summaryrefslogtreecommitdiffstats
path: root/docs/WRITING_RESOLVER_CLIENTS.md
blob: 88a873a06ed7862be7bdc64d72f7bd455add38c8 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
---
title: Writing Resolver Clients
category: Documentation for Developers
layout: default
SPDX-License-Identifier: LGPL-2.1-or-later
---

# Writing Resolver Clients

_Or: How to look up hostnames and arbitrary DNS Resource Records via \_systemd-resolved_'s bus APIs\_

_(This is a longer explanation how to use some parts of \_systemd-resolved_ bus API. If you are just looking for an API reference, consult the [bus API documentation](https://wiki.freedesktop.org/www/Software/systemd/resolved/) instead.)\_

_systemd-resolved_ provides a set of APIs on the bus for resolving DNS resource records. These are:

1. _ResolveHostname()_ for resolving hostnames to acquire their IP addresses
2. _ResolveAddress()_ for the reverse operation: acquire the hostname for an IP address
3. _ResolveService()_ for resolving a DNS-SD or SRV service
4. _ResolveRecord()_ for resolving arbitrary resource records.

Below you'll find examples for two of these calls, to show how to use them. Note that glibc offers similar (and more portable) calls in _getaddrinfo()_, _getnameinfo()_ and _res_query()_. Of these _getaddrinfo()_ and _getnameinfo()_ are directed to the calls above via the _nss-resolve_ NSS module, but _req_query()_ is not. There are a number of reasons why it might be preferable to invoke _systemd-resolved_'s bus calls rather than the glibc APIs:

1. Bus APIs are naturally asynchronous, which the glibc APIs generally are not.
2. The bus calls above pass back substantially more information about the resolved data, including where and how the data was found (i.e. which protocol was used: DNS, LLMNR, MulticastDNS, and on which network interface), and most importantly, whether the data could be authenticated via DNSSEC. This in particular makes these APIs useful for retrieving certificate data from the DNS, in order to implement DANE, SSHFP, OPENGPGKEY and IPSECKEY clients.
3. _ResolveService()_ knows no counterpart in glibc, and has the benefit of being a single call that collects all data necessary to connect to a DNS-SD or pure SRV service in one step.
4. _ResolveRecord()_ in contrast to _res_query()_ supports LLMNR and MulticastDNS as protocols on top of DNS, and makes use of _systemd-resolved_'s local DNS record cache. The processing of the request is done in the sandboxed _systemd-resolved_ process rather than in the local process, and all packets are pre-validated. Because this relies on _systemd-resolved_ the per-interface DNS zone handling is supported.

Of course, by using _systemd-resolved_ you lose some portability, but this could be handled via an automatic fallback to the glibc counterparts.

Note that the various resolver calls provided by _systemd-resolved_ will consult _/etc/hosts_ and synthesize resource records for these entries in order to ensure that this file is honoured fully.

The examples below use the _sd-bus_ D-Bus client implementation, which is part of _libsystemd_. Any other D-Bus library, including the original _libdbus_ or _GDBus_ may be used too.

## Resolving a Hostname

To resolve a hostname use the _ResolveHostname()_ call. For details on the function parameters see the [bus API documentation](https://wiki.freedesktop.org/www/Software/systemd/resolved/).

This example specifies _AF_UNSPEC_ as address family for the requested address. This means both an _AF_INET_ (A) and an _AF_INET6_ (AAAA) record is looked for, depending on whether the local system has configured IPv4 and/or IPv6 connectivity. It is generally recommended to request _AF_UNSPEC_ addresses for best compatibility with both protocols, in particular on dual-stack systems.

The example specifies a network interface index of "0", i.e. does not specify any at all, so that the request may be done on any. Note that the interface index is primarily relevant for LLMNR and MulticastDNS lookups, which distinguish different scopes for each network interface index.

