[libvirt] [PATCH] network: Let domains be restricted to local DNS
Laine Stump
laine at laine.org
Mon Dec 1 12:49:32 UTC 2014
On 11/30/2014 04:06 PM, Martin Kletzander wrote:
> On Thu, Nov 20, 2014 at 06:46:41PM -0800, Josh Stone wrote:
>> This adds a new "localOnly" attribute on the domain element of the
>> network xml. With this set to "yes", DNS requests under that domain
>> will only be resolved by libvirt's dnsmasq, never forwarded upstream.
>>
>> This was how it worked before commit f69a6b987d616, and I found that
>> functionality useful. For example, I have my host's NetworkManager
>> dnsmasq configured to forward that domain to libvirt's dnsmasq, so I can
>> easily resolve guest names from outside. But if libvirt's dnsmasq
>> doesn't know a name and forwards it to the host, I'd get an endless
>> forwarding loop. Now I can set localOnly="yes" to prevent the loop.
>>
>
> I've found 2 things in this patch I'm not sure about:
>
> 1) According to the documentation about --local= parameter, it says:
> "Setting this flag does not suppress reading of /etc/resolv.conf,
> use -R to do that." Shouldn't "no-resolv" be added as the option
> you want in the config file?
No, "-R" (aka "no-resolv") tells dnsmasq to not use the servers listed
in /etc/resolv.conf for *anything*. In this case, all that is wanted is
to resolve requests _within the configured domain_ only locally, and not
forward those requests upstream. Requests for any other domain _should_
be forwarded to whatever server is listed in /etc/resolv.conf. Based on
a discussion with Josh, I'm certain that this is the behavior he wants
(and it makes sense to have it available).
>
> 2) If your answer to the previous question is "NO", which might very
> well be the case, for example if you don't want to forward that
> one particular domain only (and I misunderstood the commit
> message), I think you should just use the ",local" subparameter of
> the domain parameter (domain=example.com,local).
Interesting idea - this requires specifying the IP/prefix of the network
in the option along with domain (which I suppose isn't a problem), and
has the same effect as adding local=/domain.com/ as well as the reverse
resolution, which is a nice plus. However, the prefix for the network
must be 8, 16, or 24 for this to work (because of the way that
*.in-addr.arpa records are defined), so we can't do this all the time.
The question is then - do we write in a special case to do it this way
when the prefix is one of the right lengths, or just not do it at all?
Having it "magically happen" for some configs and not others could lead
to confusion, but having reverse resolution when possible would be very
useful (for example, some sshd setups have a long delay if the
reverse-resolve of the client's IP doesn't return something other than
"not found"; a pointless piece of security theater, but still fairly
common).
> If you want to
> use the local= parameter, you would have to parse the domain value
> as it can be multiple domains separated by a comma and for domains
> that are actually IP addresses, you'd need to add the reverse of
> that (e.g. 122.168.192.in-addr.arpa).
I'm not following this. Although the parsing code doesn't validate it,
the domain name in libvirt's XML is only allowed to be a single
"dnsName" according to the schema, and that is also what is documented
in formatnetwork.html. And I can't think of any useful reason for
specifying an IP address as the name (even though neither libvirt nor
dnsmasq produces an error when you do that, it is simply a broken
concept from the beginning).
>
>> Signed-off-by: Josh Stone <jistone at redhat.com>
>> Cc: Laine Stump <laine at laine.org>
>> ---
>> docs/formatnetwork.html.in | 12
>> +++++++++++-
>> docs/schemas/network.rng | 3 +++
>> src/conf/network_conf.c | 5 +++++
>> src/conf/network_conf.h | 1 +
>> src/network/bridge_driver.c | 5 +++++
>> .../networkxml2confdata/nat-network-dns-local-domain.conf | 14
>> ++++++++++++++
>> tests/networkxml2confdata/nat-network-dns-local-domain.xml | 9
>> +++++++++
>> tests/networkxml2conftest.c | 1 +
>> 8 files changed, 49 insertions(+), 1 deletion(-)
>> create mode 100644
>> tests/networkxml2confdata/nat-network-dns-local-domain.conf
>> create mode 100644
>> tests/networkxml2confdata/nat-network-dns-local-domain.xml
>>
>> diff --git a/docs/formatnetwork.html.in b/docs/formatnetwork.html.in
>> index dc438aee8622..defcdba00930 100644
>> --- a/docs/formatnetwork.html.in
>> +++ b/docs/formatnetwork.html.in
>> @@ -82,7 +82,7 @@
>> <pre>
>> ...
>> <bridge name="virbr0" stp="on" delay="5"/>
>> - <domain name="example.com"/>
>> + <domain name="example.com" localOnly="no"/>
>> <forward mode="nat" dev="eth0"/>
>> ...</pre>
>>
>> @@ -113,6 +113,16 @@
>> a <code><forward></code> mode of "nat" or "route" (or an
>> isolated network with no <code><forward></code>
>> element). <span class="since">Since 0.4.5</span>
>> +
>> + <p>
>> + If the optional <code>localOnly</code> attribute on the
>> + <code>domain</code> element is "yes", then DNS requests under
>> + this domain will only be resolved by the virtual network's
>> own
>> + DNS server - they will not be forwarded to the host's
>> upstream
>> + DNS server. If <code>localOnly</code> is "no", and by
>> + default, unresolved requests <b>will</b> be forwarded.
