{"id":923,"date":"2020-01-29T13:07:01","date_gmt":"2020-01-29T11:07:01","guid":{"rendered":"https:\/\/excentiscom.webhosting.be\/?p=923"},"modified":"2022-10-12T15:46:25","modified_gmt":"2022-10-12T13:46:25","slug":"tcp-half-close-a-cool-feature-that-is-now-broken","status":"publish","type":"post","link":"https:\/\/www.excentis.com\/blog\/tcp-half-close-a-cool-feature-that-is-now-broken\/","title":{"rendered":"TCP Half-Close: a cool feature that is now broken"},"content":{"rendered":"<p><span data-contrast=\"auto\">When we released a new version of the\u00a0<\/span><span data-contrast=\"auto\">ByteBlower<\/span><span data-contrast=\"auto\">\u00a0traffic generator a few years ago\u00a0<\/span><span data-contrast=\"auto\">we started\u00a0<\/span><span data-contrast=\"auto\">getting user reports saying the TCP traffic flows mysteriously \u201cstopped\u201d after a short time.\u00a0<\/span><span data-contrast=\"auto\">Some users told us the traffic stopped after 10 to 30 seconds. Other users told us the TCP flows stopped immediately. For those users the issue was consistently\u00a0<\/span><span data-contrast=\"auto\">reproducible<\/span><span data-contrast=\"auto\">. However, we could not reproduce the issue in our own test setup.<\/span><\/p>\n<div>\n<div>\n<div>\n<div>\n<p><span data-contrast=\"auto\">After some investigation we found two things:<\/span><\/p>\n<ul>\n<li><span data-contrast=\"auto\">the problem occurred when the\u00a0<\/span><span data-contrast=\"auto\">TCP\u00a0<\/span><span data-contrast=\"auto\">c<\/span><span data-contrast=\"auto\">onnection was initiated from\u00a0<\/span><span data-contrast=\"auto\">behind a NAT<\/span><\/li>\n<li><span data-contrast=\"auto\">ByteBlower<\/span><span data-contrast=\"auto\">\u00a0was making use of the TCP\u00a0<\/span><span data-contrast=\"auto\">\u201c<\/span><span data-contrast=\"auto\">half-close\u201d feature<\/span><\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<h2><span data-contrast=\"auto\">What is\u00a0<\/span><span data-contrast=\"auto\">TCP\u00a0<\/span><span data-contrast=\"auto\">Half<\/span><span data-contrast=\"auto\">&#8211;<\/span><span data-contrast=\"auto\">Close<\/span><span data-contrast=\"auto\">?<\/span><\/h2>\n<p><span data-contrast=\"auto\">TCP allows you to close each direction of the connection independently. A TCP connection is considered to be half-closed when it\u2019s closed in one direction and still open in the other direction. It allows an application to say: &#8220;I am done sending data, so send a FIN to the other end, but I still want to receive data from the other end, until it sends me a FIN.&#8221;<\/span><\/p>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"size-full wp-image-924 aligncenter\" src=\"https:\/\/www.excentis.com\/wp-content\/uploads\/tekening_website_blog.png\" alt=\"TCP Half Close\" width=\"634\" height=\"403\" srcset=\"https:\/\/www.excentis.com\/wp-content\/uploads\/tekening_website_blog.png 634w, https:\/\/www.excentis.com\/wp-content\/uploads\/tekening_website_blog-300x191.png 300w\" sizes=\"(max-width: 634px) 100vw, 634px\" \/><\/p>\n<p><span data-contrast=\"auto\">TCP\u00a0<\/span><span data-contrast=\"auto\">half<\/span><span data-contrast=\"auto\">&#8211;<\/span><span data-contrast=\"auto\">close is sometimes used to emulate Unix-style IO redirection on TCP streams by using the FIN message as a EOF marker. The book\u00a0<\/span><em><span data-contrast=\"auto\">TCP\/IP Illustrated, Vol. 1<\/span><\/em>\u00a0<span data-contrast=\"auto\">provides a nice example for this:<\/span><\/p>\n<pre><span class=\"SpellingError SCXW180374913 BCX0\">ssh<\/span><span class=\"NormalTextRun SCXW180374913 BCX0\">\u00a0hostname sort &lt; datafile<\/span><\/pre>\n<p><span data-contrast=\"auto\">Here the \u201c<\/span><span data-contrast=\"auto\">ssh<\/span><span data-contrast=\"auto\">\u201d program starts the \u201csort\u201d program on the remote host and reads standard input from a local file named \u201cdatafile\u201d. Each line of standard input is sent to the sort program over the TCP stream. The final EOF marker from datafile is sent over the TCP stream as a FIN message and then written as EOF to the standard input of the sort program. Note that sort can\u2019t generate any output until it has received all input, so it relies on TCP\u00a0<\/span><span data-contrast=\"auto\">half<\/span><span data-contrast=\"auto\">&#8211;<\/span><span data-contrast=\"auto\">close to work.<\/span><\/p>\n<h2><span data-contrast=\"auto\">What\u2019s the problem?<\/span><\/h2>\n<p><span data-contrast=\"auto\">We suspect that some NAT devices, when seeing a FIN message, will delete the corresponding NAT entry after a short timeout even if no FIN is seen from the other side.\u00a0<\/span><span data-contrast=\"auto\">The result is that TCP connections that are in a\u00a0<\/span><span data-contrast=\"auto\">half<\/span><span data-contrast=\"auto\">&#8211;<\/span><span data-contrast=\"auto\">close state will stop working after a while.