Code change:
In the guestbook example, the redis slaves are instructed to connect to the redis-master using the proxy provided by the minion on which they reside. The minion is designated by its hostname "kubernetes-minion-N". The container cannot resolve this name to an IP address and so slave connections fail. 1) create vagrant cluster 2) create redis-master pod 3) create redis-master service 4) create redis-slave replicationController Verify redis-slave pod locations: list pods ID Image(s) Host Labels Status redis-master-2 dockerfile/redis kubernetes-minion-1/10.245.2.2 name=redis-master Running -3839-11e4-a922- brendanburns/redis-slave kubernetes-minion-1/10.245.2.2 name=redisslave,replicationController=redisSlaveController Running -3839-11e4-a922- brendanburns/redis-slave kubernetes-minion-3/10.245.2.4 name=redisslave,replicationController=redisSlaveController Running Log onto a minion containing a slave (note: vagrant hostnames are minion-N, not kubernetes-minion-N) Find the slave container docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES brendanburns/redis-slave:latest /bin/sh -c About an hour ago Up About an hour k8s--slave.---3839-11e4-a922- docker logs | tail [7] 09 Sep 17:27:08.428  Connecting to MASTER kubernetes-minion-1: [7] 09 Sep 17:27:08.430 # Unable to connect to MASTER: No such file or directory [7] 09 Sep 17:27:09.435  Connecting to MASTER kubernetes-minion-1: sudo nsenter -m -u -n -i -p -t $(docker inspect --format '{{ .State.Pid }}' ) /bin/bash [ root ]$ ping kubernetes-minion-1 ping: unknown host kubernetes-minion-1 [ root ]$ cat /etc/hosts 244.1.4 -3839-11e4-a922- 127.0.0.1 localhost ::1 localhost ip6-localhost ip6-loopback fe00::0 ip6-localnet ff00::0 ip6-mcastprefix ff02::1 ip6-allnodes ff02::2 ip6-allrouters [ root ]$ cat nameserver 10.0.2.3 cat /proc/1/environ HOME=/rootPATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/binHOSTNAME=-3839-11e4-a922-0800279696e1REDISMASTERSERVICEPORT=10000REDISMASTERPORT=tcp://kubernetes-minion-1:10000REDISMASTERPORT6379TCP=tcp://kubernetes-minion-1:10000REDISMASTERPORT6379TCPPROTO=tcpREDISMASTERPORT6379TCPPORT=10000REDISMASTERPORT6379TCPADDR=kubernetes-minion-1SERVICE_HOST=kubernetes-
Tim - are you the right guy to handle this? I think we need to resolve the name of the service host and only give IPs to the minion in env variables. We can't assume the containers have the same DNS set up as the host system.
To be clear -- this example is for the checked in Vagrant stuff? If that is the case would be the guy? I'm trying to catch up on IRC discussions from earlier today.
I can look at this, but there are two possible solutions. The first solution is in the vagrant cluster (or any other local vm setup) we could run a dns name server on the kubernetes-master that can resolve each minion by host name. We would then pass a -dns option to the docker daemon to specify that name server to use in each container. The second solution is that we pass IP address instead of host name and make no assumption or override on the containers specified name server. Is there any obvious negative to this solution? Has much thought been done on the expected Nameserver used by containers spawned by our service? - if you can weigh in, I would be happy to resolve using either approach. My preference is solution two.
So, the vagrant case is a little special as there is no sane DNS set up by default. Another suggestion (3) would be to revert the stuff and just devolve to using ip addresses directly. I would do (3) before (2) before (1). But I'm cool with any of them. As for the nameserver of the containers that are spawned -- docker currently just takes the of the host and copies that into the container. It doesn't do that for . that is the core of the issue here. If/when we do offer some level of split horizon DNS (I'd love to) we can start exposing an extra layer of DNS on top of whatever is configured for the host. That is part of the "ip per service portal" stuff that is being talked about a lot right now.
If I do (3) there is a piece of code in pod storage that mangles my hostname where I return 10.245.x.x and it asks the cloud provider to resolve the IP address for "10". Since other providers appear to also be returning IP as host, I will look to change this so kubernetes makes no modifications to hostnames that it's given. Look for PR tomorrow.
you're right I'd missed that point. This really only applies to the Vagrant cluster and so a fix to the vagrant setup is sufficient. I also agree with your priorities. Injecting a line into /etc/hosts (if that's reasonable) is the simplest solution. Either reverting or allowing a switch to pass the minion public IP rather than hostname is next. Adding a DNS service to the vagrant master and forcing DNS resolution to the master host is most complicated and least preferable. Thanks both of you for helping me think this through better. Mark
I don't grok what the difference between 2 and 3 is? I sent a PR to docker a while back () to insert arbitrary lines to /etc/hosts. I still think it is a reasonable thing to want to do.
- difference between (2) and (3) is that in the past, the vagrant cloud provider was returning the IP address as the literal hostname for the minion. I had changed this after noticing that it had some ugly side-effects in the CLI, where a call to list pods would report pod ip addresses as IP / nil. The issue (which I now believe is a bug that I want to change independent of what we do here) is that the code at this line mangles the hostname returned by the cloudprovider: This makes using the IP address as a hostname (10.245.2.2) to have the ugly side-effect of the code then asking the cloudprovider for the IP address of hostname "10", and then the cloudprovider having no idea how to proceed. Independent of what is done in this issue, I want to submit a PR tomorrow to stop that code from manipulating the reported hostname that was provided by the cloudprovider before asking that same cloudprovider to return the IP address using a potentially different input value. so with that backlog (3) means revert vagrant cloudprovider to use IPAddress as hostname, which means when we pass in what we believe is hostname in this environment, it will just work because it is in-fact the IPAddress. (2) means do not pass in hostname but pass in IPAddress by changing more core k8s code. all that being said, there is an option (4) that I am intrigued by, - after following your PR through its final conclusion, it looks like as of docker 1.2.0 you can change the /etc/hosts file of a running container. this introduces an option where the kubelet injects the /etc/hosts file of minion into the running container so it has the /etc/hosts values of the minion. I would need to play with that support a little more to know pros/cons more. I think longer term, we need a plan on what nameserver our containers are expecting to reference, and if they are inherently different in some way. I have run into a number of times the issue where doing things like running docker in docker causes the nested container to default to the nameserver 8.8.8.8, which happens to be blocked on the Red Hat network ;-) For now to unblock I will probably pursue (3) and fix the hostname bug referenced here at the same time. It does make the CLI uglier when working with this environment, but not a huge deal, at least it will function.
Thanks both for the complete explanation of what's happening and for the suggested fixes. I think in the long run using hostnames in SERVICEHOST and ensuriing either through IP injection into container /etc/hosts or by providing DNS are the best options. I'm not sure if there might still be cases where resolution of SERVICEHOST would still be best done by presenting the IP address or not.

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