In an Ubuntu 20 or 22 LTS, if I attempt to use a hostname like anyname.localhost it always seems to resolve to IPv6 ::1 address.
In an old RHEL 6, if I attempt to use a hostname like anyname.localhost it always seems to resolve to IPv4 127.0.0.1 address.
In a MS-Windows or MacOS 12.6 machine, this does not seem to happen. (It only happens if I manually edit /etc/hosts and manually add an host alias to 127.0.0.1
127.0.0.1 localhost localhost.localdomain somename.localhost
)
Is this any.sequence.of.names.localhost resolution to ::1 (localhost) always garanteed to happen ? By what reason ? In what Linux distros ?
I have a few projects where it seems useful to have many localhost aliases without having to edit /etc/hosts, but I searched about TCP and DNS quirks, and found nothing about this behaviour. (Not very sure what I should search for). Not sure if I can depend on this behaviour.
Well, thanks to the link posted in the comments above by Patrick Mevzek, I could reach a documented explanation.
For the Ubuntu 20/22 LTS, it seems to use systemd-resolved for DNS resolution.
And, as it says on
https://www.freedesktop.org/software/systemd/man/systemd-resolved.service.html
Synthetic Records
...
The hostnames "localhost" and "localhost.localdomain" as well as any hostname ending in ".localhost" or ".localhost.localdomain" are resolved to the IP addresses 127.0.0.1 and ::1.
So, this seems to explain that Linux OSes that have DNS resolution based on systemd will have this behaviour, and others (like Windows and MacOS) do not.
(So, this seemed a systemd question after all :-)
(This question is in fact a duplicate of https://serverfault.com/questions/1065513/how-does-linux-resolve-wildcard-locahost-subdomains-e-g-ping-test-localhost/1065514#1065514 - just using systemd-resolved instead of nss-myhostname , and if anyone knows how to link them both ways, could help others searching by other words).
Related
I'm not a networking guru so could use some help. I am running a RHEL7 (Red Hat Enterprise Linux) VM (Virtual Machine) where some component on the OS is sending frequent DNS queries to resolve it's own local hostname to our main DNS server (which shouldn't be happening because the DNS server won't know anything about its address). Can anyone provide guidance as to how I can find out what component/service/process this is? It's filling our logs with 19k records over just hours and I need to find a way to fix this.
The hostname for the RHEL VM is spe1.2v29999999.dev.local , there is a static IP on this VM and it is 10.70.49.61. The /etc/hosts looks like:
127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
::1 localhost localhost.localdomain localhost4 localhost4.localdomain4
I suspected it might be a java jar we have running on the VM, but I stopped it via systemctl stop MyJavaJar but after running a tcp dump via tcpdump -i any udp port 53, I could still see the queries happening.
Here are some examples from different days/times in the logs (both A and TXT records):
2020-05-05T13:53:50.189178+00:00 dns.green.blue.mycompany.com 127.0.0.1 <daemon.info> dnsmasq[20886]: 739 10.70.49.61/65078 query[A] spe1.2v29999999.dev.local from 10.70.49.61
2020-05-07T00:01:39.934899+00:00 dns.green.blue.mycompany.com 127.0.0.1 <daemon.info> dnsmasq[8615]: 27827 10.70.49.61/57348 reply spe1.2v29999999.dev.local is NXDOMAIN
2020-05-11T00:01:20.674688+00:00 dns.green.blue.mycompany.com 127.0.0.1 <daemon.info> dnsmasq[8615]: 130345 10.70.49.61/53321 query[TXT] bootstrap.spe1.2v29999999.dev.local from 10.70.49.61
Would making any changes to /etc/hostname, /etc/sysconfig, /var/named .zone files, /var/named.conf or /etc/named help? Can I do more with tcpdump? Thanks
Put:
127.0.0.1 spe1.2v29999999.dev.local
in /etc/hosts. This is done by default on many distros.
Running opensuse tumbleweed with 4.7.5-1-default kernel and gnome.
Network managed with gnome's network-manager. Network (wired and wireless) connect to my router.
ping works also to external IPs
dig works if I provide the dns ip (eg. 8.8.8.8)
nmcli dev show | grep DNS shows the right nameservers (which I set in
the network managers interface): 8.8.8.8 and 8.8.4.4,
but dig without specification of the dns IP does not work
ping etc. to internet names (e.g. google.com) does not work.
