curl and wget attempting an ipv6 connection

Question

On my dedicated server (hosted by OVH), that is running a fresh install of Ubuntu 14.04, curl and wget take approximately 10 seconds to complete a simple request.

$ curl -v google.com
* Rebuilt URL to: google.com/
* Hostname was NOT found in DNS cache

and only after 10 seconds it'll actually return something. So i've decided to run strace on this:

write(2, "Hostname was NOT found in DNS ca"..., 36) = 36
socket(PF_INET6, SOCK_DGRAM, IPPROTO_IP) = 3
close(3)                                = 0
mmap(NULL, 8392704, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0) = 0x7f0a24fb8000
mprotect(0x7f0a24fb8000, 4096, PROT_NONE) = 0
clone(child_stack=0x7f0a257b7f70, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f0a257b89d0, tls=0x7f0a257b8700, child_tidptr=0x7f0a257b89d0) = 5047
poll(0, 0, 4)                           = 0 (Timeout)
poll(0, 0, 8)                           = 0 (Timeout)
poll(0, 0, 16)                          = 0 (Timeout)
poll(0, 0, 32)                          = 0 (Timeout)
poll(0, 0, 64)                          = 0 (Timeout)
poll(0, 0, 128)                         = 0 (Timeout)
poll(0, 0, 256)                         = 0 (Timeout)
poll(0, 0, 1000)                        = 0 (Timeout)
poll(0, 0, 1000)                        = 0 (Timeout)

[...] (10 times or so)

poll(0, 0, 1000)                        = 0 (Timeout)
poll(0, 0, 1000)                        = 0 (Timeout)
socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 3
write(2, "*", 1)                        = 1
write(2, " ", 1)                        = 1
write(2, "  Trying 74.125.228.7...\n", 25) = 25

I can clearly see socket(PF_INET6) followed by socket(PF_INET) after all these timeouts. wget behaves in the same way.

More relevant information:

$ sudo sysctl -p
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

$ sudo cat /etc/default/bind9
RESOLVCONF=yes
OPTIONS="-4 -u bind"

$ cat /proc/sys/net/ipv6/conf/all/disable_ipv6
1

ifconfig shows no inet6 addresses.

How can I prevent this from happening?

Don't go out of your way to disable IPv6. This just causes more problems. If you really don't have it, it wouldn't be used anyway. If you _do_ have it, you should be using it. — Michael Hampton, Aug 15 '14 at 21:50
@MichaelHampton unfortunately that's not true as of the version of curl packaged with Ubuntu 14.04. the latest upstream libcurl does implement fallback to IPv4 if IPv6 fails, but earlier versions prefer IPv6 if it exists in DNS and don't fall back if the IPv6 network connectivity isn't actually there. — mc0e, Aug 04 '15 at 15:34

score -2 · Answer 1 · edited Apr 13 '17 at 12:22

-2

Probably you need to indicate IPv4 is preferred over IPv6 as mentioned on this link. https://askubuntu.com/questions/32298/prefer-a-ipv4-dns-lookups-before-aaaaipv6-lookups

Change setting under /etc/gai.conf.

edited Apr 13 '17 at 12:22

Community

1

answered Aug 16 '14 at 00:54

antimatter

229
1
7

Curl has its own logic for such things. – mc0e Aug 04 '15 at 15:32
Everything has default. So without specifically instructing what IP protocol to use, curl has to follow OS's preference as default. Make sense? – antimatter Aug 13 '15 at 15:26
Curl now implements logic from the [Happy Eyeballs](https://en.wikipedia.org/wiki/Happy_Eyeballs) algorithm, to smooth the transition to IPv6, though that didn't make it to release 14.04. It was [adopted](http://curl.haxx.se/bug/?i=168) in curl 7.34.0 ([release history](http://curl.haxx.se/changes.html)), so it wasn't in Ubuntu at the time of the question (it is in 15.04). Prior to that though, curl had its own logic for doing dns requests for ipv4 and ipv6 and deciding which transport to use. Check the source code if you don't believe me. – mc0e Aug 13 '15 at 16:04
Yes, let me correct my comment. Curl will make system call to resolve hostname which will be sent to getaddressinfo. The IP protocol preference of that system call is governed by gai.conf. – antimatter Aug 13 '15 at 16:04
@mc0e, can you explain what is your understanding of curl logic in resolving DNS? Reference? – antimatter Aug 13 '15 at 16:06
Actually, I might have the sequence of events wrong re ubuntu release history, but I do see a whole lot of issues with curl on ubuntu14.04 systems, where wget is fine with the same urls. You can find a reference to curl using gettaddrinfo hints [here](https://github.com/bagder/curl/blob/master/lib/asyn-thread.c#L216), and look through the history from there. I know I've seen some other relevant stuff when trying to figure out what was going on, but I'm not now sure where. There's a lot of what I've seen that I haven't understood well though. Only so much time – mc0e Aug 13 '15 at 16:49

curl and wget attempting an ipv6 connection

1 Answers1