<\/span><\/p>\n<p><span data-contrast=\"auto\">I should mention that this behavior is not allowed by\u00a0<\/span><a href=\"https:\/\/tools.ietf.org\/html\/rfc5382\" target=\"_blank\" rel=\"noreferrer noopener\"><span data-contrast=\"none\">RFC 5382<\/span><\/a><span data-contrast=\"auto\">:\u00a0<\/span><em><span data-contrast=\"auto\">The closing phase begins when both endpoints have terminated their half of the connection by sending a FIN packet<\/span><\/em><span data-contrast=\"auto\">.<\/span>\u00a0<span data-contrast=\"auto\">(The RFC also allows an idle-timeout for inactive connections after two hours and 4 minutes. But that rule did not apply in our situation because our TCP flow had only been running for a few seconds and it wasn\u2019<\/span><span data-contrast=\"auto\">t idle<\/span><span data-contrast=\"auto\">.) So maybe this was just a bug in the NAT device implementation.<\/span><\/p>\n<p><span data-contrast=\"auto\">But<\/span><span data-contrast=\"auto\">\u00a0it\u2019s not only NAT devices that can cause problems. Many firewalls also implement a TCP half-close timeout. These timeouts can be very short: Cisco decreased the minimum half-close timeout to 30 seconds to provide better DoS protection.<\/span><\/p>\n<p><span data-contrast=\"auto\">And even the Linux\u00a0operating system\u00a0implements a half-close timeout. The default value is 60 seconds. The manual for\u00a0<\/span><span class=\"TextRun BCX0 SCXW25014476\" lang=\"EN\" xml:lang=\"EN\" data-contrast=\"auto\"><span class=\"NormalTextRun BCX0 SCXW25014476\">net.ipv4.tcp_fin_timeout<\/span><\/span>\u00a0setting<span data-contrast=\"auto\">\u00a0says the following:<\/span><\/p>\n<pre><span data-contrast=\"auto\"><em>This specifies how many seconds to wait for a final FIN packet before the socket is forcibly closed.<\/em>\u00a0<\/span>\r\n<span data-contrast=\"auto\"><strong><em>This is strictly a violation of the TCP specification, but required to prevent denial-of-service attacks<\/em><\/strong><\/span>.<\/pre>\n<h2><span data-contrast=\"auto\">Conclusion<\/span><\/h2>\n<p><span data-contrast=\"auto\">Many systems have introduced a timeout mechanism that restrict<\/span><span data-contrast=\"auto\">s<\/span><span data-contrast=\"auto\">\u00a0how long a TCP connection can remain in the\u00a0<\/span><span data-contrast=\"auto\">half<\/span><span data-contrast=\"auto\">-closed state. Preventing denial of service attacks seems to be the most common reason. In other cases, like the NAT example, it may be that the reason is simply a bug in the implementation.<\/span><\/p>\n<p><span data-contrast=\"auto\"><strong>As a result the TCP half-close cannot be used reliably.<\/strong>\u00a0<\/span><span data-contrast=\"auto\">Except perhaps for a short duration, for example to allow flushing outstanding packets or sending a\u00a0protocol \u201cgoodbye<\/span><span data-contrast=\"auto\">\u201d<\/span><span data-contrast=\"auto\">\u00a0message.<\/span><\/p>\n<p><span data-contrast=\"auto\">In the end we updated the\u00a0<\/span><span data-contrast=\"auto\">ByteBlower<\/span><span data-contrast=\"auto\">\u00a0to no longer make use of the TCP half close state. We now only send a FIN message from each side once the flow has completely finished. When this version was released users reported that the issue had disappeared.<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"<p>When we released a new version of the\u00a0ByteBlower\u00a0traffic generator a few years ago\u00a0we started\u00a0getting user reports saying the TCP traffic flows mysteriously \u201cstopped\u201d after a short time.\u00a0Some users told us the traffic stopped after 10 to 30 seconds. Other users told us the TCP flows stopped immediately. For those users the issue was consistently\u00a0reproducible. However, &hellip;<\/p>\n","protected":false},"author":4,"featured_media":924,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"content-type":"","inline_featured_image":false,"_genesis_block_theme_hide_title":false,"footnotes":""},"categories":[45,61,68,69],"tags":[],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v22.4 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>TCP Half-Close: a cool feature that is now broken - Excentis<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.excentis.com\/blog\/tcp-half-close-a-cool-feature-that-is-now-broken\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"TCP Half-Close: a cool feature that is now broken - Excentis\" \/>\n<meta property=\"og:description\" content=\"When we released a new version of the\u00a0ByteBlower\u00a0traffic generator a few years ago\u00a0we started\u00a0getting user reports saying the TCP traffic flows mysteriously \u201cstopped\u201d after a short time.\u00a0Some users told us the traffic stopped after 10 to 30 seconds. 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