[root@test_android_client_download ~]# cat /etc/sysctl.conf |grep net.ipv4.ip_default_ttl
net.ipv4.ip_default_ttl = 28
[root@test_android_client_download ~]# sysctl -p

#动态修改，直接生效，这个文件保存在内存中的

[root@test_android_client_download ipv4]# ll ip_default_ttl
-rw-r--r-- 1 root root 0 Feb  9 11:15 ip_default_ttl
[root@test_android_client_download ipv4]# pwd
/proc/sys/net/ipv4
[root@test_android_client_download ipv4]#

[root@test_android_client_download ipv4]# echo 64 >/proc/sys/net/ipv4/ip_default_ttl

[root@test_android_client_download ipv4]# ping 127.0.0.1
PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.050 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.051 ms

[root@test_android_client_download ~]# ping 127.0.0.1 -c 1

PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data. 64 bytes from 127.0.0.1: icmp_seq=1 ttl=28 time=0.044 ms

--- 127.0.0.1 ping statistics --- 1 packets transmitted, 1 received, 0% packet loss, time 0ms rtt min/avg/max/mdev = 0.044/0.044/0.044/0.000 ms

#抓包  -i 指定网卡 -t 指定协议 -n不解析IP为域名 v 显示详细信息

[root@test_android_client_download ~]# tcpdump -i eth1 -n -v -t icmp
tcpdump: listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
IP (tos 0x0, ttl 28, id 0, offset 0, flags [DF], proto ICMP (1), length 84)
    112.124.111.141 > 180.149.132.47: ICMP echo request, id 62590, seq 1, length 64
IP (tos 0x14, ttl 53, id 0, offset 0, flags [DF], proto ICMP (1), length 84)
    180.149.132.47 > 112.124.111.141: ICMP echo reply, id 62590, seq 1, length 64
IP (tos 0x0, ttl 28, id 0, offset 0, flags [DF], proto ICMP (1), length 84)
    112.124.111.141 > 180.149.132.47: ICMP echo request, id 62590, seq 2, length 64
IP (tos 0x14, ttl 53, id 0, offset 0, flags [DF], proto ICMP (1), length 84)
    180.149.132.47 > 112.124.111.141: ICMP echo reply, id 62590, seq 2, length 64

如果把net.ipv4.ip_default_ttl = 1 服务器会上不去网，不能远程连接服务器，可以用内网连接，修改参数，恢复上网