Set UDP datagram source address (ticket #1239).

Previously, the source IP address of a response UDP datagram could differ from
the original datagram destination address.  This could happen if the server UDP
socket is bound to a wildcard address and the network interface chosen to output
the response packet has a different default address than the destination address
of the original packet.  For example, if two addresses from the same network are
configured on an interface.

Now source address is set explicitly if a response is sent for a server UDP
socket bound to a wildcard address.

auto/unix

diff --git a/auto/unix b/auto/unix
index 5ef74d4..52060fc 100644
--- a/auto/unix
+++ b/auto/unix
@@ -394,6 +394,19 @@
 . auto/feature
 
 
+# BSD way to set IPv4 datagram source address
+
+ngx_feature="IP_SENDSRCADDR"
+ngx_feature_name="NGX_HAVE_IP_SENDSRCADDR"
+ngx_feature_run=no
+ngx_feature_incs="#include <sys/socket.h>
+                  #include <netinet/in.h>"
+ngx_feature_path=
+ngx_feature_libs=
+ngx_feature_test="setsockopt(0, IPPROTO_IP, IP_SENDSRCADDR, NULL, 0)"
+. auto/feature
+
+
 # Linux way to get IPv4 datagram destination address
 
 ngx_feature="IP_PKTINFO"

src/os/unix/ngx_udp_sendmsg_chain.c

diff --git a/src/os/unix/ngx_udp_sendmsg_chain.c b/src/os/unix/ngx_udp_sendmsg_chain.c
index 65bde6f..5f1cfa5 100644
--- a/src/os/unix/ngx_udp_sendmsg_chain.c
+++ b/src/os/unix/ngx_udp_sendmsg_chain.c
@@ -203,6 +203,20 @@
     ngx_err_t      err;
     struct msghdr  msg;
 
+#if (NGX_HAVE_MSGHDR_MSG_CONTROL)
+
+#if (NGX_HAVE_IP_SENDSRCADDR)
+    u_char             msg_control[CMSG_SPACE(sizeof(struct in_addr))];
+#elif (NGX_HAVE_IP_PKTINFO)
+    u_char             msg_control[CMSG_SPACE(sizeof(struct in_pktinfo))];
+#endif
+
+#if (NGX_HAVE_INET6 && NGX_HAVE_IPV6_RECVPKTINFO)
+    u_char             msg_control6[CMSG_SPACE(sizeof(struct in6_pktinfo))];
+#endif
+
+#endif
+
     ngx_memzero(&msg, sizeof(struct msghdr));
 
     if (c->socklen) {
@@ -213,6 +227,82 @@
     msg.msg_iov = vec->iovs;
     msg.msg_iovlen = vec->count;
 
+#if (NGX_HAVE_MSGHDR_MSG_CONTROL)
+
+    if (c->listening && c->listening->wildcard && c->local_sockaddr) {
+
+#if (NGX_HAVE_IP_SENDSRCADDR)
+
+        if (c->local_sockaddr->sa_family == AF_INET) {
+            struct cmsghdr      *cmsg;
+            struct in_addr      *addr;
+            struct sockaddr_in  *sin;
+
+            msg.msg_control = &msg_control;
+            msg.msg_controllen = sizeof(msg_control);
+
+            cmsg = CMSG_FIRSTHDR(&msg);
+            cmsg->cmsg_level = IPPROTO_IP;
+            cmsg->cmsg_type = IP_SENDSRCADDR;
+            cmsg->cmsg_len = CMSG_LEN(sizeof(struct in_addr));
+
+            sin = (struct sockaddr_in *) c->local_sockaddr;
+
+            addr = (struct in_addr *) CMSG_DATA(cmsg);
+            *addr = sin->sin_addr;
+        }
+
+#elif (NGX_HAVE_IP_PKTINFO)
+
+        if (c->local_sockaddr->sa_family == AF_INET) {
+            struct cmsghdr      *cmsg;
+            struct in_pktinfo   *pkt;
+            struct sockaddr_in  *sin;
+
+            msg.msg_control = &msg_control;
+            msg.msg_controllen = sizeof(msg_control);
+
+            cmsg = CMSG_FIRSTHDR(&msg);
+            cmsg->cmsg_level = IPPROTO_IP;
+            cmsg->cmsg_type = IP_PKTINFO;
+            cmsg->cmsg_len = CMSG_LEN(sizeof(struct in_pktinfo));
+
+            sin = (struct sockaddr_in *) c->local_sockaddr;
+
+            pkt = (struct in_pktinfo *) CMSG_DATA(cmsg);
+            ngx_memzero(pkt, sizeof(struct in_pktinfo));
+            pkt->ipi_spec_dst = sin->sin_addr;
+        }
+
+#endif
+
+#if (NGX_HAVE_INET6 && NGX_HAVE_IPV6_RECVPKTINFO)
+
+        if (c->local_sockaddr->sa_family == AF_INET6) {
+            struct cmsghdr       *cmsg;
+            struct in6_pktinfo   *pkt6;
+            struct sockaddr_in6  *sin6;
+
+            msg.msg_control = &msg_control6;
+            msg.msg_controllen = sizeof(msg_control6);
+
+            cmsg = CMSG_FIRSTHDR(&msg);
+            cmsg->cmsg_level = IPPROTO_IPV6;
+            cmsg->cmsg_type = IPV6_PKTINFO;
+            cmsg->cmsg_len = CMSG_LEN(sizeof(struct in6_pktinfo));
+
+            sin6 = (struct sockaddr_in6 *) c->local_sockaddr;
+
+            pkt6 = (struct in6_pktinfo *) CMSG_DATA(cmsg);
+            ngx_memzero(pkt6, sizeof(struct in6_pktinfo));
+            pkt6->ipi6_addr = sin6->sin6_addr;
+        }
+
+#endif
+    }
+
+#endif
+
 eintr:
 
     n = sendmsg(c->fd, &msg, 0);