musl - Question about tcp failure in udp truncated scenarios

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Message-ID:
 <SE1P216MB248429552230F8F04DB93F8F9E652@SE1P216MB2484.KORP216.PROD.OUTLOOK.COM>
Date: Fri, 13 Sep 2024 13:56:49 +0000
From: JinCheng Li <naiveli233@...look.com>
To: musl <musl@...ts.openwall.com>
Subject: Question about tcp failure in udp truncated scenarios

Hi

    I have a question for __res_msend_rc in dns query. In the case of multiple nameservers, if the first vaild udp query result is truncated,  musl will discard the udp query result and turn to tcp query. But I found many dns nameservers do not response to the tcp connection or do not respond to tcp query request which will finally trigger timeout. As a result, the success rate of DNS queries is greatly reduced when udp is truncated, for example, in the case of only IPV4.
    In bionic, the dns query in multiple nameservers is one by one. The first udp is truncated, if the tcp also fail, it will turn to the second udp and do it again. This may be slower than musl in terms of speed, but is more stable in truncation cases.
    In musl, if the udp truncated case has happened, we will convert to tcp which is more likely not supported by the nameserver and discard all other udp response which may be also valid from other nameserver . Do you have any good suggestions to optimize this?

Best
Li

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