Technical Analysis of Underscores in Domain Names and Hostnames: RFC Standards and Practical Applications

In internet infrastructure, the Domain Name System (DNS) serves as a critical component, with its naming conventions directly impacting the interoperability and compatibility of network services. A common technical question arises: Are underscores "_" allowed in subdomains? This article provides an in-depth technical analysis based on relevant RFC standards, clarifying the important distinctions between domain names and hostnames.

Domain Name Syntax in DNS Standards

According to RFC 2181, Section 11 "Name syntax," the DNS protocol itself imposes minimal restrictions on labels used to identify resource records. The standard explicitly states: "The DNS itself places only one restriction on the particular labels that can be used to identify resource records. That one restriction relates to the length of the label and the full name. [...] Implementations of the DNS protocols must not place any restrictions on the labels that can be used. In particular, DNS servers must not refuse to serve a zone because it contains labels that might not be acceptable to some DNS client programs." This means that, at the protocol level, domain name labels can include various characters, including underscores, as long as the length does not exceed 63 octets.

Further referencing RFC 1034, Section 3.5 "Preferred name syntax," this document describes traditional naming conventions but does not exclude underscores as valid characters. In practice, domain names with underscores are widely used, such as _jabber._tcp.gmail.com and _sip._udp.apnic.net, often seen in scenarios like service discovery (SRV records).

Restrictions and Distinctions for Hostnames

It is essential to clarify that domain names and hostnames have different definitions and restrictions in standards. Hostnames, as a special type of domain name identifying internet hosts, are governed by RFC 952 and RFC 1123. RFC 1123, Section 2.1 "Host Names and Numbers," specifies that hostnames must adhere to the letter-digit-hyphen (LDH) rule, allowing only ASCII letters, digits, and the hyphen "-", with the hyphen prohibited at the beginning or end. Therefore, using underscores in hostnames is non-compliant with standards.

This distinction is also emphasized in RFC 2181: "...[the fact that] any binary label can have an MX record does not imply that any binary name can be used as the host part of an e-mail address..." This further illustrates the different requirements for domain names (e.g., used in DNS records) versus hostnames (e.g., used in URLs or email addresses) at the application level.

Impact of Internationalized Domain Names (IDNA)

With the globalization of the internet, Internationalized Domain Name (IDNA) standards have emerged, allowing non-ASCII characters in domain names. Relevant RFCs such as RFC 5890 to RFC 5895 define the IDNA framework and protocols. RFC 5890 introduces the concept of LDH labels for hostnames, aligning with the "preferred name syntax" defined in RFC 1034 and RFC 1123, emphasizing that only letters, digits, and hyphens are permitted.

Notably, even in the context of internationalization, underscores remain disallowed in hostnames. Early proposals like "RACE" encoding attempted to handle internationalized characters but explicitly excluded underscores in hostnames. This highlights the standard's considerations for backward compatibility and security.

Practical Considerations

In real-world network environments, although standards prohibit underscores in hostnames, such configurations may occasionally be encountered due to legacy systems or misconfigurations. Following the "Robustness Principle"—"Be conservative in what you send, liberal in what you accept"—systems should attempt to handle various inputs when parsing, but when generating domain names or hostnames, strict adherence to standards is advised to avoid potential issues.

For developers and network administrators, it is recommended to distinguish usage in the following scenarios:

• In DNS records (e.g., SRV, TXT records), underscores can be safely used, such as in _service._proto.example.com.
• In hostnames (used for URLs, email addresses, or server identification), underscores should be avoided to comply with RFC 1123 standards.
• When implementing internationalized domain names, refer to IDNA-related RFCs to ensure encoding and processing align with the latest specifications.

In summary, underscores are permissible in domain names but should be avoided in hostnames. Understanding this distinction aids in building more compatible and stable network applications.