While running nmap against localhost I started to see open ports in the dynamic range (>1024). Kind of odd, knowing netstat and ss did not show any listeners on the port.

With tcpdump, I confirmed system was returning S/ACK for ports that did not have a listener enabled. The "issue" or feature only occurs if source port matches the destination port.

# netstat -nap | grep 5000
#

$ nc localhost -p 5000 5000
a
a

# tcpdump -i lo port 5000
14:28:18.059708 IP 127.0.0.1.5000 > 127.0.0.1.5000: Flags [S], seq 2005295207, win 43690, options [mss 65495,sackOK,TS val 4481790 ecr 0,nop,wscale 7], length 0
14:28:18.059721 IP 127.0.0.1.5000 > 127.0.0.1.5000: Flags [S.], seq 2005295207, ack 2005295208, win 43690, options [mss 65495,sackOK,TS val 4481790 ecr 4481790,nop,wscale 7], length 0
14:28:18.059729 IP 127.0.0.1.5000 > 127.0.0.1.5000: Flags [.], ack 2005295208, win 342, options [nop,nop,TS val 4481790 ecr 4481790], length 0
14:28:19.121392 IP 127.0.0.1.5000 > 127.0.0.1.5000: Flags [P.], seq 2005295208:2005295210, ack 2005295208, win 342, options [nop,nop,TS val 4482056 ecr 4481790], length 2
14:28:19.121407 IP 127.0.0.1.5000 > 127.0.0.1.5000: Flags [.], ack 2005295210, win 342, options [nop,nop,TS val 4482056 ecr 4482056], length 0

# hping3 -S 127.0.0.1 -p 5000 -s 5000
HPING 127.0.0.1 (lo 127.0.0.1): S set, 40 headers + 0 data bytes 
len=40 ip=127.0.0.1 ttl=64 id=2036 sport=5000 flags=S seq=0 win=512 rtt=3.8 ms             << SYN
DUP! len=52 ip=127.0.0.1 ttl=64 DF id=670 sport=5000 flags=A seq=0 win=342 rtt=3.8 ms      << SYN/ACK
len=40 ip=127.0.0.1 ttl=64 DF id=43435 sport=5000 flags=RA seq=1 win=0 rtt=3.7 ms

I confirmed it with nmap, nc, and hping3; granted they build on same c libraries, so I am not even sure if this should be filed as a kernel bug (or even a bug);
just did not expect to see this behavior // 

Expected to see a RST instead.