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19:25:47.629351 IP 2.2.2.2.80 > 1.1.1.1.31889: Flags [.], seq 2372487272:2372492544, ack 2808407165, win 122, options [nop,nop,TS val 911640439 ecr 235835291], length 5272
19:25:47.631626 IP 1.1.1.1.31889 > 2.2.2.2.80: Flags [.], ack 2372480682, win 3876, options [nop,nop,TS val 235835291 ecr 911640428], length 0
19:25:47.631638 IP 1.1.1.1.31889 > 2.2.2.2.80: Flags [.], ack 2372483318, win 3793, options [nop,nop,TS val 235835291 ecr 911640428], length 0
19:25:47.631677 IP 1.1.1.1.31889 > 2.2.2.2.80: Flags [.], ack 2372483318, win 4096, options [nop,nop,TS val 235835292 ecr 911640428], length 0
19:25:47.674712 IP 1.1.1.1.31889 > 2.2.2.2.80: Flags [.], ack 2372485954, win 4054, options [nop,nop,TS val 235835335 ecr 911640428], length 0
What is the meaning of the ecr in the options list in tcpdump's output?
I google it but failed.
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tcpdump allows you to specify network packets that are either using some port X as source or destination. For example, to capture DNS traffic, you can use port 53 . You could prefix the port keyword with src/dst as src port 53 or dst port 53 and filter it even further.
The "-r" option lets you read the output of a file. All you have to do is use the "-r" option with tcpdump command and specify the path of the file you want to read.
One of the most useful topics covered is verbosity, which allows you to control the level of output from the tcpdump command. This is the final article in the series, so be sure you have read parts one and two.
mss 1460 is the maximum segment size, or maximum IP datagram size that can be handled without using fragmentation. Both sides of the connection must agree on a value; if they are different, the lower value is used. sackOK means "selective acknowledgments," or allow the receiver to acknowledge packets out of sequence.
ECR - Echo reply --- that was sent with the acknowledgement field. Its a timestamp value calculated based upon the TSval sent in the sync packet...
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