Is there any point to full host names in /etc/hosts ?

This is the situation: a friend and I both use a broadband router with a few
machines behind them, at our respective homes. We each picked a name for our
'domain'. His is fictional. Mine also exists on the net, but with only the
www and ftp of rikishi42.net defined in the DNS, not my home machines.

He's added 2 of my PC's in the /etc/hosts of his laptop, for use when he
visits and connects it to my LAN.

This would be a sample from that file:

# Home net
192.168.108.101 desktop.myDomain desktop
192.168.108.124 laptop.myDomain laptop
192.168.108.101 server.myDomain server
#Rikishi's net
192.168.8.142 gramps.rikishi42.net gramps
192.168.8.181 pigmy.rikishi42.net pigmy


We've allways used this, works just fine.
When connectiong to my machines, he's allways used the short (host only)
name. Last time, he accidentally tried with the full name. It failed.


My guess is that, upon finding that rikishi42.net exists, there is an
attempt to get the address from the DNS, skipping hosts all together.

But do I presume correctly, or is there more to it?

--
There is an art, it says, or rather, a knack to flying.
The knack lies in learning how to throw yourself at the ground and miss.
Douglas Adams

On Fri, 2 Nov 2007 12:44:11 +0100, Rikishi 42 <skunkworks@rikishi42.net> wrote:
| This is the situation: a friend and I both use a broadband router with a few
| machines behind them, at our respective homes. We each picked a name for our
| 'domain'. His is fictional. Mine also exists on the net, but with only the
| www and ftp of rikishi42.net defined in the DNS, not my home machines.
|
| He's added 2 of my PC's in the /etc/hosts of his laptop, for use when he
| visits and connects it to my LAN.
|
| This would be a sample from that file:
|
| # Home net
| 192.168.108.101 desktop.myDomain desktop
| 192.168.108.124 laptop.myDomain laptop
| 192.168.108.101 server.myDomain server
| #Rikishi's net
| 192.168.8.142 gramps.rikishi42.net gramps
| 192.168.8.181 pigmy.rikishi42.net pigmy
|
|
| We've allways used this, works just fine.
| When connectiong to my machines, he's allways used the short (host only)
| name. Last time, he accidentally tried with the full name. It failed.
|
|
| My guess is that, upon finding that rikishi42.net exists, there is an
| attempt to get the address from the DNS, skipping hosts all together.
|
| But do I presume correctly, or is there more to it?


/etc/nsswitch.conf determines whether DNS or /etc/hosts is checked
first, and whether the other is checked at all.

Sounds like your friend might have DNS first. The second choice is
generally only checked if the first fails (i.e DNS is unreachable). If
DNS returns a "host does not exist" error, that does not trigger a fall
back to /etc/hosts.


--
Reverend Paul Colquhoun, ULC. http://andor.dropbear.id.au/~paulcol
Asking for technical help in newsgroups? Read this first:
http://catb.org/~esr/faqs/smart-questions.html#intro

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,--- Rikishi 42 writes:
| This is the situation: a friend and I both use a broadband router with a few
| machines behind them, at our respective homes. We each picked a name for our
| 'domain'. His is fictional. Mine also exists on the net, but with only the
| www and ftp of rikishi42.net defined in the DNS, not my home machines.

| He's added 2 of my PC's in the /etc/hosts of his laptop, for use when he
| visits and connects it to my LAN.

| This would be a sample from that file:

| # Home net
| 192.168.108.101 desktop.myDomain desktop
| 192.168.108.124 laptop.myDomain laptop
| 192.168.108.101 server.myDomain server
| #Rikishi's net
| 192.168.8.142 gramps.rikishi42.net gramps
| 192.168.8.181 pigmy.rikishi42.net pigmy


| We've allways used this, works just fine.
| When connectiong to my machines, he's allways used the short (host only)
| name. Last time, he accidentally tried with the full name. It failed.

