6 +1461,8 @@ really_buffer_update(struct buffered *buf, struct interface *ifp,
 
     if(v4) {
         if(!ifp->ipv4) {
+            if((ifp->flags & IF_V4VIAV6) == 0)
+                return;
             ae = AE_V4VIAV6;
         } else {
             ae = AE_IPV4;