[Qemu-devel] [PATCH] slirp: Read host DNS config on demand

[Ed Swierk <eswierk@aristanetworks.com>]

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 10 seconds to avoid unnecessary overhead
if name lookups occur in rapid succession.

Signed-off-by: Ed Swierk <eswierk@aristanetworks.com>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>

---
diff --git a/slirp/ip_icmp.c b/slirp/ip_icmp.c
index 95a4b39..751a8e2 100644
--- a/slirp/ip_icmp.c
+++ b/slirp/ip_icmp.c
@@ -127,7 +127,8 @@ icmp_input(struct mbuf *m, int hlen)
           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;
 	}
diff --git a/slirp/libslirp.h b/slirp/libslirp.h
index 93087ed..67c70e3 100644
--- a/slirp/libslirp.h
+++ b/slirp/libslirp.h
@@ -8,6 +8,8 @@
 struct Slirp;
 typedef struct Slirp Slirp;
 
+int get_dns_addr(struct in_addr *pdns_addr);
+
 Slirp *slirp_init(int restricted, struct in_addr vnetwork,
                   struct in_addr vnetmask, struct in_addr vhost,
                   const char *vhostname, const char *tftp_path,
diff --git a/slirp/main.h b/slirp/main.h
index 28d92d8..e87b068 100644
--- a/slirp/main.h
+++ b/slirp/main.h
@@ -32,7 +32,6 @@ extern u_int curtime;
 extern fd_set *global_readfds, *global_writefds, *global_xfds;
 extern struct in_addr our_addr;
 extern struct in_addr loopback_addr;
-extern struct in_addr dns_addr;
 extern char *username;
 extern char *socket_path;
 extern int towrite_max;
diff --git a/slirp/slirp.c b/slirp/slirp.c
index e883f82..987aad9 100644
--- a/slirp/slirp.c
+++ b/slirp/slirp.c
@@ -29,8 +29,6 @@
 
 /* host address */
 struct in_addr our_addr;
-/* host dns address */
-struct in_addr dns_addr;
 /* host loopback address */
 struct in_addr loopback_addr;
 
@@ -51,9 +49,12 @@ static int do_slowtimo;
 TAILQ_HEAD(slirp_instances, Slirp) slirp_instances =
     TAILQ_HEAD_INITIALIZER(slirp_instances);
 
+struct in_addr dns_addr = { 0 };
+u_int dns_addr_time = 0;
+
 #ifdef _WIN32
 
-static int get_dns_addr(struct in_addr *pdns_addr)
+int get_dns_addr(struct in_addr *pdns_addr)
 {
     FIXED_INFO *FixedInfo=NULL;
     ULONG    BufLen;
@@ -61,6 +62,11 @@ static int get_dns_addr(struct in_addr *pdns_addr)
     IP_ADDR_STRING *pIPAddr;
     struct in_addr tmp_addr;
 
+    if (dns_addr.s_addr != 0 && (curtime - dns_addr_time) < 10000) {
+        *pdns_addr = dns_addr;
+        return 0;
+    }
+
     FixedInfo = (FIXED_INFO *)GlobalAlloc(GPTR, sizeof(FIXED_INFO));
     BufLen = sizeof(FIXED_INFO);
 
@@ -84,6 +90,8 @@ static int get_dns_addr(struct in_addr *pdns_addr)
     pIPAddr = &(FixedInfo->DnsServerList);
     inet_aton(pIPAddr->IpAddress.String, &tmp_addr);
     *pdns_addr = tmp_addr;
+    dns_addr = tmp_addr;
+    dns_addr_time = curtime;
     if (FixedInfo) {
         GlobalFree(FixedInfo);
         FixedInfo = NULL;
@@ -98,7 +106,7 @@ static void winsock_cleanup(void)
 
 #else
 
-static int get_dns_addr(struct in_addr *pdns_addr)
+int get_dns_addr(struct in_addr *pdns_addr)
 {
     char buff[512];
     char buff2[257];
@@ -106,6 +114,11 @@ static int get_dns_addr(struct in_addr *pdns_addr)
     int found = 0;
     struct in_addr tmp_addr;
 
+    if (dns_addr.s_addr != 0 && (curtime - dns_addr_time) < 10000) {
+        *pdns_addr = dns_addr;
+        return 0;
+    }
+
     f = fopen("/etc/resolv.conf", "r");
     if (!f)
         return -1;
@@ -120,8 +133,11 @@ static int get_dns_addr(struct in_addr *pdns_addr)
             if (tmp_addr.s_addr == loopback_addr.s_addr)
                 tmp_addr = our_addr;
             /* If it's the first one, set it to dns_addr */
-            if (!found)
+            if (!found) {
                 *pdns_addr = tmp_addr;
+                dns_addr = tmp_addr;
+                dns_addr_time = curtime;
+            }
 #ifdef DEBUG
             else
                 lprint(", ");
@@ -177,11 +193,6 @@ static void slirp_init_once(void)
     if (our_addr.s_addr == 0) {
         our_addr = loopback_addr;
     }
-
-    /* FIXME: This address may change during runtime */
-    if (get_dns_addr(&dns_addr) < 0) {
-        dns_addr = loopback_addr;
-    }
 }
 
 static void slirp_state_save(QEMUFile *f, void *opaque);
diff --git a/slirp/socket.c b/slirp/socket.c
index d8fbe89..207109c 100644
--- a/slirp/socket.c
+++ b/slirp/socket.c
@@ -552,7 +552,8 @@ 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;
 	  }
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;
       }