attach, or bind to ports on a system.
The first column, PROTO, gives the protocol that this particular connection or listening port is using. We have several TCP ports open, as well as a few UDP ports. The Recv-Q and Send-Q columns show how many bits are waiting to be handled on this connection. If you see that your system has Recv-Q numbers continually, you know that it cannot process incoming data quickly enough. Similarly, if the Send-Q column keeps having entries, you know that either the network or the other system in the connection cannot accept data as quickly as you can send it. While occasional bursts of either Send-Q or Recv-Q entries are normal, individual entries in these columns should disappear quickly. You need to watch your own system to learn what is normal and what isn’t. The Local Address column is, as you might guess, an open IP address and port number on the local system. The first four numbers are the IP address, and the port number is appended with a period. For example, 192.168.1.250.22 is port 22 on the IP address 192.168.1.250. If this entry is an asterisk, a period, and a port number, it means the system is listening on all available IP addresses for an incoming connection on that port. That particular line does not show any active connections, but the system is ready to accept one. The Foreign Address column shows the address and port number on the remote end of any connection. Finally, the (state) column shows the status of the TCP handshake. You don’t need to know all of the possible TCP connection states right now; just become familiar with what’s normal. ESTABLISHED means that a connection is complete, and data is quite probably flowing over that connection. LAST_ACK, FIN_WAIT_1, and FIN_WAIT_2 mean that the connection is closing. SYN_RCVD, ACK, and SYN+ACK are all parts of the normal connection creation process. Here we look at the netstat output on a brand-new, out-of-the-box OpenBSD install. I’m using SSH to connect to it, but it has no custom services running. # netstat -na -f inet Active Internet connections (including servers) Proto Recv-Q Send-Q Local Address Foreign Address (state) 1 tcp 0 0 192.168.1.250.22 192.168.1.200.49182 ESTABLISHED tcp 0 0 192.168.1.250.22 192.168.1.200.49181 2 TIME_WAIT tcp 0 0 3 127.0.0.1.587 *.* LISTEN tcp 0 0 127.0.0.1.25 *.* LISTEN tcp 0 0 *.22 4 *.* LISTEN tcp 0 0 *.37 *.* LISTEN tcp 0 0 *.13 *.* LISTEN tcp 0 0 *.113 *.* LISTEN tcp 0 0 127.0.0.1.111 *.* LISTEN tcp 0 0 *.111 *.* LISTEN Active Internet connections (including servers) Proto Recv-Q Send-Q Local Address Foreign Address (state) udp 0 0 *.700 5 *.* udp 0 0 *.798 *.* udp 0 0 *.512 *.* udp 0 0 127.0.0.1.111 *.* udp 0 0 *.514 *.* udp 0 0 *.111 *.* # Page 170
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