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Currently, on multihomed host, when chrony is not bound to a specific
IP address, a query is sent to an interface and the default source IP
hint for the back route differs, the reply will have a source IP
different than where the query was destinied to. This will cause
problems because connection tracking firewalls will drop the replies
and most likely the client program will get confused too.

This patch uses the IP_PKTINFO mechanism to get the IP address where
received packets where targetted to and use that IP address as source
hint when sending a reply.
---
 addressing.h |    1 +
 broadcast.c  |    1 +
 cmdmon.c     |    3 ++
 conf.c       |    1 +
 ntp_io.c     |   92 +++++++++++++++++++++++++++++++++++++++++----------------
 5 files changed, 72 insertions(+), 26 deletions(-)

diff --git a/addressing.h b/addressing.h
index aa20ed9..05152f4 100644
--- a/addressing.h
+++ b/addressing.h
@@ -36,6 +36,7 @@
 typedef struct {
   unsigned long ip_addr;
   unsigned short port;
+  unsigned long local_ip_addr;
 } NTP_Remote_Address;
 
 #if 0
diff --git a/broadcast.c b/broadcast.c
index be217e7..c979741 100644
--- a/broadcast.c
+++ b/broadcast.c
@@ -146,6 +146,7 @@ BRD_AddDestination(unsigned long addr, unsigned short port, int interval)
 
   destinations[n_destinations].addr.ip_addr = addr;
   destinations[n_destinations].addr.port = port;
+  destinations[n_destinations].addr.local_ip_addr = 0;
   destinations[n_destinations].interval = interval;
 
   SCH_AddTimeoutInClass((double) interval, 1.0,
diff --git a/cmdmon.c b/cmdmon.c
index 819977c..8affb0b 100644
--- a/cmdmon.c
+++ b/cmdmon.c
@@ -1097,6 +1097,7 @@ handle_add_server(CMD_Request *rx_message, CMD_Reply *tx_message)
   
   rem_addr.ip_addr = ntohl(rx_message->data.ntp_source.ip_addr);
   rem_addr.port = (unsigned short)(ntohl(rx_message->data.ntp_source.port));
+  rem_addr.local_ip_addr = 0;
   params.minpoll = ntohl(rx_message->data.ntp_source.minpoll);
   params.maxpoll = ntohl(rx_message->data.ntp_source.maxpoll);
   params.presend_minpoll = ntohl(rx_message->data.ntp_source.presend_minpoll);
@@ -1133,6 +1134,7 @@ handle_add_peer(CMD_Request *rx_message, CMD_Reply *tx_message)
   
   rem_addr.ip_addr = ntohl(rx_message->data.ntp_source.ip_addr);
   rem_addr.port = (unsigned short)(ntohl(rx_message->data.ntp_source.port));
+  rem_addr.local_ip_addr = 0;
   params.minpoll = ntohl(rx_message->data.ntp_source.minpoll);
   params.maxpoll = ntohl(rx_message->data.ntp_source.maxpoll);
   params.presend_minpoll = ntohl(rx_message->data.ntp_source.presend_minpoll);
@@ -1167,6 +1169,7 @@ handle_del_source(CMD_Request *rx_message, CMD_Reply *tx_message)
   
   rem_addr.ip_addr = ntohl(rx_message->data.del_source.ip_addr);
   rem_addr.port = 0;
+  rem_addr.local_ip_addr = 0;
   
   status = NSR_RemoveSource(&rem_addr);
   switch (status) {
diff --git a/conf.c b/conf.c
index e34927e..ddd13f1 100644
--- a/conf.c
+++ b/conf.c
@@ -949,6 +949,7 @@ CNF_AddSources(void) {
   for (i=0; i<n_ntp_sources; i++) {
     server.ip_addr = ntp_sources[i].ip_addr;
     server.port = ntp_sources[i].port;
+    server.local_ip_addr = 0;
 
     switch (ntp_sources[i].type) {
       case SERVER:
diff --git a/ntp_io.c b/ntp_io.c
index afb6ad1..db89758 100644
--- a/ntp_io.c
+++ b/ntp_io.c
@@ -118,6 +118,12 @@ NIO_Initialise(void)
     LOG(LOGS_ERR, LOGF_NtpIO, "Could not set broadcast socket options");
     /* Don't quit - we might survive anyway */
   }
+  /* We want the local IP info too */
+  if (setsockopt(sock_fd, IPPROTO_IP, IP_PKTINFO, (char *)&on_off, sizeof(on_off)) < 0) {
+    LOG(LOGS_ERR, LOGF_NtpIO, "Could not request packet info using socket option");
+    /* Don't quit - we might survive anyway */
+  }
+
 
   /* Bind the port */
   my_addr.sin_family = AF_INET;
@@ -182,22 +188,30 @@ read_from_socket(void *anything)
 
   int status;
   ReceiveBuffer message;
-  int message_length;
   struct sockaddr_in where_from;
-  socklen_t from_length;
   unsigned int flags = 0;
   struct timeval now;
   NTP_Remote_Address remote_addr;
   double local_clock_err;
+  char cmsgbuf[256];
+  struct cmsghdr *cmsg;
+  struct msghdr msg;
+  struct iovec iov;
 
   assert(initialised);
 