How is he trying connect to your box ? And are you sure, it failed at
name resolution step, not something like connection etc. ? To resolve
a hostname yourself, you can try "getent hosts <hostname>" .

| My guess is that, upon finding that rikishi42.net exists, there is an
| attempt to get the address from the DNS, skipping hosts all together.

| But do I presume correctly, or is there more to it?

It all depends on "/etc/nsswitch.conf"'s "hosts" entry. Usually
"hosts" entry has "files" first and then "dns". But anyways, even if
order is reverse, then also it should work, if it is not able to
resolve from DNS.

Anyways I added your records in my /etc/hosts and it is able to
resolve 'gramps.rikishi42.net' without any problems.

| --
| There is an art, it says, or rather, a knack to flying.
| The knack lies in learning how to throw yourself at the ground and miss.
| Douglas Adams

Anyways, you can instead of adding each other's boxes in /etc/hosts,
there is one more way, which you can try. i.e. Avahi
( http://avahi.org/ ) .

HTH
- --
Ashish Shukla आशीष शुक्ल http://wahjava.wordpress.com/
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On 2007-11-02, Paul Colquhoun <postmaster@andor.dropbear.id.au> wrote:

> /etc/nsswitch.conf determines whether DNS or /etc/hosts is checked
> first, and whether the other is checked at all.
Didn't know that one, thanks. I'll check it out.

> Sounds like your friend might have DNS first. The second choice is
> generally only checked if the first fails (i.e DNS is unreachable). If
> DNS returns a "host does not exist" error, that does not trigger a fall
> back to /etc/hosts.
Seems exactly what I feared. I'll check his config, next time he visits.


--
There is an art, it says, or rather, a knack to flying.
The knack lies in learning how to throw yourself at the ground and miss.
Douglas Adams

On 2007-11-02, Ashish Shukla ???? ????? <wahjava@gmail.com> wrote:

>| We've allways used this, works just fine.
>| When connectiong to my machines, he's allways used the short (host only)
>| name. Last time, he accidentally tried with the full name. It failed.
>
> How is he trying connect to your box ? And are you sure, it failed at
> name resolution step, not something like connection etc. ? To resolve
> a hostname yourself, you can try "getent hosts <hostname>" .
Connection is OK, since the short names worked.
Thanks for getent, I leared something.


>| My guess is that, upon finding that rikishi42.net exists, there is an
>| attempt to get the address from the DNS, skipping hosts all together.
>
>| But do I presume correctly, or is there more to it?
>
> It all depends on "/etc/nsswitch.conf"'s "hosts" entry. Usually
> "hosts" entry has "files" first and then "dns". But anyways, even if
> order is reverse, then also it should work, if it is not able to
> resolve from DNS.
>
> Anyways I added your records in my /etc/hosts and it is able to
> resolve 'gramps.rikishi42.net' without any problems.
That would confirm that his nsswitch is not set correctly.
Thanks for the info.

--
There is an art, it says, or rather, a knack to flying.
The knack lies in learning how to throw yourself at the ground and miss.
Douglas Adams

On Fri, 2 Nov 2007, in the Usenet newsgroup comp.os.linux.networking, in article
<b5frv4-0qn.ln1@whisper.very.softly>, Rikishi 42 wrote:

>He's added 2 of my PC's in the /etc/hosts of his laptop, for use when he
>visits and connects it to my LAN.
>
>This would be a sample from that file:
>
># Home net
>192.168.108.101 desktop.myDomain desktop
>192.168.108.124 laptop.myDomain laptop
>192.168.108.101 server.myDomain server

Minor quibble - a given hostname OR IP address should appear on one
line only.

>#Rikishi's net
>192.168.8.142 gramps.rikishi42.net gramps
>192.168.8.181 pigmy.rikishi42.net pigmy
>
>We've allways used this, works just fine.
>When connectiong to my machines, he's allways used the short (host only)
>name. Last time, he accidentally tried with the full name. It failed.

