I found the issue. It checks for "ipv6=" token in #if (NGX_HAVE_INET6) block. so if NGX_HAVE_INET6 is not enabled, it treats it as a host instead of ignoring it.
#if (NGX_HAVE_INET6)
if (ngx_strncmp(names[i].data, "ipv6=", 5) == 0) {
if (ngx_strcmp(&names[i].data[5], "on") == 0) {
r->ipv6 = 1;
} else if (ngx_strcmp(&names[i].data[5], "off") == 0) {
r->ipv6 = 0;
} else {
ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
"invalid parameter: %V", &names[i]);
return NULL;
}
continue;
}
#endif
should have been:
if (ngx_strncmp(names[i].data, "ipv6=", 5) == 0) {
#if (NGX_HAVE_INET6)
if (ngx_strcmp(&names[i].data[5], "on") == 0) {
r->ipv6 = 1;
} else if (ngx_strcmp(&names[i].data[5], "off") == 0) {
r->ipv6 = 0;
} else {
ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
"invalid parameter: %V", &names[i]);
return NULL;
}
#endif
continue;
}