how do I know if I have a firewall

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https://bbs.archlinux.org – oh, ok. Well, I have two archlinux machines behind a router, and some other windows ones. At times I can't ping between them (local ips), at times I can. Sometimes I saw that if run "ping arch2" from my arch 1 machine, then while it's printing "destination host unreachable" I run "ping arch1" from my arch 2 machine, suddenly pings start to be successful - but it doesn't allways work. Also it's not limited to pings - when ping isn't working I can't ssh or see samba shares neither.Then the noip2 (for use with www.no-ip.org) daemon is gi (HowTos)