Maintained by: NLnet Labs

LibUnbound Tutorial part 6

By W.C.A. Wijngaards, NLnet Labs, February 2008.

6. DNSSEC Validate

This example program performs DNSSEC (secure DNS) validation of a lookup. It is a modification of the example program from part 2.

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <arpa/inet.h>
#include <unbound.h>

int main(void)
{
	struct ub_ctx* ctx;
	struct ub_result* result;
	int retval;

	/* create context */
	ctx = ub_ctx_create();
	if(!ctx) {
		printf("error: could not create unbound context\n");
		return 1;
	}
	/* read /etc/resolv.conf for DNS proxy settings (from DHCP) */
	if( (retval=ub_ctx_resolvconf(ctx, "/etc/resolv.conf")) != 0) {
		printf("error reading resolv.conf: %s. errno says: %s\n", 
			ub_strerror(retval), strerror(errno));
		return 1;
	}
	/* read /etc/hosts for locally supplied host addresses */
	if( (retval=ub_ctx_hosts(ctx, "/etc/hosts")) != 0) {
		printf("error reading hosts: %s. errno says: %s\n", 
			ub_strerror(retval), strerror(errno));
		return 1;
	}

	/* read public keys for DNSSEC verification */
	if( (retval=ub_ctx_add_ta_file(ctx, "keys")) != 0) {
		printf("error adding keys: %s\n", ub_strerror(retval));
		return 1;
	}

	/* query for webserver */
	retval = ub_resolve(ctx, "www.nlnetlabs.nl", 
		1 /* TYPE A (IPv4 address) */, 
		1 /* CLASS IN (internet) */, &result);
	if(retval != 0) {
		printf("resolve error: %s\n", ub_strerror(retval));
		return 1;
	}

	/* show first result */
	if(result->havedata)
		printf("The address is %s\n", 
			inet_ntoa(*(struct in_addr*)result->data[0]));
	/* show security status */
	if(result->secure)
		printf("Result is secure\n");
	else if(result->bogus)
		printf("Result is bogus: %s\n", result->why_bogus);
	else 	printf("Result is insecure\n");

	ub_resolve_free(result);
	ub_ctx_delete(ctx);
	return 0;
}

download example code

Invocation of this program yields the following

First testrun
$ touch keys
$ example_6
The address is 213.154.224.1
Result is insecure

The first testrun uses an empty keyfile, and since there is no security configured for nlnetlabs.nl, the result is insecure. For a secure result, DNSSEC security must be configured on both the server and the client, and in this example run, the nlnetlabs.nl server has security configured, but the key file is empty on the client.

Second testrun
$ dig nlnetlabs.nl DNSKEY > keys
$ example_6
The address is 213.154.224.1
Result is secure

The second testrun obtains the current DNSKEY information for nlnetlabs.nl using dig (from the named utilities). This is not a secure method to obtain keys, check keys carefully before you trust them and enter them into your application (for example RIPE distributes key files with added PGP signatures). But it is very easy, and useful for this tutorial. The lookup result is secure, because it is signed with the correct keys.

Third testrun
$ echo 'nlnetlabs.nl. 3528 IN DNSKEY ( 256 3 5 
	AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
	AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
	AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
	AAAAAA== )' > keys
$ example_6
The address is 213.154.224.1
Result is bogus: validation failure <www.nlnetlabs.nl. A IN>: 
signatures from unknown keys from 213.154.224.254 for trust anchor 
nlnetlabs.nl. while building chain of trust

The third example puts a key into the keyfile that is not going to match any signatures. The echo command is wrapped onto multiple lines on this page for presentation, put the text onto one line. Because the key and the signatures on the data do not match, verification fails and the result is bogus.

The program starts like part 2 of the tutorial, creates the unbound context and reads in /etc/resolv.conf and /etc/hosts. Then it adds the contents of the keys file from the current directory as trusted keys. It continues to resolve www.nlnetlabs.nl and prints the result. It also prints the security status of the result.

The function ub_ctx_add_ta_file adds trusted keys. The keys file contains text in the zone file format (output from dig or drill tools, or a copy and paste from the DNS zone file). It can contain DNSKEY and DS entries, for any number of domain names. If any of the keys matches the signatures on lookup results, the result->secure is set true.

The function ub_ctx_add_ta (not shown in example) can be used to add a trusted key from a string. A single DNSKEY or DS key entry, on a single line, is expected. Multiple keys can be given with multiple calls to ub_ctx_add_ta. For example

	if( (retval=ub_ctx_add_ta(ctx, "jelte.nlnetlabs.nl. DS 31560 "
		"5 1 1CFED84787E6E19CCF9372C1187325972FE546CD")) != 0)
	{ /* print error */ }

It is also possible to read in named (BIND-style) key config files. These files contain trusted-key{} clauses. The function ub_ctx_trustedkeys (not shown in example) adds the keys from a bind-style config file. ub_ctx_set_option(ctx, "auto-trust-anchor-file:", "keys") (not shown in example) can be used to use auto-updated keys (with RFC5011), the file is read from and written to when the keys change. The probes have to be frequent enough to not lose track, about every 15 days.

It is worth noting that with DNSSEC it is possible to verify nonexistence of data. So, if the example above is modified to query for foobar.nlnetlabs.nl and with correct keys in the keys file, the output is no data, but the result is secure.

DNSSEC has complicated verification procedures. The result is distilled into two booleans, secure and bogus. Either the result is secure, the result is bogus, or the result is neither of the two, called insecure. Insecure happens when no DNSSEC security is configured for the domain name (or you simply forgot to add the trusted key). Secure means that one of the trusted keys verifies the signatures on the data. Bogus (security failed) can have many reasons, DNSSEC protects against alteration of the data in transit, signatures can expire, the trusted keys can be rolled over to fresh trusted keys, and many others. The functions ub_ctx_debugout (sets a stream to log to) and ub_ctx_debuglevel (try level 2) can give more information about a security failure. The why_bogus string as printed in the example above attempts to give a detailed reason for the failure. An e-commerce application can simply look at result->secure for its shopping server, and only continue if the result is secure.

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