Logging from one Docker Container to Another

Question

When trying to connect two containers on my ubuntu 16.04 host, I can not send messages from one container to the other, while the adress is available at the host.

I started one container (which provides a syslog-service by syslog-ng) with:

docker run -d -p 127.0.0.1:515:514/udp --name syslog-ng bobrik/syslog-ng

This container is defined in: https://github.com/bobrik/docker-syslog-ng . According to https://www.balabit.com/documents/syslog-ng-ose-latest-guides/en/syslog-ng-ose-guide-admin/html/configuring-sources-network.html the udp(ip(0.0.0.0), port(514)); in the syslog-ng.conf should make it possible to accept all ip-connections.

Now I can log from the host to the container using:

logger -n 127.0.0.1 -P 515 test123

The second container was started with (one time with the --link, once without)

docker run -it --link syslog-ng ubuntu /bin/bash

and in both cases, the logging is not available to the container, i.e.

logger -n 127.0.0.3 -P 515

returns no error message, but no message is added to the log either. (Tried it also with 127.0.0.1 and syslog-ng in case of the linked container).

So the question is: Why is logging in the container not possible?

If I start another container, both are on the same network, docker network inspect bridge returns:

[
    {
        "Name": "bridge",
        "Id": "6836c8a52555f30f27001daf9b111ad41a035a31783250e043c34602ea83cfe3",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "172.17.0.0/16",
                    "Gateway": "172.17.0.1"
                }
            ]
        },
        "Internal": false,
        "Containers": {
            "3acef55e018280571410b63b0cb7314ba354b67fb6523662b48ad09be8424423": {
                "Name": "syslog-ng",
                "EndpointID": "d6f2ad9cc9b5f6a5030e6061c4abb600ff7b0f16711f169954dd446f1351cb08",
                "MacAddress": "02:42:ac:11:00:03",
                "IPv4Address": "172.17.0.3/16",
                "IPv6Address": ""
            },
            "4f7d0101f97dca63b160e16a5e298c45f2ff51aa35085d0cdec96497a598be1a": {
                "Name": "goofy_lamarr",
                "EndpointID": "e357d6a99f67964bac4619d1ac984cd361a9c39137ea8d67a1ebc641e498919b",
                "MacAddress": "02:42:ac:11:00:05",
                "IPv4Address": "172.17.0.5/16",
                "IPv6Address": ""
            }
        },
        "Options": {
            "com.docker.network.bridge.default_bridge": "true",
            "com.docker.network.bridge.enable_icc": "true",
            "com.docker.network.bridge.enable_ip_masquerade": "true",
            "com.docker.network.bridge.host_binding_ipv4": "0.0.0.0",
            "com.docker.network.bridge.name": "docker0",
            "com.docker.network.driver.mtu": "1500"
        },
        "Labels": {}
    }
]

This is /etc/hosts on the ubuntu container where I try to log:

cat /etc/hosts 
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
172.17.0.3  syslog-ng 3acef55e0182
172.17.0.5  4f7d0101f97d

So the syslog-ng should be available with syslog-ng or 172.17.0.3.

If I understood https://docs.docker.com/engine/userguide/networking/default_network/dockerlinks/ correctly, than linking should make it possible to log directly to syslog-ng, and without the link, I should still be able to send messages from the one container to the container which exposed a port by ip, according to https://docs.docker.com/engine/userguide/networking/.

It seems like this is the same as: Logging from one docker container to another, but there are no concrete containers provided.

Has anyone an idea why this logging isn't possible?

Answer 1

It works. The problem was with the port number(515), with this command:

docker run -d -p 127.0.0.1:515:514/udp --name syslog-ng bobrik/syslog-ng

you say that the internal port 514 can be reached from the host with the 127.0.0.1:515. But if you want to reach the container directly from another container, you can reach it with syslog-ng:514 or with 172.17.0.2:514:

so the correct logger will be:

logger -n syslog-ng -P 514 test123345

I hope it helps

Br,

Micek