Edit File: SSL_stateless.3
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Always turn off hyphenation; it makes .\" way too many mistakes in technical documents. .if n .ad l .nh .SH "NAME" SSL_stateless, DTLSv1_listen \&\- Statelessly listen for incoming connections .SH "SYNOPSIS" .IX Header "SYNOPSIS" .Vb 1 \& #include <openssl/ssl.h> \& \& int SSL_stateless(SSL *s); \& int DTLSv1_listen(SSL *ssl, BIO_ADDR *peer); .Ve .SH "DESCRIPTION" .IX Header "DESCRIPTION" \&\fBSSL_stateless()\fR statelessly listens for new incoming TLSv1.3 connections. \&\fBDTLSv1_listen()\fR statelessly listens for new incoming \s-1DTLS\s0 connections. If a ClientHello is received that does not contain a cookie, then they respond with a request for a new ClientHello that does contain a cookie. If a ClientHello is received with a cookie that is verified then the function returns in order to enable the handshake to be completed (for example by using \fBSSL_accept()\fR). .SH "NOTES" .IX Header "NOTES" Some transport protocols (such as \s-1UDP\s0) can be susceptible to amplification attacks. Unlike \s-1TCP\s0 there is no initial connection setup in \s-1UDP\s0 that validates that the client can actually receive messages on its advertised source address. An attacker could forge its source \s-1IP\s0 address and then send handshake initiation messages to the server. The server would then send its response to the forged source \s-1IP.\s0 If the response messages are larger than the original message then the amplification attack has succeeded. .PP If \s-1DTLS\s0 is used over \s-1UDP\s0 (or any datagram based protocol that does not validate the source \s-1IP\s0) then it is susceptible to this type of attack. TLSv1.3 is designed to operate over a stream-based transport protocol (such as \s-1TCP\s0). If \s-1TCP\s0 is being used then there is no need to use \fBSSL_stateless()\fR. However, some stream-based transport protocols (e.g. \s-1QUIC\s0) may not validate the source address. In this case a TLSv1.3 application would be susceptible to this attack. .PP As a countermeasure to this issue TLSv1.3 and \s-1DTLS\s0 include a stateless cookie mechanism. The idea is that when a client attempts to connect to a server it sends a ClientHello message. The server responds with a HelloRetryRequest (in TLSv1.3) or a HelloVerifyRequest (in \s-1DTLS\s0) which contains a unique cookie. The client then resends the ClientHello, but this time includes the cookie in the message thus proving that the client is capable of receiving messages sent to that address. All of this can be done by the server without allocating any state, and thus without consuming expensive resources. .PP OpenSSL implements this capability via the \fBSSL_stateless()\fR and \fBDTLSv1_listen()\fR functions. The \fBssl\fR parameter should be a newly allocated \s-1SSL\s0 object with its read and write BIOs set, in the same way as might be done for a call to \&\fBSSL_accept()\fR. Typically, for \s-1DTLS,\s0 the read \s-1BIO\s0 will be in an \*(L"unconnected\*(R" state and thus capable of receiving messages from any peer. .PP When a ClientHello is received that contains a cookie that has been verified, then these functions will return with the \fBssl\fR parameter updated into a state where the handshake can be continued by a call to (for example) \fBSSL_accept()\fR. Additionally, for \fBDTLSv1_listen()\fR, the \fB\s-1BIO_ADDR\s0\fR pointed to by \fBpeer\fR will be filled in with details of the peer that sent the ClientHello. If the underlying \&\s-1BIO\s0 is unable to obtain the \fB\s-1BIO_ADDR\s0\fR of the peer (for example because the \s-1BIO\s0 does not support this), then \fB*peer\fR will be cleared and the family set to \&\s-1AF_UNSPEC.