Might be interesting to know what is in /etc/resolv.conf - domain or
search lines?

>My guess is that, upon finding that rikishi42.net exists, there is an
>attempt to get the address from the DNS, skipping hosts all together.

No - how would it know that a domainname exists in DNS without asking
the DNS?

>But do I presume correctly, or is there more to it?

There are two files that control the order in which the hosts file or
DNS is consulted. Older applications used /etc/host.conf, and this
has a simple line that reads

order hosts,bind

which translates to 'check the hosts file, and if the hostname is not
found, check DNS'. The second (more modern file) is /etc/nsswitch.conf
which has it's own man page

[compton ~]$ whatis nsswitch.conf
nsswitch.conf [nsswitch] (5) - System Databases and Name Service Switch
configuration file
[compton ~]$

and a common configuration might be

hosts: files nisplus nis dns

which translates to 'check the hosts file, and if the hostname is not
found, check NIS+ and if the hostname is not found check NIS, and if the
hostname is not found check DNS". This line might be shortened by
eliminating the NIS terms if you don't have NIS or NIS+ servers on your
LAN (most don't).

The other possible problem is the '/etc/resolv.conf' file, and the
brain-dead use of 'domain' and/or 'search' lines. These lines allow
use of short names in DNS - which may or may not be a massive security
hole. There are a number of man pages you can look at - start with
'gethostbyname', 'resolver (5)' and 'hostname'.

Perhaps the best way to troubleshoot DNS problems is to simply use a
packet sniffer and LOOK at the questions (and answers) you are tossing
at the name server.

/usr/sbin/tcpdump -ni eth0 port 53

might give the clues needed. The DNS-HOWTO offers some assistance if
you don't know what this dialog should look like.

Old guy

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,--- Paul Colquhoun writes:

[...]

| Sounds like your friend might have DNS first. The second choice is
| generally only checked if the first fails (i.e DNS is unreachable). If
| DNS returns a "host does not exist" error, that does not trigger a fall
| back to /etc/hosts.

If 'dns' returns 'host doesn't exist' (i.e. NOTFOUND, for details checkout
nsswitch.conf(5)), then the default action will be to continue, unless
your friend's 'hosts' entry in '/etc/nsswitch.conf' is similar to following
entry:

hosts: dns [NOTFOUND=return] files

According to above entry, if 'dns' returns NOTFOUND, name resolution
should end there only, didn't proceed to 'files' .

HTH
- --
Ashish Shukla आशीष शुक्ल http://wahjava.wordpress.com/
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Rikishi 42 <skunkworks@rikishi42.net> writes:

>This is the situation: a friend and I both use a broadband router with a few
>machines behind them, at our respective homes. We each picked a name for our
>'domain'. His is fictional. Mine also exists on the net, but with only the
>www and ftp of rikishi42.net defined in the DNS, not my home machines.

>He's added 2 of my PC's in the /etc/hosts of his laptop, for use when he
>visits and connects it to my LAN.

>This would be a sample from that file:

># Home net
>192.168.108.101 desktop.myDomain desktop
>192.168.108.124 laptop.myDomain laptop
>192.168.108.101 server.myDomain server
>#Rikishi's net
>192.168.8.142 gramps.rikishi42.net gramps
>192.168.8.181 pigmy.rikishi42.net pigmy


>We've allways used this, works just fine.
>When connectiong to my machines, he's allways used the short (host only)
>name. Last time, he accidentally tried with the full name. It failed.


>My guess is that, upon finding that rikishi42.net exists, there is an
>attempt to get the address from the DNS, skipping hosts all together.

It depends on whether he told his system to look for the address in hosts
or in dns first.
/etc/hosts.conf and /etc/nsswitch.conf


>But do I presume correctly, or is there more to it?

