Amazon Virtual Private Cloud by Amazon Web Services

Author:Amazon Web Services
Language: eng
Format: mobi
Publisher: Amazon Web Services
Published: 2020-07-15T00:00:00+00:00

TCP flag sequence

The following is an example of a custom flow log that captures the following fields in the following order.

version vpc-id subnet-id instance-id interface-id account-id type srcaddr dstaddr srcport dstport pkt-srcaddr pkt-dstaddr protocol bytes packets start end action tcp-flags log-status

The tcp-flags field can help you identify the direction of the traffic, for example, which server initiated the connection. In the following records (starting at 7:47:55 PM and ending at 7:48:53 PM), two connections were started by a client to a server running on port 5001. Two SYN flags (2) were received by server from the client from different source ports on the client (43416 and 43418). For each SYN, a SYN-ACK was sent from the server to the client (18) on the corresponding port.

3 vpc-abcdefab012345678 subnet-aaaaaaaa012345678 i-01234567890123456 eni-1235b8ca123456789 123456789010 IPv4 52.213.180.42 10.0.0.62 43416 5001 52.213.180.42 10.0.0.62 6 568 8 1566848875 1566848933 ACCEPT 2 OK

3 vpc-abcdefab012345678 subnet-aaaaaaaa012345678 i-01234567890123456 eni-1235b8ca123456789 123456789010 IPv4 10.0.0.62 52.213.180.42 5001 43416 10.0.0.62 52.213.180.42 6 376 7 1566848875 1566848933 ACCEPT 18 OK

3 vpc-abcdefab012345678 subnet-aaaaaaaa012345678 i-01234567890123456 eni-1235b8ca123456789 123456789010 IPv4 52.213.180.42 10.0.0.62 43418 5001 52.213.180.42 10.0.0.62 6 100701 70 1566848875 1566848933 ACCEPT 2 OK

3 vpc-abcdefab012345678 subnet-aaaaaaaa012345678 i-01234567890123456 eni-1235b8ca123456789 123456789010 IPv4 10.0.0.62 52.213.180.42 5001 43418 10.0.0.62 52.213.180.42 6 632 12 1566848875 1566848933 ACCEPT 18 OK

In the second aggregation interval, one of the connections that was established during the previous flow is now closed. The client sent a FIN flag (1) to the server for the connection on port 43418. The server sent a FIN to the client on port 43418.

3 vpc-abcdefab012345678 subnet-aaaaaaaa012345678 i-01234567890123456 eni-1235b8ca123456789 123456789010 IPv4 10.0.0.62 52.213.180.42 5001 43418 10.0.0.62 52.213.180.42 6 63388 1219 1566848933 1566849113 ACCEPT 1 OK

3 vpc-abcdefab012345678 subnet-aaaaaaaa012345678 i-01234567890123456 eni-1235b8ca123456789 123456789010 IPv4 52.213.180.42 10.0.0.62 43418 5001 52.213.180.42 10.0.0.62 6 23294588 15774 1566848933 1566849113 ACCEPT 1 OK

For short connections (for example, a few seconds) that are opened and closed within a single aggregation interval, the flags might be set on the same line in the flow log record for traffic flow in the same direction. In the following example, the connection is established and finished within the same aggregation interval. In the first line, the TCP flag value is 3, which indicates that there was a SYN and a FIN message sent from the client to the server. In the second line, the TCP flag value is 19, which indicates that there was SYN-ACK and a FIN message sent from the server to the client.

3 vpc-abcdefab012345678 subnet-aaaaaaaa012345678 i-01234567890123456 eni-1235b8ca123456789 123456789010 IPv4 52.213.180.42 10.0.0.62 43638 5001 52.213.180.42 10.0.0.62 6 1260 17 1566933133 1566933193 ACCEPT 3 OK

3 vpc-abcdefab012345678 subnet-aaaaaaaa012345678 i-01234567890123456 eni-1235b8ca123456789 123456789010 IPv4 10.0.0.62 52.213.180.42 5001 43638 10.0.0.62 52.213.180.42 6 967 14 1566933133 1566933193 ACCEPT 19 OK

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