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Understanding 0.0.0.0 in /etc/hosts

ldt
ldt Posts: 8
in LFS201 Class Forum

I'm a little confused about the meaning of the address 0.0.0.0 in Lab 35.2. Looking at the list of addresses in hosts.txt I get the impression that adding these to /etc/hosts should block them. But it doesn't. Taking adgrx.com as an example, I can ping it and its IP address is 144.217.74.222 and I can view it in a browser. After appending hosts.txt to /etc/hosts it pings at 127.0.0.1. But it still comes up in the browser, even after I clear the cache and do a hard reload. The ping at 127.0.0.1 I kind of get, but not completely. But I would expect to get a "page not found" in the browser. Can somebody explain this to me? Thanks. I'm using Firefox in a virtual Ubuntu 18.04.

Comments

  • GRO 108
    GRO 108 Posts: 46

    It could be DNS caching. This can be setup as a service on the system or the browser could be using it's own.

    There's some helpful info in this CentOS forum thread https://forums.centos.org/viewtopic.php?t=60089

  • lee42x
    lee42x Posts: 380

    Setting an address to 0.0.0.0 in /etc/hosts will cause the site to be inaccessible as you mentioned.

    In your environment, use the "host " command to check the IP address, if it resolves to 0.0.0.0 there will be no output from the "host" command. This will confirm you system is looking at the /etc/hosts file for name resolution as expected.
    Mine looked like this:

    [email protected]:/tmp$ host adgrx.com
    adgrx.com has address 144.217.74.222
    [email protected]:/tmp$ sudo sh -c "echo 0.0.0.0 adgrx.com >> /etc/hosts "
    [email protected]:/tmp$ host adgrx.com
    [email protected]:/tmp$

    When I tried the text browser with the name resolving to 0.0.0.0 :

    [email protected]:/tmp$ elinks --dump http://adgrx.com
    ELinks: Connection refused

    Removing the entry from /etc/hosts and rerunning the web request and counting the number of lines of text returned:

    [email protected]:/tmp$ sudo awk -i inplace '!/0.0.0.0 adgrx.com/' /etc/hosts
    [email protected]:/tmp$ elinks --dump http://adgrx.com | wc -l
    322

    As you can see many lines of information were returned.

    If your results are different then:
    Are you using one of the Linux Foundation VM's ?

    Lee

  • ldt
    ldt Posts: 8

    GRO 108:
    Thanks for your suggestions.

    [email protected]:~$ cat /etc/resolv.conf
    nameserver 127.0.0.53
    options edns0
    search localdomain

    Not sure what this means or how to go about clearing the DNS cache or even if it is possible. I'm pobably just being dense, but I found the CentOS discussion to be inconclusive. Also, see below, I have the same problem with an address that Firefox never would have had a chance to cache.

    lee42x:
    Playing around with your very helpful comments, I’m getting a mixed bag of results. I’ve tried this with a couple of virtual machines. I’m using VMware Workstation 15 Player. Ubuntu 18.04 is the Linux Foundation pre-built virtual machine. CentOS 8 I installed from its .iso file.

    Ubuntu:
    I double checked everything. Using “host” and “elinks” I get the same result you do before and after the /etc/hosts update. ("sudo apt install elinks" worked out of the box) However, Firefox still insists on loading the actual website page. I’ve cleared the cache and retyped adgrx.com (because it had redirected). Nothing seems to block it. I also tried another address, adperium.com, which never was in the Firefox cache and even though elinks returns “Connection refused”, Firefox gets the real web page from the Internet in both cases.

    [email protected]:~/temp$ host adgrx.com
    adgrx.com has address 144.217.74.222
    [email protected]:~/temp$ elinks -dump adgrx.com
    … lots of stuff …
    [email protected]:~/temp$ host adperium.com
    adperium.com has address 188.166.49.116
    [email protected]:~/temp$ elinks -dump adperium.com
    … lots of stuff …
    [email protected]:~/temp$ sudo sh -c "cat hosts.txt >> /etc/hosts"
    [email protected]:~/temp$ host adgrx.com
    [email protected]:~/temp$ host adperium.com
    [email protected]:~/temp$ elinks --dump adgrx.com
    ELinks: Connection refused
    [email protected]:~/temp$ elinks --dump adperium.com
    ELinks: Connection refused

    But Firefox, after clearing the cache, can reach both of these pages. On this machine I’ve never tried the adperium.com address, so it can’t be a cache problem. ???

    CentOS 8:
    yum can’t find elinks, so I don’t have this utility here. But what is weird here is that “host” returns the IP address 144.217.74.222 both before and after updating “/etc/hosts” with hosts.txt. But Firefox, after clearing the cache, now returns “unable to connect” as expected.