This examples makes no use of either the input flags parameter, nor the output flags parameter. See the _ResolveRecord()_ example below for information how to make use of the _SD_RESOLVED_AUTHENTICATED_ bit in the returned flags parameter.

```
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <systemd/sd-bus.h>

int main(int argc, char*argv[]) {
        sd_bus_error error = SD_BUS_ERROR_NULL;
        sd_bus_message *reply = NULL;
        const char *canonical;
        sd_bus *bus = NULL;
        uint64_t flags;
        int r;

        r = sd_bus_open_system(&bus);
        if (r < 0) {
                fprintf(stderr, "Failed to open system bus: %s\n", strerror(-r));
                goto finish;
        }

        r = sd_bus_call_method(bus,
                               "org.freedesktop.resolve1",
                               "/org/freedesktop/resolve1",
                               "org.freedesktop.resolve1.Manager",
                               "ResolveHostname",
                               &error,
                               &reply,
                               "isit",
                               0,                                        /* Network interface index where to look (0 means any) */
                               argc >= 2 ? argv[1] : "www.github.com",   /* Hostname */
                               AF_UNSPEC,                                /* Which address family to look for */
                               UINT64_C(0));                             /* Input flags parameter */
        if (r < 0) {
               fprintf(stderr, "Failed to resolve hostnme: %s\n", error.message);
                sd_bus_error_free(&error);
                goto finish;
        }

        r = sd_bus_message_enter_container(reply, 'a', "(iiay)");
        if (r < 0)
                goto parse_failure;

        for (;;) {
                char buf[INET6_ADDRSTRLEN];
                int ifindex, family;
                const void *data;
                size_t length;

                r = sd_bus_message_enter_container(reply, 'r', "iiay");
                if (r < 0)
                        goto parse_failure;
                if (r == 0)  /* Reached end of array */
                        break;
                r = sd_bus_message_read(reply, "ii", &ifindex, &family);
                if (r < 0)
                        goto parse_failure;
                r = sd_bus_message_read_array(reply, 'y', &data, &length);
                if (r < 0)
                        goto parse_failure;
                r = sd_bus_message_exit_container(reply);
                if (r < 0)
                        goto parse_failure;

                printf("Found IP address %s on interface %i.\n", inet_ntop(family, data, buf, sizeof(buf)), ifindex);
        }

        r = sd_bus_message_exit_container(reply);
        if (r < 0)
                goto parse_failure;
        r = sd_bus_message_read(reply, "st", &canonical, &flags);
        if (r < 0)
                goto parse_failure;

        printf("Canonical name is %s\n", canonical);
        goto finish;

parse_failure:
        fprintf(stderr, "Parse failure: %s\n", strerror(-r));

finish:
        sd_bus_message_unref(reply);
        sd_bus_flush_close_unref(bus);
        return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
}
```

Compile this with a command line like the following (under the assumption you save the sources above as `addrtest.c`):

```
gcc addrtest.c -o addrtest -Wall `pkg-config --cflags --libs libsystemd`
```

## Resolving an Arbitrary DNS Resource Record

Use `ResolveRecord()` in order to resolve arbitrary resource records. The call will return the binary RRset data. This calls is useful to acquire resource records for which no high-level calls such as ResolveHostname(), ResolveAddress() and ResolveService() exist. In particular RRs such as MX, SSHFP, TLSA, CERT, OPENPGPKEY or IPSECKEY may be requested via this API.

This example also shows how to determine whether the acquired data has been authenticated via DNSSEC (or another means) by checking the `SD_RESOLVED_AUTHENTICATED` bit in the
returned `flags` parameter.

This example contains a simple MX record parser. Note that the data comes pre-validated from `systemd-resolved`, hence we allow the example to parse the record slightly sloppily, to keep the example brief. For details on the MX RR binary format, see [RFC 1035](https://www.rfc-editor.org/rfc/rfc1035.txt).