>--
>There is an art, it says, or rather, a knack to flying.
>The knack lies in learning how to throw yourself at the ground and miss.
>Douglas Adams

> This is the situation: a friend and I both use a broadband router with a few
> machines behind them, at our respective homes. We each picked a name for our
> 'domain'. His is fictional. Mine also exists on the net, but with only the
> www and ftp of rikishi42.net defined in the DNS, not my home machines.

> He's added 2 of my PC's in the /etc/hosts of his laptop, for use when he
> visits and connects it to my LAN.

This will not answer your question but you may want to try and use DHCP
with `dnsmasq' on your home network. This way you can setup your local
dns proxy to serve names for your local network. E.g. I have my router
use a fictional ".home" domain and all the machines that connect to it
get a name there and can lookup each other without having to twiddle any
/etc/hosts file.

Better yet: the name they get is determined by the machine's name
(passed to the DHCP server), so I don't even have a centralized database
that maps names to IP either: it's all setup dynamically (although you
do have to ask GNU/Linux's dhcp client to pass this name explicitly
because it doesn't do it by default contrary to Mac OS X's).


Stefan

Moe Trin wrote:
> On Fri, 2 Nov 2007, in the Usenet newsgroup comp.os.linux.networking, in article
> <b5frv4-0qn.ln1@whisper.very.softly>, Rikishi 42 wrote:
>
>> He's added 2 of my PC's in the /etc/hosts of his laptop, for use when he
>> visits and connects it to my LAN.
>>
>> This would be a sample from that file:
>>
>> # Home net
>> 192.168.108.101 desktop.myDomain desktop
>> 192.168.108.124 laptop.myDomain laptop
>> 192.168.108.101 server.myDomain server
>
> Minor quibble - a given hostname OR IP address should appear on one
> line only.

That's is correct the format is

[internet address] [domain name] [alias]

>
>> #Rikishi's net
>> 192.168.8.142 gramps.rikishi42.net gramps
>> 192.168.8.181 pigmy.rikishi42.net pigmy
>>
>> We've allways used this, works just fine.
>> When connectiong to my machines, he's allways used the short (host only)
>> name. Last time, he accidentally tried with the full name. It failed.
>
> Might be interesting to know what is in /etc/resolv.conf - domain or
> search lines?
>
>> My guess is that, upon finding that rikishi42.net exists, there is an
>> attempt to get the address from the DNS, skipping hosts all together.
>
> No - how would it know that a domainname exists in DNS without asking
> the DNS?
>
>> But do I presume correctly, or is there more to it?
>
> There are two files that control the order in which the hosts file or
> DNS is consulted. Older applications used /etc/host.conf, and this
> has a simple line that reads
>
> order hosts,bind
>
> which translates to 'check the hosts file, and if the hostname is not
> found, check DNS'. The second (more modern file) is /etc/nsswitch.conf
> which has it's own man page
>
> [compton ~]$ whatis nsswitch.conf
> nsswitch.conf [nsswitch] (5) - System Databases and Name Service Switch
> configuration file
> [compton ~]$
>
> and a common configuration might be
>
> hosts: files nisplus nis dns
>
> which translates to 'check the hosts file, and if the hostname is not
> found, check NIS+ and if the hostname is not found check NIS, and if the
> hostname is not found check DNS". This line might be shortened by
> eliminating the NIS terms if you don't have NIS or NIS+ servers on your
> LAN (most don't).
>
> The other possible problem is the '/etc/resolv.conf' file, and the
> brain-dead use of 'domain' and/or 'search' lines. These lines allow
> use of short names in DNS - which may or may not be a massive security
> hole. There are a number of man pages you can look at - start with
> 'gethostbyname', 'resolver (5)' and 'hostname'.
>
> Perhaps the best way to troubleshoot DNS problems is to simply use a
> packet sniffer and LOOK at the questions (and answers) you are tossing
> at the name server.
>
> /usr/sbin/tcpdump -ni eth0 port 53
>
> might give the clues needed. The DNS-HOWTO offers some assistance if
> you don't know what this dialog should look like.
>
> Old guy
>