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tech:notes_client_dns [2025/04/26 11:09] – Jean-Baptistetech:notes_client_dns [2026/06/11 10:16] (Version actuelle) – Jean-Baptiste
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 +<!DOCTYPE markdown>
 +{{tag>Brouillon DNS CA}}
 +
 +# Notes client DNS
 +
 +Voir aussi les clients DNS :
 +* [Unbound](https://doc.ubuntu-fr.org/unbound)
 +
 +Voir aussi :
 +* https://linuxfr.org/news/happydomain-on-devrait-tous-avoir-un-nom-de-domaine#toc-%C3%80-quoi-sert-happydomain
 +
 +
 +### /etc/resolv.conf
 +
 +Voir aussi:
 +* /etc/nsswitch.conf
 +
 +#### Hard limits for the /etc/resolv.conf file
 +
 +NOTE : Le comportement est différent avec **musl libc** (alternative à glibc)
 +
 +You need to be aware of the hard limits for name servers, domain name, and search domains when you create or edit the `/etc/resolv.conf` file.
 +
 +The hard limits for the `/etc/resolv.conf` file are as follows:
 +  *     Maximum number of name servers is 3. (is hardcoded as the 4th will never be used)
 +  *     Maximum line size is 256. (avant glibc 2.26 ?)
 +  *     Maximum domain name length is 256 characters. (avant glibc 2.26 ?)
 +  *     Maximum search domains limit is 6.
 +    * In glibc 2.25 and earlier, the search list is limited to six domains with a total of 256 characters.  Since glibc 2.26, the search list is unlimited (RHEL 6 ; sous RHEL 7 et 8 ça dépend de la version de la glibc)
 +  *     Total number of characters for all search domains is 256. (avant glibc 2.26)
 +  *     Note: You should use only tab or space to separate host names in the search domain list.
 +
 +
 +Si plus de 3 serveurs dans /etc/resolv.conf : "Nameserver limits were exceeded, some nameservers have been omitted..."
 +
 +
 +Note: There is no limit on file size.
 +
 +
 +Le mot-clé **search** du fichier `resolv.conf` du système  peut  être  surchargé  indépendamment
 +pour  chaque  processus  en  remplissant  la variable d'environnement **LOCALDOMAIN** avec une
 +liste de domaines de recherche séparés par des espaces. \\
 +
 +Le mot-clé **options** du fichier `resolv.conf` du système peut  être  surchargé  indépendamment
 +pour  chaque processus en remplissant la variable d'environnement **RES_OPTIONS** en une liste
 +d'options de la bibliothèque resolver (séparées par  des  espaces)...
 +
 +
 +
 +### Redirection de port
 +
 +~~~bash
 +sudo socat -v -v udp4-listen:53,reuseaddr,fork udp4:8.8.8.8:53
 +~~~
 +
 +
 +## Test connexions
 +
 +~~~bash
 +# UDP
 +#host -U -W 1 127.0.0.1 8.8.8.8
 +host -W 1 127.0.0.1 8.8.8.8
 +
 +# TCP
 +host -T -W 1 127.0.0.1 8.8.8.8
 +~~~
 +
 +~~~bash
 +echo "127.0.0.1,192.168.1.1,8.8.8.8" | tr "," "\n" | xargs -L1 -I% bash -c "host -W 1 plop.plop % | grep -q 'not found' && echo OK % || echo NOK %"
 +~~~
 +
 +~~~yaml
 +- name: Check DNS connectivity - UDP
 +  command: "host -W 1 127.0.0.1 {{ item | quote }}"
 +  check_mode: false
 +  changed_when: false
 +  register: cmd_host_udp
 +  failed_when: not ( cmd_host_udp.rc == 0 or ( cmd_host_udp.rc == 1 and cmd_host_udp.stdout_lines is search('not found') ) )
 +  with_items: "{{ resolv_new_nameservers }}"
 +
 +- name: Check DNS connectivity - TCP
 +  command: "host -T -W 1 127.0.0.1 {{ item | quote }}"
 +  check_mode: false
 +  changed_when: false
 +  register: cmd_host_tcp
 +  failed_when: not ( cmd_host_tcp.rc == 0 or ( cmd_host_tcp.rc == 1 and cmd_host_tcp.stdout_lines is search('not found') ) )