\s0 Typically user code is expected to \*(L"connect\*(R" the underlying socket to the peer and continue the handshake in a connected state. .PP Prior to calling \fBDTLSv1_listen()\fR user code must ensure that cookie generation and verification callbacks have been set up using \&\fBSSL_CTX_set_cookie_generate_cb\fR\|(3) and \fBSSL_CTX_set_cookie_verify_cb\fR\|(3) respectively. For \fBSSL_stateless()\fR, \fBSSL_CTX_set_stateless_cookie_generate_cb\fR\|(3) and \fBSSL_CTX_set_stateless_cookie_verify_cb\fR\|(3) must be used instead. .PP Since \fBDTLSv1_listen()\fR operates entirely statelessly whilst processing incoming ClientHellos it is unable to process fragmented messages (since this would require the allocation of state). An implication of this is that \fBDTLSv1_listen()\fR \&\fBonly\fR supports ClientHellos that fit inside a single datagram. .PP For \fBSSL_stateless()\fR if an entire ClientHello message cannot be read without the \&\*(L"read\*(R" \s-1BIO\s0 becoming empty then the \fBSSL_stateless()\fR call will fail. It is the application's responsibility to ensure that data read from the \*(L"read\*(R" \s-1BIO\s0 during a single \fBSSL_stateless()\fR call is all from the same peer. .PP \&\fBSSL_stateless()\fR will fail (with a 0 return value) if some \s-1TLS\s0 version less than TLSv1.3 is used. .PP Both \fBSSL_stateless()\fR and \fBDTLSv1_listen()\fR will clear the error queue when they start. .SH "RETURN VALUES" .IX Header "RETURN VALUES" For \fBSSL_stateless()\fR a return value of 1 indicates success and the \fBssl\fR object will be set up ready to continue the handshake. A return value of 0 or \-1 indicates failure. If the value is 0 then a HelloRetryRequest was sent. A value of \-1 indicates any other error. User code may retry the \fBSSL_stateless()\fR call. .PP For \fBDTLSv1_listen()\fR a return value of >= 1 indicates success. The \fBssl\fR object will be set up ready to continue the handshake. the \fBpeer\fR value will also be filled in. .PP A return value of 0 indicates a non-fatal error. This could (for example) be because of nonblocking \s-1IO,\s0 or some invalid message having been received from a peer. Errors may be placed on the OpenSSL error queue with further information if appropriate. Typically user code is expected to retry the call to \fBDTLSv1_listen()\fR in the event of a non-fatal error. .PP A return value of <0 indicates a fatal error. This could (for example) be because of a failure to allocate sufficient memory for the operation. .PP For \fBDTLSv1_listen()\fR, prior to OpenSSL 1.1.0, fatal and non-fatal errors both produce return codes <= 0 (in typical implementations user code treats all errors as non-fatal), whilst return codes >0 indicate success. .SH "SEE ALSO" .IX Header "SEE ALSO" \&\fBSSL_CTX_set_cookie_generate_cb\fR\|(3), \fBSSL_CTX_set_cookie_verify_cb\fR\|(3), \&\fBSSL_CTX_set_stateless_cookie_generate_cb\fR\|(3), \&\fBSSL_CTX_set_stateless_cookie_verify_cb\fR\|(3), \fBSSL_get_error\fR\|(3), \&\fBSSL_accept\fR\|(3), \fBssl\fR\|(7), \fBbio\fR\|(7) .SH "HISTORY" .IX Header "HISTORY" The \fBSSL_stateless()\fR function was added in OpenSSL 1.1.1. .PP The \fBDTLSv1_listen()\fR return codes were clarified in OpenSSL 1.1.0. The type of \*(L"peer\*(R" also changed in OpenSSL 1.1.0. .SH "COPYRIGHT" .IX Header "COPYRIGHT" Copyright 2015\-2020 The OpenSSL Project Authors. All Rights Reserved. .PP Licensed under the OpenSSL license (the \*(L"License\*(R"). You may not use this file except in compliance with the License. You can obtain a copy in the file \s-1LICENSE\s0 in the source distribution or at <https://www.openssl.org/source/license.html>.