support EDNS0 (RFC2671) buffer size notification on DNS queries.

"options edns0" in /etc/resolv.conf will enable the behavior.  no behavior
change if you don't have the line.  see resolv.conf(5) for more details.

EDNS0 is useful for avoiding TCP DNS queries/replies on larger DNS responses.
also, draft-ietf-dnsext-message-size-* plans to mandate EDNS0 support for DNS
clients that support IPv6 transport.

1 files changed, 39 insertions, 2 deletions

diff --git a/src/lib/libc/net/res_mkquery.c b/src/lib/libc/net/res_mkquery.c
index 3e7e2ae5d3..61595a6e8c 100644
--- a/src/lib/libc/net/res_mkquery.c
+++ b/src/lib/libc/net/res_mkquery.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: res_mkquery.c,v 1.8 1997/04/13 22:37:21 provos Exp $  */
+/*      $OpenBSD: res_mkquery.c,v 1.9 2001/06/11 10:06:00 itojun Exp $  */
 
 /*
  * ++Copyright++ 1985, 1993
@@ -60,7 +60,7 @@
 static char sccsid[] = "@(#)res_mkquery.c       8.1 (Berkeley) 6/4/93";
 static char rcsid[] = "$From: res_mkquery.c,v 8.5 1996/08/27 08:33:28 vixie Exp $";
 #else
-static char rcsid[] = "$OpenBSD: res_mkquery.c,v 1.8 1997/04/13 22:37:21 provos Exp $";
+static char rcsid[] = "$OpenBSD: res_mkquery.c,v 1.9 2001/06/11 10:06:00 itojun Exp $";
 #endif
 #endif /* LIBC_SCCS and not lint */
 
@@ -193,3 +193,40 @@ res_mkquery(op, dname, class, type, data, datalen, newrr_in, buf, buflen)
         }
         return (cp - buf);
 }
+
+/* attach OPT pseudo-RR, as documented in RFC2671 (EDNS0). */
+int
+res_opt(n0, buf, buflen, anslen)
+        int n0;
+        u_char *buf;            /* buffer to put query */
+        int buflen;             /* size of buffer */
+        int anslen;             /* answer buffer length */
+{
+        register HEADER *hp;
+        register u_char *cp;
+
+        hp = (HEADER *) buf;
+        cp = buf + n0;
+        buflen -= n0;
+
+        if (buflen < 1 + RRFIXEDSZ)
+                return -1;
+
+        *cp++ = 0;      /* "." */
+        buflen--;
+
+        __putshort(T_OPT, cp);  /* TYPE */
+        cp += INT16SZ;
+        __putshort(anslen & 0xffff, cp);        /* CLASS = UDP payload size */
+        cp += INT16SZ;
+        *cp++ = NOERROR;        /* extended RCODE */
+        *cp++ = 0;              /* EDNS version */
+        __putshort(0, cp);      /* MBZ */
+        cp += INT16SZ;
+        __putshort(0, cp);      /* RDLEN */
+        cp += INT16SZ;
+        hp->arcount = htons(ntohs(hp->arcount) + 1);
+        buflen -= RRFIXEDSZ;
+
+        return cp - buf;
+}