Linux – yum HTTP Error 403 – Forbidden behind NAT on AWS

amazon-web-servicescentoslinuxnat;yum

I had some weird issues while trying to run yum update on a centos 7 server on AWS behind an instance NAT.

Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=genclo error was
14: HTTP Error 403 - Forbidden


One of the configured repositories failed (Unknown),
and yum doesn't have enough cached data to continue. At this point the only
safe thing yum can do is fail. There are a few ways to work "fix" this:

 1. Contact the upstream for the repository and get them to fix the problem.

 2. Reconfigure the baseurl/etc. for the repository, to point to a working
    upstream. This is most often useful if you are using a newer
    distribution release than is supported by the repository (and the
    packages for the previous distribution release still work).

 3. Disable the repository, so yum won't use it by default. Yum will then
    just ignore the repository until you permanently enable it again or use
    --enablerepo for temporary usage:

        yum-config-manager --disable <repoid>

 4. Configure the failing repository to be skipped, if it is unavailable.
    Note that yum will try to contact the repo. when it runs most commands,
    so will have to try and fail each time (and thus. yum will be be much
    slower). If it is a very temporary problem though, this is often a nice
    compromise:

        yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=true

Cannot find a valid baseurl for repo: base/7/x86_64

Now trying with curl from the instance that I use as a NAT instance I get the correct response:

curl -vvv 'http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=genclo 

http://mirror.strencom.net/centos/7.2.1511/os/x86_64/
http://ftp.heanet.ie/pub/centos/7.2.1511/os/x86_64/
http://www.mirrorservice.org/sites/mirror.centos.org/7.2.1511/os/x86_64/
http://mirrors.ukfast.co.uk/sites/ftp.centos.org/7.2.1511/os/x86_64/
http://anorien.csc.warwick.ac.uk/mirrors/centos/7.2.1511/os/x86_64/
http://mirrors.vooservers.com/centos/7.2.1511/os/x86_64/
http://centos.serverspace.co.uk/centos/7.2.1511/os/x86_64/
http://mirror.ox.ac.uk/sites/mirror.centos.org/7.2.1511/os/x86_64/
http://mirrors.melbourne.co.uk/sites/ftp.centos.org/centos/7.2.1511/os/x86_64/
 http://mirror.vorboss.net/centos/7.2.1511/os/x86_64/

The date is correct so I'm very confused now. And I'm starting to consider moving to AWS Linux.

Thanks for you help 🙂

UPDATE: I used an EIP to the same instance and yum is working so I guess its an NAT issue. I still don't understand why.

Best Answer

For internet access to work behind a NAT instance, your default route in your private subnets needs to be the internal ENI of your NAT instance - see docs.

Also, your NAT EC2 instance needs to have the Source/Destination Check disabled - see here. I would also make sure you have followed and understood the Amazon documentation on using a NAT EC2 instance.

For ease of use, I would recommend using an AWS NAT Gateway, which provides the same functionality as an EC2 instance, without the hassle of setting up the OS firewall.

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