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title: Isolated IP Management |
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description: > |
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For routers, a management interface should always work, even if the |
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firewall is broken, or the IP address conflicts with other interfaces. |
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Isolating the interface to a vnet jail and using unix domain sockets fixes |
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this problem. |
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posted: !!timestamp '2023-08-17' |
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created: !!timestamp '2023-08-17' |
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time: 12:00 PM |
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tags: |
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- FreeBSD |
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- jails |
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--- |
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The latest iteration of my home firewall has a spare interface. Many |
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switches these days have a dedicated management interface, and I wanted |
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something similar for my firewall. I want an interface that I can plug in, |
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get an IP via DHCP, and I can ssh into and it'll "just work" even if |
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there's a misconfiguration, or an IP conflict. |
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On FreeBSD, |
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[vnet jails](https://wiki.freebsd.org/Jails#VNET-based_networking_for_Jails) |
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are a way to isolate an interface such that it won't interfere (or receive |
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interference) from other interfaces. Some may have just used epair and |
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assigned IPs, but this could cause conflict, while using a unix domain |
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socket keeps things isolated, and FreeBSD ships w/ everything needed to |
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make this work. A package like `socat` isn't needed. |
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Configuring the host was straight forward, make a directory for the socket: |
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``` |
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mkdir /var/mgmt |
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``` |
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And then add a line to `/etc/inetd.conf` to listen to incoming connections |
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on the socket and launch `sshd`: |
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``` |
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/var/mgmt/mgmt.ssh.sock stream unix nowait root /usr/sbin/sshd sshd -i |
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``` |
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The `-i` option to `sshd` tells it to run in "`inetd`" mode, which means that |
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the stdin and stdout of the process is the socket to use for communication. |
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The next harder part was getting a jail configured that would accept |
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incoming connections and forward them to the unix domain socket. |
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First the jail configuration, which goes in `/etc/jail.conf` or similar |
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location: |
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``` |
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mgmt { |
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host.hostname = mgmt; # Hostname |
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vnet ="new"; |
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vnet.interface="mgmt0"; |
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path = "/usr/jails/mgmt/root"; # Path to the jail |
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mount.fstab="/usr/jails/mgmt/fstab"; # mount spec |
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mount.devfs; # Mount devfs inside the jail |
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devfs_ruleset = "101"; |
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exec.start = "/bin/sh /etc/rc"; # Start command |
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exec.stop = "/bin/sh /etc/rc.shutdown"; # Stop command |
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} |
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``` |
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There isn't anything special in this. It's pretty standard jail |
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configuration, the differences are the vnet configuration, and the |
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`devfs_ruleset`. |
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The `devfs_ruleset` is necessary in order to expose the bpf interface |
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used by dhclient. This required the following lines in `/etc/devfs.rules`: |
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``` |
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[mydevfsrules_jail=100] |
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add include $devfsrules_hide_all |
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add include $devfsrules_unhide_basic |
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add include $devfsrules_unhide_login |
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[devfsrules_jail_dhcp=101] |
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add include $mydevfsrules_jail |
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add path 'bpf*' unhide |
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``` |
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Note that after adding the above lines, you need to run: |
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``` |
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service devfs start |
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``` |
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to load the rules (per |
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[devfs(8)](https://man.freebsd.org/cgi/man.cgi?query=devfs&apropos=0&sektion=8&manpath=FreeBSD+13.2-RELEASE+and+Ports&arch=default&format=html) |
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man page). |
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Note: I learned my mistake not to number my blocks immediately after |
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the standard defaults (from `/etc/defaults/devfs.rules`). I had done that |
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befure but there's now a conflict, so I skip ahead a bit to get a unique range. |
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Now I needed to make a number of directories for the jail: |
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``` |
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mkdir -p /usr/jails/mgmt/root |
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mkdir -p /usr/jails/mgmt/etc |
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mkdir -p /usr/jails/mgmt/var/mgmt |
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mkdir -p /usr/jails/mgmt/tmp |
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``` |
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I needed to setup the `fstab` for the jail: |
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``` |
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# Device Mountpoint FStype Options Dump Pass# |
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/ /usr/jail/mgmt/root nullfs ro 0 0 |
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/usr/jail/mgmt/etc /usr/jail/mgmt/root/etc nullfs rw 0 0 |
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/usr/jail/mgmt/var /usr/jail/mgmt/root/var nullfs rw 0 0 |
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/var/mgmt /usr/jail/mgmt/root/var/mgmt nullfs ro 0 0 |
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/usr/jail/mgmt/tmp /usr/jail/mgmt/root/tmp nullfs rw 0 0 |
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``` |
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This is a little bit more tricky, It first `nullfs` mounts the root system. |
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I'm using ZFS boot environments, so this is a pretty clean FreeBSD install |
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without much host specific data. It then mounts some jail specific directories |
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for `etc`, `tmp` and `var` and finally mounts the shared directory w/ the |
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unix domain socket to the host system. Also note that a couple of the mounts |
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are read-only to prevent the jail from modifying the system. |
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The `etc` directory was populated from the system `etc` via: |
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``` |
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tar -cf - -C /etc . | tar -xf - -C /usr/jails/mgmt/etc |
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``` |
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Then the jail was configured, first `/usr/jails/mgmt/etc/rc.conf`: |
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``` |
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hostname="mgmt.funkthat.com" |
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sendmail_enable="NONE" |
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sendmail_submit_enable="NO" |
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sendmail_outbound_enable="NO" |
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sendmail_msp_queue_enable="NO" |
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# Management port |
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ifconfig_mgmt0="DHCP" |
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# necessary as devd can't be run in a jail |
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synchronous_dhclient="YES" |
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inetd_enable="YES" # Run the network daemon dispatcher (YES/NO). |
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``` |
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The key part of this configuration that took me a while to figure out was |
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the `synchronous_dhclient` line. It used to be that `netif` would start |
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dhclient, but in order to better handle USB ethernet devices and other |
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removable interfaces, it was moved to `devd`. The only problem is that |
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`devd` hasn't been jail'ified, and you can't run it to get things like |
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link notifications that would normally launch dhclient. Setting this to |
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yes, makes sure it gets launched when the jail starts. |
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And then the following line was added to `/usr/jails/mgmt/etc/inetd.conf`: |
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``` |
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ssh stream tcp nowait root /usr/bin/nc nc -N -U /var/mgmt/mgmt.ssh.sock |
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``` |
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This is the part that will forward incoming connections to the ssh port |
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on to the unix domain socket. |
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Now that everything is configured, a simple `jail -c mgmt` will get |
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the jail running and accepting connections. |
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This was testing and deployed on a FreeBSD 14-CURRENT build as of August |
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8th, 2023, or more specifically, from `main-n264621-09c20a29328`, but it |
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should work on all currently supported FreeBSD releases. |
