External Data Representation (XDR) is a standard for describing and encoding data so computers with different architectures can exchange it consistently. It defines the data’s wire format—not the message’s purpose, how it is transported, or how it is secured. Protocols such as ONC RPC and NFS use XDR to represent data within their own communications.
What XDR defines—and what it does not
XDR specifies how supported data types are laid out as bytes. An application or protocol gives those values meaning and defines the surrounding message and transport. As RFC 4506 puts it, “XDR is a standard for the description and encoding of data.” It is not itself a network protocol, programming language, application, or security mechanism.
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That distinction matters in practice: two systems can decode an XDR value the same way, but they still need an agreed schema or protocol to know what the value represents and what to do with it. ONC RPC and NFS are examples of protocols that use XDR; the protocols provide the broader communication context.
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How XDR represents data
XDR defines values as byte sequences using a four-byte (32-bit) basic unit. Encoded items occupy multiples of four bytes. The standard uses a fixed big-endian wire convention, rather than a sender’s or receiver’s native in-memory byte order, so implementations may need to convert values when encoding or decoding.
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Variable-length items include a length and are padded with zero bytes when necessary to reach the next four-byte boundary. The padding is part of the encoded representation; it is not additional application data. XDR’s fixed byte order and alignment rules help systems with different processor architectures interpret the same encoded values without negotiating those details for each payload.
Example: a padded string
Consider the four-character string cat!. Its characters occupy four bytes, so no padding is needed. A five-character string such as hello occupies five data bytes and needs three zero padding bytes to reach an eight-byte boundary. For a variable-length string, the encoded form also carries its length. The protocol using XDR determines what that string means.
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Data types in the standard
XDR covers common high-level data shapes, including:
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- Signed and unsigned integers, floating-point values, booleans, and enumerations
- Fixed- and variable-length arrays, strings, and opaque byte sequences
- Structures, discriminated unions, and optional data
A declaration can constrain a value’s length. RFC 4506 advises protocols to define explicit limits where feasible. The standard does not aim to represent every possible machine-level format: it does not define bit fields, bitmaps, or packed or binary-coded decimals.
A worked structure: file metadata and data
RFC 4506 illustrates XDR with a record containing a filename, file kind, owner, and opaque file data. The example shows how a variable-length value is encoded with its length and how zero padding brings the filename and file data to a four-byte boundary. It demonstrates the representation rules, not a complete file-transfer mechanism: a protocol still has to define the record’s meaning and how it is exchanged.
Where XDR came from and its standard status
Sun Microsystems introduced XDR in RFC 1014, published in June 1987, as a way to describe and encode data for exchange among different computer architectures. RFC 1832 followed in 1995. The operative specification is RFC 4506, edited by M. Eisler and published in May 2006 as STD 67; it obsoletes RFC 1832 and states that it makes no technical changes to that earlier specification.
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XDR’s regular layout does not make untrusted input safe. RFC 4506 warns that oversized variable-length values can overflow receiver buffers; embedded NUL bytes can be mishandled by software using NUL-terminated strings; and characters that are valid in the encoding may still be invalid for a particular application. Recursive data structures can also consume excessive stack or other resources.
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- Check declared lengths against application limits and available receiver buffers before allocating or copying data.
- Validate decoded strings for the application’s expected character set and reject or handle embedded NUL bytes deliberately.
- Bound recursive decoding with a depth or list limit, or use a non-recursive decoder where appropriate.
- Use the surrounding protocol’s security protections. XDR itself does not encrypt data, authenticate peers, or provide other security services.
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