{"id":7548,"date":"2021-03-11T00:34:04","date_gmt":"2021-03-11T05:34:04","guid":{"rendered":"https:\/\/www.rushworth.us\/lisa\/?p=7548"},"modified":"2021-03-11T00:44:16","modified_gmt":"2021-03-11T05:44:16","slug":"mtu-probing","status":"publish","type":"post","link":"https:\/\/www.rushworth.us\/lisa\/?p=7548","title":{"rendered":"MTU Probing"},"content":{"rendered":"<p>We&#8217;ve had a number of <em>very<\/em> strange network problems lately &#8212; Zoneminder cannot talk to cameras, clients veg out talking to Myth, Twonky is non-functional (even the web page &#8212; you get enough of the header to have a title, but the page just hangs, Scott cannot get to our Discourse site. And, more frustratingly, he cannot SSH to some of our hosts. Using &#8220;ssh -v&#8221; and throwing on a bunch of flags to <em>not<\/em> attempt key auth (-o PasswordAuthentication=yes -o PreferredAuthentications=keyboard-interactive,password -o PubkeyAuthentication=no) and his connection still hung. But, at least, I could see <em>something<\/em>. The last thing the SSH connection reported is:<\/p>\n<pre class=\"\">debug1: expecting SSH2_MSG_KEX_ECDH_REPLY<\/pre>\n<p>Which I&#8217;ve seen before &#8230; fortunately when I had a great Unix support guy working in the same office building that I did. Who let me stop over and bounce really oddball problems off of him. He told me to enable mtu probing.<\/p>\n<pre>echo 1 &gt;\/proc\/sys\/net\/ipv4\/tcp_mtu_probing<\/pre>\n<p>And, if that doesn&#8217;t work, use &#8220;echo 2&#8221;. Which &#8230;. yeah, wouldn&#8217;t have been any of my first thirty guesses. <a href=\"https:\/\/blog.cloudflare.com\/path-mtu-discovery-in-practice\/\" target=\"_blank\" rel=\"noopener\">Cloudflare published a good article on what exactly MTU path discovery <em>is<\/em><\/a>, and I can RTFM enough to figure out what I&#8217;ve set here. But no idea what&#8217;s got a smaller MTU than our computers.<\/p>\n<p>&nbsp;<\/p>\n<pre class=\"_3GnarIQX9tD_qsgXkfSDz1\"><code class=\"_34q3PgLsx9zIU5BiSOjFoM\">tcp_mtu_probing - INTEGER\r\n\tControls TCP Packetization-Layer Path MTU Discovery.  \r\n\t  0 - Disabled\r\n\t  1 - Disabled by default, enabled when an ICMP black hole detected\r\n\t  2 - Always enabled, use initial MSS of tcp_base_mss.<\/code><\/pre>\n","protected":false},"excerpt":{"rendered":"<p>We&#8217;ve had a number of very strange network problems lately &#8212; Zoneminder cannot talk to cameras, clients veg out talking to Myth, Twonky is non-functional (even the web page &#8212; you get enough of the header to have a title, but the page just hangs, Scott cannot get to our Discourse site. And, more frustratingly, &hellip;<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[30],"tags":[1311,1312,1294,1310],"class_list":["post-7548","post","type-post","status-publish","format-standard","hentry","category-system-administration","tag-ipv4","tag-mtu-path-probing","tag-networking","tag-tcp"],"_links":{"self":[{"href":"https:\/\/www.rushworth.us\/lisa\/index.php?rest_route=\/wp\/v2\/posts\/7548","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rushworth.us\/lisa\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rushworth.us\/lisa\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rushworth.us\/lisa\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rushworth.us\/lisa\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=7548"}],"version-history":[{"count":2,"href":"https:\/\/www.rushworth.us\/lisa\/index.php?rest_route=\/wp\/v2\/posts\/7548\/revisions"}],"predecessor-version":[{"id":7550,"href":"https:\/\/www.rushworth.us\/lisa\/index.php?rest_route=\/wp\/v2\/posts\/7548\/revisions\/7550"}],"wp:attachment":[{"href":"https:\/\/www.rushworth.us\/lisa\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7548"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rushworth.us\/lisa\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7548"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rushworth.us\/lisa\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7548"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}