resolved: packet - ensure there is space for IP+UDP headers

Currently we only make sure our links can handle the size of the payload witohut taking the headers into account.
systemd · Jul 14, 2015 · a016660 · a016660
1 parent ff89f8b
commit a016660
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Showing 3 changed files with 6 additions and 3 deletions.
diff --git a/src/resolve/resolved-dns-packet.c b/src/resolve/resolved-dns-packet.c
@@ -32,10 +32,10 @@ int dns_packet_new(DnsPacket **ret, DnsProtocol protocol, size_t mtu) {
 
         assert(ret);
 
-        if (mtu <= 0)
+        if (mtu <= UDP_PACKET_HEADER_SIZE)
                 a = DNS_PACKET_SIZE_START;
         else
-                a = mtu;
+                a = mtu - UDP_PACKET_HEADER_SIZE;
 
         if (a < DNS_PACKET_HEADER_SIZE)
                 a = DNS_PACKET_HEADER_SIZE;

diff --git a/src/resolve/resolved-dns-packet.h b/src/resolve/resolved-dns-packet.h
@@ -21,6 +21,8 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  ***/
 
+#include <netinet/udp.h>
+#include <netinet/ip.h>
 
 #include "macro.h"
 #include "sparse-endian.h"
@@ -53,6 +55,7 @@ struct DnsPacketHeader {
 };
 
 #define DNS_PACKET_HEADER_SIZE sizeof(DnsPacketHeader)
+#define UDP_PACKET_HEADER_SIZE (sizeof(struct iphdr) + sizeof(struct udphdr))
 
 /* The various DNS protocols deviate in how large a packet can grow,
    but the TCP transport has a 16bit size field, hence that appears to

diff --git a/src/resolve/resolved-dns-scope.c b/src/resolve/resolved-dns-scope.c
@@ -160,7 +160,7 @@ int dns_scope_emit(DnsScope *s, DnsPacket *p) {
                 if (p->size > DNS_PACKET_UNICAST_SIZE_MAX)
                         return -EMSGSIZE;
 
-                if (p->size > mtu)
+                if (p->size + UDP_PACKET_HEADER_SIZE > mtu)
                         return -EMSGSIZE;
 
                 if (family == AF_INET)