Docker routing/reverse proxy issue, can't curl other container

Question

I have a single docker host running 2 web apps inside of individual containers. I have an nginx container setup in front of both of them acting as a reverse proxy. There are two dns entries for different subdomains pointing to this single host so I can reach app 1 with app1.domain.com and app2 with app2.domain.com. This setup is working fine, and each app is accessible to the broader universe.

However, app2 also needs to be able to make an http call to webservices provided by app1. For some reason, the http calls to http://app1.domain.com can't be resolved from within the app2 container. curl http://app1.domain.com returns Failed to connect to app1.domain.com port 80: No route to host. Oddly, I can ping app1.domain.com from within app2's container and it successfully resolves to the hosts url. I have tried disabling iptables with service iptables stop on the docker host and that causes the both the curl and ping commands to simply hang for a while before finally returning an error about unknown host for ping and could not resolve host for curl.

Finally, I can curl from app2's container to app1 using the docker ip address and port, though that is not an ideal solution given that it would require changing how this app is deployed and configured so that this ip address and port can be discovered.

UPDATE: Output of iptables -n -L -v -x

    Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
    pkts      bytes target     prot opt in     out     source               destination
       0        0 ACCEPT     tcp  --  eth1   *       10.191.192.0/18      0.0.0.0/0
     124     6662 ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0
       3      120 ACCEPT     tcp  --  eth0   *       0.0.0.0/0            0.0.0.0/0           tcp dpt:3306
  141668 14710477 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp dpt:5432
  252325 512668022 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
      31     2635 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0
       0        0 ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0
    5496   331240 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           state NEW tcp dpt:22
     623    37143 REJECT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           reject-with icmp-host-prohibited

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
    pkts      bytes target     prot opt in     out     source               destination
  437791 334335762 DOCKER     all  --  *      docker0  0.0.0.0/0            0.0.0.0/0
  438060 347940196 ACCEPT     all  --  *      docker0  0.0.0.0/0            0.0.0.0/0           ctstate RELATED,ESTABLISHED
  680992 61107377 ACCEPT     all  --  docker0 !docker0  0.0.0.0/0            0.0.0.0/0
     356    24168 ACCEPT     all  --  docker0 docker0  0.0.0.0/0            0.0.0.0/0
       0        0 REJECT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           reject-with icmp-host-prohibited

Chain OUTPUT (policy ACCEPT 604 packets, 125207 bytes)
    pkts      bytes target     prot opt in     out     source               destination
       0        0 ACCEPT     tcp  --  *      eth1    0.0.0.0/0            10.191.192.0/18
     124     6662 ACCEPT     all  --  *      lo      0.0.0.0/0            0.0.0.0/0

Chain DOCKER (1 references)
    pkts      bytes target     prot opt in     out     source               destination
       0        0 ACCEPT     tcp  --  !docker0 docker0  0.0.0.0/0            172.17.0.2          tcp dpt:81
       0        0 ACCEPT     tcp  --  !docker0 docker0  0.0.0.0/0            172.17.0.2          tcp dpt:443
    2191   156283 ACCEPT     tcp  --  !docker0 docker0  0.0.0.0/0            172.17.0.2          tcp dpt:80
       0        0 ACCEPT     tcp  --  docker0 docker0  172.17.0.60          172.17.0.7          tcp dpt:3000
       0        0 ACCEPT     tcp  --  docker0 docker0  172.17.0.7           172.17.0.60         tcp spt:3000

app1 docker ip: 172.17.0.7 app2 docker ip: 172.17.0.60

Answer 1

You can link your docker containers and then use the link to directly talk to app1 from within app2. In this way you can avoid dns resolution, and hence would be faster.

Assuming you are running the containers in the following way:

docker run --name app1 app1-image

docker run --name app2 --link app1 app2-image

Now from within app2 container you can access app1 with the hostname 'app1'