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Set larger UDP buffer to avoid TCP fallback if possible

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This commit sets the UDP buffer to a larger size, as well as adds
an EDNS block to the DNS request passing this size. This allows
DNS servers to return larger responses, and in turn allow c-ares
to avoid TCP fallback due to requests failing because of the lack
of buffer size.
This commit is contained in:
Tim Wojtulewicz 2022-04-03 16:56:17 -07:00
parent f9f37b11c6
commit ea7cea12c4
1 changed files with 17 additions and 6 deletions
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23
src/DNS_Mgr.cc
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@ -52,6 +52,10 @@ constexpr int DNS_TIMEOUT = 5;
// The maximum allowed number of pending asynchronous requests.
constexpr int MAX_PENDING_REQUESTS = 20;
// The maximum number of bytes requested via UDP. TCP fallback won't happen on
// requests until a response is larger than this.
constexpr int MAX_UDP_BUFFER_SIZE = 4096;
// This unfortunately doesn't exist in c-ares, even though it seems rather useful.
static const char* request_type_string(int request_type)
{
@ -268,9 +272,9 @@ void DNS_Request::MakeRequest(ares_channel channel, DNS_Mgr* mgr)
std::unique_ptr<unsigned char, ares_deleter> query_str;
int len = 0;
int status = ares_create_query(query_host.c_str(), C_IN, request_type,
DNS_Request::request_id, 1,
out_ptr<unsigned char*>(query_str), &len, 0);
int status = ares_create_query(
query_host.c_str(), C_IN, request_type, DNS_Request::request_id, 1,
out_ptr<unsigned char*>(query_str), &len, MAX_UDP_BUFFER_SIZE);
if ( status != ARES_SUCCESS )
return;
@ -587,11 +591,18 @@ void DNS_Mgr::InitSource()
ares_options options;
int optmask = 0;
// Don't close the socket for the server even if we have no active
// requests.
options.flags = ARES_FLAG_STAYOPEN;
// Enable an EDNS option to be sent with the requests. This allows us to set
// a bigger UDP buffer size in the request, which prevents fallback to TCP
// at least up to that size.
options.flags = ARES_FLAG_EDNS;
optmask |= ARES_OPT_FLAGS;
options.ednspsz = MAX_UDP_BUFFER_SIZE;
optmask |= ARES_OPT_EDNSPSZ;
options.socket_receive_buffer_size = MAX_UDP_BUFFER_SIZE;
optmask |= ARES_OPT_SOCK_RCVBUF;
// This option is in milliseconds.
options.timeout = DNS_TIMEOUT * 1000;
optmask |= ARES_OPT_TIMEOUTMS;