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The short answer: one DNS TXT character-string can contain at most 255 data octets, but a TXT record can contain several character-strings. The practical amount you can retrieve reliably is usually smaller because DNS transports the entire response—including the name, headers, question, record metadata, other answers, and sometimes DNSSEC data—inside a packet. Classic DNS over UDP allows 512 octets; EDNS(0) can negotiate larger UDP payloads, commonly starting at 4096 octets, but fragmentation, firewalls and resolver behavior make large TXT responses less portable.
What the 255-byte limit actually means
DNS TXT data is defined as one or more character-strings. Each character-string starts with a one-octet length field followed by up to 255 data octets. The length octet is transport overhead, so the maximum payload in one string is 255 octets (256 octets when that length octet is included).
That is a limit on an individual string, not automatically on the complete TXT resource record or on all TXT text published at a name. A record can legally contain multiple strings, each no longer than 255 data octets. This is why the answer to “Can a DNS TXT record be bigger than 255 characters?” is yes—but only by splitting the value into separate strings.
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“Octet” is the standards term for an eight-bit byte. The limit is therefore measured in bytes, not visual characters. A value encoded with multibyte characters can consume several octets per displayed character, and binary-looking data still counts one octet per byte. Treat 255 as a byte budget unless the protocol that owns the TXT record specifies a different encoding.
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How multiple strings are represented
A zone-file entry can show several quoted strings on one TXT record, for example:
example.com. IN TXT ("first part" "second part" "third part")Each quoted item is a separate character-string. DNS returns the strings in order; the application decides how to interpret their boundaries. Protocols commonly concatenate them into one logical value, while other software may expose an array of strings. Never assume that inserting a new string inserts a space or another delimiter. Follow the specification for the protocol using the TXT record.
Why the whole DNS response is the real constraint
A resolver does not fetch TXT text in isolation. It receives a DNS message containing a header, the question, the owner name, record type and class, TTL, RDATA length fields, and the TXT RDATA itself. The response may also contain other records or DNSSEC signatures. Every byte consumes the response budget.
Consequently, the maximum reliable TXT payload depends on the queried name and the rest of the answer. A short owner name leaves more room than a long one. An answer containing several TXT records uses more space than an answer containing one. Name compression can reduce repeated names, but it does not eliminate the other fields or the TXT string length octets.
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Classic DNS over UDP: 512 octets
RFC 1035 sets the classic maximum DNS message size over UDP at 512 octets. That figure covers the complete message, not 512 octets of TXT text. After subtracting the header, question and resource-record fields, the available TXT data is lower and varies by name and answer composition.
For SPF, RFC 7208 gives a practical rule: the published SPF record should remain small enough for the DNS response to fit within 512 octets. It also says that when the DNS message plus the queried name and the text of all records of that type is below 450 octets, the response ought to fit in a 512-octet UDP answer. The 450-octet value is a guideline, not a promise that every name and resolver will use exactly that amount.
What a 600-byte value looks like in practice
Suppose an application needs to publish 600 data octets. It cannot place them in one string. It could split them into three strings of 255, 255 and 90 data octets. The TXT RDATA then also contains three one-octet length fields, and the complete DNS response adds the header, question and record metadata. The application therefore does not receive a 600-byte value merely because the strings add up to 600; the packet must still fit the transport path.
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EDNS(0), specified by RFC 6891, lets a resolver advertise a larger UDP payload size to a server. A 4096-octet maximum is described as a possible starting value. It is a negotiated transport setting, not a guaranteed storage allowance for every TXT record, resolver or network.
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The negotiated number can be lower, and the path between the authoritative server and resolver can impose a smaller safe size. RFC 6891 discusses falling back around 1280–1410 octets when larger packets are not reliable. Packets near or above common network maximums can fragment; firewalls and other middleboxes may drop fragments or reject unfamiliar EDNS behavior. A resolver may then retry over TCP, truncate the UDP response, or return an error depending on what happened.
What happens when UDP is too small
- The authoritative server prepares a response based on the resolver’s advertised UDP size.
- If the answer cannot fit, the server may set the truncation (TC) flag and return an incomplete UDP response.
- A standards-compliant resolver can retry the query over TCP and obtain the larger answer.
- The client application ultimately sees whatever the resolver can retrieve and validate; it does not control whether the path supports large fragmented UDP packets.
TCP fallback makes larger answers possible, but it adds a connection and creates another compatibility dependency. Some lightweight DNS clients, embedded stacks and poorly implemented integrations do not retry correctly. A design that only works after TCP fallback is less interoperable than one that fits in a compact UDP response.
Per-string, record and response limits compared
| What is being measured | Value or rule | What it means for TXT data |
|---|---|---|
| One TXT character-string | 255 data octets | Hard maximum for one string; the string’s one-octet length field is additional. |
| Classic DNS UDP message | 512 octets | Includes header, question, names, record metadata and all answer data, so TXT payload is smaller. |
| SPF operational target | Keep the response within 512 octets; below 450 octets is an RFC 7208 fitting guideline | A conservative deployment rule for SPF, not a universal limit for every TXT use. |
| EDNS(0) UDP payload | 4096 octets may be offered initially; lower fallback sizes are discussed | A negotiated packet size, not guaranteed TXT storage or delivery. |
| TCP response | No 512-octet UDP ceiling | Can carry a truncated answer after fallback, but adds interoperability and latency risk. |
How applications should handle multiple TXT strings
DNS preserves the sequence of character-strings, but TXT itself does not define one universal concatenation rule for every application. A protocol specification may require the strings to be joined, may treat each as a separate field, or may impose its own escaping and encoding rules.
