Mastering resolvectl Domains: Eliminating Timeouts and Understanding DNS Resolution #

In the intricate world of network configuration, understanding how your system resolves domain names is paramount. Modern Linux systems, particularly those utilizing systemd’s resolved service, offer sophisticated tools for managing DNS resolution. Among these, resolvectl stands out as the primary interface for interacting with systemd-resolved. However, some users encounter frustrating issues where configuring domain suffixes via resolvectl leads to DNS query timeouts and unexpected NXDOMAIN responses for seemingly valid domains. This comprehensive guide from revWhiteShadow aims to demystify the resolvectl domain command, explain the underlying mechanisms, and provide actionable strategies to overcome these resolution challenges, ensuring seamless internet connectivity. We will delve deep into the nuances of domain suffix configuration, exploring potential pitfalls and offering precise solutions to achieve robust DNS resolution across your network interfaces.

Understanding the Role of resolvectl domain #

The resolvectl domain command is a powerful tool within the systemd-resolved ecosystem. Its primary purpose is to associate specific DNS search domains with particular network interfaces. When you configure a domain using resolvectl domain <interface> <domain>, you are essentially instructing systemd-resolved to append this domain to any unqualified hostnames queried on the specified <interface>. For instance, if you set sudo resolvectl domain eth0 internal.corp, and then attempt to resolve server1, systemd-resolved will automatically try to resolve server1.internal.corp using the DNS servers configured for the eth0 interface. This feature is analogous to the search directive in traditional /etc/resolv.conf files but offers a more granular, interface-specific approach, allowing for differentiated DNS resolution policies across various network connections.

The intention behind this functionality is to simplify the resolution of internal hostnames within private networks or corporate environments. Instead of having to type the fully qualified domain name (FQDN) for every internal resource, administrators can configure short, easily memorable names, relying on the system to automatically append the necessary domain suffixes. This mirrors the behavior expected from a properly functioning DNS client, where queries for unqualified names are intelligently transformed into FQDNs based on configured search paths. The expectation is that if a DNS server knows about a specific subdomain, such as subdomain.example.com, and the example.com domain is configured as a search domain, then a query for subdomain should successfully resolve to subdomain.example.com.

Diagnosing resolvectl domain Timeout Issues #

When resolvectl domain configurations lead to timeouts and NXDOMAIN errors for valid domain queries, it points to a breakdown in the DNS resolution process. This can stem from several interconnected factors, often related to how systemd-resolved interprets and applies the configured search domains.

The Mechanics of Domain Suffix Appending #

When a query for an unqualified hostname, such as myserver, is issued on an interface with internal.corp as a search domain, systemd-resolved generates a sequence of FQDNs to attempt resolution. The primary FQDN will be myserver.internal.corp. If this fails, and other search domains are configured or if the initial query for myserver itself (without any appended domain) fails, the system may enter a state where it continues to probe for variations that do not exist or are misconfigured.

The critical point of failure often lies in the interaction between the appended domain and the DNS servers associated with that interface. If the DNS servers configured for the interface do not possess the necessary records or are not properly configured to handle queries for the suffixed names, resolution will fail. This could manifest as timeouts if the DNS server is unresponsive to those specific query types, or NXDOMAIN (Non-Existent Domain) if the server explicitly states that the domain does not exist.

Common Causes of Timeouts and NXDOMAIN Errors #

Several specific issues can precipitate these unwanted resolution behaviors:

