Happy Eyeballs
Testing: Late DNS Answers Test

This test verifies whether the browser dynamically incorporates new DNS answers into its connection candidate list while connection attempts are already in progress.

Our DNS server immediately returns the AAAA record, pointing to an IPv6 address that carries an artificial network delay (configured below). The A record is withheld for the duration shown in each column. The browser starts a TCP connection to the slow IPv6 address; the A record for the fast IPv4 address arrives later.

A browser that dynamically adds the late IPv4 address to its candidate list will establish an IPv4 connection before the delayed IPv6 handshake completes — shown as IPv4 in the result. A browser that ignores the late answer and commits to the initial IPv6 attempt will show IPv6 (but with a high total fetch duration reflecting the netem delay). The column at which the result flips from IPv4 to IPv6 reveals the browser's effective late-answer tolerance window relative to the configured IPv6 netem delay.

Note: at A delay = 0 ms both records arrive simultaneously, so that column reflects standard Happy Eyeballs fallback behavior rather than late-answer incorporation.