Skip to content

Commit

Permalink
docs: Add debugging notes for DNS/toFQDNs systems
Browse files Browse the repository at this point in the history 
Debugging FQDN-related issues comes up often, and when developing the
issues are more subtle. We don't have a section for developer debugging
so we now do, and it includes a walkthrough of how to investigate the
DNS proxy and fqdn DNS and policy components. It also touches briefly on
how to interrogate the datapath Policy Map.

Signed-off-by: Ray Bejjani <ray@isovalent.com>
  • Loading branch information
@raybejjani
raybejjani committed Apr 27, 2020
1 parent 0b02fc1 commit 6fcfd39
Show file tree
Hide file tree
Showing 2 changed files with 328 additions and 0 deletions.
327 changes: 327 additions & 0 deletions Documentation/contributing/development/debugging.rst
View file
Original file line number Diff line number Diff line change
@@ -0,0 +1,327 @@
.. only:: not (epub or latex or html)

WARNING: You are looking at unreleased Cilium documentation.
Please use the official rendered version released here:
https://docs.cilium.io

#########
Debugging
#########


toFQDNs and DNS Debugging
-------------------------

The interactions of L3 toFQDNs and L7 DNS rules can be difficult to debug
around. Unlike many other policy rules, these are resolved at runtime with
unknown data. Pods may create large numbers of IPs in the cache or the IPs
returned returned may not be compatible with our datapath implementation.
Sometimes we also just have bugs.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Isolating the source of toFQDNs issues
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

While there is no common culprit when debugging, the DNS Proxy shares the least
code with other system and so is more likely the least audited in this chain.
The cascading caching scheme is also complex in its behaviour. Determining
whether an issue is caused by the DNS components, in the policy layer or in the
datapath is often the first step when debugging toFQDNs related issues.
Generally, working top-down is easiest as the information needed to verify
low-level correctness can be collected in the initial debug invocations.


REFUSED vs NXDOMAIN responses
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The proxy uses REFUSED DNS responses to indicate a denied request. Some libc
implementations, notably musl which is common in Alpine Linux images, terminate
the whole DNS search in these cases. This often manifests as a connect error in
applications, as the libc lookup returns no data.
To work around this, denied responses can be configured to be NXDOMAIN by
setting the ``--tofqdns-dns-reject-response-code`` command line argument.


