1=pod 2 3=head1 NAME 4 5SSL_stateless, 6DTLSv1_listen 7- Statelessly listen for incoming connections 8 9=head1 SYNOPSIS 10 11 #include <openssl/ssl.h> 12 13 int SSL_stateless(SSL *s); 14 int DTLSv1_listen(SSL *ssl, BIO_ADDR *peer); 15 16=head1 DESCRIPTION 17 18SSL_stateless() statelessly listens for new incoming TLSv1.3 connections. 19DTLSv1_listen() statelessly listens for new incoming DTLS connections. If a 20ClientHello is received that does not contain a cookie, then they respond with a 21request for a new ClientHello that does contain a cookie. If a ClientHello is 22received with a cookie that is verified then the function returns in order to 23enable the handshake to be completed (for example by using SSL_accept()). 24 25=head1 NOTES 26 27Some transport protocols (such as UDP) can be susceptible to amplification 28attacks. Unlike TCP there is no initial connection setup in UDP that 29validates that the client can actually receive messages on its advertised source 30address. An attacker could forge its source IP address and then send handshake 31initiation messages to the server. The server would then send its response to 32the forged source IP. If the response messages are larger than the original 33message then the amplification attack has succeeded. 34 35If DTLS is used over UDP (or any datagram based protocol that does not validate 36the source IP) then it is susceptible to this type of attack. TLSv1.3 is 37designed to operate over a stream-based transport protocol (such as TCP). 38If TCP is being used then there is no need to use SSL_stateless(). However, some 39stream-based transport protocols (e.g. QUIC) may not validate the source 40address. In this case a TLSv1.3 application would be susceptible to this attack. 41 42As a countermeasure to this issue TLSv1.3 and DTLS include a stateless cookie 43mechanism. The idea is that when a client attempts to connect to a server it 44sends a ClientHello message. The server responds with a HelloRetryRequest (in 45TLSv1.3) or a HelloVerifyRequest (in DTLS) which contains a unique cookie. The 46client then resends the ClientHello, but this time includes the cookie in the 47message thus proving that the client is capable of receiving messages sent to 48that address. All of this can be done by the server without allocating any 49state, and thus without consuming expensive resources. 50 51OpenSSL implements this capability via the SSL_stateless() and DTLSv1_listen() 52functions. The B<ssl> parameter should be a newly allocated SSL object with its 53read and write BIOs set, in the same way as might be done for a call to 54SSL_accept(). Typically, for DTLS, the read BIO will be in an "unconnected" 55state and thus capable of receiving messages from any peer. 56 57When a ClientHello is received that contains a cookie that has been verified, 58then these functions will return with the B<ssl> parameter updated into a state 59where the handshake can be continued by a call to (for example) SSL_accept(). 60Additionally, for DTLSv1_listen(), the B<BIO_ADDR> pointed to by B<peer> will be 61filled in with details of the peer that sent the ClientHello. If the underlying 62BIO is unable to obtain the B<BIO_ADDR> of the peer (for example because the BIO 63does not support this), then B<*peer> will be cleared and the family set to 64AF_UNSPEC. Typically user code is expected to "connect" the underlying socket to 65the peer and continue the handshake in a connected state. 66 67Prior to calling DTLSv1_listen() user code must ensure that cookie generation 68and verification callbacks have been set up using 69L<SSL_CTX_set_cookie_generate_cb(3)> and L<SSL_CTX_set_cookie_verify_cb(3)> 70respectively. For SSL_stateless(), L<SSL_CTX_set_stateless_cookie_generate_cb(3)> 71and L<SSL_CTX_set_stateless_cookie_verify_cb(3)> must be used instead. 72 73Since DTLSv1_listen() operates entirely statelessly whilst processing incoming 74ClientHellos it is unable to process fragmented messages (since this would 75require the allocation of state). An implication of this is that DTLSv1_listen() 76B<only> supports ClientHellos that fit inside a single datagram. 77 78For SSL_stateless() if an entire ClientHello message cannot be read without the 79"read" BIO becoming empty then the SSL_stateless() call will fail. It is the 80application's responsibility to ensure that data read from the "read" BIO during 81a single SSL_stateless() call is all from the same peer. 82 83SSL_stateless() will fail (with a 0 return value) if some TLS version less than 84TLSv1.3 is used. 85 86Both SSL_stateless() and DTLSv1_listen() will clear the error queue when they 87start. 88 89=head1 RETURN VALUES 90 91For SSL_stateless() a return value of 1 indicates success and the B<ssl> object 92will be set up ready to continue the handshake. A return value of 0 or -1 93indicates failure. If the value is 0 then a HelloRetryRequest was sent. A value 94of -1 indicates any other error. User code may retry the SSL_stateless() call. 95 96For DTLSv1_listen() a return value of >= 1 indicates success. The B<ssl> object 97will be set up ready to continue the handshake. the B<peer> value will also be 98filled in. 99 100A return value of 0 indicates a non-fatal error. This could (for 101example) be because of nonblocking IO, or some invalid message having been 102received from a peer. Errors may be placed on the OpenSSL error queue with 103further information if appropriate. Typically user code is expected to retry the 104call to DTLSv1_listen() in the event of a non-fatal error. 105 106A return value of <0 indicates a fatal error. This could (for example) be 107because of a failure to allocate sufficient memory for the operation. 108 109For DTLSv1_listen(), prior to OpenSSL 1.1.0, fatal and non-fatal errors both 110produce return codes <= 0 (in typical implementations user code treats all 111errors as non-fatal), whilst return codes >0 indicate success. 112 113=head1 SEE ALSO 114 115L<SSL_CTX_set_cookie_generate_cb(3)>, L<SSL_CTX_set_cookie_verify_cb(3)>, 116L<SSL_CTX_set_stateless_cookie_generate_cb(3)>, 117L<SSL_CTX_set_stateless_cookie_verify_cb(3)>, L<SSL_get_error(3)>, 118L<SSL_accept(3)>, L<ssl(7)>, L<bio(7)> 119 120=head1 HISTORY 121 122The SSL_stateless() function was added in OpenSSL 1.1.1. 123 124The DTLSv1_listen() return codes were clarified in OpenSSL 1.1.0. 125The type of "peer" also changed in OpenSSL 1.1.0. 126 127=head1 COPYRIGHT 128 129Copyright 2015-2020 The OpenSSL Project Authors. All Rights Reserved. 130 131Licensed under the OpenSSL license (the "License"). You may not use 132this file except in compliance with the License. You can obtain a copy 133in the file LICENSE in the source distribution or at 134L<https://www.openssl.org/source/license.html>. 135 136=cut 137