Slow HTTP downloads through Cisco ASA 5500

Recently we noticed weird behavior downloading files from certain sites. The transfer would start out fast (around 10 MB/s), then after a couple of seconds it would plummet to around 9 KB/s. It didn’t happen for every file or every site: downloads from S3 buckets were still particularly fast. But some files that I remember being particularly fast were now showing this weird fast/slow/fast/slow behavior, for example the Sun JDK and ISOs from that used to saturate our pipe were now getting all cRAzY.

After some poking around I decided to test HTTP versus FTP to see if it could be an application/protocol-level issue. The easiest way to do this was to find a file available via both FTP and HTTP and download it via both protocols. This is where came in handy. I used cURL to download it and noticed that via HTTP it was much slower than over FTP:

[evan@boba 16:07:03 ~]$ curl -O
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  227M  100  227M    0     0   9.8M      0  0:00:22  0:00:22 --:--:-- 7816k
[evan@boba 16:07:33 ~]$ rm CentOS-6.2-x86_64-netinstall.iso 
[evan@boba 16:07:39 ~]$ curl -O
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  227M  100  227M    0     0  5686k      0  0:00:40  0:00:40 --:--:-- 6269k

22 seconds via FTP at 9.8MB/s average, 40 seconds over HTTP at 5.6 MB/s average (which was one of the better HTTP runs).

This was affecting all machines on our network, and had nothing to do with the per-machine iptables rules (verified by flushing all rules). The only thing I could think of that might affect all machines, but only HTTP and not FTP would be something like packet inspection. Well, turns out that http packet inspection is on by default on the ASA. So I disabled it as described here:

Zeus(config)# conf t
Zeus(config)# policy-map global_policy
Zeus(config-pmap)# class inspection_default
Zeus(config-pmap-c)# no inspect http
Zeus(config-pmap-c)# write mem
Building configuration...

Since then HTTP transfers have been consistently fast.


One thought on “Slow HTTP downloads through Cisco ASA 5500”

  1. Hello,

    Do you realize that the ASA is a firewall appliance; and turning off inspection on http (the protocol most commonly used for malicious attacks) disables the primary function the device was designed to perform?

    You should compare the delay in download for the ASA vs another product both with http inspection on. The ASA will win. If it doesn’t perform to your liking, upgrade the flash, software, and ram; another feature of the ASA.

    Otherwise buy a dlink or Linksys with a much dumber level of packet inspection. It depends on how important the data is behind the ASA. If you don’t mind the possibility of it all being compromised turn off all packet inspection and it will fly.

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