SCP and SFTP on AIX: Complete File Transfer Guide

Secure file transfer is a fundamental task in AIX system administration. This guide covers SCP (Secure Copy) and SFTP (SSH File Transfer Protocol) on AIX, including practical examples, batch operations, and troubleshooting.

SCP vs SFTP: Which to Use?

Rule of thumb: Use SCP for quick transfers, SFTP for interactive work or when you need to browse remote directories.

SCP on AIX

Basic SCP Syntax

scp [options] source destination

Copy File to Remote Server

# Copy local file to remote server
scp /tmp/data.tar user@remote-aix:/home/user/

# Copy to specific remote directory
scp /tmp/config.cfg [email protected]:/etc/myapp/

# Copy with different remote filename
scp /tmp/backup.tar user@remote:/backups/backup_20240101.tar

Copy File from Remote Server

# Copy remote file to local directory
scp user@remote-aix:/var/log/app.log /tmp/

# Copy to current directory
scp user@remote-aix:/etc/hosts .

# Copy with different local filename
scp root@remote:/etc/ssh/sshd_config ./sshd_config.backup

Copy Directories Recursively

# Copy entire directory to remote
scp -r /home/user/project user@remote:/backup/

# Copy remote directory to local
scp -r user@remote:/var/www/html /tmp/website_backup/

SCP with Different Port

# Use port 2222 instead of default 22
scp -P 2222 file.txt user@remote:/tmp/

SCP with Identity File (Key)

# Use specific SSH key
scp -i ~/.ssh/my_private_key file.txt user@remote:/tmp/

# Combine with other options
scp -i ~/.ssh/aix_key -r /data user@aix-server:/backup/

Limit Bandwidth

# Limit to 1000 Kbit/s (useful for slow links)
scp -l 1000 large_file.tar user@remote:/tmp/

# Limit to ~500 KB/s
scp -l 4000 huge_backup.tar user@remote:/backups/

Preserve File Attributes

# Preserve modification times, access times, and modes
scp -p /etc/important.conf user@remote:/etc/

Compress During Transfer

# Enable compression (good for text files, slow links)
scp -C large_text_file.log user@remote:/logs/

# Combine with recursive
scp -Cr /var/log/ user@remote:/backup/logs/

Copy Between Two Remote Servers

# Copy from server1 to server2 (via local machine)
scp user@server1:/data/file.txt user@server2:/backup/

# Note: This routes through your local machine
# For direct server-to-server, SSH into server1 and scp from there

SCP with Verbose Output

# Show progress and debug info
scp -v large_file.tar user@remote:/tmp/

# Very verbose (for debugging)
scp -vvv file.txt user@remote:/tmp/

Batch SCP Examples

# Copy multiple files
scp file1.txt file2.txt file3.txt user@remote:/data/

# Copy using wildcards
scp /logs/*.log user@remote:/backup/logs/

# Copy all .conf files from /etc
scp /etc/*.conf user@remote:/backup/etc/

SFTP on AIX

Starting an SFTP Session

# Connect to remote server
sftp user@remote-aix

# Connect on different port
sftp -P 2222 user@remote-aix

# Connect with specific key
sftp -i ~/.ssh/my_key user@remote-aix

SFTP Interactive Commands

Once connected, you have access to these commands:

# Navigation
pwd                 # Print remote working directory
lpwd                # Print local working directory
cd /path            # Change remote directory
lcd /local/path     # Change local directory
ls                  # List remote directory
lls                 # List local directory

# File Transfer
get remote_file           # Download file
get remote_file local     # Download with different name
put local_file            # Upload file
put local_file remote     # Upload with different name
mget *.log                # Download multiple files
mput *.txt                # Upload multiple files

# Directory Transfer
get -r remote_dir         # Download directory recursively
put -r local_dir          # Upload directory recursively

# File Operations
rm file                   # Delete remote file
rmdir directory           # Delete remote directory
mkdir directory           # Create remote directory
rename old new            # Rename remote file
chmod 755 file            # Change remote file permissions
chown uid file            # Change remote file owner
chgrp gid file            # Change remote file group

# Information
df -h                     # Remote disk usage
!command                  # Run local shell command
help                      # Show all commands
exit / quit / bye         # End session

SFTP Practical Examples

Interactive session example:

$ sftp admin@aix-server
Connected to aix-server.
sftp> cd /var/log
sftp> ls -la
-rw-r--r--    1 root     system     123456 Feb 12 09:00 syslog
-rw-r--r--    1 root     system      54321 Feb 12 08:30 messages
sftp> get syslog /tmp/syslog_backup
Fetching /var/log/syslog to /tmp/syslog_backup
sftp> lcd /home/admin/uploads
sftp> put newconfig.conf /etc/myapp/
Uploading newconfig.conf to /etc/myapp/newconfig.conf
sftp> exit

SFTP Batch Mode

For scripting and automation, use batch mode:

Create a batch file (sftp_commands.txt):

cd /var/log
get syslog /backup/syslog
get messages /backup/messages
get secure /backup/secure
quit

Execute batch:

sftp -b sftp_commands.txt user@remote-aix

Inline batch commands:

echo "get /var/log/syslog /tmp/syslog" | sftp user@remote

SFTP in Scripts

Backup script example:

#!/bin/ksh
# backup_logs.sh - Backup remote AIX logs via SFTP

REMOTE_HOST="aix-server"
REMOTE_USER="backup"
BACKUP_DIR="/backup/$(date +%Y%m%d)"
KEY_FILE="/home/backup/.ssh/backup_key"

# Create local backup directory
mkdir -p "$BACKUP_DIR"

# Create SFTP batch file
cat > /tmp/sftp_batch_$$ << EOF
cd /var/log
get syslog $BACKUP_DIR/syslog
get messages $BACKUP_DIR/messages
get -r audit $BACKUP_DIR/audit
quit
EOF

# Execute SFTP with batch
sftp -i "$KEY_FILE" -b /tmp/sftp_batch_$$ ${REMOTE_USER}@${REMOTE_HOST}

# Cleanup
rm -f /tmp/sftp_batch_$$

echo "Backup completed to $BACKUP_DIR"

SFTP Server Configuration

The SFTP subsystem is configured in /etc/ssh/sshd_config:

# Default SFTP subsystem
Subsystem sftp /usr/sbin/sftp-server

# With logging
Subsystem sftp /usr/sbin/sftp-server -l INFO

# Restrict user to SFTP only (chroot)
Match User sftpuser
    ChrootDirectory /sftp/%u
    ForceCommand internal-sftp
    AllowTcpForwarding no
    X11Forwarding no

Create SFTP-Only User on AIX

# Create user
mkuser -a home=/sftp/sftpuser shell=/bin/false sftpuser

# Set password
passwd sftpuser

# Create chroot directory structure
mkdir -p /sftp/sftpuser/upload
chown root:system /sftp/sftpuser
chmod 755 /sftp/sftpuser
chown sftpuser:staff /sftp/sftpuser/upload
chmod 755 /sftp/sftpuser/upload

# Add to sshd_config
cat >> /etc/ssh/sshd_config << EOF

Match User sftpuser
    ChrootDirectory /sftp/%u
    ForceCommand internal-sftp
    AllowTcpForwarding no
EOF

# Restart SSH
stopsrc -s sshd && startsrc -s sshd

Troubleshooting File Transfers

Permission Denied Errors

# Check remote directory permissions
ssh user@remote "ls -la /path/to/directory"

# Check if user can write
ssh user@remote "touch /path/to/directory/test_write && rm /path/to/directory/test_write"

# For SCP, ensure parent directory exists
ssh user@remote "mkdir -p /path/to/directory"

Connection Issues

# Test SSH connection first
ssh -v user@remote

# Check if SFTP subsystem is enabled
ssh user@remote "grep -i sftp /etc/ssh/sshd_config"

# Verify sftp-server exists
ssh user@remote "ls -la /usr/sbin/sftp-server"

Transfer Interrupted

# SFTP can resume (use reget/reput)
sftp user@remote
sftp> reget partially_downloaded_file
sftp> reput partially_uploaded_file

# For SCP, you need rsync instead
rsync -avP --progress user@remote:/path/to/file /local/path/

Slow Transfer Speeds

# Disable compression if already compressed files
scp -o Compression=no file.tar.gz user@remote:/tmp/

# Use faster cipher
scp -c aes128-ctr file.tar user@remote:/tmp/

# Check for network issues
ping -c 5 remote-server
traceroute remote-server

"Received message too long" Error

Usually indicates the remote server is outputting text before SFTP can start. Check:

# Look for echo/print statements in shell profile
ssh user@remote "cat ~/.profile ~/.bashrc ~/.kshrc 2>/dev/null"

# Fix: Wrap output in login shell check
# In remote user's profile:
if [ -t 0 ]; then
    echo "Welcome message"  # Only shows for interactive shells
fi

SCP/SFTP Best Practices on AIX

Use Key-Based Authentication

# Generate key
ssh-keygen -t ed25519 -f ~/.ssh/transfer_key -N ""

# Copy to remote
ssh-copy-id -i ~/.ssh/transfer_key.pub user@remote

# Use in transfers
scp -i ~/.ssh/transfer_key file user@remote:/path/

Create SSH Config for Convenience

Add to ~/.ssh/config:

Host aix-prod
    HostName 192.168.1.100
    User admin
    IdentityFile ~/.ssh/aix_key
    Port 22

Host aix-backup
    HostName 192.168.1.101
    User backup
    IdentityFile ~/.ssh/backup_key

Then simply:

scp file.txt aix-prod:/tmp/
sftp aix-backup

Log All Transfers

For audit purposes, create wrapper scripts:

#!/bin/ksh
# /usr/local/bin/scp-logged
LOG="/var/log/scp_transfers.log"
echo "$(date '+%Y-%m-%d %H:%M:%S') USER=$USER CMD=scp ARGS=$*" >> "$LOG"
/usr/bin/scp "$@"

Related Guides

• AIX SSH Configuration - Complete SSH setup guide
• SSH Tunneling on AIX - Port forwarding guide
• How to Restart SSH in AIX - SSH service management

Summary

SCP and SFTP provide secure file transfer capabilities on AIX:

• SCP: Quick, command-line transfers with scp source dest
• SFTP: Interactive or batch file management
• Use -r for directories, -P for different ports, -i for keys
• SFTP batch mode (-b) enables scripted automation
• Always verify permissions and test SSH connectivity first