 +  with_items: "{{ resolv_new_nameservers }}"
 +
 +~~~
 +
 +
 +Ou encore :
 +~~~bash
 +curl --dns-servers 192.168.0.1,192.168.0.2 https://example.com
 +curl --dns-servers 10.0.0.1:53 https://example.com
 +
 +systemd-resolve HOSTNAME SERVERNAME
 +~~~
 +
 +
 +## Conf Docker
 +
 +### Docker-compose
 +
 +~~~yaml
 +  my-app:
 +     build: my-app
 +     dns:
 +       - 10.20.20.1  # dns server 1
 +       - 10.21.21.2  # dns server 2
 +     dns_search: ibm-edv.ibmnet.int
 +~~~
 +
 +
 +~~~yaml
 +spec:
 +  containers:
 +  - name: test
 +    image: nginx
 +    dnsConfig:
 +      options:
 +      - name: timeout
 +        value: "2"
 +~~~
 +
 +
 +~~~yaml
 +extra_hosts:
 + - "somehost:162.242.195.82"
 + - "otherhost:50.31.209.229"
 +~~~
 +
 +
 +
 +
 +
 +## Cache DNS coté client
 +
 +Voir aussi :
 +* https://geekflare.com/fr/setup-dns-caching-dnsmasq-on-ubuntu/
 +* https://doc.ubuntu-fr.org/unbound
 +* Rescached
 +  * https://wiki.archlinux.org/title/rescached
 +  * https://github.com/shuLhan/rescached-go
 +
 +
 +`/etc/systemd/resolved.conf`
 +~~~ini
 +DNS=1.1.1.1 1.0.0.1
 +~~~
 +
 +~~~bash
 +sudo mv /etc/resolv.conf /etc/resolv.conf.orig
 +
 +sudo ln -s /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf
 +~~~
 +
 +`/etc/resolv.conf`
 +~~~bash
 +nameserver 127.0.0.53
 +options edns0
 +~~~
 +
 +**Voir aussi `/etc/nsswitch.conf`**
 +
 +~~~bash
 +sudo pkill -USR1 systemd-resolve
 +sudo systemd-resolve --flush-caches
 +sudo resolvectl flush-caches
 +
 +sudo journalctl -u systemd-resolved > ~/resolved.txt
 +
 +sudo systemd-resolve --statistics
 +~~~
 +
 +
 +
 +### Flush DNS / Clear cache
 +
 +
 +Vider le cache DNS SystemD
 +~~~bash
 +systemd-resolve --flush-caches
 +~~~
 +
 +Clear BIND Server DNS Cache
 +~~~bash
 +sudo rndc restart
 +~~~
 +
 +Clear DNS cache for a particular domain, use the below command.
 +~~~bash
 +sudo rndc flushname <Domain_Name>
 +~~~
 +
 +Clear the dnsmasq Server DNS Cache
 +~~~bash
 +sudo systemctl restart dnsmasq
 +~~~
 +
 +Flush nscd server DNS Cache
 +~~~bash
 +sudo systemctl restart nscd
 +~~~
 +
 +pdnsd
 +~~~bash
 +pdnsd-ctl empty-cache
 +~~~
 +
 +
 +## Options
 +
 +No ipv6 AAAA
 +~~~
 +# since glibc 2.36
 +options         no-aaaa
 +~~~
 +
 +Voir aussi AAAA vs A6
 +
 +no-tld-query (since glibc 2.14)
 +
 +
 +## Pb
 +
 +### Pb Résolution noms courts
 +
 +`/etc/resolv.conf`
 +~~~bash
 +# options rotate timeout:1 retries:1
 +# options rotate timeout:2 attempts: 1
 +options         timeout:0 attempts:1
 +nameserver      192.168.10.10
 +nameserver      192.168.11.10
 +search          domain1.local domain2.local
 +domain          domain2.local
 +~~~
 +
 +Résolution FQDN OK
 +~~~
 +# getent hosts srv01.domain1.local
 +192.168.1.101  srv01.domain1.local
 +~~~
 +
 +Résolution FQDN NOK
 +~~~
 +# getent hosts srv01
 +#
 +~~~
 +
 +
 +#### Solution
 +
 +Placer `search` après `domain` ou alors commenter `domain`
 +
 +According to the implementation of Glibc, if `domain` and `search` directives are both used, only the last instance will be used for DNS queries. \\
 +Source : https://access.redhat.com/solutions/5958611
 +
 +The `domain` directive is an obsolete name for the `search` directive that handles one search list entry only. \\
 +Source : https://man7.org/linux/man-pages/man5/resolv.conf.5.html
 +
 +`/etc/resolv.conf`
 +~~~bash
 +options rotate timeout:1 retries:1
