slirp: Read host DNS config on demand

Currently the qemu user-mode networking stack reads the host DNS
configuration (/etc/resolv.conf or the Windows equivalent) only once
when qemu starts.  This causes name lookups in the guest to fail if the
host is moved to a different network from which the original DNS servers
are unreachable, a common occurrence when the host is a laptop.

This patch changes the slirp code to read the host DNS configuration on
demand, caching the results for at most 1 second to avoid unnecessary
overhead if name lookups occur in rapid succession.  On non-Windows
hosts, /etc/resolv.conf is re-read only if the file has been replaced or
if its size or mtime has changed.

Signed-off-by: Ed Swierk <eswierk@aristanetworks.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/socket.c b/socket.c
index 35e980e..b974267 100644
--- a/socket.c
+++ b/socket.c
@@ -557,7 +557,8 @@ int sosendto(struct socket *so, struct mbuf *m)
         slirp->vnetwork_addr.s_addr) {
         /* It's an alias */
         if (so->so_faddr.s_addr == slirp->vnameserver_addr.s_addr) {
-            addr.sin_addr = dns_addr;
+            if (get_dns_addr(&addr.sin_addr) < 0)
+                addr.sin_addr = loopback_addr;
         } else {
             addr.sin_addr = loopback_addr;
         }