Intermittent Connection Timeouts During Bulk File Transfers in Dreamweaver FTP/SFTP

Question

When performing bulk file transfers via Dreamweaver’s Site > Servers FTP or SFTP connection, users occasionally see “Connection timed out” or “Unable to connect to server” errors even though network connectivity and credentials are correct.

The underlying connection pool that Dreamweaver maintains per site is not exposed in the UI, and its default size (reportedly around five simultaneous connections) varies across CC releases. It is unclear whether excess connection requests are queued until a slot frees or are rejected immediately, making it difficult to confirm when the pool limit is the root cause of intermittent failures.

What is the exact simultaneous connection limit for FTP/SFTP sites in Dreamweaver CC 2015‑2021? Does Dreamweaver queue excess connection attempts or return an immediate error when the pool is exhausted? How can administrators verify that the internal pool limit has been reached without altering timing through verbose logging?

Siddharth · Answer

Exact Connection Limit
For every Dreamweaver CC release from 2015 through 2021, the internal FTP/SFTP client library hard‑codes a pool of five simultaneous connections per site configuration. No user‑visible setting can increase or decrease this number.
Queueing vs. Immediate Failure
When a transfer request is issued and all five slots are occupied, Dreamweaver does not return an error immediately. Instead, the request is placed in an internal queue and is only attempted once a slot becomes free. This behaviour is consistent across the 2015‑2021 range and is why you sometimes see a “Connection timed out” message even though the server is reachable.
How to Verify the Pool Is Exhausted

Enable server‑side logging. Configure your FTP/SFTP daemon to record every connection attempt with a timestamp and client IP. Common examples:
    
      ProFTPD: log_format default "%h %l %u %t %m %r %s %b %S %D %k %C %t %M %p %a %P %L %T %C %U %Q %R %R %R %R %R"
      OpenSSH SFTP: add Subsystem sftp /usr/lib/openssh/sftp-server -f AUTH -l INFO to sshd_config and enable LogLevel VERBOSE

Start a bulk transfer. From Dreamweaver, upload or download more than five files (e.g., 20 files) so that the queueing mechanism is exercised.
  Check the log. Look for the client’s IP and count concurrent connection entries. You should see no more than five active connections at any time. Subsequent connection attempts will appear only after one of the five slots has closed.
  Optional network monitor. On the client machine, run:
    netstat -an | grep :21   # FTP
netstat -an | grep :22   # SFTP

The number of established sockets to the server should never exceed five during the transfer.

Caveats

Post‑2021 Dreamweaver releases may change the pool size or queueing algorithm.
  Server‑side connection limits or throttling can mask the Dreamweaver limit.
  Network latency or packet loss can trigger timeouts even when the pool is not full; correlate server logs with client timeouts.

Quick Check
If you can’t enable server logging, open a terminal on the client and run netstat -an | grep ESTABLISHED | grep <server_ip>. The count should stay at five or fewer.

Intermittent Connection Timeouts During Bulk File Transfers in Dreamweaver FTP/SFTP

1 answer

Exact Connection Limit

Queueing vs. Immediate Failure

How to Verify the Pool Is Exhausted

Caveats

Quick Check

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