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Other people's firewalls

The more appropriate title would be “ingress and egress firewalls that affect your connectivity but which you have no visibility and limited control of” but it was too long. I’ve experienced this tale many times with just one minor variation to the beginning of the plot and the rest of the details remaining the same. It’s either a:

  • corporate firewall: owned by another team or group in a much larger organisation.
  • vendor firewall: owned by a *aaS provider that you have other services with.

The usual premise is that they are providing some value-add security for you, possibly other tenants, and indeed themselves. However in reality it usually winds up having the opposite effect on yourself.

Lost in translation

To configure your own firewall you’ll use a domain-specific configuration syntax. You might not be particularly fond of the syntax (iptables, I’m looking at you) but you can have confidence that it clearly describes the thing that you have asked for and it will quickly draw your attention to any syntactical mistakes. It might look something like:

pass in on $ext_if proto { udp, tcp } from any to port 53
pass out on $ext_if proto icmp from to any icmp-type 8 code 0

The same doesn’t normally exist for other people’s firewalls. Your interface is a free-form text field in a change request application or email. The syntax is a plain English description of your changes, because you don’t know what technology their firewalls are or what else their engineers are familiar with. Everyone will have their own specific style and the level of detail will range somewhere between:

  • “Please open inbound DNS and outbound ping”
  • “Please allow UDP/53 and TCP/53 to and ICMP echo-reply from 10/8”

All of which harbour some degree of ambiguity. And even if you manage to describe your rule perfectly, it still has to be transcribed by a human into the right syntax without any typos and placed at the correct order in the ruleset. All in all, there’s a pretty good chance that you won’t get the thing you wanted first time.


Like raw configuration, it’s very rare that you’re able to get hold of logs or packet captures from devices under other people’s control. Which means that when a change request doesn’t behave as intended your means of debugging the problem are very much hindered. At best you can probably tell whether a connection is being blocked or dropped if you own both sides of the connection. Though the latter, if drop is the default policy, can be indistinguishable from a routing or addressing mistake.

This reminds me of something that my colleague Cal said when we were talking with someone else about their corporate firewall woes, with some paraphrasing: “surely you’d put a logging rule in place for a few days and see what traffic matched first”. Which is when it struck me that I’d forgotten that this was a Done Thing. I’d been dealing with devices that I couldn’t get logs from for so long that it probably wouldn’t cross my mind any longer. Which is a depressing realisation.

There is of course a special place reserved for people that break utilities like ping(1) and traceroute(1) that can prove essential when debugging network issues.

Default deny egress

Something that “other people” seem incredibly fond of is default deny policies on outbound traffic. A whitelist of protocols that you are allowed to connect out on, usually limited to DNS, HTTP and HTTPS.

The biggest problem is that this goes against the principle of least astonishment. Most people are used to working with networks that have restrictive ingress and permissive egress. People are frequently surprised when things don’t work that way; they deploy a new tool or service and find that it doesn’t work in a given environment, eventually track down the problem and raise a firewall change request, forget the solution and get on with their life, then repeat the same process a month or two later.

I’ve often heard this justified in the corporate context as a means to prevent an attacker from leaking data out of the network if a machine within was compromised - for example FTPing all of your customer database to a host in $OTHER_COUNTRY. Except some important facts are often disregarded:

  1. Outbound initiated HTTP and HTTPS are themselves perfectly good means of data transfer.
  2. TCP is stateful two-way protocol. If an attacker is able to initiate an inbound connection and subvert that service then they can send traffic back out on the very same channel.
  3. Unless there is some DPI (in which case there should be a better IPS than default deny) then there’s no guarantee that TCP/80 is actually HTTP. It any could be any protocol that an attacker wishes to stand up on that port. And even with DPI, TCP/443 could be anything encapsulated in TLS.

It is sometimes driven by a concern that their users/tenants will become a platform to attack others, such as email spam or DoS reflection attacks. Blocking everything might be an appropriate defence for cheap shared hosting. But I feel like at some stage you need to differentiate users that know what they’re doing and are prepared to take responsibility for themselves.

NB: I don’t mean BCP46 which includes guidelines about preventing your network from emitting traffic that doesn’t belong to you. Something that everyone should implement.

Refactoring and rationalisation

Something that sysadmins and developers often do when working on configuration or code is refactor to ensure that it reads well and executes efficiently, in addition to rationalising old content that is no longer relevant.

In the process of adding a new firewall rule it’s not uncommon to discover existing rules in the same vicinity that duplicate or contradict each other, or that are simply no longer required due to services being decommissioned.

The trouble with being abstracted away from the real config and having someone else implement it for you is that these things rarely happen. The person editing the config doesn’t have enough context to make those decisions for you and there is limited incentive for them to liaise back-and-forth with you to clean it up. So technical debt is left to accumulate.

Over compensation

Every one of those frustrations inherently increases the time it takes for a change to be correctly implemented and degrades the feedback loop.

A cumulative effect is that users/tenants often react by over compensating with their changes. They make requests for rules that are more permissive than they need to be. They don’t make requests to remove rules that are no longer needed.

This has exactly the opposite effect of what you want from a firewall. You’re less secure because too much is allowed through. Yet because nobody can see the detail some people will continue to either:

  • think that they are secure regardless
  • not ask any questions because it’s too hard


I’m not sure that I have a good solution to offer from this rant. Other than try to take ownership of the things that matter to you. The answer to “who owns my security?” is usually the same as the answer to “who owns my availability?".