{"id":711,"date":"2022-10-29T12:20:18","date_gmt":"2022-10-29T20:20:18","guid":{"rendered":"https:\/\/www.nathanbak.com\/?p=711"},"modified":"2026-01-02T19:59:39","modified_gmt":"2026-01-03T03:59:39","slug":"getting-local-address-in-go","status":"publish","type":"post","link":"https:\/\/nathanbak.com\/?p=711","title":{"rendered":"Getting local address in Go"},"content":{"rendered":"\n

Awhile ago I had some tests that would spin up a server and then hit some “localhost:8080” endpoints and verify the responses. The tests worked great in my IDE, but when the tests were invoked as a step during a Docker build, they could not find the server. This post is about how I got my tests to run within a Docker container.<\/p>\n\n\n\n

There are a variety of ways to get the local address in a Docker container, but many of the solutions I found online involved running one or more command line utilities and I wanted something that “just worked” in Go. After various kicking and swearing (and searching), this is what I came up with:<\/p>\n\n\n\n

\n\n
package main
\n
\nimport (
\n    "fmt"
\n    "net"
\n    "os"
\n)
\n
\nfunc main() {
\n    fmt.Println(getLocalAddress())
\n}
\n
\nfunc getLocalAddress() (string, error) {
\n    var address string
\n    _, err := os.Stat("\/.dockerenv") \/\/ 1
\n    if err == nil {
\n        conn, err := net.Dial("udp", "8.8.8.8:80") \/\/ 2
\n        if err != nil {
\n            return address, err
\n        }
\n        defer conn.Close()
\n        address = conn.LocalAddr().(*net.UDPAddr).IP.String() \/\/ 3
\n    } else {
\n        address = "localhost" \/\/ 4
\n    }
\n    return address, nil
\n}<\/div><\/div>\n\n<\/pre>\n\n\n\n
The above is a complete main.go which can be compiled and ran, but the most important piece is the getLocalAddress() function which returns the local address (or an error if something goes wrong, but I haven’t had problems).  Here are some of the key points of the code:<\/p>\n\n\n\n
  1. Checking if the “\/.dockerenv” file exists is an easy way to check if the code is running in a Docker container.<\/li>
  2. We create a net.Conn instance<\/li>
  3. I first tried using\n\n
    address = conn.LocalAddr().String()<\/div><\/div>\n\nto get the local IP address, but it included a port that I didn’t want.  Instead of doing string manipulation, I found  the returned LocalAddr was a pointer to a net.UDPAddr and so a simple casting provided easy access to the IP.<\/li>
  4. If not running in Docker, “localhost” is good enough.<\/li><\/ol>\n\n\n\n
    And that’s about it.  When I have checked, the local IP address of the container has been in the 172.17.0.x range which I believe is a Docker default.  Since doing this, there haven’t been any problems running the test, but I’m not sure if this is an industrial strength solution suitable for production environments.<\/p>\n","protected":false},"excerpt":{"rendered":"
    Awhile ago I had some tests that would spin up a server and then hit some “localhost:8080” endpoints and verify the responses. The tests worked great in my IDE, but when the tests were invoked as a step during a Docker build, they could not find the server. This post is about how I got … Continue reading “Getting local address in Go”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[8,19],"class_list":["post-711","post","type-post","status-publish","format-standard","hentry","category-uncategorized","tag-docker","tag-golang"],"_links":{"self":[{"href":"https:\/\/nathanbak.com\/index.php?rest_route=\/wp\/v2\/posts\/711","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/nathanbak.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/nathanbak.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/nathanbak.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/nathanbak.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=711"}],"version-history":[{"count":1,"href":"https:\/\/nathanbak.com\/index.php?rest_route=\/wp\/v2\/posts\/711\/revisions"}],"predecessor-version":[{"id":713,"href":"https:\/\/nathanbak.com\/index.php?rest_route=\/wp\/v2\/posts\/711\/revisions\/713"}],"wp:attachment":[{"href":"https:\/\/nathanbak.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=711"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/nathanbak.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=711"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/nathanbak.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=711"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}