Implementation note: IPAddr#to_s string encoding

This note documents why Otto normalizes IP strings produced by its masking helpers. It is an implementation detail, not a claim about every Ruby 3 release or every version of the ipaddr default gem.

Verified behavior

Otto's blocking compatibility targets are Ruby 3.2, 3.3, and 3.4. The following behavior was reproduced locally with representative installed patch releases:

Ruby	ipaddr	IPv4 IPAddr#to_s	IPv6 IPAddr#to_s
3.2.4	1.2.5	US-ASCII	UTF-8
3.3.5	1.2.7	US-ASCII	UTF-8
3.4.10	1.2.7	US-ASCII	UTF-8

Ruby 4.0.6 with ipaddr 1.2.8 showed the same result, but Ruby 4.0 is a provisional, non-blocking Otto target. See the runtime and dependency security policy for the current support matrix.

You can check the active runtime directly:

require 'ipaddr'

puts RUBY_DESCRIPTION
puts IPAddr::VERSION if defined?(IPAddr::VERSION)
p IPAddr.new('192.168.1.1').to_s.encoding
p IPAddr.new('::1').to_s.encoding

The result comes from the ipaddr implementation's separate IPv4 and IPv6 formatting paths. Treat it as version-specific behavior and rerun the check when changing Ruby or overriding the default ipaddr gem.

Otto's normalization

An IP address string contains only ASCII bytes, so relabeling a generated US-ASCII address as UTF-8 does not change its bytes. Otto uses this at the boundary where IPPrivacy.mask_ip creates IPv4 and IPv6 strings:

IPAddr.new(masked_integer, address_family).to_s.force_encoding(Encoding::UTF_8)

This gives downstream Rack code one encoding for Otto-generated masked addresses. A US-ASCII string is normally compatible with UTF-8 text; the encoding difference alone does not imply an error. Normalization prevents code that requires an explicit UTF-8 label from receiving different labels for IPv4 and IPv6.

Use force_encoding here only because Otto constructed the value from an IP address and therefore knows every byte is ASCII. Do not apply the same operation to arbitrary external bytes without validating or transcoding them first.