Monitor Events
~~~~~~~~~~~~~~

The DNS Proxy emits multiple L7 DNS monitor events. One for the request and one
for the response (if allowed). Often the L7 DNS rules are paired with L3
toFQDNs rules and events relating to those rules are also relevant.

.. Note::

Be sure to run cilium monitor on the same node as the pod being debugged!

.. code:: bash
$ kubectl exec pod/cilium-sbp8v -n cilium -- cilium monitor --related-to 3459
Listening for events on 4 CPUs with 64x4096 of shared memory
Press Ctrl-C to quit
level=info msg="Initializing dissection cache..." subsys=monitor
-> Request dns from 3459 ([k8s:org=alliance k8s:io.kubernetes.pod.namespace=default k8s:io.cilium.k8s.policy.serviceaccount=default k8s:io.cilium.k8s.policy.cluster=default k8s:class=xwing]) to 0 ([k8s:io.cilium.k8s.policy.serviceaccount=kube-dns k8s:io.kubernetes.pod.namespace=kube-system k8s:k8s-app=kube-dns k8s:io.cilium.k8s.policy.cluster=default]), identity 323->15194, verdict Forwarded DNS Query: cilium.io. A
-> endpoint 3459 flow 0xe6866e21 identity 15194->323 state reply ifindex lxc84b58cbdabfe orig-ip 10.60.1.115: 10.63.240.10:53 -> 10.60.0.182:42132 udp
-> Response dns to 3459 ([k8s:org=alliance k8s:io.kubernetes.pod.namespace=default k8s:io.cilium.k8s.policy.serviceaccount=default k8s:io.cilium.k8s.policy.cluster=default k8s:class=xwing]) from 0 ([k8s:io.cilium.k8s.policy.cluster=default k8s:io.cilium.k8s.policy.serviceaccount=kube-dns k8s:io.kubernetes.pod.namespace=kube-system k8s:k8s-app=kube-dns]), identity 323->15194, verdict Forwarded DNS Query: cilium.io. A TTL: 486 Answer: '104.198.14.52'
-> endpoint 3459 flow 0xe6866e21 identity 15194->323 state reply ifindex lxc84b58cbdabfe orig-ip 10.60.1.115: 10.63.240.10:53 -> 10.60.0.182:42132 udp
Policy verdict log: flow 0x614e9723 local EP ID 3459, remote ID 16777217, dst port 80, proto 6, ingress false, action allow, match L3-Only, 10.60.0.182:41510 -> 104.198.14.52:80 tcp SYN
-> stack flow 0x614e9723 identity 323->16777217 state new ifindex 0 orig-ip 0.0.0.0: 10.60.0.182:41510 -> 104.198.14.52:80 tcp SYN
-> 0: 10.60.0.182:41510 -> 104.198.14.52:80 tcp SYN
-> endpoint 3459 flow 0x7388921 identity 16777217->323 state reply ifindex lxc84b58cbdabfe orig-ip 104.198.14.52: 104.198.14.52:80 -> 10.60.0.182:41510 tcp SYN, ACK
-> stack flow 0x614e9723 identity 323->16777217 state established ifindex 0 orig-ip 0.0.0.0: 10.60.0.182:41510 -> 104.198.14.52:80 tcp ACK
-> 0: 10.60.0.182:41510 -> 104.198.14.52:80 tcp ACK
-> stack flow 0x614e9723 identity 323->16777217 state established ifindex 0 orig-ip 0.0.0.0: 10.60.0.182:41510 -> 104.198.14.52:80 tcp ACK
-> 0: 10.60.0.182:41510 -> 104.198.14.52:80 tcp ACK
-> endpoint 3459 flow 0x7388921 identity 16777217->323 state reply ifindex lxc84b58cbdabfe orig-ip 104.198.14.52: 104.198.14.52:80 -> 10.60.0.182:41510 tcp ACK
-> 0: 10.60.0.182:41510 -> 104.198.14.52:80 tcp ACK
-> stack flow 0x614e9723 identity 323->16777217 state established ifindex 0 orig-ip 0.0.0.0: 10.60.0.182:41510 -> 104.198.14.52:80 tcp ACK, FIN
-> 0: 10.60.0.182:41510 -> 104.198.14.52:80 tcp ACK, FIN
-> endpoint 3459 flow 0x7388921 identity 16777217->323 state reply ifindex lxc84b58cbdabfe orig-ip 104.198.14.52: 104.198.14.52:80 -> 10.60.0.182:41510 tcp ACK, FIN
-> stack flow 0x614e9723 identity 323->16777217 state established ifindex 0 orig-ip 0.0.0.0: 10.60.0.182:41510 -> 104.198.14.52:80 tcp ACK
The above is for a simple ``curl cilium.io`` in a pod. The L7 DNS request is
the first set of message and the subsequent L3 connection is the HTTP
component. AAAA DNS lookups commonly happen but were removed to simplify the
example.

- If no L7 DNS requests appear, the proxy redirect is not in place. This may
mean that the policy does not select this endpoint or there is an issue with
the proxy redirection. Whether any redirects exist can be checked with
``cilium status --all-redirects``.
In the past, a bug occurred with more permissive L3 rules overriding the
proxy redirect, causing the proxy to never see the requests.
- If the L7 DNS request is blocked, with an explicit denied message, then the
requests are not allowed by the proxy. This may be due to a typo in the
network policy, or the matchPattern rule not allowing this domain. It may
also be due to a bug in policy propagation to the DNS Proxy.
- If the DNS request is allowed, with an explicit message, and it should not
be, this may be because a more general policy is in place that allows the
request. ``matchPattern: "*"`` visibility policies are commonly in place and
would supersede all other, more restrictive, policies.
If no other policies are in place, incorrect allows may indicate a bug when
passing policy information to the proxy. There is no way to dump the rules in
the proxy, but a debug log is printed when a rule is added. Look for
``DNS Proxy updating matchNames in allowed list during UpdateRules``.
The pkg/proxy/dns.go file contains the DNS proxy implementation.

If L7 DNS behaviour seems correct, see the sections below to further isolate
the issue. This can be verified with ``cilium fqdn cache list``. The IPs in the
response should appear in the cache for the appropriate endpoint. The lookup
time is included in the json output of the command.

.. code:: bash
$ kubectl exec pod/cilium-sbp8v -n cilium -- cilium fqdn cache list
Endpoint Source FQDN TTL ExpirationTime IPs
3459 lookup cilium.io. 3600 2020-04-21T15:04:27.146Z 104.198.14.52
DNS Proxy Errors
~~~~~~~~~~~~~~~~

REFUSED responses are returned when the proxy encounters an error during
processing. This can be confusing to debug as that is also the response when a
DNS request is denied. An error log is always printed in these cases. Some are
callbacks provided by other packages via daemon in cilium-agent.

