Why yum resolve dl.google.com to an IPv6 address?

fedora yum chrome

40,792

Solution 1

By default, yum will resolve both IPv4 & IPv6 addresses of host/domain name, see the wireshark screenshot below (I don't know python program, so network traffic capture is the only way to find out how yum resolve host name).

yum resolve IP address, capture

To resolve IPv4 address only, just add ip_resolve=4 or ip_resolve=ipv4 config to yum.conf (see man yum.conf for more help). For Fedora, execute the following command from terminal using root account:

echo "ip_resolve=4" >> /etc/yum.conf

To resolve IPv6 address only, change ip_resolve=4 above to ip_resolve=6.

Other programs

wget and curl

# To resolve IPv4 address only
wget -4
curl -4

# To resolve IPv6 address only
wget -6
curl -6

host

host also have -4 and -6 options, but they don't have same meaning wget & curl, they force host to use IPv4/IPv6 transport to query host name.

# To resolve IPv4 address only
host -t A

# To resolve IPv6 address only
host -t AAAA

Solution 2

= EDIT =

It turns out yum was correctly trying both ipv4 and ipv6. And the dns resolver in glibc correctly detected that your computer does not have a routable ipv6 address, so it preferred the ipv4 address. However glibc still returned the ipv6 address. It it just put the ipv6 address at the bottom of the preference list.

So yum tried the ipv6 address last. Unfortunately, it seems that yum only showed the last error. So yum only showed the error for ipv6 - which you already expected would fail! - and did not show what the error was for ipv4.

= Previous answer - which was completely wrong about the problem =

I don't know about yum. The error sounds like it's missing an implementation of Happy Eyeballs, i.e. fallback to ipv4. There is an ipv6 address...

$ host dl.google.com
dl.google.com is an alias for dl.l.google.com.
dl.l.google.com has address 173.194.34.132
...many randomly permuted addresses...
dl.l.google.com has address 173.194.34.137
dl.l.google.com has IPv6 address 2a00:1450:400c:c06::5d

Apparently there is a workaround for when this goes wrong, which is to edit /etc/gai.conf and uncomment the line precedence ::ffff:0:0/96 100. This gives precedence to ipv4 addresses.

I think normally it should just work. You'll have a link-local ipv6 address, but a site-local ipv4 address (or a public one without NAT). The libc DNS resolver should then prefer ipv4 destinations. This is specified in RFC 3484.

I wonder if this is a Teredo-style problem. Has your router (e.g. an Apple Airport) assigned you a global IPV6 address through an unreliable tunnel? My computer (no global ipv6) looks like this:

$ ip addr |grep inet6
    inet6 ::1/128 scope host 
    inet6 fe80::215:afff:fe9f:fcd2/64 scope link

Solution 3

You can completely disable IPv6 by adding

net.ipv6.conf.all.disable_ipv6 = 1

to a file in /etc/sysctl.d, for example /etc/sysctl.d/disable-ipv6.conf.

40,792

LiuYan 刘研

// unsigned char *p; // Map<String,Object> map; -- SELECT Name FROM ab -- INNER JOIN cdr ON ab.PhoneNumber=cdr.PhoneNumber /* pre {font-family: 'Ubuntu Mono';} */  // function f() {} # echo -e "your message:\n$(fortune)" | \ # gpg -e --clearsign -o - -r [email protected] REM if ""%1"" == """" goto default ; exten => 911,1,Answer() svn commit -m "to be continue"

Updated on September 18, 2022

Comments

LiuYan 刘研 almost 2 years
I installed Google chrome browser on Fedora 18. Now when I try update softwares using yum upgrade, it report the following error and I can't get chrome updated.
```
http://dl.google.com/linux/chrome/rpm/stable/x86_64/repodata/repomd.xml: [Errno 14] curl#7 - "Failed to connect to 2404:6800:4005:c00::88: Network is unreachable"
```
It looks like dl.google.com is resolved to an IPv6 address, but I don't use IPv6, and ping dl.google.com returns an IPv4 address as expected.
```
# ping dl.google.com
PING dl.l.google.com (74.125.128.93) 56(84) bytes of data.
```
What's the possible reason, and how can I fix this?
- Admin over 11 years
  
  Not sure about other implementations of ping, but at least the ping from iputils only uses IPV4, ping6 uses IPv6.
LiuYan 刘研 over 11 years

weird, yum provides "/etc/gai.conf" said it's in glibc package, I've already installed glibc, but I don't have gai.conf file under /etc. This's a gai.conf file under /usr/share/doc/glibc-common-2.16 which comes from glibc-common package
LiuYan 刘研 over 11 years

my router is TP-Link WRT841N, it only understand IPv4. ip addr shows inet6 entries, but they're already there before (i probably refuse to use IPv6 in my life, i don't need it).
LiuYan 刘研 over 11 years

created /etc/gai.conf manually with the following content: reload yes precedence ::ffff:0:0/96 100, but i still got same error
LiuYan 刘研 over 11 years

well, i added this config, restart computer, and ip addr does not show inet6 entries anymore, but the error still occurs.
LiuYan 刘研 almost 11 years

the actual reason is our "great firewall" blocked the ip addresses which resolved by the DNS server, so yum (or the backend libcurl?) may try every IPv4 address until the last IPv6 address failed to connect. I'll choose this answer which closest to the reason. after an IP address of dl.google.com which is not blocked by GFW to /etc/hosts, 'yum update' works fine now.
user2948306 almost 11 years

@LiuYan刘研, that explanation about Yum's behaviour seems quite useful to me. It would be nice to have it as an answer - I would upvote it :). I expect there are more scenarios where google.com IPs would be inaccessible, beyond the content filtering applied by one particular nation.
LiuYan 刘研 over 10 years

just added my answer with wireshark screenshot.