What is FTP ALG in my router?

Kahlo kahlow · Dec 14, 2016

Hello, I have set up my router (WR940N) and connected one of the LAN ports to the WAN ports of my ISP router because my ISP's router has a phone port and I need it to make the phone functional. Although it does work, receive and send calls, it randomly hangs up after like 3-5 minutes. I've red somewhere that it could be my firewall and it in fact was my firewall. I have SPI firewall, PPTP PASSTHROUGH, L2TP PASSTHROUGH, IPSEC PASSTHROUGH, TFTP ALG, H323 ALG, RTSP ALG, SIP ALG put to ''enabled''. It seem that only FTP ALG makes it hang up randomly after a couple of minute. Why is it? What is FTP ALG and is it bad if I disabled it? Also I cannot see my NAT type. Whether it's open or strict.

Thank you.

The Paladin · Dec 15, 2016

The File Transfer Protocol (FTP) is a widely and commonly used method of exchanging files over IP networks. In addition to the main control connection, data connections are also made for any data transfer between the client and the server; and the host, port, and direction are negotiated through the control channel. For active mode FTP, the firewall service scans the client-to-server application data for the PORT command, which provides the IP address and port number to which the server connects. For passive-mode FTP, the firewall service scans the client-to-server application data for the PASV command and then scans the server-to-client responses for the 227 response, which contains the IP address and port number to which the client connects. FTP represents the addresses and port numbers in ASCII. As a result, when addresses and ports are rewritten, the TCP sequence number might be changed, and thereafter the NAT service needs to maintain this delta in SEQ and ACK numbers by performing sequence NAT on all subsequent packets.
The FTP ALG supports the following:
Automatically allocates data ports and firewall permissions for dynamic data connection
Monitors the control connection in both active and passive modes
Rewrites the control packets with the appropriate NAT address and port information
Network Address Translation, Protocol Translation (NAT-PT)
Transport Layer Security (TLS) as the security mechanism

do you need it, probably not, but more to the point of your question most routers need to have SIP ALG disabled for your voice over IP to work properly.

Kahlo kahlow · Dec 15, 2016

The Paladin :

The File Transfer Protocol (FTP) is a widely and commonly used method of exchanging files over IP networks. In addition to the main control connection, data connections are also made for any data transfer between the client and the server; and the host, port, and direction are negotiated through the control channel. For active mode FTP, the firewall service scans the client-to-server application data for the PORT command, which provides the IP address and port number to which the server connects. For passive-mode FTP, the firewall service scans the client-to-server application data for the PASV command and then scans the server-to-client responses for the 227 response, which contains the IP address and port number to which the client connects. FTP represents the addresses and port numbers in ASCII. As a result, when addresses and ports are rewritten, the TCP sequence number might be changed, and thereafter the NAT service needs to maintain this delta in SEQ and ACK numbers by performing sequence NAT on all subsequent packets.
The FTP ALG supports the following:
Automatically allocates data ports and firewall permissions for dynamic data connection
Monitors the control connection in both active and passive modes
Rewrites the control packets with the appropriate NAT address and port information
Network Address Translation, Protocol Translation (NAT-PT)
Transport Layer Security (TLS) as the security mechanism

do you need it, probably not, but more to the point of your question most routers need to have SIP ALG disabled for your voice over IP to work properly.

Thank you very much and it's been a day now but the phone seem to work fine and does not hang up anymore since I disabled FTP ALG and still have SIP ALG enabled. If I ever get into the same problem I will disabled it and test.

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What is FTP ALG in my router?

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