include/netd_resolv/resolv.h

/*
 * Copyright (C) 2014 The Android Open Source Project
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *  * Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *  * Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#pragma once

#include <android-base/format.h>
#include <android-base/strings.h>
#include <arpa/nameser.h>
#include <netinet/in.h>

/*
 * Passing NETID_UNSET as the netId causes system/netd/resolv/DnsProxyListener.cpp to
 * fill in the appropriate default netId for the query.
 */
#define NETID_UNSET 0u

/*
 * MARK_UNSET represents the default (i.e. unset) value for a socket mark.
 */
#define MARK_UNSET 0u

/**
 * Internal ResNsendFlags to query every configured DNS server to find an answer, even
 * if some servers indicate the name cannot be resolved.
 *
 * Any reply that is not NOERROR, such as NXDOMAIN, or does not have answer records
 * (NODATA), will cause the resolver to try other usable servers (servers that do not
 * have a high failure rate), instead of returning the error immediately.
 *
 * This is not expected to be used by apps (it is currently restricted to UIDs below
 * FIRST_APP_UID), so it is not part of the NDK.
 */
constexpr int RESOLV_TRY_ALL_USABLE_SERVERS = 1 << 31;

/*
 * Passing APP_SOCKET_NONE as the app_socket in getaddrinfo, gethostbyname,
 * gethostbyaddr, res_nsend means that the query is not tied to a listening socket
 * in a querying process, and it will not be cancellable based on a socket state.
 */
constexpr std::optional<int> APP_SOCKET_NONE = std::nullopt;

#define NET_CONTEXT_INVALID_UID ((uid_t)-1)
#define NET_CONTEXT_INVALID_PID ((pid_t)-1)

/*
 * A struct to capture context relevant to network operations.
 *
 * Application and DNS netids/marks can differ from one another under certain
 * circumstances, notably when a VPN applies to the given uid's traffic but the
 * VPN network does not have its own DNS servers explicitly provisioned.
 *
 * The introduction of per-UID routing means the uid is also an essential part
 * of the evaluation context. Its proper uninitialized value is
 * NET_CONTEXT_INVALID_UID.
 */
struct android_net_context {
    unsigned app_netid;
    unsigned app_mark;
    unsigned dns_netid;
    unsigned dns_mark;
    uid_t uid = NET_CONTEXT_INVALID_UID;
    unsigned flags;
    // Variable to store the pid of the application sending DNS query.
    pid_t pid = NET_CONTEXT_INVALID_PID;

    std::string toString() const {
        return fmt::format("{} {} {} {} {} {}", app_netid, app_mark, dns_netid, dns_mark, uid,
                           flags);
    }
};

#define NET_CONTEXT_FLAG_USE_LOCAL_NAMESERVERS 0x00000001
#define NET_CONTEXT_FLAG_USE_EDNS 0x00000002
#define NET_CONTEXT_FLAG_USE_DNS_OVER_TLS 0x00000004

// TODO: investigate having the resolver check permissions itself, either by adding support to
// libbinder_ndk or by converting IPermissionController into a stable AIDL interface.
typedef bool (*check_calling_permission_callback)(const char* permission);
typedef void (*get_network_context_callback)(unsigned netid, uid_t uid,
                                             android_net_context* netcontext);
typedef void (*log_callback)(const char* msg);
typedef int (*tagSocketCallback)(int sockFd, uint32_t tag, uid_t uid, pid_t pid);

// The DnsResolver module invokes this callback once before starting each DNS
// lookup. The callback receives the android_net_context associated with the
// request, and the (possibly unqualified) hostname requested by the app via
// getaddrinfo() or gethostbyname().
//
// If the callback returns false, the DnsResolver will abort the request
// returning EAI_SYSTEM. If the callback returns true, the query will proceed as
// usual.
//
// If this callback is not present (i.e. set to nullptr), the effect is the same
// of returning true.
//
// This callback *will* be invoked concurrently from multiple threads. It must
// peform its own locking when accessing shared data structures. Furthermore,
// the callback must not sleep nor perform RPC requests.
//
// Be mindful that hostnames could contain sensitive user data. Do not log them
// and do not transmit them to third parties without explicit user
// authorization.
//
typedef bool (*evaluate_domain_name_callback)(
    const android_net_context &netcontext, const char *host);

/*
 * Some functions needed by the resolver (e.g. checkCallingPermission()) live in
 * libraries with no ABI stability guarantees, such as libbinder.so.
 * As a temporary workaround, we keep these functions in netd and call them via
 * function pointers.
 */
struct ResolverNetdCallbacks {
    check_calling_permission_callback check_calling_permission;
    get_network_context_callback get_network_context;
    log_callback log;
    tagSocketCallback tagSocket;
    evaluate_domain_name_callback evaluate_domain_name;
};

#define TAG_SYSTEM_DNS 0xFFFFFF82

#define LIBNETD_RESOLV_PUBLIC extern "C" [[gnu::visibility("default")]]

LIBNETD_RESOLV_PUBLIC bool resolv_has_nameservers(unsigned netid);

// Set callbacks and bring DnsResolver up.
LIBNETD_RESOLV_PUBLIC bool resolv_init(const ResolverNetdCallbacks* callbacks);

// Function that performs RDNS in local cache. The |domain_name_size| is the size of domain_name
// buffer, which is recommended to NS_MAXDNAME. Function return false if hostname not found or
// domain_name_size > NS_MAXDNAME.
LIBNETD_RESOLV_PUBLIC bool resolv_gethostbyaddr_from_cache(unsigned netId, char domain_name[],
                                                           size_t domain_name_size,
                                                           const char* ip_address, int af);