- ``Rejecting DNS query from endpoint due to error``: This is the "normal"
policy-reject message. It is a debug log.
- ``cannot extract endpoint IP from DNS request``: The proxy cannot read the
socket information to read the source endpoint IP. This could mean an
issue with the datapath routing and information passing.
- ``cannot extract endpoint ID from DNS request``: The proxy cannot use the
source endpoint IP to get the cilium-internal ID for that endpoint. This is
different from the Security Identity. This could mean that cilium is not
managing this endpoint and that something has gone awry. It could also mean a
routing problem where a packet has arrived at the proxy incorrectly.
- ``cannot extract destination IP:port from DNS request``: The proxy cannot
read the socket information of the original request to obtain the intended
target IP:Port. This could mean an issue with the datapath routing and
information passing.
- ``cannot find server ip in ipcache``: The proxy cannot resolve a Security
Identity for the target IP of the DNS request. This should always succeed, as
world catches all IPs not set by more specific entries. This can mean a
broken ipcache BPF table.
- ``Rejecting DNS query from endpoint due to error``: While checking if the DNS
request was allowed (based on Endpoint ID, destination IP:Port and the DNS
query) an error occurred. These errors would come from the internal rule
lookup in the proxy, the ``allowed`` field.
- ``Timeout waiting for response to forwarded proxied DNS lookup``: The proxy
forwards requests 1:1 and does not cache. It applies a 5s timeout on
responses to those requests, as the client will retry within this period
(usually). Bursts of these errors can happen if the DNS target server
misbehaves and many pods see DNS timeouts. This isn't an actual problem with
cilium or the proxy although it can be caused by policy blocking the DNS
target server if it is in-cluster.
- ``Timed out waiting for datapath updates of FQDN IP information; returning
response``: When the proxy updates the DNS caches with response data, it
needs to allow some time for that information to get into the datapath.
Otherwise, pods would attempt to make the outbound connection (the thing that
caused the DNS lookup) before the datapath is ready. Many stacks retry the
SYN in such cases but some return an error and some apps further crash as a
response. This delay is configurable by setting the
``--tofqdns-proxy-response-max-delay`` command line argument but defaults to
100ms. It can be exceeded if the system is under load.


Identities and Policy
~~~~~~~~~~~~~~~~~~~~~

Once a DNS response has been passed back through the proxy and is placed in the
DNS cache ``toFQDNs`` rules can begin using the IPs in the cache. There are
multiple layers of cache:

- A per-Endpoint ``DNSCache`` stores the lookups for this endpoint. It is
restored on cilium startup with the endpoint. Limits are applied here for
``--tofqdns-endpoint-max-ip-per-hostname`` and TTLs are tracked. The
``--tofqdns-min-ttl`` is not used here.
- A per-Endpoint ``DNSZombieMapping`` list of IPs that have expired from the
per-Endpoint cache but are waiting for the Connection Tracking GC to mark
them in-use or not. This can take up to 12 hours to occur. This list is
size-limited by ``--tofqdns-max-deferred-connection-deletes``.
- A global ``DNSCache`` where all endpoint and poller DNS data is collected. It
does apply the ``--tofqdns-min-ttl`` value but not the
``--tofqdns-endpoint-max-ip-per-hostname`` value.

If an IP exists in the FQDN cache (check with ``cilium fqdn cache list``) then
``toFQDNs`` rules that select a domain name, either explicitly via
``matchName`` or via ``matchPattern``, should cause IPs for that domain to have
allocated Security Identities. These can be listed with:

.. code:: bash
$ kubectl exec pod/cilium-sbp8v -n cilium -- cilium identity list
ID LABELS
1 reserved:host
2 reserved:world
3 reserved:unmanaged
4 reserved:health
5 reserved:init
6 reserved:remote-node
323 k8s:class=xwing
k8s:io.cilium.k8s.policy.cluster=default
k8s:io.cilium.k8s.policy.serviceaccount=default
k8s:io.kubernetes.pod.namespace=default
k8s:org=alliance
...
16777217 cidr:104.198.14.52/32
reserved:world
Note that CIDR identities are allocated locally on the node and have a high-bit set so they are often in the 16-million range.
Note that this is the identity in the monitor output for the HTTP connection.

In cases where there is no matching identity for an IP in the fqdn cache it may
simply be because no policy selects an associated domain. The policy system
represents each ``toFQDNs:`` rule with a ``FQDNSelector`` instance. These
receive updates from a global ``NameManage`` in the daemon.
They can be listed along with other selectors (roughly corresponding to any L3 rule):

.. code:: bash
$ kubectl exec pod/cilium-sbp8v -n cilium -- cilium policy selectors
SELECTOR USERS IDENTITIES
MatchName: , MatchPattern: * 1 16777217
&LabelSelector{MatchLabels:map[string]string{},MatchExpressions:[]LabelSelectorRequirement{},} 2 1
2
3
4
5
6
323
6188
15194
18892
25379
29200
32255
33831
16777217
&LabelSelector{MatchLabels:map[string]string{reserved.none: ,},MatchExpressions:[]LabelSelectorRequirement{},} 1
In this example 16777217 is used by two selectors, one with ``matchPattern: "*"``
and another empty one. This is because of the policy in use:

.. code:: yaml
apiVersion: cilium.io/v2
kind: CiliumNetworkPolicy
metadata:
name: "tofqdn-dns-visibility"
spec:
endpointSelector:
matchLabels:
any:org: alliance
egress:
- toPorts:
- ports:
- port: "53"
protocol: ANY
rules:
dns:
- matchPattern: "*"
- toFQDNs:
- matchPattern: "*"
The L7 DNS rule has an implicit L3 allow-all because it defines only L4 and L7
sections. This is the second selector in the list, and includes all possible L3
identities known in the system. In contrast, the first selector, which
corresponds to the ``toFQDNS: matchName: "*"`` rule would list all identities
for IPs that came from the DNS Proxy. Other CIDR identities would not be
included.

Datapath Plumbing
~~~~~~~~~~~~~~~~~

