Ubuntu传输TTL 0多播数据包

TTL为0的IP数据包不得离开主机。

但是,当我启动使用TTL 0多播UDP数据包的应用程序时,我看到TTL 0的数据包离开主机几秒钟,并达到TTL 0的正常行为。这很可能在重新启动和首次启动应用程序后发生。

我确认了TTL 0的数据包通过tcpdump离开主机:

05:31:39.048304 IP (tos 0x0, id 14487, offset 0, flags [DF], proto UDP (17), length 1344)
    192.168.1.200.46968 > 239.0.0.1.1234: UDP, length 1316
05:31:39.049594 IP (tos 0x0, id 14488, offset 0, flags [DF], proto UDP (17), length 1344)
    192.168.1.200.46968 > 239.0.0.1.1234: UDP, length 1316
05:31:39.051601 IP (tos 0x0, id 14489, offset 0, flags [DF], proto UDP (17), length 1344)
    192.168.1.200.46968 > 239.0.0.1.1234: UDP, length 1316
05:31:39.053584 IP (tos 0x0, id 14490, offset 0, flags [DF], proto UDP (17), length 1344)
    192.168.1.200.46968 > 239.0.0.1.1234: UDP, length 1316

As we can see ttl is not displayed which means TTL 0, as confirmed from tcpdump man page: https://www.tcpdump.org/manpages/tcpdump.1.html (search ttl, it clearly indicated: ttl is the time-to-live; it is not reported if it is zero).

There are no any iptables rules running.

uname -a: Linux mydevice 4.15.0-101-generic #102-Ubuntu SMP Mon May 11 10:07:26 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

lsb_release -a:

No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.4 LTS
Release:        18.04
Codename:       bionic

造成这种现象的原因是什么,我该如何解决?