by Fredrik Hagstran » Thu, 18 Oct 2001 18:54:03
Does anyone know if there is a good way to find or to get assigned free
TCP/UDP ports in Linux?
/Fredrik
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by David Efflan » Fri, 19 Oct 2001 10:36:06
First check /etc/services and inetd.conf or xinetd to see what standardQuote:> Does anyone know if there is a good way to find or to get assigned free
> TCP/UDP ports in Linux?
Then either check 'netstat -at', or if a script or program, just try
opening successive ports until you get one that does not return an error
(you get a bind error if a port is in use). Instead of dieing, you just
need to keep looping through ports until successful.
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1. ipchains/TCP/UDP, Why should I open UDP ports so that my TCP ports can work?
Hi,
I have set up firewall to accept some ports.
/sbin/ipchains -F input
/sbin/ipchains -P input ACCEPT
/sbin/ipchains -A input -p TCP -s 0/0 --dport ssh -j ACCEPT
/sbin/ipchains -A input -p UDP -s 0/0 --dport ssh -j ACCEPT
....
Then I want to lock down all other ports.
/sbin/ipchains -A input -p TCP -s ! 192.168.0.0/24 -j DENY
#??? what's wrong with this UDP ???
/sbin/ipchains -A input -p UDP -s ! 192.168.0.0/24 -j DENY
If I comment out the UDP line, I can ssh into firewall from outside.
If I don't comment out that line, I can no longer ssh into firewall from
outside.
Why should I open UDP ports so that my TCP ports can work?
Thanks for any help.
Ed Wu
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