-  from_length = sizeof(where_from);
-  message_length = sizeof(message);
+  iov.iov_base = message.arbitrary;
+  iov.iov_len = sizeof(message);
+  msg.msg_name = &where_from;
+  msg.msg_namelen = sizeof(where_from);
+  msg.msg_iov = &iov;
+  msg.msg_iovlen = 1;
+  msg.msg_control = (void *) cmsgbuf;
+  msg.msg_controllen = sizeof(cmsgbuf);
+  msg.msg_flags = 0;
 
   LCL_ReadCookedTime(&now, &local_clock_err);
-  status = recvfrom(sock_fd, (char *)&message, message_length, flags,
-                    (struct sockaddr *)&where_from, &from_length);
+  status = recvmsg(sock_fd, &msg, flags);
 
   /* Don't bother checking if read failed or why if it did.  More
      likely than not, it will be connection refused, resulting from a
@@ -209,6 +223,13 @@ read_from_socket(void *anything)
   if (status > 0) {
     remote_addr.ip_addr = ntohl(where_from.sin_addr.s_addr);
     remote_addr.port = ntohs(where_from.sin_port);
+    remote_addr.local_ip_addr = 0;
+
+    for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
+      if (cmsg->cmsg_level == IPPROTO_IP && cmsg->cmsg_type == IP_PKTINFO)
+        remote_addr.local_ip_addr =
+          ntohl(((struct in_pktinfo *) CMSG_DATA(cmsg))->ipi_spec_dst.s_addr);
+    }
 
     if (status == NTP_NORMAL_PACKET_SIZE) {
 
@@ -229,21 +250,45 @@ read_from_socket(void *anything)
 }
 
 /* ================================================== */
-/* Send an unauthenticated packet to a given address */
+/* Send a packet to given address */
 
-void
-NIO_SendNormalPacket(NTP_Packet *packet, NTP_Remote_Address *remote_addr)
+static void
+NIO_SendPacket(NTP_Packet *packet, int packetlen, NTP_Remote_Address *remote_addr)
 {
   struct sockaddr_in remote;
+  struct msghdr msg;
+  struct iovec iov;
+  struct {
+    struct cmsghdr cm;
+    struct in_pktinfo ipi;
+  } cmsg;
 
   assert(initialised);
 
   remote.sin_family = AF_INET;
   remote.sin_port = htons(remote_addr->port);
   remote.sin_addr.s_addr = htonl(remote_addr->ip_addr);
+  iov.iov_base = (void *) packet;
+  iov.iov_len = packetlen;
+  msg.msg_name = &remote;
+  msg.msg_namelen = sizeof(remote);
+  msg.msg_iov = &iov;
+  msg.msg_iovlen = 1;
+  if (remote_addr->local_ip_addr) {
+    cmsg.cm.cmsg_len = sizeof(cmsg);
+    cmsg.cm.cmsg_level = IPPROTO_IP;
+    cmsg.cm.cmsg_type = IP_PKTINFO;
+    memset(&cmsg.ipi, 0, sizeof(cmsg.ipi));
+    cmsg.ipi.ipi_spec_dst.s_addr = htonl(remote_addr->local_ip_addr);
+    msg.msg_control = (void *) &cmsg;
+    msg.msg_controllen = sizeof(cmsg);
+  } else {
+    msg.msg_control = NULL;
+    msg.msg_controllen = 0;
+  }
+  msg.msg_flags = 0;
 
-  if (sendto(sock_fd, (void *) packet, NTP_NORMAL_PACKET_SIZE, 0,
-             (struct sockaddr *) &remote, sizeof(remote)) < 0) {
+  if (sendmsg(sock_fd, &msg, 0) < 0) {
     LOG(LOGS_WARN, LOGF_NtpIO, "Could not send to %s:%d : %s",
         UTI_IPToDottedQuad(remote_addr->ip_addr), remote_addr->port, strerror(errno));
   }
@@ -252,26 +297,21 @@ NIO_SendNormalPacket(NTP_Packet *packet, NTP_Remote_Address *remote_addr)
 }
 
 /* ================================================== */
-/* Send an authenticated packet to a given address */
+/* Send an unauthenticated packet to a given address */
 
 void
-NIO_SendAuthenticatedPacket(NTP_Packet *packet, NTP_Remote_Address *remote_addr)
+NIO_SendNormalPacket(NTP_Packet *packet, NTP_Remote_Address *remote_addr)
 {
-  struct sockaddr_in remote;
-
-  assert(initialised);
-
-  remote.sin_family = AF_INET;
-  remote.sin_port = htons(remote_addr->port);
-  remote.sin_addr.s_addr = htonl(remote_addr->ip_addr);
+  NIO_SendPacket(packet, NTP_NORMAL_PACKET_SIZE, remote_addr);
+}
 
-  if (sendto(sock_fd, (void *) packet, sizeof(NTP_Packet), 0,
-             (struct sockaddr *) &remote, sizeof(remote)) < 0) {
-    LOG(LOGS_WARN, LOGF_NtpIO, "Could not send to %s:%d : %s",
-        UTI_IPToDottedQuad(remote_addr->ip_addr), remote_addr->port, strerror(errno));
-  }
+/* ================================================== */
+/* Send an authenticated packet to a given address */
 
-  return;
+void
+NIO_SendAuthenticatedPacket(NTP_Packet *packet, NTP_Remote_Address *remote_addr)
+{
+  NIO_SendPacket(packet, sizeof(NTP_Packet), remote_addr);
 }
 
 /* ================================================== */
-- 
1.5.6.3