The meaning of the TTL value and the Difference from the DNS TTL value of the domain name

The meaning of the TTL value and the difference from the TTL value of the domain name

This article originates from the time drifting bottle http://www.9usb.net, the original address: http://www.9usb.net/201004/ttl-yuyuming-ttl.html

TTL isTime to LiveAbbreviation that specifies the maximum number of network segments that an IP packet is allowed to pass before it is discarded by the router. The TTL is a 8 bit field in the IPV4 header.

what is TTL?
The TTL is a value in the IP protocol package that specifies the number of network segments that the datagram is allowed to pass before it is discarded by the router.
In many cases, packets cannot be delivered to the destination for a certain amount of time. The solution is to discard the package after a period of time, and then give the sender a message that the sender decides whether to resend. The TTL is set by the sending host to prevent packets from continually looping on the IP internetwork forever. When forwarding IP packets, the router is required to reduce the TTL by at least 1. When counted to 0 o'clock, the router decides to discard the packet and sends an ICMP message to the original sender.
The TTL value helps us roughly identify the operating system type of the host.
The TTL field value for Unix and Unix-like operating system ICMP Echo replies is 255
The TTL field value of the Compaq Tru64 5.0 ICMP Echo answer is 64
The TTL field value of the Microsoft Windows nt/2k operating system ICMP echo answer is 128
The TTL field value for the Microsoft Windows 95 operating system ICMP echo answer is 32
Special cases:
LINUX Kernel 2.2.x & 2.4.x The TTL field value of the ICMP Echo reply is 64
FreeBSD 4.1, 4.0, 3.4;
Sun Solaris 2.5.1, 2.6, 2.7, 2.8;
OpenBSD 2.6, 2.7,
NetBSD
HP UX 10.20
The TTL field value of the ICMP echo answer is 255
Windows 95/98/98SE
Windows ME
The TTL field value of the ICMP echo answer is 32
Windows NT4 Wrks
Windows NT4 Server
Windows 2000
Windows XP
The TTL field value of the ICMP echo answer is 128
What is the TTL value for a domain name?
TTL (time-to-live), It simply means that a domain name resolution record is cached on the DNS server for the time. When the DNS servers in each region accept the resolution request, the DNS server specified by the domain name will be issued a resolution request to obtain the resolution record, after obtaining this record, the record will be stored in the DNS server for a period of time, if you receive this domain name resolution Request, the DNS server will no longer make a request to the DNS server, but instead directly return the record that was just obtained, and the time that this record is retained on the DNS server is the TTL value.
Set the TTL value of the domain name reasonably:
One. Increase the TTL value to save domain name resolution time.
In general, domain name resolution records are rarely changed. We can increase the TTL value of the domain record by increasing the time that the record is cached in the DNS server, so that for a longer period of time, when we visit this website, the local ISP's DNS server does not need to make a resolution request to the NS server of the domain name, but returns the domain name resolution record directly from the local cache.
The TTL value is in seconds, and the usual default value is 3600, which is the default cache of 1 hours. We can increase the TTL value according to the actual need, for example, to cache the day set to 86400.
Two. Decrease the TTL value to reduce the inaccessible time when changing space.
DNS records are modified when the domain name space is changed, because the DNS record cache problem, the new domain name record may be in place in some places, but in some places may wait two days or even longer to take effect, only then some users will not be able to access the site for a period of time.
In order to minimize the resolution time difference around the world, it is reasonable to:
1. First check the current TTL value of the domain name.
2. Modify the TTL value to a settable minimum value, 60 seconds is recommended.
3. Wait a day to ensure that DNS server caches are expired and records are updated everywhere.
4. Set the change DNS resolution to the new record, at which point the DNS can update to the new record as quickly as possible.
5. After confirming that DNS has been updated, the TTL value is set to a common value (for example: ttl=86400).

The meaning of the TTL value and the Difference from the DNS TTL value of the domain name