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feat(atc): enforce RFC 1123 TLD all-numeric rejection in IsHostName #365

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Merged
davidkallesen merged 1 commit into from
Jun 24, 2026
Merged

feat(atc): enforce RFC 1123 TLD all-numeric rejection in IsHostName #365

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19 changes: 17 additions & 2 deletions src/Atc/Extensions/StringHasIsExtensions.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -598,11 +598,26 @@ public static bool IsUriOpcTcp(this string value)
/// Accepts single-label names (e.g. <c>localhost</c>) and an optional trailing dot (e.g. <c>example.com.</c>).
/// Each label is 1-63 ASCII alphanumeric/hyphen characters and may not start or end with a hyphen;
/// the total length is limited to 253 characters. Underscores and raw Unicode (non-punycode IDN) are not allowed.
/// Per RFC 1123 §2.1 the top-level (right-most) label must not be all-numeric, which also disambiguates
/// host names from IPv4 addresses (e.g. <c>192.168.0.27</c> is rejected).
/// This is a purely syntactic check and does not perform any DNS resolution.
/// </remarks>
public static bool IsHostName(this string value)
=> !string.IsNullOrEmpty(value) &&
RxHostName.Value.IsMatch(value);
{
if (string.IsNullOrEmpty(value) ||
!RxHostName.Value.IsMatch(value))
{
return false;
}

var host = value.EndsWith('.')
? value[..^1]
: value;

var topLevelLabel = host[(host.LastIndexOf('.') + 1)..];

return !topLevelLabel.All(char.IsDigit);
}

/// <summary>
/// Determines whether the specified value is a valid IPv4 address.
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5 changes: 5 additions & 0 deletions test/Atc.Tests/Extensions/StringHasIsExtensionsTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -384,6 +384,7 @@ public void IsUriOpcTcp(
[InlineData(true, "example.com.")]
[InlineData(true, "a.b.c.d.e.f")]
[InlineData(true, "xn--mnchen-3ya.de")]
[InlineData(true, "host123")]
[InlineData(false, "")]
[InlineData(false, " ")]
[InlineData(false, "-leadinghyphen.com")]
Expand All @@ -392,6 +393,10 @@ public void IsUriOpcTcp(
[InlineData(false, "double..dot.com")]
[InlineData(false, "space in.host")]
[InlineData(false, "münchen.de")]
[InlineData(false, "192.168.0.27")]
[InlineData(false, "1.2.3.4")]
[InlineData(false, "123")]
[InlineData(false, "example.123")]
public void IsHostName(
bool expected,
string input)
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