• Incorrect Domain Suffix Configuration: A typographical error in the domain name entered with resolvectl domain is a surprisingly common culprit. Even a single misplaced character can render the entire suffix invalid, leading the DNS resolver on a wild goose chase. For example, example.com versus example.co.m would fundamentally break resolution.
• DNS Server Misconfiguration: The DNS servers associated with the network interface might be incorrectly configured or unable to resolve the target domain or its subdomains. This could be due to outdated DNS records, incorrect zone file entries, or firewall rules blocking DNS queries. If the DNS server responsible for example.com doesn’t know about subdomain.example.com, a query for subdomain (which resolvectl might transform into subdomain.example.com) will result in NXDOMAIN.
• Network Interface Specificity: resolvectl allows domain suffixes to be scoped to specific interfaces. If the domain is set for an interface that is not the active or preferred route for reaching the target DNS server, or if the DNS server for that interface isn’t authoritative for the domain, resolution will fail. This is particularly relevant in multi-homed systems.
• Conflicting DNS Settings: In complex network setups, there might be conflicting DNS configurations. This could arise from interactions between systemd-resolved, DHCP-assigned DNS servers, static DNS configurations, or even other DNS management tools. These conflicts can disrupt the expected behavior of domain suffix appending.
• Firewall Restrictions: Network firewalls, both on the client and server sides, can block DNS query traffic (typically UDP or TCP port 53). If the DNS server for the configured domain is behind a firewall that is inadvertently blocking queries originating from your system’s IP address or directed towards the specific suffixed names, timeouts will occur.
• DNSSEC Issues: While less common for simple internal domains, DNS Security Extensions (DNSSEC) can introduce validation failures. If the configured DNS servers or the client’s DNS resolver have issues with DNSSEC validation for the domain in question, it can lead to resolution failures.
• TTL (Time To Live) Values: Extremely low or unusually high TTL values on DNS records can sometimes contribute to transient resolution problems, although they are less likely to cause consistent timeouts unless there are underlying connectivity or server issues.
• Systemd-resolved Caching Behavior: Although designed to enhance performance, systemd-resolved’s caching mechanism could, in rare circumstances, serve stale or incorrect information, leading to unexpected resolution behavior.

Strategies for Effective resolvectl domain Configuration #

To successfully implement domain suffixes with resolvectl and avoid the frustrating timeouts, a methodical approach to configuration and troubleshooting is essential.

Verifying Interface and Domain Associations #

The first step is to ensure that the domain suffix is correctly associated with the intended network interface.

1. Identify the Correct Network Interface #

Use ip a or nmcli device status to identify the active network interface(s) you are using for DNS resolution. Common interface names include eth0, enp2s0, wlan0, etc.

2. Check Current resolvectl Configurations #

To view the current DNS settings managed by systemd-resolved, including domains associated with interfaces, use:

resolvectl status

This command provides a wealth of information, including the DNS servers for each interface and any configured search domains. Look for the section corresponding to your network interface to verify existing domain settings.

3. Correctly Set the Domain Suffix #

To set a domain suffix for a specific interface, use the following command. Replace <interface> with your actual interface name and <your.domain.com> with the domain you wish to append.

sudo resolvectl domain <interface> <your.domain.com>

For example, to set internal.corp for enp2s0:

sudo resolvectl domain enp2s0 internal.corp

If you need to set multiple domains, you can repeat the command for each domain or, in some systemd versions, specify them comma-separated. However, using the command iteratively is generally safer and more explicit.

4. Applying Changes #

After making changes with resolvectl, it is often beneficial to restart the systemd-resolved service or at least flush its cache to ensure the new settings are immediately effective.

sudo systemctl restart systemd-resolved

Alternatively, you can try flushing the cache:

sudo resolvectl flush-caches

Troubleshooting DNS Resolution Issues #

When timeouts persist, a systematic diagnostic approach is crucial.

1. Test Resolution Directly with dig or nslookup #

Bypass systemd-resolved’s client-side appending temporarily to isolate the issue.

• Test with the FQDN: Use dig to query the fully qualified domain name directly.

  dig @<dns_server_ip> <fully.qualified.subdomain.example.com>
  

  Replace <dns_server_ip> with the IP address of a DNS server authoritative for the domain (or one of the servers listed for your interface in resolvectl status). This verifies if the DNS server itself has the record.

• Test with resolvectl’s Query Tool: resolvectl offers a way to query through its interface.

  resolvectl query --interface=<interface> <unqualified.hostname>
  

  For example:

  resolvectl query --interface=enp2s0 myserver
  

  Observe the output for any errors or timeouts.

2. Analyze DNS Server Responsiveness #

If dig to the specific DNS server works for the FQDN but resolvectl query for the unqualified name fails, the issue likely lies in the domain suffix appending or the DNS server’s handling of those queries.

• Check DNS Server Configuration: Access the DNS server (or consult your network administrator) and verify that the zone files for example.com correctly include records for subdomain.example.com or the specific hostname you are trying to resolve.

• Examine DNS Server Logs: DNS server logs can provide detailed information about why a query was rejected or timed out. Look for entries related to the queries being made.

3. Examine Network Connectivity and Firewalls #

Ensure that your system can reach the configured DNS servers without interruption.

• Ping the DNS Server:

  ping <dns_server_ip>
  

• Trace Route to the DNS Server:

  traceroute <dns_server_ip>
  

• Check Firewall Rules: Inspect local firewall rules (e.g., ufw, iptables) and any network firewalls to ensure that UDP and TCP traffic on port 53 to your DNS servers is permitted.

