本文探讨了Linux系统中修改`net.ipv4.ip_default_ttl`参数对网络连接的影响，当设置为1时导致服务器无法远程访问，但内网连接仍然可行。通过示例展示了如何查看和修改该参数，并使用`ping`和`tcpdump`工具验证其在网络通信中的作用。
[[email protected]_android_client_download ~]# cat /etc/sysctl.conf |grep net.ipv4.ip_default_ttl
net.ipv4.ip_default_ttl = 28
[[email protected]_android_client_download ~]# sysctl -p
[[email protected]_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 -nv 显示详细信息
[[email protected]_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
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