For a policy to be fully realized the datapath for an Endpoint must be updated.
In the case of a new DNS-source IP, the CIDR identity associated with it must
propagate from the selectors to the Endpoint specific policy. Unless a new
policy is being added, this often only involves updating the Policy Map of the
Endpoint with the new CIDR Identity of the IP. This can be verified:

.. code:: bash
$ kubectl exec pod/cilium-sbp8v -n cilium -- cilium bpf policy get 3459
DIRECTION LABELS (source:key[=value]) PORT/PROTO PROXY PORT BYTES PACKETS
Ingress reserved:unknown ANY NONE 1367 7
Ingress reserved:host ANY NONE 0 0
Egress reserved:unknown 53/TCP 36447 0 0
Egress reserved:unknown 53/UDP 36447 138 2
Egress cidr:104.198.14.52/32 ANY NONE 477 6
reserved:world
Note that the labels for identities are resolved here. This can be skipped, or
there may be cases where this doesn't occur:

.. code:: bash
$ kubectl exec pod/cilium-sbp8v -n cilium -- cilium bpf policy get -n 3459
DIRECTION IDENTITY PORT/PROTO PROXY PORT BYTES PACKETS
Ingress 0 ANY NONE 1367 7
Ingress 1 ANY NONE 0 0
Egress 0 53/TCP 36447 0 0
Egress 0 53/UDP 36447 138 2
Egress 16777217 ANY NONE 477 6
L3 ``toFQDNs`` rules are egress only, so we would expect to see an ``Egress``
entry with Security Identity ``16777217``. The L7 rule, used to redirect to the
DNS Proxy is also present with a populated ``PROXY PORT``. It has a 0
``IDENTITY`` as it is an L3 wildcard, ie the policy allows any peer on the
specified port.

An identity missing here can be an error in various places:

- Policy doesn't actually allow this Endpoint to connect. A sanity check is to
use ``cilium endpoint list`` to see if cilium thinks it should have policy
enforcement.
- Endpoint regeneration is slow and the Policy Map has not been updated yet.
This can occur in cases where we have leaked IPs from the DNS cache (i.e.
they were never deleted correctly) or when there are legitimately many IPs.
It can also simply mean an overloaded node or even a deadlock within cilium.
- A more permissive policy has removed the need to include this identity. This
is likely a bug, however, as the IP would still have an identity allocated
and it would be included in the Policy Map. In the past, a similar bug
occurred with the L7 redirect and that would stop this whole process at the
beginning.
1 change: 1 addition & 0 deletions Documentation/contributing/development/index.rst
View file
Original file line number Diff line number Diff line change
Expand Up @@ -22,3 +22,4 @@ understand the workflow of getting these changes reviewed and merged upstream.
dev_setup
images
codeoverview
debugging

0 comments on commit 6fcfd39

Please sign in to comment.