n0-computer/iroh · error · MaxLengthExceededError

MaxLengthExceededError

MaxLengthExceededError

Error message

max length exceeded

What it means

MaxLengthExceededError from iroh-dns's UserData. UserData::try_from(String) validates that the string does not exceed UserData::MAX_LENGTH before wrapping it; longer strings are rejected. This bound exists because user data is embedded in DNS discovery records with strict size limits.

Solutions

1. Shorten the user data string to at most UserData::MAX_LENGTH bytes before constructing.
2. Compress or serialize the payload more compactly (e.g. minimal JSON, base64 of packed data).
3. Truncate or split the data and store the remainder out-of-band (e.g. in your own service, referenced by a short ID).
4. Check the length in application code before calling try_from to fail gracefully.

Example fix

// before
let user_data = UserData::try_from(long_status_string)?; // panics on error path / MaxLengthExceededError
// after
let s = &long_status_string[..UserData::MAX_LENGTH.min(long_status_string.len())];
let user_data = UserData::try_from(s.to_string())?;

fn fits_user_data(s: &str) -> Result<(), &'static str> {
    if s.len() > UserData::MAX_LENGTH { Err("user data too long for DNS record") } else { Ok(()) }
}

match UserData::try_from(value) {
    Ok(ud) => ud,
    Err(_) => UserData::try_from(truncate(&value, UserData::MAX_LENGTH))?,
}

When it happens

Trigger: Calling UserData::try_from(some_string) where some_string.len() > UserData::MAX_LENGTH.

Common situations: Publishing endpoint info to iroh DNS discovery with overly long user-data payloads (large JSON blobs, long descriptions); user input passed straight through into discovery metadata.

Understand the failure class

Background: "File too large" / "file size exceeds limit" errors: why libraries cap file sizes and how to fix them — this error's family across 46 libraries.

Related errors

• InvalidLength
• EmptyPacket
• InvalidBucketConfig
• InvalidAlpn

AI-assisted analysis of n0-computer/iroh@2b4de030ce (2026-09-08). Data as JSON: /api/errors/ef896470a67054ac. Report an issue: GitHub.

Appendix: source

Thrown at iroh-dns/src/endpoint_info.rs:327

    /// The max byte length allowed for user-defined data.
    ///
    /// In DNS discovery services, the user-defined data is stored in a TXT record character string,
    /// which has a max length of 255 bytes. We need to subtract the `user-data=` prefix,
    /// which leaves 245 bytes for the actual user-defined data.
    pub const MAX_LENGTH: usize = 245;
}

/// Error returned when an input value is too long for [`UserData`].
#[allow(missing_docs)]
#[stack_error(derive, add_meta)]
#[error("max length exceeded")]
pub struct MaxLengthExceededError {}

impl TryFrom<String> for UserData {
    type Error = MaxLengthExceededError;

    fn try_from(value: String) -> Result<Self, Self::Error> {
        ensure!(value.len() <= Self::MAX_LENGTH, MaxLengthExceededError);
        Ok(Self(value))
    }
}

impl FromStr for UserData {
    type Err = MaxLengthExceededError;

    fn from_str(s: &str) -> std::result::Result<Self, Self::Err> {
        ensure!(s.len() <= Self::MAX_LENGTH, MaxLengthExceededError);
        Ok(Self(s.to_string()))
    }
}

impl fmt::Display for UserData {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.0)
    }
}

View on GitHub (pinned to 2b4de030ce)