Golang program cannot find other containers in Docker

This is a creation in Article, where the information may have evolved or changed.

Reading objects

Let's say readers understand docker,docker-compose and go syntax

Problem description

I have three applications called Mysql,goapp,javaapp, respectively. Their dependencies are as follows:

Image.png

• Goapp completes the logic by invoking the Javaapp service.

• Javaapp directly with MySQL database.

In order to make them three easy to run in the Docker container I used the docker-compose. The specific configuration file is as follows:

version: '2'services:  mysql:    container_name: mysql    image: mysql:5.7    restart: always    hostname: mysql    environment:       MYSQL_DATABASE: ${MYSQL_DATABASE}       MYSQL_USER: ${MYSQL_USER}       MYSQL_PASSWORD: ${MYSQL_PASSWORD}       MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD}  goApp:    container_name: goApp    image: goApp:0.1    hostname: goApp    ports:      - "8080:8080"    javaApp:    container_name: javaApp    image: javaApp:0.1    hostname: javaApp    ports:      - "6031:6031"  

The structure runs smoothly, and it's always stable. Suddenly one day, I gave my server modified a hostname modified to app.crop.cn, and then ran away. The specific phenomenon is as follows:

• The Javaapp program can be accessed normally. Be able to access the MySQL database.

• Goapp cannot access the Javaapp application through Javaapp.

• Enter Javaapp and Goapp this container, can ping each other through.

Problem analysis

There are two key points in the phenomenon:

• In a container, all pings can be interconnected. Explains that the DNS inside Docker is working.

• Only the Go program cannot find the corresponding container based on the hostname.

According to the second doubtful guess: The DNS parsing mechanism of the go language is different from that of Java. I found it along this road, and finally found the following in the official documentation.

On Unix systems, the resolver has two options for resolving names. It can use a pure Go resolver that sends DNS requests directly to the servers listed in /etc/resolv.conf, or it can use a cgo-based resolver that calls C library routines such as getaddrinfo and getnameinfo.

This means that go has implemented a set of methods to request DNS resolution. Other programs should use the base C standard library getaddrinfo. So they do not behave the same. What else to say, go and see the code. The implementation was found in the Dnsconfig_unix.go file.

func dnsDefaultSearch() []string {    hn, err := getHostname()    if err != nil {        // best effort        return nil    }    if i := byteIndex(hn, '.'); i >= 0 && i < len(hn)-1 {        return []string{ensureRooted(hn[i+1:])}    }    return nil}func ensureRooted(s string) string {    if len(s) > 0 && s[len(s)-1] == '.' {        return s    }    return s + "."}

As you can see from the code, the default Search is to get the contents of the current host's hostname, the first point (".") later in the section. For example, I have changed the hostname to app.crop.cn, then the default search he gets is crop.cn. The So when I asked for Javaapp, he would javaApp.crop.cn the domain like a docker built-in DNS request. Of course, the request is not.

Problem Solving Solutions

After finding the reason, the following is relatively simple. It is only necessary to set the Dns-search parameter to "." When starting the container. You can do it.

Last modified docker-compose.yml to:

version: '2'services:  mysql:    container_name: mysql    image: mysql:5.7    restart: always    hostname: mysql    environment:       MYSQL_DATABASE: ${MYSQL_DATABASE}       MYSQL_USER: ${MYSQL_USER}       MYSQL_PASSWORD: ${MYSQL_PASSWORD}       MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD}  goApp:    container_name: goApp    image: goApp:0.1    dns-search: .    hostname: goApp    ports:      - "8080:8080"        javaApp:    container_name: javaApp    image: javaApp:0.1    hostname: javaApp    ports:      

In this way, this seemingly wonderful problem has been taken care of.

Resources

1. NAMERESOLVER-GLIBC Wiki

2. https://golang.org/pkg/net/#hdr-name_resolution

3. https://docs.docker.com/engine/userguide/networking/default_network/configure-dns/