.\" $OpenBSD: async_resolver.3,v 1.10 2013/04/01 15:49:54 deraadt Exp $ .\" .\" Copyright (c) 2012, Eric Faurot .\" .\" Permission to use, copy, modify, and distribute this software for any .\" purpose with or without fee is hereby granted, provided that the above .\" copyright notice and this permission notice appear in all copies. .\" .\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES .\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR .\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES .\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. .\" .Dd $Mdocdate: April 1 2013 $ .Dt ASYN_RESOLVER 3 .Os .Sh NAME .Nm async_resolver , .Nm async_resolver_done , .Nm async_run , .Nm async_run_sync , .Nm async_abort , .Nm res_send_async , .Nm res_query_async , .Nm res_search_async , .Nm getrrsetbyname_async , .Nm gethostbyname_async , .Nm gethostbyname2_async , .Nm gethostbyaddr_async , .Nm getnetbyname_async , .Nm getnetbyaddr_async , .Nm freenetent , .Nm getaddrinfo_async , .Nm getnameinfo_async .Nd asynchronous resolver functions .Sh SYNOPSIS .Fd #include .Ft struct asr * .Fn async_resolver "const char *conf" .Ft void .Fn async_resolver_done "struct asr *asr" .Ft int .Fn async_run "struct async *as" "struct async_res *ar" .Ft int .Fn async_run_sync "struct async *as" "struct async_res *ar" .Ft void .Fn async_abort "struct async *as" .Ft struct async * .Fn res_send_async "const unsigned char *pkt" "int pktlen" "unsigned char *ans" "int anslen" "struct asr *asr" .Ft struct async * .Fn res_query_async "const char *name" "int class" "int type" "unsigned char *ans" "int anslen" "struct asr *asr" .Ft struct async * .Fn getrrsetbyname_async "const char *hostname" "unsigned int rdclass" "unsigned int rdtype" "unsigned int flags" "struct asr *asr" .Ft struct async * .Fn gethostbyname_async "const char *name" "struct asr *asr" .Ft struct async * .Fn gethostbyname2_async "const char *name" "int af" "struct asr *asr" .Ft struct async * .Fn gethostbyaddr_async "const void *addr" "socklen_t len" "int af" "struct asr *asr" .Ft struct async * .Fn getnetbyname_async "const char *name" "struct asr *asr" .Ft struct async * .Fn getnetbyaddr_async "in_addr_t net" "int type" "struct asr *asr" .Ft struct async * .Fn getaddrinfo_async "const char *hostname" "const char *servname" "const struct addrinfo *hints" "struct asr *asr" .Ft struct async * .Fn getnameinfo_async "const struct sockaddr *sa" "socklen_t salen" "char *host" "size_t hostlen" "char *serv" "size_t servlen" "int flags" "struct asr *asr" .Sh DESCRIPTION The .Nm asr functions provide a simple interface for asynchronous address resolution and nameserver querying. They should be used in place of the classical resolver functions of libc when blocking is not desirable. .Pp The principle of operation is as follows: All async requests are made against an .Nm asr context which basically defines a list of sources to query and a strategy to do so. The user creates a query through one of the dedicated functions. A query is a state-machine that can be run to try to fulfill a particular request. This is done by calling in a generic API that performs the state transitions until it needs to give the control back to the user, either because a result is available, or because the next transition implies a blocking call (a file descriptor needs to be read from or written to). The user is responsible for dealing with the situation (fetch the result, or wait until the fd conditions are met), and call back into the resolving machinery when it is ready to proceed. .Pp .Fn async_resolver is the function used to create a new resolver context. The .Fa conf argument is a path to the resolver configuration file as described in .Xr resolv.conf 5 . If NULL, the default .Pa /etc/resolv.conf file is used. The context tracks file changes to automatically update its configuration if needed, replacing the current setup if a valid one can be reloaded from the file. If the configuration file cannot be loaded at context creation time, it falls back to: .Bd -literal -offset indent lookup file .Ed .Pp If the first character of the .Fa conf string is a '!', the configuration is read from the rest of the string rather than loaded from a file. No further update occurs in this case. .Pp .Fn async_resolver_done is used to discard the .Fa asr context when it is not used anymore. Once called, that context is invalidated and cannot be used to create new queries. Internally, the context is refcounted, so that existing queries made against it will be able to complete safely. All relevant resources are effectively freed when all such queries are cleared. .Pp The .Fn async_run function drives the resolving process. It runs the .Fa as asynchronous query until an answer is available, or until it cannot continue without blocking. The results are returned to the user through the .Fa ar parameter, which must be a valid pointer to user allocated memory. .Fa ar is defined as: .Bd -literal -offset indent struct async_res { int ar_cond; int ar_fd; int ar_timeout; int ar_errno; int ar_h_errno; int ar_gai_errno; int ar_rrset_errno; int ar_count; int ar_rcode; void *ar_data; int ar_datalen; union { struct sockaddr sa; struct sockaddr_in sain; struct sockaddr_in6 sain6; } ar_sa; struct addrinfo *ar_addrinfo; struct rrsetinfo *ar_rrsetinfo; struct hostent *ar_hostent; struct netent *ar_netent; }; .Ed .Pp The function returns one of the following values: .Bl -tag -width "ASYNC_YIELD " -offset indent .It ASYNC_COND The query cannot be processed further until a specific condition on a file descriptor becomes true. The following members of the .Fa ar structure are