IO: Support nonlocal bind in socket interface

Add option to socket interface for nonlocal binding, i.e. binding to an
IP address that is not present on interfaces. This behaviour is enabled
when SKF_FREEBIND socket flag is set. For Linux systems, it is
implemented by IP_FREEBIND socket flag.

Minor changes done by commiter.

3 files changed, 29 insertions, 0 deletions

diff --git a/sysdep/bsd/sysio.h b/sysdep/bsd/sysio.h
index c757960a..f1887fb4 100644
--- a/sysdep/bsd/sysio.h
+++ b/sysdep/bsd/sysio.h
@@ -271,3 +271,9 @@ sk_set_priority(sock *s, int prio UNUSED)
 {
   ERR_MSG("Socket priority not supported");
 }
+
+static inline int
+sk_set_freebind(sock *s)
+{
+  ERR_MSG("Freebind is not supported");
+}
diff --git a/sysdep/linux/sysio.h b/sysdep/linux/sysio.h
index e21ff487..f13eda7c 100644
--- a/sysdep/linux/sysio.h
+++ b/sysdep/linux/sysio.h
@@ -10,6 +10,10 @@
 #define IPV6_MINHOPCOUNT 73
 #endif
 
+#ifndef IPV6_FREEBIND
+#define IPV6_FREEBIND 78
+#endif
+
 #ifndef TCP_MD5SIG_EXT
 #define TCP_MD5SIG_EXT 32
 #endif
@@ -266,3 +270,18 @@ sk_set_priority(sock *s, int prio)
   return 0;
 }
 
+static inline int
+sk_set_freebind(sock *s)
+{
+  int y = 1;
+
+  if (sk_is_ipv4(s))
+    if (setsockopt(s->fd, SOL_IP, IP_FREEBIND, &y, sizeof(y)) < 0)
+      ERR("IP_FREEBIND");
+
+  if (sk_is_ipv6(s))
+    if (setsockopt(s->fd, SOL_IPV6, IPV6_FREEBIND, &y, sizeof(y)) < 0)
+      ERR("IPV6_FREEBIND");
+
+  return 0;
+}
diff --git a/sysdep/unix/io.c b/sysdep/unix/io.c
index 3d67d0a7..4fd77453 100644
--- a/sysdep/unix/io.c
+++ b/sysdep/unix/io.c
@@ -1436,6 +1436,10 @@ sk_open(sock *s)
 	if (sk_set_high_port(s) < 0)
 	  log(L_WARN "Socket error: %s%#m", s->err);
 
+    if (s->flags & SKF_FREEBIND)
+      if (sk_set_freebind(s) < 0)
+        log(L_WARN "Socket error: %s%#m", s->err);
+
     sockaddr_fill(&sa, s->af, bind_addr, s->iface, bind_port);
     if (bind(fd, &sa.sa, SA_LEN(sa)) < 0)
       ERR2("bind");