Fixing TCP Connection Timeout During Large FTP Transfers in Virtualized Environments

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When transferring large files via FTP in virtualized environments (especially between Linux VMs and external FTP servers), the TCP connection timeout issue typically stems from these technical factors:

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TCP keepalive settings not being properly configured

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Network Address Translation (NAT) timeout thresholds being too aggressive

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Firewall or router dropping idle connections

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VM network adapter configurations interfering with long-running transfers

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Before implementing solutions, verify where the timeout occurs:

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># Check current TCP keepalive settings
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>cat /proc/sys/net/ipv4/tcp_keepalive_time
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>cat /proc/sys/net/ipv4/tcp_keepalive_intvl
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>cat /proc/sys/net/ipv4/tcp_keepalive_probes
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># Monitor established connections
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>watch -n 5 "netstat -tn | grep ESTABLISHED"
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1. Adjust TCP Keepalive Parameters

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Temporarily modify these values during your FTP session:

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># Set keepalive to 300 seconds (5 minutes)
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>echo 300 > /proc/sys/net/ipv4/tcp_keepalive_time
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>echo 60 > /proc/sys/net/ipv4/tcp_keepalive_intvl
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>echo 5 > /proc/sys/net/ipv4/tcp_keepalive_probes
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For permanent changes, add to /etc/sysctl.conf:

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>net.ipv4.tcp_keepalive_time = 300
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>net.ipv4.tcp_keepalive_intvl = 60
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>net.ipv4.tcp_keepalive_probes = 5
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2. Use FTP Client with Built-in Keepalive

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Configure ncftpget with keepalive:

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>ncftpget -R -v -T -k 60 ftp://user:pass@server.com/path/to/largefile
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Where -k 60 sends keepalive packets every 60 seconds.

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3. Alternative: lftp with Robust Transfer

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lftp handles interrupted transfers better:

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>lftp -e "set net:timeout 30; set net:max-retries 10; set net:reconnect-interval-base 60; \
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>mirror --use-pget-n=5 /remote/path/ /local/path/; quit" ftp://user:pass@server.com
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4. Router/Firewall Configuration

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If you control the network equipment:

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># For Linux iptables:
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>iptables -I INPUT -p tcp --dport 21 -m state --state ESTABLISHED,RELATED -j ACCEPT
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>iptables -I INPUT -p tcp --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
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># Increase NAT timeout (Cisco example):
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>ip nat translation timeout 86400
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For very large files (>1GB), adjust window scaling:

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># Linux VM settings
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>echo "4096 87380 4194304" > /proc/sys/net/ipv4/tcp_rmem
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>echo "4096 87380 4194304" > /proc/sys/net/ipv4/tcp_wmem
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After implementing changes, verify with:

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># Show current TCP parameters
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>sysctl -a | grep tcp
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># Test transfer with verbose output
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>curl -v -O ftp://server.com/largefile.zip
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When transferring large files via FTP to a Linux VM, that dreaded timeout error "Could not read reply from control connection -- timed out" typically appears mid-transfer. Based on your setup (VMWare/VirtualBox with bridged/NAT networking), we're likely dealing with TCP keepalive issues at multiple layers.

The core issue stems from intermediate devices (routers/NAT) dropping idle connections. Here's how to verify and configure keepalive settings:

# Check current keepalive settings (Linux VM)
cat /proc/sys/net/ipv4/tcp_keepalive_time 
cat /proc/sys/net/ipv4/tcp_keepalive_intvl
cat /proc/sys/net/ipv4/tcp_keepalive_probes

# Temporary settings (survives until reboot)
sudo sysctl -w net.ipv4.tcp_keepalive_time=300
sudo sysctl -w net.ipv4.tcp_keepalive_intvl=60
sudo sysctl -w net.ipv4.tcp_keepalive_probes=5

For ncftpget, add these parameters:

ncftpget -R -T -t 300 -d /path/to/debug.log ftp://user:pass@host/largefile.ext

Where:
-t 300 sets timeout to 300 seconds
-T enables auto-resume
-d enables debug logging

lftp handles interrupted transfers better:

lftp -e "set net:timeout 300; set net:max-retries 5; \
set net:reconnect-interval-base 30; \
get /remote/largefile.ext -o localfile.ext; quit" ftp://user:pass@host

Add these registry tweaks to prevent host OS interference:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters]
"KeepAliveTime"=dword:000493e0
"KeepAliveInterval"=dword:00007530
"MaxDataRetransmissions"=dword:0000000a

For VMWare/VirtualBox, ensure these VMX settings exist:

ethernet0.connectionType = "bridged"
ethernet0.virtualDev = "vmxnet3"
isolation.tools.ghi.longrunning.enable = "TRUE"

When all else fails, consider these more resilient protocols:

# Using rsync over SSH
rsync -avzP --timeout=300 user@host:/path/to/largefile.ext .

# HTTP with aria2 (supports resume)
aria2c -x16 -s16 -k1M --timeout=300 http://host/largefile.ext

Use these commands to identify where packets get dropped:

# Continuous ping test
ping -t ftp.host.com

# TCP connection test
sudo tcpdump -i eth0 'host ftp.host.com and port 21'

# Check for MTU issues
tracepath -n ftp.host.com