Bug #15773

Net::HTTP doesn't try next IP address in case of timeout

Added by nicolasnoble (Nicolas Noble) 2 months ago. Updated 2 months ago.

Target version:


This example requires to have a working IPv6 address. Since IPv6 is used in first priority, I am using it to demonstrate the problem, but it exists with plain IPv4, which will be more round-robin-style, so less deterministic to show a reproduction case. I have made two URLs that have both IPv4 and IPv6 addresses: and - both URLs should work in an IPv6-enabled web browser, as well as curl or wget for instance. The difference is that the bad-ipv6 subdomain doesn't have an IPv6 that will actually connect. Therefore, browsers, curl and wget will fallback to using the IPv4 when the initial IPv6 connection attempt failed:

$ wget -T1 -O - # demonstrating using wget because its output is more clear than curl's verbose
--2019-04-16 15:56:52--
Resolving ( 2001:bc8:3690:200::2,
Connecting to (|2001:bc8:3690:200::2|:80... failed: Connection timed out.
Connecting to (||:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 45 [text/html]
Saving to: ‘STDOUT’

-                                      0%[                                                                      ]       0  --.-KB/s               <html><body><h1>It works!</h1></body></html>
-                                    100%[=====================================================================>]      45  --.-KB/s    in 0s

2019-04-16 15:56:53 (6.55 MB/s) - written to stdout [45/45]

However, in Ruby, using Net::HTTP (or OpenURI), that's not going to be the case:

$ ruby bad-ipv6.rb
<html><body><h1>It works!</h1></body></html>
Net::OpenTimeout: execution expired

Contents of my test file:

require 'open-uri'

['ipv6', 'bad-ipv6'].each do |s|
  url = '' %[s]
  puts url
    puts open(url).read
  rescue StandardError => e
    puts "#{e.class}: #{e.message}"

The proper behavior should be to retry the next IP address and exhaust all of the IPs in the DNS resolution results before throwing out an error.


Updated by naruse (Yui NARUSE) 2 months ago

  • Status changed from Open to Rejected

In general resolving DNS is done by libc (getaddrinfo) and Ruby just uses their result.
Therefore it is not a bug and won't be a small feature.

Updated by nicolasnoble (Nicolas Noble) 2 months ago

I thought long and hard about how to reply to this.

Let's put it this way: the fact that you are using glibc isn't the problem at all. The fact that you are badly using its results is.

Any other big piece of code out there that is doing any sort of abstraction is going to be establishing an internet connection will retry on all the entries returned by the DNS, in a round robin fashion. Ruby is the only one that I know of that doesn't do that. In fact, if you run my reproduction case on jruby, it will work correctly. If anything, the bug should be that jruby and ruby are acting differently.

It is important to realize that when one is attempting to write any abstraction layer, there's an expectation of correctness. In this one case, the current chosen model of abstraction means that the http downloader system will catastrophically fail to work in certain conditions without any sort of recourse by the user of the abstraction.

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