Am completely lost by now. Help greatly appreciated.
(Matt stated the solution in his comment but I failed to understand it.)
Solution found on opensuse forum: the /etc/resolve.conf file was corrupted (empty). After deleting it and rebooting, the system (network-manager?) generated a new, working version. It contains the ip of the nameserver.
I am using a LAMP stack. Will there be problems if I update the /etc/host file to reflect 127.0.0.1 as somename from localhost?
Thanks!
Edit:
I sometimes work in remote sites with no network. I have the same setup on different machines and I need the server name to know dynamically where to do changes, etc.
I edited /etc/hosts to show
127.0.0.1 localhost somename
Now, with my Wifi off I am trying http://somename and it is not connecting. If I turn my wifi on, it works. But I need it to work with no connection. How can I resolve this?
No. Just add somename after localhost separated by space in /etc/hosts. You can add as many aliases as you like, as long as you don't delete localhost.
I was using nodejs + nano + couchdb for my application successfully up until today. For some reason all of a sudden I'm getting ECONNREFUSED when I try to run my application. If I try to query the database using the web browser or using a different application (java application) it works fine. I'm uncertain why just in this scenario it stopped working. I've been researching for the past 2 days and can't find any help. I believe this might have something to do with too many open connections, but that's a little bit out of my realm of knowledge. Can anyone provide me with any insight on debugging this issue or any direction I could go in? I should mention this couchdb lives on iriscouch
Add more information about stack that you're using. But basically it's server machine doesn't want to allow connecting. Also try run your app with DEBUG=*, nano will log via console.log almost everything.
E.g. change in package.json start command to node changetoyourapp.js DEBUG=*
I faced yesterday same issue with nodejitsu/iriscouch. Issue disappeared after some restarts.
Check the version of your node vs the expected node version of nano. It is possible that nano does not work with node > 16.
This is down to Node v18 now preferring an IPv6 address over and IPv4 address if two exist for the same hostname.
i.e. if your /etc/hosts contains entries like this:
127.0.0.1 localhost
::1 localhost
Node v16 will say that "localhost" resolves to 127.0.0.1 where Node v18 will say "localhost" resolves to ::1, the IPv6 equivalent. As CouchDB doesn't listen on an IPv6 port by default, then a connection to ::1 will be refused.
Solutions:
Use 127.0.0.1 instead of localhost in your URLs.
Use a domain name that resolves unambiguously to an IPv4 address e.g. 127.0.0.1 my.pretend.host in your /etc/hosts file.
Revert to Nodev16 which preferred IPv4 addresses in its dns lookup.
Make CouchDB bind to an IPv6 address by changing bind_address = ::1 in couchdb.ini. You can then do curl 'http://USER:PASS#[::1]:5984/.
See
https://github.com/apache/couchdb-nano/issues/313#issuecomment-1321760360
Linux terminal showing hostname dhcppc4 that i never configured. As per configuration files hostname must be localhost :
[root#dhcppc4 ~]# cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=localhost.localdomain
[root#dhcppc4 ~]# cat /etc/hosts
127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
Note : I attached a new monitor to my laptop.
Don't know how but sometimes terminal shows localhost as hostname.
what is dhcppc4? How to resolve the issue?
This question is not suitable for stackoverflow (this should be on serverfault i guess)
Anyway, DHCP server could even assign an hostname to your machine
Look here: RFC 1553 - DHCP
Maybe this is your case?
You are most assuredly allowed to edit the HOSTNAME line of that config' file you refer to in your question just fine.
It will not show up until you logout & back in again. You will also want to run /etc/init.d/rsyslogd restart (or whatever syslog daemon you are using) to make that name take effect in the log files.
You may or may not need to run a vi /etc/hosts to adjust the lines in there as you need for your specific applications. Most will be fine, but some are very very picky, like the otherwise very awesome software, Zimbra, for one example, wont work if those lines are not perfect. Asterisk could be another one that might require that file to be perfect match with your configuration.
You can or can not work with your DHCP server to set this, but most people dont, most edit the HOSTNAME and move on.
It is actually important, though, and do in fact resolve this, dont just leave it, as we sysadmins do not want to be typing mission critical commands at the BASH prompt on the wrong machine when all 6 of your servers all say "localhost".