Limit SSH to IPv6 on FreeBSD

When I first set up my server, I got numerous daily attempts to login via SSH almost immediately. None of them were successful, because I use public key authentication, of course, but the log spam was annoying nonetheless. The problem is with only 2³² addresses, scanning all possible IPv4 addresses for open ports is quite easy.

But, yet again, IPv6 to the rescue. It has 2¹²⁸ possible addresses, so scanning all of them for open ports is doable but takes a significant amount of time. My server has a /64 subnet, so even if someone knows the prefix, there are still 2⁶⁴ addresses to scan. Both my home internet and my cell phone provider support IPv6 (as they should), so limiting SSH to a spare IPv6 address was the obvious solution.

This is not an approach for security, but merely to get rid of the annoying login attempts. The server still has to be secured. No one should believe that trying to “hide” the IP address does anything for security. If anyone really wants to find it, he will. But it helped to reduce the random failed login attempts, from a couple of hundreds on bad days, to not a single one in more than a year. So without all this noise in the logs, I can more easily see if anyone is actually trying to get into my server now.

The first step is to add another IPv6 address to the network interface, which can be used for SSH. Let’s assume the IPv6 prefix for the /64 subnet is 2001:10:20:30. All regular services might use 2001:10:20:30::1, so 2001:10:20:30::2222 could be reserved for SSH only. The additional address can be added in the /etc/rc.conf, assuming the network interface is em0:

ipv6_default_interface="em0"
ifconfig_em0_ipv6="inet6 2001:10:20:30::1/64"
ifconfig_em0_alias0_ipv6="inet6 2001:10:20:30::2222/64"

The networking can be restartet with:

# service netif restart

Alternatively, the server can also be rebooted. Now ifconfig should list the additional address:

# ifconfig em0
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=6c07bb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,TSO4,TSO6,LRO,VLAN_HWTSO,LINKSTATE,RXCSUM_IPV6,TXCSUM_IPV6>
ether 96:00:00:aa:bb:cc
hwaddr 96:00:00:aa:bb:cc
inet6 2001:10:20:30::1 prefixlen 64 
inet6 fe80::9400:ff:feaa:bbcc%vtnet0 prefixlen 64 scopeid 0x1 
inet6 2001:10:20:30::2222 prefixlen 64 
inet 10.20.30.40 netmask 0xffffffff broadcast 10.20.30.40 
nd6 options=8021<PERFORMNUD,AUTO_LINKLOCAL,DEFAULTIF>
media: Ethernet 10Gbase-T <full-duplex>
status: active

You can also try to ping the new address and it should work fine. If the services on your server are not bound to specific IP addresses, but to any, now is the time to fix that. The other services should only use the 2001:10:20:30::1 address, and 2001:10:20:30::2222 is for SSH exclusively. sockstat is your friend.

With the new IPv6 address up and running, SSH can be configured to only listen to it and nothing else. This is done by having only a single ListenAddress parameter in /etc/ssh/sshd_config:

ListenAddress 2001:10:20:30::2222

Save the config file and be sure you can access your server directly or via a serial console, in case you made a mistake and lock yourself out. Then restart SSH to apply the changes:

# service sshd restart

You can check with sockstat if sshd is only listening to the new IPv6 address:

# sockstat -l | grep sshd
root sshd 34490 3 tcp6 2001:10:20:30::2222:22 *:*

If it is listening to anything else, check your sshd_config file.

The only way to access the server via SSH is by using the new dedicated IPv6 address:

$ ssh 2001:10:20:30::2222

This is inconvenient, so I recommend adding a new subdomain, like myssh.myserver.com, with a single AAAA record:

$ host myssh.myserver.com
myssh.myserver.com has IPv6 address 2001:10:20:30::2222

With these little changes the amount of random login attempts can be significantly reduced. However, it only works when all of your devices from which you manage the server support and use IPv6. But that should be the standard nowadays.