DNS TXT record splitter: split a long value into 255-octet strings
In short: Paste a TXT value, such as a long DKIM key, and get it cut into strings of at most 255 octets, written as a quoted zone file line. Paste an already split record and it is joined into the single value a reader sees. Limit: It splits text; it does not check that the record's content is a valid DKIM, SPF or DMARC record.
One DNS character-string holds at most 255 octets, so a long TXT value such as a 2048-bit DKIM key has to be published as two or more quoted strings in one record. Readers join them with nothing between. Paste the value and this tool cuts it, quotes it and writes the zone file line.
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Only used in the zone file line.
255 is the limit; lower it only if your host asks.
Sample result
A 410-octet DKIM value becomes two strings of 255 and 155 octets in one TXT record.
- Strings
- 2
- Value
- 410 octets
- Longest string
- 255 octets
What this means for you
Readers join the strings with nothing between them, so the value is unchanged.
- 255-octet strings
- Quoted zone file line
- Joins split records back
- Copy and download
- DNS
- TXT
- DKIM
By SimpleVerifier Team · Last updated · Runs in your browser; nothing you paste is sent to us
A published record is only half of a sending setup.
- Publish the record, then check that the domain's mailboxes exist before you send from it.
- Upload or paste it at /verify. Every address gets a live mailbox check, and the first 100 email checks are free with a Google sign-in, no card.
How do I publish a TXT value longer than 255 characters?
- Paste the whole TXT value, quoted or not. A value already split into quoted strings is joined first.
- Set the record name and, if you want it in the line, a TTL. Leave the string size at 255.
- Copy the split strings or the zone file line. If your DNS host has one box for the value, paste the strings exactly as shown, quotes included, unless the host says otherwise.
- Paste the published record back in to join it, and check the length matches.
Example
- You enter
- A DKIM record
v=DKIM1; k=rsa; p=...with a 2048-bit RSA public key, 410 octets long, namedselector1._domainkey. - You get
- The value needs splitting. It becomes 2 strings of 255 and 155 octets, written as one TXT line with both strings quoted, and the joined value is the original 410 octets.
- Limits
- Runs in your browser; nothing is uploaded.
Why is a TXT value limited to 255 characters?
In DNS a TXT record holds one or more character-strings. RFC 1035 defines a character-string as a length octet followed by that many characters, so one string carries at most 255 octets of data. A record with more text than that is not one longer string; it is several strings in the same record.
How do readers put the strings back together?
The standards for SPF and DKIM both say the strings are joined with nothing between them. RFC 7208 says the strings are treated as if concatenated without adding spaces, and RFC 6376 says the same for DKIM key records. So the split points can fall anywhere, even inside a key, as long as no space is added.
Which records are usually long?
A DKIM key record with a 2048-bit RSA key is longer than 255 characters; the DKIM key length checker shows the length of one you have. SPF records that list many services can also pass it. Tell the tool your value and it reports the length in octets, not characters, because the limit is in octets.
Who uses this tool, and for what?
- Cold email senders
- Publish the DKIM key for a new sending domain when the DNS host rejects a long single value.
- Deliverability consultants
- Rewrite a record a client pasted as one long string so it is valid.
- Sales ops
- Check the exact value a record resolves to after it was entered in pieces.
What can’t this tool tell you?
- It splits text; it does not check that the record's content is a valid DKIM, SPF or DMARC record.
- Some DNS hosts split long values themselves, and some have their own entry rules; follow your host's instructions for the field.
- The limit is counted in octets of UTF-8; records are normally plain ASCII, where one character is one octet.
- Nothing is looked up in live DNS.
Common questions
- What is the maximum length of one TXT string?
- 255 octets of data, set by the length octet in RFC 1035. A TXT record can hold several strings, so the record as a whole can be longer.
- Does it matter where I split the value?
- No, as long as nothing is added between the strings. Readers join them directly. The tool splits at exactly 255 octets, or at the size you set.
- Do I need the quotes?
- In a zone file, yes, around each string. Many DNS panels add them or split long values for you, so check how your host wants a long value entered.
- What if the value contains a quote or a backslash?
- The tool puts a backslash in front of each one, as the zone file format requires.
- Can I join strings that are already split?
- Yes. Paste them with their quotes and the joined value appears, with its length.
- Is the value uploaded anywhere?
- No. The splitting happens in your browser.
Sources
A tidy list can still be full of dead addresses.
Extracting, deduplicating and syntax checks all work on the text of an address. None of them can tell whether the mailbox exists. Only the server that hosts it can answer that, through an SMTP check, and that is what the paid product does.
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