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Commit f3c42075 authored by Ken Chen's avatar Ken Chen
Browse files

Don't use 5353 source port for mDNS queries 

RFC 6762 section 5.1 says that one-shot Multicast DNS queries MUST NOT
be sent using UDP source port 5353, since using UDP source port 5353
signals the presence of a fully compliant Multicast DNS querier.

DNS resolver works as a one-shot Multicast DNS querier, which send
queries from random port and does not fully implement Multicast DNS
features. The 5353 source port should be excluded from random port
selection.

Bug: 227147672
Test: atest
Change-Id: I05d426c179182d3f08b037cb1bab3a43ea501063
parent 637d7718

res_send.cpp

+9 −3
Original line number Diff line number Diff line
@@ -253,7 +253,7 @@ static struct timespec evNowTime(void) { 
// END: Code copied from ISC eventlib



/* BIONIC-BEGIN: implement source port randomization */

static int random_bind(int s, int family) {

static int random_bind(int s, int family, bool isMdns) {

    sockaddr_union u;

    int j;

    socklen_t slen;
@@ -279,6 +279,10 @@ static int random_bind(int s, int family) { 
    for (j = 0; j < 10; j++) {

        /* find a random port between 1025 .. 65534 */

        int port = 1025 + (arc4random_uniform(65535 - 1025));

        // RFC 6762 section 5.1: Don't use 5353 source port on one-shot Multicast DNS queries. DNS

        // resolver does not fully compliant mDNS.

        if (isMdns && port == 5353) continue;



        if (family == AF_INET)

            u.sin.sin_port = htons(port);

        else
@@ -775,7 +779,7 @@ same_ns: 
            }

        }

        errno = 0;

        if (random_bind(statp->tcp_nssock, nsap->sa_family) < 0) {

        if (random_bind(statp->tcp_nssock, nsap->sa_family, false /* isMdns */) < 0) {

            *terrno = errno;

            dump_error("bind/vc", nsap);

            statp->closeSockets();
@@ -1078,7 +1082,9 @@ static int setupUdpSocket(ResState* statp, const sockaddr* sockap, unique_fd* fd 
        }

    }



    if (random_bind(*fd_out, sockap->sa_family) < 0) {

    const auto addr = IPSockAddr::toIPSockAddr(*sockap);

    const bool isMdns = (addr == mdns_addrs[0] || addr == mdns_addrs[1]);

    if (random_bind(*fd_out, sockap->sa_family, isMdns) < 0) {

        *terrno = errno;

        dump_error("bind", sockap);

        return 0;