4. Investigate Conflicting DNS Settings #

If your system obtains DNS settings via DHCP, there might be conflicts if you are also trying to set static domains.

• DHCP-Provided DNS: Check your DHCP client configuration or network manager settings to see if DNS servers are being provided automatically.

• Static DNS Configuration: If you have manually configured DNS servers in files like /etc/resolv.conf (though systemd-resolved often manages this), ensure they don’t conflict with systemd-resolved’s settings. Typically, systemd-resolved is designed to work with or override /etc/resolv.conf by providing a stub resolver at 127.0.0.53. However, improper configurations can still cause issues.

5. Rebuilding resolvectl Configuration (Advanced) #

In rare cases, the systemd-resolved state might become corrupted. While not usually necessary, you could consider resetting its configuration. Use this with extreme caution as it affects all network name resolution.

• Temporarily Disable systemd-resolved:

  sudo systemctl stop systemd-resolved
  sudo systemctl disable systemd-resolved
  

• Manually Configure /etc/resolv.conf (for testing): Create a basic /etc/resolv.conf pointing to a known good DNS server.

  echo "nameserver 8.8.8.8" | sudo tee /etc/resolv.conf
  

• Test Resolution:

  dig google.com
  

• Re-enable systemd-resolved:

  sudo systemctl enable systemd-resolved
  sudo systemctl start systemd-resolved
  

  Ensure your network manager or other services that rely on systemd-resolved are also restarted.

Advanced Considerations for Domain Suffixes #

The way systemd-resolved handles multiple search domains and specific domain types can be nuanced.

• Search Domain Precedence: If multiple search domains are configured, systemd-resolved will try them in the order they are listed. It’s crucial to ensure the most relevant domains are listed first, especially if there’s a possibility of name collisions.

• Search Domain vs. Domain for IP Address: resolvectl domain is specifically for search domains. There’s also resolvectl domain <interface> "" which effectively removes search domains for that interface, and resolvectl domain <interface> <domain> <domain> to set multiple. It’s important not to confuse this with setting specific DNS servers for particular domains using /etc/dnsmasq.d or similar configurations if you are using a local DNS forwarder.

• Understanding systemd-resolved’s Stub Resolver: systemd-resolved typically runs a stub resolver on 127.0.0.53. All DNS queries from applications are directed to this stub. The stub then forwards the query to the appropriate backend resolver managed by systemd-resolved, taking into account interface-specific configurations. Understanding this hop is key to debugging.

• The search Directive Analogy: While the functionality is similar to /etc/resolv.conf’s search directive, systemd-resolved’s implementation is more dynamic and aware of network interface states. This means it can react to interface changes (e.g., disconnecting and reconnecting a network cable) and reapply the correct DNS configurations, including search domains. The key difference is the namespacing by network interface, which is precisely what allows for granular control but also introduces potential complexity if not managed correctly.

Optimizing for Performance and Reliability #

Once the resolution issues are resolved, consider these points for ongoing reliability.

• Use Specific DNS Servers: If possible, configure systemd-resolved to use specific, reliable DNS servers that are authoritative for your domains. This can be done through DHCP settings or static configuration for the network interface.

• Monitor DNS Server Health: Ensure the DNS servers you rely on are healthy and responsive. Tools like pingdom or internal monitoring systems can help track DNS server availability.

• Keep System Updated: Regularly update your system and systemd packages to benefit from bug fixes and performance improvements in systemd-resolved.

Conclusion #

Successfully configuring domain suffixes with resolvectl is essential for seamless internal name resolution on modern Linux systems. When faced with timeouts and NXDOMAIN errors, a methodical approach involving verification of interface and domain associations, direct DNS queries, analysis of DNS server configurations, and network diagnostics is key. By understanding the nuances of how systemd-resolved appends search domains and by meticulously troubleshooting potential misconfigurations in domain suffixes, DNS server settings, or network connectivity, you can eliminate these resolution issues. The power of resolvectl domain lies in its ability to provide interface-specific DNS policies, but this granularity demands precision in its application. By following the strategies outlined by revWhiteShadow, users can master resolvectl configurations, ensuring reliable and efficient DNS resolution across their networks, ultimately outranking common resolution problems and providing a robust foundation for network operations. This detailed understanding empowers you to manage your network’s DNS resolution with confidence, resolving even the most elusive of issues.