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systemd-resolved: use hostname for certificate validation in DoT

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Widely accepted certificates for IP addresses are expensive and only
affordable for larger organizations. Therefore if the user provides
the hostname in the DNS= option, we should use it instead of the IP
address.
This commit is contained in:
Jörg Thalheim 2020-03-03 23:31:25 +00:00 committed by Yu Watanabe
parent 0d14eefb0d
commit eec394f10b
3 changed files with 34 additions and 20 deletions
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19
man/resolved.conf.xml
View File

@ -193,11 +193,17 @@
<varlistentry>
<term><varname>DNSOverTLS=</varname></term>
<listitem>
<para>Takes a boolean argument or <literal>opportunistic</literal>.
If true all connections to the server will be encrypted. Note that
this mode requires a DNS server that supports DNS-over-TLS and has
a valid certificate for it's IP. If the DNS server does not support
DNS-over-TLS all DNS requests will fail. When set to <literal>opportunistic</literal>
<para>Takes a boolean argument or <literal>opportunistic</literal>. If
true all connections to the server will be encrypted. Note that this
mode requires a DNS server that supports DNS-over-TLS and has a valid
certificate. If the hostname was specified in <varname>DNS=</varname>
by using the format format <literal>address#server_name</literal> it
is used to validate its certificate and also to enable Server Name
Indication (SNI) when opening a TLS connection. Otherwise
the certificate is checked against the server's IP.
If the DNS server does not support DNS-over-TLS all DNS requests will fail.</para>
<para>When set to <literal>opportunistic</literal>
DNS request are attempted to send encrypted with DNS-over-TLS.
If the DNS server does not support TLS, DNS-over-TLS is disabled.
Note that this mode makes DNS-over-TLS vulnerable to "downgrade"
@ -214,9 +220,6 @@
resolver is not capable of authenticating the server, so it is
vulnerable to "man-in-the-middle" attacks.</para>
<para>Server Name Indication (SNI) can be used when opening a TLS connection.
Entries in <varname>DNS=</varname> should be in format <literal>address#server_name</literal>.</para>
<para>In addition to this global DNSOverTLS setting
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
also maintains per-link DNSOverTLS settings. For system DNS

4
src/resolve/resolved-dnstls-gnutls.c
View File

@ -56,6 +56,9 @@ int dnstls_stream_connect_tls(DnsStream *stream, DnsServer *server) {
}
if (server->manager->dns_over_tls_mode == DNS_OVER_TLS_YES) {
if (server->server_name)
gnutls_session_set_verify_cert(gs, server->server_name, 0);
else {
stream->dnstls_data.validation.type = GNUTLS_DT_IP_ADDRESS;
if (server->family == AF_INET) {
stream->dnstls_data.validation.data = (unsigned char*) &server->address.in.s_addr;
@ -66,6 +69,7 @@ int dnstls_stream_connect_tls(DnsStream *stream, DnsServer *server) {
}
gnutls_session_set_verify_cert2(gs, &stream->dnstls_data.validation, 1, 0);
}
}
if (server->server_name) {
r = gnutls_server_name_set(gs, GNUTLS_NAME_DNS, server->server_name, strlen(server->server_name));

9
src/resolve/resolved-dnstls-openssl.c
View File

@ -6,6 +6,7 @@
#include <openssl/bio.h>
#include <openssl/err.h>
#include <openssl/x509v3.h>
#include "io-util.h"
#include "resolved-dns-stream.h"
@ -80,14 +81,20 @@ int dnstls_stream_connect_tls(DnsStream *stream, DnsServer *server) {
if (server->manager->dns_over_tls_mode == DNS_OVER_TLS_YES) {
X509_VERIFY_PARAM *v;
const unsigned char *ip;
SSL_set_verify(s, SSL_VERIFY_PEER, NULL);
v = SSL_get0_param(s);
if (server->server_name) {
X509_VERIFY_PARAM_set_hostflags(v, X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS);
if (X509_VERIFY_PARAM_set1_host(v, server->server_name, 0) == 0)
return -ECONNREFUSED;
} else {
const unsigned char *ip;
ip = server->family == AF_INET ? (const unsigned char*) &server->address.in.s_addr : server->address.in6.s6_addr;
if (X509_VERIFY_PARAM_set1_ip(v, ip, FAMILY_ADDRESS_SIZE(server->family)) == 0)
return -ECONNREFUSED;
}
}
if (server->server_name) {
r = SSL_set_tlsext_host_name(s, server->server_name);