[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>

---
Index: qemu-kvm-0.10.5/slirp/ip_icmp.c
===================================================================
--- qemu-kvm-0.10.5.orig/slirp/ip_icmp.c
+++ qemu-kvm-0.10.5/slirp/ip_icmp.c
@@ -140,7 +140,8 @@ icmp_input(m, hlen)
 	/* It's an alias */
 	switch(ntohl(so->so_faddr.s_addr) & 0xff) {
 	case CTL_DNS:
-	  addr.sin_addr = dns_addr;
+	  if (get_dns_addr(&addr.sin_addr) < 0)
+	    addr.sin_addr = loopback_addr;
 	  break;
 	case CTL_ALIAS:
 	default:
Index: qemu-kvm-0.10.5/slirp/libslirp.h
===================================================================
--- qemu-kvm-0.10.5.orig/slirp/libslirp.h
+++ qemu-kvm-0.10.5/slirp/libslirp.h
@@ -5,6 +5,8 @@
 extern "C" {
 #endif
 
+int get_dns_addr(struct in_addr *pdns_addr);
+
 void slirp_init(int restrict, char *special_ip);
 
 void slirp_select_fill(int *pnfds,
Index: qemu-kvm-0.10.5/slirp/main.h
===================================================================
--- qemu-kvm-0.10.5.orig/slirp/main.h
+++ qemu-kvm-0.10.5/slirp/main.h
@@ -37,7 +37,6 @@ extern struct in_addr special_addr;
 extern struct in_addr alias_addr;
 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;
Index: qemu-kvm-0.10.5/slirp/slirp.c
===================================================================
--- qemu-kvm-0.10.5.orig/slirp/slirp.c
+++ qemu-kvm-0.10.5/slirp/slirp.c
@@ -28,8 +28,6 @@
 
 /* host address */
 struct in_addr our_addr;
-/* host dns address */
-struct in_addr dns_addr;
 /* host loopback address */
 struct in_addr loopback_addr;
 
@@ -61,9 +59,12 @@ fd_set *global_readfds, *global_writefds
 
 char slirp_hostname[33];
 
+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;
@@ -71,6 +72,11 @@ static int get_dns_addr(struct in_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);
 
@@ -94,6 +100,8 @@ static int get_dns_addr(struct in_addr *
     pIPAddr = &(FixedInfo->DnsServerList);
     inet_aton(pIPAddr->IpAddress.String, &tmp_addr);
     *pdns_addr = tmp_addr;
+    dns_addr = tmp_addr;
+    dns_addr_time = curtime;
 #if 0
     printf( "DNS Servers:\n" );
     printf( "DNS Addr:%s\n", pIPAddr->IpAddress.String );
@@ -113,7 +121,7 @@ static int get_dns_addr(struct in_addr *
 
 #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];
@@ -121,6 +129,11 @@ static int get_dns_addr(struct in_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;
@@ -135,8 +148,11 @@ static int get_dns_addr(struct in_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(", ");
@@ -195,11 +211,6 @@ void slirp_init(int restrict, char *spec
     /* set default addresses */
     inet_aton("127.0.0.1", &loopback_addr);
 
-    if (get_dns_addr(&dns_addr) < 0) {
-        dns_addr = loopback_addr;
-        fprintf (stderr, "Warning: No DNS servers found\n");
-    }
-
     if (special_ip)
         slirp_special_ip = special_ip;
 
Index: qemu-kvm-0.10.5/slirp/socket.c
===================================================================
--- qemu-kvm-0.10.5.orig/slirp/socket.c
+++ qemu-kvm-0.10.5/slirp/socket.c
@@ -567,7 +567,8 @@ sosendto(so, m)
 	  /* It's an alias */
 	  switch(ntohl(so->so_faddr.s_addr) & 0xff) {
 	  case CTL_DNS:
-	    addr.sin_addr = dns_addr;
+	    if (get_dns_addr(&addr.sin_addr) < 0)
+	      addr.sin_addr = loopback_addr;
 	    break;
 	  case CTL_ALIAS:
 	  default:
Index: qemu-kvm-0.10.5/slirp/tcp_subr.c
===================================================================
--- qemu-kvm-0.10.5.orig/slirp/tcp_subr.c
+++ qemu-kvm-0.10.5/slirp/tcp_subr.c
@@ -398,7 +398,8 @@ int tcp_fconnect(so)
       /* It's an alias */
       switch(ntohl(so->so_faddr.s_addr) & 0xff) {
       case CTL_DNS:
-	addr.sin_addr = dns_addr;
+	if (get_dns_addr(&addr.sin_addr) < 0)
+	  addr.sin_addr = loopback_addr;
 	break;
       case CTL_ALIAS:
       default: