Wrong URL parsing with url.ResolveReference() if http:// is missing

Question

I've build a web crawler that searches a website for all links on that page and take this links and search on them for more links until the whole page is crawled. Worked perfectly until I came across a special site.

Problem with their linking:

Normal case 1: absolute path like 'http://www.example.com/test'

Normal case 2: relative path like '/test'

Problematic new case: absolute path without the http:// - just 'www.example.com'

Example code that shows the problem:

package main

import (
    "fmt"
    "log"
    "net/url"
)

func main() {

    u, err := url.Parse("http://www.example.com")
    if err != nil {
        log.Fatal(err)
    }
    base, err := url.Parse("http://example.com/directory/")
        if err != nil {
            log.Fatal(err)
        }

    u2, err := url.Parse("www.example.com")
    if err != nil {
        log.Fatal(err)
    }
    base2, err := url.Parse("http://example.com/directory/")
        if err != nil {
            log.Fatal(err)
        }

    fmt.Println(base.ResolveReference(u))
    fmt.Println(base2.ResolveReference(u2))
}

http://www.example.com
http://example.com/test/www.example.com

As you can see the second line gives back a wrong URL because the test for an absolute URL is u.IsAbs() = false if the http:// is missing ...

Any ideas how to fix that? I have to test 100.000 - 1.000.000 links on a daily base, maybe more and it needs to be performant.

Answer 1

Unfortunately there's no real "fix" for this, because if you get a link with an href like this:

www.example.com

In the general case it's ambiguous between:

http://host.tld/path/to/www.example.com
http://www.example.com

In fact, most browsers treat a link like this:

<a href="www.example.com">

As this:

<a href="/current/path/www.example.com">

I'd suggest doing the same (since this is a bug with the person's website), and if you get a 404 just treat it as you would any other.