filled: .Pp .Bl -tag -width "ar_timeout " -compact .It Fa ar_cond One of ASYNC_READ or ASYNC_WRITE. .It Fa ar_fd The file descriptor waiting for an IO operation. .It Fa ar_timeout The timeout, expressed in milliseconds. .El .Pp The caller is expected to call .Fn async_run again once the condition holds or the timeout expires. .It ASYNC_DONE The query is completed. The members relevant to the actual async query type are set accordingly, including error conditions. In any case, the query is cleared and its address is invalidated. .It ASR_YIELD A partial result is available. This code is used for async queries that behave as iterators over the result set. The query-specific members of .Fa ar are set accordingly and the resolving process can be resumed by calling .Fn async_run . .El .Pp Note that although the query itself may fail (the error being properly reported in the .Fa ar structure), the .Fn async_run function itself cannot fail and it always preserves errno. .Pp The .Fn async_run_sync function is a wrapper around .Fn async_run that handles the read/write conditions, thus falling back to a blocking interface. It only returns partial and complete results through ASYNC_YIELD and ASYNC_DONE respectively. It also preserves errno. .Pp The .Fn async_abort function clears a running query. It can be called after a partial result has been retrieved or when the query is waiting on a file descriptor. Note that a completed query is already cleared when .Fn async_run returns, so .Fn async_abort must not be called in this case. .Pp The remaining functions are used to initiate different kinds of query on the .Fa asr resolver context. The specific operational details for each of them are described below. All functions return NULL if they could not allocate the necessary resources to initiate the query. All other errors (especially invalid parameters) are reported when calling .Fn async_run . They usually have the same interface as an exisiting resolver function, with an additional .Ar asr contex argument, which specifies the context to use for this request. If NULL, the default thread-local context is used. .Pp The .Fn res_send_async , .Fn res_query_async and .Fn res_search_async functions are asynchronous versions of the standard libc resolver routines. Their interface is very similar, except that they take a resolver context as last argument, and the return value is found upon completion in the .Fa ar_datalen member of the response structure. In addition, the .Fa ar_sa union contains the address of the DNS server that sent the response, .Fa ar_rcode contains the code returned by the server in the DNS response packet, and .Fa ar_count contains the number of answers in the packet. If no answer buffer is provided, a new one is allocated to fit the response and returned as the .Fa ar_data member. This buffer must be freed by the caller. On error, the .Fa ar_errno and .Fa ar_h_errno members are set accordingly. .Pp The .Fn getrrsetbyname_async function is an asynchronous version of .Xr getrrsetbyname 3 . Upon completion, the return code is found in .Fa ar_rrset_errno and the address to the newly allocated result set is set in .Fa ar_rrsetinfo . As for the blocking function, it must be freed by calling .Xr freerrset 3 . .Pp The .Fn gethostbyname_async , .Fn gethostbyname2_async and .Fn gethostbyaddr_async functions provide an asynchronous version of the network host entry functions. Upon completion, .Ar ar_h_errno is set and the resulting hostent address, if found, is set in the .Ar ar_hostent field. Note that unlike their blocking counterparts, these functions always return a pointer to newly allocated memory, which must be released by the caller using .Xr free 3 . .Pp Similarly, the .Fn getnetbyname_async and .Fn getnetbyaddr_async functions provide an asynchronous version of the network entry functions. Upon completion, .Ar ar_h_errno is set and the resulting netent address, if found, is set in the .Ar ar_netent field. The memory there is also allocated for the request, and it must be freed by .Xr free 3 . .Pp The .Fn getaddrinfo_async function is an asynchronous version of the .Xr getaddrinfo 3 call. It provides a chain of addrinfo structures with all valid combinations of socket address for the given .Fa hostname , .Fa servname and .Fa hints . Those three parameters have the same meaning as for the blocking counterpart. Upon completion the return code is set in .Fa ar_gai_errno . The .Fa ar_errno member may also be set. On success, the .Fa ar_addrinfo member points to a newly allocated list of addrinfo. This list must be freed with .Xr freeaddrinfo 3 . The .Fa ar_count contains the number of elements in the list. .Sh WORKING WITH THREADS This implementation of the asynchronous resolver interface is thread-safe and lock-free internally, but the following restriction applies: Two different threads must not create queries on the same context or run queries originating from the same context at the same time. If they want to do that, all calls must be protected by a mutex around that context. .Pp It is generally not a problem since the main point of the asynchronous resolver is to multiplex queries within a single thread of control, so sharing a resolver among threads is not useful. .Pp Note that this restriction only applies to resolver contexts explicitly created by the caller, as a thread-local context is used by default. .Sh SEE ALSO .Xr getaddrinfo 3 , .Xr gethostbyname 3 , .Xr getnameinfo 3 , .Xr getnetbyname 3 , .Xr getrrsetbyname 3 , .Xr res_send 3 , .Xr resolv.conf 5 .Sh CAVEATS This DNS resolver implementation doesn't support the EDNS0 protocol extension yet.