For details on the function parameters see the [bus API documentation](https://wiki.freedesktop.org/www/Software/systemd/resolved/).

```
#include <assert.h>
#include <endian.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <systemd/sd-bus.h>

#define DNS_CLASS_IN 1U
#define DNS_TYPE_MX 15U

#define SD_RESOLVED_AUTHENTICATED (UINT64_C(1) << 9)

static const uint8_t* print_name(const uint8_t* p) {
        bool dot = false;
        for (;;) {
                if (*p == 0)
                        return p + 1;
                if (dot)
                        putchar('.');
                else
                        dot = true;
                printf("%.*s", (int) *p, (const char*) p + 1);
                p += *p + 1;
        }
}

static void process_mx(const void *rr, size_t sz) {
        uint16_t class, type, rdlength, preference;
        const uint8_t *p = rr;
        uint32_t ttl;

        fputs("Found MX: ", stdout);
        p = print_name(p);

        memcpy(&type, p, sizeof(uint16_t));
        p += sizeof(uint16_t);
        memcpy(&class, p, sizeof(uint16_t));
        p += sizeof(uint16_t);
        memcpy(&ttl, p, sizeof(uint32_t));
        p += sizeof(uint32_t);
        memcpy(&rdlength, p, sizeof(uint16_t));
        p += sizeof(uint16_t);
        memcpy(&preference, p, sizeof(uint16_t));
        p += sizeof(uint16_t);

        assert(be16toh(type) == DNS_TYPE_MX);
        assert(be16toh(class) == DNS_CLASS_IN);
        printf(" preference=%u ", be16toh(preference));

        p = print_name(p);
        putchar('\n');

        assert(p == (const uint8_t*) rr + sz);
}

int main(int argc, char*argv[]) {
        sd_bus_error error = SD_BUS_ERROR_NULL;
        sd_bus_message *reply = NULL;
        sd_bus *bus = NULL;
        uint64_t flags;
        int r;

        r = sd_bus_open_system(&bus);
        if (r < 0) {
                fprintf(stderr, "Failed to open system bus: %s\n", strerror(-r));
                goto finish;
        }

        r = sd_bus_call_method(bus,
                               "org.freedesktop.resolve1",
                               "/org/freedesktop/resolve1",
                               "org.freedesktop.resolve1.Manager",
                               "ResolveRecord",
                               &error,
                               &reply,
                               "isqqt",
                               0,                                  /* Network interface index where to look (0 means any) */
                               argc >= 2 ? argv[1] : "gmail.com",  /* Domain name */
                               DNS_CLASS_IN,                       /* DNS RR class */
                               DNS_TYPE_MX,                        /* DNS RR type */
                               UINT64_C(0));                       /* Input flags parameter */
        if (r < 0) {
                fprintf(stderr, "Failed to resolve record: %s\n", error.message);
                sd_bus_error_free(&error);
                goto finish;
        }

        r = sd_bus_message_enter_container(reply, 'a', "(iqqay)");
        if (r < 0)
                goto parse_failure;

        for (;;) {
                uint16_t class, type;
                const void *data;
                size_t length;
                int ifindex;

                r = sd_bus_message_enter_container(reply, 'r', "iqqay");
                if (r < 0)
                        goto parse_failure;
                if (r == 0)  /* Reached end of array */
                        break;
                r = sd_bus_message_read(reply, "iqq", &ifindex, &class, &type);
                if (r < 0)
                        goto parse_failure;
                r = sd_bus_message_read_array(reply, 'y', &data, &length);
                if (r < 0)
                        goto parse_failure;
                r = sd_bus_message_exit_container(reply);
                if (r < 0)
                        goto parse_failure;

                process_mx(data, length);
        }

        r = sd_bus_message_exit_container(reply);
        if (r < 0)
                goto parse_failure;
        r = sd_bus_message_read(reply, "t", &flags);
        if (r < 0)
                goto parse_failure;

        printf("Response is authenticated: %s\n", flags & SD_RESOLVED_AUTHENTICATED ? "yes" : "no");
        goto finish;

parse_failure:
        fprintf(stderr, "Parse failure: %s\n", strerror(-r));

finish:
        sd_bus_message_unref(reply);
        sd_bus_flush_close_unref(bus);
        return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
   }
```

Compile this with a command line like the following (under the assumption you save the sources above as `rrtest.c`):

```
gcc rrtest.c -o rrtest -Wall `pkg-config --cflags --libs libsystemd`
```