{"id":2125,"date":"2010-09-15T15:35:00","date_gmt":"2010-09-15T15:35:00","guid":{"rendered":"http:\/\/www.b.shuttle.de\/hayek\/Hayek\/Jochen\/wp\/blog-en\/2010\/09\/15\/the-isp-doesnt-let-you-ssh\/"},"modified":"2010-09-15T15:35:00","modified_gmt":"2010-09-15T15:35:00","slug":"the-isp-doesnt-let-you-ssh","status":"publish","type":"post","link":"https:\/\/wp.jochen.hayek.name\/blog-en\/2010\/09\/15\/the-isp-doesnt-let-you-ssh\/","title":{"rendered":"the ISP doesn&#8217;t let you ssh"},"content":{"rendered":"<p>\t\t\t\tDid that occur to you before? Your (new) Internet connection seems to work instantly, but then you recognize, that you do not succeed with ssh connections. (Search engine question: &#8220;Why does my SSH connection not work?&#8221;) That may have different reasons.<\/p>\n<ol>\n<li>The ISP block connections on certain destination ports like 22.<br \/>\nIf you can, get the sshd you want to connect to, to also answer on port 23 e.g. . You achieve this with an additional line like this in sshd_config there: &#8220;Port 23&#8221;.<br \/>\nUsually the &#8220;Port 22&#8221; has not been explicit there so far, so if you add another Port line, you would also have to make that &#8220;Port 22&#8221; explicit, otherwise that sshd will not answer on port 22 any longer.<\/li>\n<li>The ISP does not like TOS flags in IP packets, that do not equal 0, or they just don&#8217;t like the TOS flag chosen by your openssh.<br \/>\nThe following bullets should actually be &#8220;2.1&#8221; \u2026, but blogger doesn&#8217;t easily support this, and I am just to lazy to enforce it HTML-wise.<\/li>\n<\/ol>\n<ul>\n<li>If you are using Linux and iptables, this may be your quickiest way to achieve this:<br \/>\n<span>iptables -t mangle -A PREROUTING -p TCP &#8211;dport 22 -j TOS &#8211;set-tos 0x0<\/span><br \/>\nIt did not work for me though, so I had to find another way.<\/li>\n<li>I decided to adapt openssh to my needs, and here is the recipe:<br \/>\nGet yourself an openssh TAR ball, unpack it, configure it!<br \/>\nAdd this line to config.h : &#8220;#define IP_TOS_IS_BROKEN 1&#8221; in order to leave the TOS flag untouched!<br \/>\nBuild it, and install it to a safe place, i.e. not necessarily to the standard place, so you will still be able to use the standard installation!<\/li>\n<li>A friend of mine built himself a utility to run on the router (a FRITZ!Box 7050 router) and selectively adapt the TOS flag according to his needs.<br \/>\nMigrating the utility to our FRITZ!Box 7390 models cost us too much time though, so we abandoned that solution.<br \/>\nIf you are still interested in this approach, he might forward his sources to you, so let us know!<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Did that occur to you before? Your (new) Internet connection seems to work instantly, but then you recognize, that you do not succeed with ssh connections. (Search engine question: &#8220;Why does my SSH connection not work?&#8221;) That may have different reasons. The ISP block connections on certain destination ports like 22. If you can, get [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false,"_share_on_mastodon":"0"},"categories":[666],"tags":[1110],"class_list":["post-2125","post","type-post","status-publish","format-standard","hentry","category-uncategorized","tag-isp-problems"],"share_on_mastodon":{"url":"","error":""},"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/paO0kP-yh","jetpack_likes_enabled":true,"amp_enabled":true,"_links":{"self":[{"href":"https:\/\/wp.jochen.hayek.name\/blog-en\/wp-json\/wp\/v2\/posts\/2125","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/wp.jochen.hayek.name\/blog-en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/wp.jochen.hayek.name\/blog-en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/wp.jochen.hayek.name\/blog-en\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/wp.jochen.hayek.name\/blog-en\/wp-json\/wp\/v2\/comments?post=2125"}],"version-history":[{"count":0,"href":"https:\/\/wp.jochen.hayek.name\/blog-en\/wp-json\/wp\/v2\/posts\/2125\/revisions"}],"wp:attachment":[{"href":"https:\/\/wp.jochen.hayek.name\/blog-en\/wp-json\/wp\/v2\/media?parent=2125"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wp.jochen.hayek.name\/blog-en\/wp-json\/wp\/v2\/categories?post=2125"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wp.jochen.hayek.name\/blog-en\/wp-json\/wp\/v2\/tags?post=2125"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}