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/slirp/tcp_subr.c b/slirp/tcp_subr.c
index 51b3834..0417345 100644
--- a/slirp/tcp_subr.c
+++ b/slirp/tcp_subr.c
@@ -340,7 +340,8 @@ int tcp_fconnect(struct socket *so)
         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;
       }