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1// SPDX-License-Identifier: GPL-2.02// Test what happens when client does not provide MD5 on SYN,3// but then does on the ACK that completes the three-way handshake.4 5`./defaults.sh`6 7// Establish a connection.8 0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 39 +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 010 +0 bind(3, ..., ...) = 011 +0 listen(3, 1) = 012 13 +0 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 10>14 +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>15// Ooh, weird: client provides MD5 option on the ACK:16 +.01 < . 1:1(0) ack 1 win 514 <md5 000102030405060708090a0b0c0d0e0f,nop,nop>17 +.01 < . 1:1(0) ack 1 win 514 <md5 000102030405060708090a0b0c0d0e0f,nop,nop>18 19// The TCP listener refcount should be 2, but on buggy kernels it can be 0:20 +0 `grep " 0A " /proc/net/tcp /proc/net/tcp6 | grep ":1F90"`21 22// Now here comes the legit ACK:23 +.01 < . 1:1(0) ack 1 win 51424 25// Make sure the connection is OK:26 +0 accept(3, ..., ...) = 427 28 +.01 write(4, ..., 1000) = 100029