Limiting the number of connections
As a network-facing service, sslh
can be attacked by very
simple denial-of-service attacks by creating a lot of
concurrent connections. Protecting against such attacks is
complicated from the server side. This attack will cause
sslh
to create a large number of file handles, which can
cause exgagerated resource consumption.
This threat can be mitigated using several limitation mechanisms. Keep in mind that limiting the number of connections means that in case of an attack, the server resources are protected, but this might be at the expense of serving legitimate connections. In particular, in some cases this might mean SSH is no longer available.
There are several mechanisms to limit the number of connections:
- Use
ulimit
(see bash(3) or your shell’s man page) to limit the number of file descriptors.
Then, sslh
provides several mechanisms to limit the number
of concurrent connections, which in turns limits the number
of file descriptors used.
Essentially there are two ways to do this:
-
you can set
max_connections
per protocol, andsslh
will drop connections after probing if the count is exceeded. This should help in keeping SSH connections available even if an attacker is stuffing other protocols. Currently this does not work for forking protocols (support is planned). -
you can set
max_connections
for eachlisten
entry. So thesslh
process for each port will limit the number of concurrent connections to that port. This is similar to theudp_max_connections
setting, but for TCP.
sslh-select
and sslh-ev
support both limit types.
sslh-fork
has an explicit design goal to be as simple as
possible, which makes it impossible to implement limits per
protocol (because protocol probing is performed after the
process has forked). Limits will only work for listen
entries.
As of sslh 2.2.5, this is an experimental feature and not all use cases have been tested. If 2.2.5 does not exist yet, you must be on the Git’s head, so it is even more experimtental! Do not hesitate to send feedback if you notice inconsistent behaviour.