On Thu, 2019-04-18 at 16:17 +0000, Daniel Miller wrote: Thanks for this report! You're right, Ncat currently refuses to attempt a connection to port 0. I will remove that restriction, since in my testing it appears to be capable of making a client connection to server port 0. You said that ncat -lkp 0 works, but if you check the output of netstat, you'll see that Ncat is not actually listening on port 0. While the TCP spec does not specify any special handling for port 0, most socket implementations treat the port value 0 as a request to assign an ephemeral port when calling bind(2). Nmap does not appear to have any problems using port 0 as a source port with -g0. Did you mean something else? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub, or mute the thread. {"api_version":"1.0","publisher":{"api_key":"05dde50f1d1a384dd78767c5 5493e4bb","name":"GitHub"},"entity":{"external_key":"github/nmap/nmap ","title":"nmap/nmap","subtitle":"GitHub repository","main_image_url":"https://github.githubassets.com/images/ email/message_cards/header.png","avatar_image_url":"https://github.gi thubassets.com/images/email/message_cards/avatar.png","action":{"name ":"Open in GitHub","url":"https://github.com/nmap/nmap"}},"updates":{ ***@***.*** in #1560: Thanks for this report! You're right, Ncat currently refuses to attempt a connection to port 0. I will remove that restriction, since in my testing it appears to be capable of making a client connection to server port 0.\r\n\r\nYou said that `ncat -lkp 0` works, but if you check the output of `netstat`, you'll see that Ncat is not actually listening on port 0. While the [TCP spec](https://tools.ietf .org/html/rfc793) does not specify any special handling for port 0, most socket implementations treat the port value 0 as a request to assign an ephemeral port when calling `bind(2)`.\r\n\r\nNmap does not appear to have any problems using port 0 as a source port with `-g0`. Did you mean something else?"}],"action":{"name":"View Issue","url":" #1560 (comment)"}}} [ { ***@***.***": "http://schema.org", ***@***.***": "EmailMessage", "potentialAction": { ***@***.***": "ViewAction", "target": "#1560 (comment) 78662", "url": "#1560 (comment) 62", "name": "View Issue" }, "description": "View this Issue on GitHub", "publisher": { ***@***.***": "Organization", "name": "GitHub", "url": "https://github.com" } } ]