>> + <span class="since">Since 1.2.11</span>
>> + </p>
>> </dd>
>> <dt><code>forward</code></dt>
>> <dd>Inclusion of the <code>forward</code> element indicates that
>> diff --git a/docs/schemas/network.rng b/docs/schemas/network.rng
>> index 4546f8037580..a1da28092375 100644
>> --- a/docs/schemas/network.rng
>> +++ b/docs/schemas/network.rng
>> @@ -225,6 +225,9 @@
>> <optional>
>> <element name="domain">
>> <attribute name="name"><ref name="dnsName"/></attribute>
>> + <optional>
>> + <attribute name="localOnly"><ref
>> name="virYesNo"/></attribute>
>> + </optional>
>> </element>
>> </optional>
>>
>> diff --git a/src/conf/network_conf.c b/src/conf/network_conf.c
>> index 067334e87cb0..61451c39805f 100644
>> --- a/src/conf/network_conf.c
>> +++ b/src/conf/network_conf.c
>> @@ -2083,6 +2083,11 @@ virNetworkDefParseXML(xmlXPathContextPtr ctxt)
>>
>> /* Parse network domain information */
>> def->domain = virXPathString("string(./domain[1]/@name)", ctxt);
>> + tmp = virXPathString("string(./domain[1]/@localOnly)", ctxt);
>> + if (tmp) {
>> + def->domain_local = STRCASEEQ(tmp, "yes");
>> + VIR_FREE(tmp);
>> + }
>>
>> if ((bandwidthNode = virXPathNode("./bandwidth", ctxt)) != NULL &&
>> (def->bandwidth = virNetDevBandwidthParse(bandwidthNode, -1))
>> == NULL)
>> diff --git a/src/conf/network_conf.h b/src/conf/network_conf.h
>> index 660cd2d10cd1..6308a7dcfbf7 100644
>> --- a/src/conf/network_conf.h
>> +++ b/src/conf/network_conf.h
>> @@ -232,6 +232,7 @@ struct _virNetworkDef {
>>
>> char *bridge; /* Name of bridge device */
>> char *domain;
>> + bool domain_local; /* Choose not to forward dns for this domain */
>> unsigned long delay; /* Bridge forward delay (ms) */
>> bool stp; /* Spanning tree protocol */
>> virMacAddr mac; /* mac address of bridge device */
>> diff --git a/src/network/bridge_driver.c b/src/network/bridge_driver.c
>> index 6cb421c52850..dfa375d3aa72 100644
>> --- a/src/network/bridge_driver.c
>> +++ b/src/network/bridge_driver.c
>> @@ -912,6 +912,11 @@ networkDnsmasqConfContents(virNetworkObjPtr
>> network,
>> }
>>
>> if (network->def->domain) {
>> + if (network->def->domain_local) {
>> + virBufferAsprintf(&configbuf,
>> + "local=/%s/\n",
>> + network->def->domain);
>> + }
>> virBufferAsprintf(&configbuf,
>> "domain=%s\n"
>> "expand-hosts\n",
>> diff --git
>> a/tests/networkxml2confdata/nat-network-dns-local-domain.conf
>> b/tests/networkxml2confdata/nat-network-dns-local-domain.conf
>> new file mode 100644
>> index 000000000000..5f41b9186cbc
>> --- /dev/null
>> +++ b/tests/networkxml2confdata/nat-network-dns-local-domain.conf
>> @@ -0,0 +1,14 @@
>> +##WARNING: THIS IS AN AUTO-GENERATED FILE. CHANGES TO IT ARE LIKELY
>> TO BE
>> +##OVERWRITTEN AND LOST. Changes to this configuration should be
>> made using:
>> +## virsh net-edit default
>> +## or other application using the libvirt API.
>> +##
>> +## dnsmasq conf file created by libvirt
>> +strict-order
>> +local=/example.com/
>> +domain=example.com
>> +expand-hosts
>> +except-interface=lo
>> +bind-dynamic
>> +interface=virbr0
>> +addn-hosts=/var/lib/libvirt/dnsmasq/default.addnhosts
>> diff --git
>> a/tests/networkxml2confdata/nat-network-dns-local-domain.xml
>> b/tests/networkxml2confdata/nat-network-dns-local-domain.xml
>> new file mode 100644
>> index 000000000000..a92d71f1f2f6
>> --- /dev/null
>> +++ b/tests/networkxml2confdata/nat-network-dns-local-domain.xml
>> @@ -0,0 +1,9 @@
>> +<network>
>> + <name>default</name>
>> + <uuid>81ff0d90-c91e-6742-64da-4a736edb9a9c</uuid>
>> + <forward dev='eth0' mode='nat'/>
>> + <bridge name='virbr0' stp='on' delay='0' />
>> + <domain name='example.com' localOnly='yes'/>
>> + <ip address='192.168.122.1' netmask='255.255.255.0'>
>> + </ip>
>> +</network>
>> diff --git a/tests/networkxml2conftest.c b/tests/networkxml2conftest.c
>> index 4f1d9345ffe4..d2aa8c62cfcd 100644
>> --- a/tests/networkxml2conftest.c
>> +++ b/tests/networkxml2conftest.c
>> @@ -146,6 +146,7 @@ mymain(void)
>> DO_TEST("nat-network-dns-hosts", full);
>> DO_TEST("nat-network-dns-forward-plain", full);
>> DO_TEST("nat-network-dns-forwarders", full);
>> + DO_TEST("nat-network-dns-local-domain", full);
>> DO_TEST("dhcp6-network", dhcpv6);
>> DO_TEST("dhcp6-nat-network", dhcpv6);
>> DO_TEST("dhcp6host-routed-network", dhcpv6);
>> --
>> 2.1.0
>>
>> --
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