 +# options rotate timeout:2 attempts: 1
 +nameserver      192.168.10.10
 +nameserver      192.168.11.10
 +domain          domain2.local
 +search          domain1.local domain2.local
 +~~~
 +
 +~~~
 +# getent hosts srv01
 +192.168.1.101  srv01.domain1.local
 +
 +# getent hosts srv01.domain1.local
 +192.168.1.101  srv01.domain1.local
 +~~~
 +
 +
 +### Pb client DNS Linux résolution FQDN KO
 +
 +~~~bash
 +$ getent hosts plop
 +192.168.21.25    plop.acme.local
 +
 +$ getent hosts plop.acme.local
 +$
 +
 +$ dig +short plop.acme.local
 +192.168.21.25
 +~~~
 +
 +
 +#### Solution
 +
 +Vérif avec dig / nslookup et si OK le pb est ailleurs, par exemple dans la conf **libc**
 +
 +`/etc/nsswitch.conf`
 +~~~
 +#hosts:          files mdns4_minimal [NOTFOUND=return] dns myhostname
 +hosts:          files dns myhostname
 +~~~
 +
 +
 +
 +## Outils
 +
 +
 +### Dig
 +
 +the step-by-step name resolution, you can do this:
 +~~~bash
 +dig +add +trace @8.8.8.8 www.google.com
 +~~~
 +
 +### kdig 
 +
 +kdig client (part of Knot): 
 +
 +kdig +tls @localhost -p 8353 foobar.test
 +
 +
 +### check-soa
 +
 +https://framagit.org/bortzmeyer/check-soa
 +
 +### blaeu-resolve
 +
 +~~~bash
 +blaeu-resolve -r 100 - -type NS meteofrance.com
 +~~~
 +
 +
 +--------------
 +
 +## Notes client DNS & diag
 +
 +~~~bash
 +# /etc/systemd/resolved.conf
 +sudo systemctl restart systemd-resolved.service
 +~~~
 +
 +Conf
 +~~~bash
 +systemd-resolve --set-dns=192.168.1.1 -i wlan1
 +~~~
 +
 +Status
 +~~~bash
 +systemd-resolve --status
 +~~~
 +
 +
 +
 +Disable and stop the systemd-resolved service
 +~~~bash
 +sudo systemctl disable systemd-resolved.service
 +sudo systemctl stop systemd-resolved
 +~~~
 +
 +`/etc/NetworkManager/NetworkManager.conf`
 +~~~ini
 +[main]
 +dns=default
 +~~~
 +
 +~~~bash
 +unlink /etc/resolv.conf
 +sudo service network-manager restart
 +~~~
 +
 +DHCP DNS ?
 +
 +`/etc/systemd/network/enp0s31f6.network`
 +~~~ini
 +[DHCP]
 +UseDNS=true
 +~~~
 +
 +Autre 
 +~~~bash
 +ps aux | grep dnsmasq 
 +~~~
 +
 +
 +
 +### Diag port ?
 +
 +`resolv.py`
 +~~~python
 +import socket
 +for x in range(5):
 +   print(socket.getaddrinfo('gnu.org', 80))
 +~~~
 +
 +~~~
 +# strace -e trace=connect python resolv.py 2>&1 | grep 53
 +connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.10.10")}, 16) = 0
 +connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.20.20")}, 16) = 0
 +connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.1.5")}, 16) = 0
 +connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.10.10")}, 16) = 0
 +connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.20.20")}, 16) = 0
 +~~~
 +
 +
 +
 +~~~bash
 +lsof -i udp:53
 +time tcpdump udp port 53 -Qout -n | nl
 +time strace -f -e trace=%network -p 29976 2>&1 | grep 'htons(53)' | nl
 +~~~
 +
 +~~~
 +# sysdig -c lsof "'fd.l4proto=udp and fd.port=53'"
 +COMMAND             PID     TID     USER    FD      TYPE        NAME
 +systemd-resolve     2870470 2870470 systemd 12      ipv4        127.0.0.53:53
 +~~~
 +
 +~~~bash
 +csysdig -v connections fd.ip=10.172.93.224
 +iftop -f "host 10.172.93.224"
 +~~~
 +
 +
 +## Autres
 +
 +windows
 +
 +~~~
 +nslookup
 +set detail
 +~~~
 +
 +
 +## Todo
 +
 +Root serveur.
 +MX
 +Transfer de zone.
 +[[audit]]
 +
 +-------
 +
 +https://github.com/bortzmeyer/check-soa
 +
 +~~~bash
 +check-soa -i dz
 +~~~
 +
  
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