From e2e9517607f67c1e23c059769b19bf4270d22123 Mon Sep 17 00:00:00 2001 From: Rich Felker Date: Thu, 22 Sep 2022 12:41:23 -0400 Subject: [PATCH 2/3] res_send: use a temp buffer if caller's buffer is under 512 bytes for extremely small buffer sizes, the DNS query core in __res_msend may malfunction completely, being unable to get even the headers to determine the response code. but there is also a problem for reasonable sizes under 512 bytes: __res_msend is unable to determine if the udp answer was truncated at the recv layer, in which case it may be incomplete, and res_send is then unable to honor its contract to return the length of the full, non-truncated answer. at present, res_send does not honor that contract anyway when the full answer would exceed 512 bytes, since there is no tcp fallback, but this change at least makes it consistent in a context where this is the only "full answer" to be had. --- src/network/res_send.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/src/network/res_send.c b/src/network/res_send.c index ee4abf1f..9593164d 100644 --- a/src/network/res_send.c +++ b/src/network/res_send.c @@ -1,8 +1,16 @@ #include +#include int __res_send(const unsigned char *msg, int msglen, unsigned char *answer, int anslen) { - int r = __res_msend(1, &msg, &msglen, &answer, &anslen, anslen); + int r; + if (anslen < 512) { + unsigned char buf[512]; + r = __res_send(msg, msglen, buf, sizeof buf); + if (r >= 0) memcpy(answer, buf, r < anslen ? r : anslen); + return r; + } + r = __res_msend(1, &msg, &msglen, &answer, &anslen, anslen); return r<0 || !anslen ? -1 : anslen; } -- 2.21.0