On 6/10/2021 4:04 AM, Fernando Gont wrote:
Hi, Blake,
Thanks a lot for your comments! In-line....
On Fri, 2021-06-04 at 11:13 -0500, Blake Hudson wrote:
Current gen Cisco ASA firewalls have logic so that if the connection from a private host originated from a privileged source port, the NAT translation to public IP also uses an unprivileged source port (not necessarily the same source port though). Did you actaully mean "...also uses a *privileged port*"?
Yes I did. Thanks.
I found out that this behavior can cause issues when you have devices on your network that implement older DNS libraries or configs using UDP 53 as a source and destination port for their DNS lookups. Occasionally the source port gets translated to one that ISC BIND servers have in a blocklist (chargen, echo, time, and a few others) and the query is ignored. As I recall, this behavior is hard coded so patching and recompiling BIND is required to work around it.
I forget what the older ASA behavior was. It may have been to leave the source port unchanged through the NAT process (I think this is what you mean by "not translated"). In that case the client doesn't implement source port randomization and the NAT doesn't "upgrade" the connection to a random source port so I don't really see it as an issue. The issue would be that if the port is not translated, and multiple systems in the internal real of the NAT try to use the same privileged port (say, 123) simultaneously, things wouldn't work.
Not quite. If multiple devices behind a NAT use SRC=123 & DST = 123 for connections, their connections will still work most of the time. First, if the connections are to different destinations there would be no NAT conflict. Second, if the connections occur at different times, there would be no NAT conflict. Third, if there was a NAT conflict (meaning connections using the same SRC port, DST port, and DST host at the same time) the NAT device would see this and would either adjust the translation to use a different SRC port (in which case the connection succeeds) or may drop/reject the connection (in which case the client would eventually retry). So saying "things wouldn't work" or "you won't be able to have multiple NTP clients behind the same firewall" is not true. Saying that "the number of simultaneous connections is limited" when behind a NAT would be more accurate. _But this statement is true of any one-to-many NAT_. Most NTP clients utilize multiple NTP servers, often from a pool of available servers, and initiate connections rather infrequently so I do not expect this to be a problem in practice unless there are thousands of NTP clients behind a single NAT accessing a common NTP server (and that NAT does not do a good job of dealing with collisions). I do, however, agree that clients should probably use ephemeral ports when making any outbound connections as this provides more entropy for NAT as well as for connection security. This extends to NTP.