BSD: Use MSG_DONTROUTE for unicast packets on FreeBSD

BSD systems cannot use SO_DONTROUTE, because it does not work properly
with multicast packets (perhaps it tries to find iface based on multicast
group address). But we can use MSG_DONTROUTE sendmsg() flag for unicast
packets. Works on FreeBSD, is ignored on OpenBSD and is broken on NetBSD
(i guess due to integrated routing table and ARP table).

lib/ip.h

@@ -241,6 +241,9 @@ static inline int ip6_is_v4mapped(ip6_addr a)
 #define ipa_classify(x) ip6_classify(&(x))
 #define ipa_is_link_local(x) ip6_is_link_local(x)
 
+static inline int ip4_is_unicast(ip4_addr a)
+{ return _I(a) < 0xe0000000; }
+
 /* XXXX remove */
 static inline int ipa_classify_net(ip_addr a)
 { return ipa_zero2(a) ? (IADDR_HOST | SCOPE_UNIVERSE) : ipa_classify(a); }

sysdep/bsd/sysio.h

@@ -12,6 +12,11 @@
 #include <sys/param.h>
 
 
+#ifdef __FreeBSD__
+/* Should be defined in sysdep/cf/bsd.h, but it is flavor-specific */
+#define CONFIG_DONTROUTE_UNICAST
+#endif
+
 #ifdef __NetBSD__
 
 #ifndef IP_RECVTTL

sysdep/cf/README

@@ -11,6 +11,7 @@ CONFIG_MC_PROPER_SRC	Multicast packets have source address according to socket s
 CONFIG_SKIP_MC_BIND	Don't call bind on multicast socket (def for *BSD)
 CONFIG_NO_IFACE_BIND	Bind to iface is not available, use workarounds (def for *BSD)
 CONFIG_UNIX_DONTROUTE	Use setsockopts DONTROUTE (undef for *BSD)
+CONFIG_DONTROUTE_UNICAST Use MSG_DONTROUTE flag for unicast packets (def for FreeBSD)
 CONFIG_USE_HDRINCL	Use IP_HDRINCL instead of control messages for source address on raw IP sockets.
 
 CONFIG_RESTRICTED_PRIVILEGES	Implements restricted privileges using drop_uid()

sysdep/unix/io.c

@@ -1541,6 +1541,7 @@ sk_sendmsg(sock *s)
   struct iovec iov = {s->tbuf, s->tpos - s->tbuf};
   byte cmsg_buf[CMSG_TX_SPACE];
   sockaddr dst;
+  int flags = 0;
 
   sockaddr_fill(&dst, s->af, s->daddr, s->iface, s->dport);
 
@@ -1551,6 +1552,11 @@ sk_sendmsg(sock *s)
     .msg_iovlen = 1
   };
 
+#ifdef CONFIG_DONTROUTE_UNICAST
+  if (ipa_is_ip4(s->daddr) && ip4_is_unicast(ipa_to_ip4(s->daddr)))
+    flags = MSG_DONTROUTE;
+#endif
+
 #ifdef CONFIG_USE_HDRINCL
   byte hdr[20];
   struct iovec iov2[2] = { {hdr, 20}, iov };
@@ -1566,7 +1572,7 @@ sk_sendmsg(sock *s)
   if (s->flags & SKF_PKTINFO)
     sk_prepare_cmsgs(s, &msg, cmsg_buf, sizeof(cmsg_buf));
 
-  return sendmsg(s->fd, &msg, 0);
+  return sendmsg(s->fd, &msg, flags);
 }
 
 static inline int