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SPF
SPF is a common example of a value that can exceed one 255-octet string. RFC 7208 explicitly notes that multiple strings are useful when an SPF record would exceed the single-string maximum. SPF implementations interpret the published strings as one logical SPF record, subject to SPF’s own syntax and lookup limits. Splitting at an arbitrary point is safe only when the resulting sequence still forms valid SPF syntax.
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Verification tokens and policy records
For a verification token, certificate policy, or other vendor-defined value, use the exact string-handling rules documented by that protocol. Some validators concatenate strings without separators; others expect one complete token in one string. If a provider gives a value longer than 255 bytes, copy its requested split exactly rather than adding spaces, line breaks or punctuation.
Binary or encoded content
Base64, hashes and other encoded values still count their encoded bytes. Encoding often makes the published value longer than the underlying binary object. DNS TXT is therefore a poor transport for large blobs: splitting is syntactically possible, but packet size, caching, truncation and parser behavior become increasingly fragile.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.DNSSEC and other records reduce the available room
When DNSSEC records are included in a response, signatures and related data consume additional bytes. A response containing multiple TXT records, aliases, authority information or other sections also has less room for TXT text. Whether those records appear in the particular answer depends on the query and resolver behavior, so there is no single universal “maximum TXT bytes per name.”
For a conservative design, measure the complete response produced by the authoritative service, not just the length of the TXT value in a zone editor. Test the name through the resolver types and network paths your users actually employ, including DNSSEC-validating resolvers and IPv4 and IPv6 paths where applicable.
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A practical sizing method
- Define the logical value. Count bytes after the protocol’s required encoding and escaping, not the number of characters shown in a user interface.
- Split at protocol-safe boundaries. Keep every character-string at or below 255 data octets. Do not insert separators unless the consuming protocol requires them.
- Estimate response overhead. Include the owner name, question, answer fields, length octets, other TXT records and any records your server adds to the response.
- Target the classic path when possible. For SPF, follow RFC 7208’s 512-octet rule and its approximately 450-octet guideline. For other TXT uses, choose a similarly conservative packet size based on your clients.
- Query the deployed name. Inspect the actual answer, including its size and whether the TC flag appears. Test from more than one recursive resolver if the record is business-critical.
- Plan a fallback or redesign. If the response requires large EDNS packets or TCP, consider moving the data to an HTTPS endpoint, a purpose-built DNS record type, or a smaller indirection token.
Diagnosing oversized TXT answers
The value is rejected when saved
The zone editor may enforce its own per-string or total-record limit. Those implementation limits vary by DNS service and are separate from the protocol limits. Split the value into standards-compliant strings and consult that service’s documentation rather than assuming a 255-byte cap applies to the entire record.
Some resolvers return only part of the value
Check whether the UDP response is truncated and whether the resolver retries over TCP. A client that reads only the first UDP response can appear to lose strings even though the authoritative server published them. Reduce the response size, enable a standards-compliant TCP fallback, or change the application to retrieve a compact token.
It works on one network but not another
That pattern commonly indicates a path-MTU, fragmentation or middlebox problem. Lower the advertised EDNS payload where you control the resolver, avoid relying on fragmented packets, and test across networks with different firewalls and access links. The fact that a 4096-octet EDNS value works in one location does not make it universally safe.
The application sees unexpected spaces or breaks
Inspect how the client library exposes TXT data. Some libraries return an array of strings; others concatenate them. Confirm the protocol’s required join behavior and ensure that your zone-file quoting did not add literal spaces or line breaks.
Design guidance: when TXT is the wrong container
TXT is appropriate for short, human-readable policies, ownership proofs and configuration hints. It becomes a poor choice when the value is large, changes frequently, must be fetched by many constrained clients, or needs reliable delivery across unknown networks.
Quick Recap
- Prefer one compact token that points to data retrieved over HTTPS when the payload is large.
- Publish only the fields the protocol requires; remove redundant whitespace and unused parameters.
- Use multiple strings to cross the 255-octet per-string boundary, not as a way to ignore whole-response limits.
- Keep SPF especially compact because its DNS response and lookup behavior have explicit operational constraints.
- Retest after adding DNSSEC, additional TXT records or long owner names, since the available packet space can change.
The numbers to remember
- 255 data octets: maximum payload in one TXT character-string under RFC 1035.
- 512 octets: maximum classic DNS-over-UDP message, including all DNS overhead.
- About 450 octets: RFC 7208’s guideline for the queried name and text of records of one type to fit comfortably in a 512-octet response.
- 4096 octets: an EDNS(0) UDP payload size that may be negotiated initially under RFC 6891—not a universal TXT-record capacity.
- TCP fallback: the mechanism for retrieving a response that cannot fit in the selected UDP payload, with added compatibility and latency considerations.
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