    [[email protected] temp]$ host adgrx.com
    adgrx.com has address 144.217.74.222
    [[email protected] temp]$ host adperium.com
    adperium.com has address 188.166.49.116
    [[email protected] temp]$ sudo sh -c "cat hosts.txt >> /etc/hosts"
    [sudo] password for student:
    [[email protected] temp]$ host adgrx.com
    adgrx.com has address 144.217.74.222
    [[email protected] temp]$ host adperium.com
    adperium.com has address 188.166.49.116
    [[email protected] temp]$ ping adgrx.com
    PING adgrx.com (127.0.0.1) 56(84) bytes of data.
    64 bytes from localhost (127.0.0.1): icmp_seq=1 ttl=64 time=0.061 ms
    [[email protected] temp]$ ping adperium.com
    PING adperium.com (127.0.0.1) 56(84) bytes of data.
    64 bytes from localhost (127.0.0.1): icmp_seq=1 ttl=64 time=0.055 ms

    But Firefox now returns “Connection refused” for both of these pages. ???

    So I’m getting different results from the two OSs but neither one of the is exactly what I expected.

  • luisviveropena
    luisviveropena Posts: 1,043
    edited August 2020

    Hi @ldt ,

    1.- I did a test on Ubuntu 18.04 from the command line, with the "host" command and it worked. So, for Ubuntu 18.04, is it only the browser that's not working as expected?

    2.- For installing elinks on CentOS 8 I did the following:

    [[email protected] ~]# dnf search elinks
Last metadata expiration check: 1 day, 15:14:50 ago on Mon 03 Aug 2020 07:42:21 AM -05.
No matches found.
[[email protected] ~]# dnf repolist 
repo id                            repo name
AppStream                          CentOS-8 - AppStream
BaseOS                             CentOS-8 - Base
extras                             CentOS-8 - Extras

    That's all the repos enabled by default. But "elinks" is in the PowerTools one. So let's use it:

    [[email protected] ~]# dnf --enablerepo=PowerTools install elinks
CentOS-8 - PowerTools                            71 kB/s | 1.9 MB     00:27    
Last metadata expiration check: 0:00:23 ago on Tue 04 Aug 2020 10:59:03 PM -05.
Dependencies resolved.
================================================================================
 Package       Architecture  Version                    Repository         Size
================================================================================
Installing:
 elinks        x86_64        0.12-0.58.pre6.el8         PowerTools        916 k

Transaction Summary
================================================================================
Install  1 Package

Total download size: 916 k
Installed size: 2.9 M
Is this ok [y/N]: y
Downloading Packages:
elinks-0.12-0.58.pre6.el8.x86_64.rpm             45 kB/s | 916 kB     00:20    
--------------------------------------------------------------------------------
Total                                            43 kB/s | 916 kB     00:21     
Running transaction check
Transaction check succeeded.
Running transaction test
Transaction test succeeded.
Running transaction
  Preparing        :                                                        1/1 
  Installing       : elinks-0.12-0.58.pre6.el8.x86_64                       1/1 
  Running scriptlet: elinks-0.12-0.58.pre6.el8.x86_64                       1/1 
  Verifying        : elinks-0.12-0.58.pre6.el8.x86_64                       1/1 
Installed products updated.

Installed:
  elinks-0.12-0.58.pre6.el8.x86_64                                              

Complete!

    You can check on the available repos on CentOS 8 here:

    https://wiki.centos.org/AdditionalResources/Repositories

    Many regards,
    Luis.

  • lee42x
    lee42x Posts: 380

    Please confirm the contents of /etc/resolv.conf (cat /etc/hosts) and the "hosts" entry from /etc/nsswitch.conf ( grep hosts /etc/nsswitch).
    Thanks Lee

  • ldt
    ldt Posts: 8

    luis:
    1)-Yes, on Ubuntu the only problem is with the browser so I'm going to let that one go. I think Firefox does some of its own dns work but I'm not going to spend any time looking into that.

    2)-I got elinks working on CentOS 8. Thanks for your help. Love the tool.

    3)-My remaining problem is that on CentOS 8, host returns the actual URL both with and without 0.0.0.0. That is:

    [[email protected] temp]$ host adgrx.com
adgrx.com has address 144.217.74.222
[[email protected] temp]$ sudo sh -c "cat hosts.txt >> /etc/hosts"
[sudo] password for student: 
[[email protected] temp]$ host adgrx.com
adgrx.com has address 144.217.74.222
[[email protected] temp]$ cat /etc/hosts | grep adgrx.com
0.0.0.0 adgrx.com

    This I still don't understand.

    lee42x:

    [[email protected] temp]$ cat /etc/resolv.conf
# Generated by NetworkManager
search localdomain
nameserver 192.168.211.2

[[email protected] temp]$ grep hosts /etc/nsswitch.conf
#     hosts: files dns
#     hosts: files dns  # from user file
# Valid databases are: aliases, ethers, group, gshadow, hosts,
hosts:      files dns myhostname

    Not sure what I'm looking for here or what this means.

  • lee42x
    lee42x Posts: 380

    I can help there, line 5 contains the name server to use for name
    resolution.
    Line 10 says look in the file, /etc/hosts first then ask the dns name server.

    Lee

  • lee42x
    lee42x Posts: 380

    ldt, Luis,

    I can confirm that CentOS-8 is not behaving as expected for this lab. The "host" program does not appear to be looking in the /etc/hosts file for name resolution, effectively ignoring the /etc/nsswitch order of resolution and using only dns services.

    Many things need to be verified before declaring a bug, so for now please be aware.

    Lee

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