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Why do popular TCP-using services have UDP as well as TCP entries in /etc/services?

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6

I'm reading a book on network programming with Go. One of the chapters deals with the /etc/services file. Something I noticed while exploring this file is that certain popular entries like HTTP and SSH, both of which use TCP at the transport layer, have a second entry for UDP. For example on Ubuntu 14.04:

ubuntu@vm1:~$ grep ssh /etc/services 
ssh 22/tcp # SSH Remote Login Protocol 
ssh 22/udp

ubuntu@vm1:~$ grep http /etc/services 
http 80/tcp www # WorldWideWeb HTTP 
http 80/udp # HyperText Transfer Protocol

Anyone know why these have two entries? I don't believe SSH or HTTP ever use UDP (confirmed by this question for SSH).

linux networking services protocols

share|improve this question

edited 5 hours ago

sixty4bit

asked 8 hours ago

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• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  22/udp was removed in Debian in 2016. IANA still lists 22/udp and lists both udp and tcp for most protocols that are only usually implemented on either one of them. Could just be that it means 22 is reserved for ssh in case somebody wants to implement ssh over udp some day?
  
  – Stéphane Chazelas
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  See also section 7.1 of rfc6335
  
  – Stéphane Chazelas
  7 hours ago
  
  

  
  
  
  

add a comment
 | 

6

I'm reading a book on network programming with Go. One of the chapters deals with the /etc/services file. Something I noticed while exploring this file is that certain popular entries like HTTP and SSH, both of which use TCP at the transport layer, have a second entry for UDP. For example on Ubuntu 14.04:

ubuntu@vm1:~$ grep ssh /etc/services 
ssh 22/tcp # SSH Remote Login Protocol 
ssh 22/udp

ubuntu@vm1:~$ grep http /etc/services 
http 80/tcp www # WorldWideWeb HTTP 
http 80/udp # HyperText Transfer Protocol

Anyone know why these have two entries? I don't believe SSH or HTTP ever use UDP (confirmed by this question for SSH).

linux networking services protocols

share|improve this question

edited 5 hours ago

sixty4bit

asked 8 hours ago

sixty4bitsixty4bit

13355 bronze badges

New contributor

sixty4bit is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  22/udp was removed in Debian in 2016. IANA still lists 22/udp and lists both udp and tcp for most protocols that are only usually implemented on either one of them. Could just be that it means 22 is reserved for ssh in case somebody wants to implement ssh over udp some day?
  
  – Stéphane Chazelas
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  See also section 7.1 of rfc6335
  
  – Stéphane Chazelas
  7 hours ago
  
  

  
  
  
  

add a comment
 | 

I'm reading a book on network programming with Go. One of the chapters deals with the /etc/services file. Something I noticed while exploring this file is that certain popular entries like HTTP and SSH, both of which use TCP at the transport layer, have a second entry for UDP. For example on Ubuntu 14.04:

ubuntu@vm1:~$ grep ssh /etc/services 
ssh 22/tcp # SSH Remote Login Protocol 
ssh 22/udp

ubuntu@vm1:~$ grep http /etc/services 
http 80/tcp www # WorldWideWeb HTTP 
http 80/udp # HyperText Transfer Protocol

Anyone know why these have two entries? I don't believe SSH or HTTP ever use UDP (confirmed by this question for SSH).

linux networking services protocols

share|improve this question

edited 5 hours ago

sixty4bit

asked 8 hours ago

sixty4bitsixty4bit

13355 bronze badges

New contributor

sixty4bit is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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ubuntu@vm1:~$ grep ssh /etc/services 
ssh 22/tcp # SSH Remote Login Protocol 
ssh 22/udp

ubuntu@vm1:~$ grep http /etc/services 
http 80/tcp www # WorldWideWeb HTTP 
http 80/udp # HyperText Transfer Protocol

share|improve this question

edited 5 hours ago

sixty4bit

asked 8 hours ago

sixty4bitsixty4bit

13355 bronze badges

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sixty4bit is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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share|improve this question

edited 5 hours ago

sixty4bit

asked 8 hours ago

sixty4bitsixty4bit

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sixty4bit is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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asked 8 hours ago

sixty4bitsixty4bit

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asked 8 hours ago

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4

Basically, it's because that was the tradition from way back when port numbers started being assigned through approximately 2011. See, for example, §7.1 “Past Principles” of RFC 6335. It's possible they will be un-alloced someday, of course, as they're a limited resource (only 1023 available, total).

And, by the way, HTTP/3 runs over UDP. Though it can use any UDP port, not just 80/443. So really those are still unused.

As far as Debian is concerned, its /etc/services already had 22/udp in 1.0 (buzz 1996).

It was however removed in this commit in 2016, first released in version 5.4 of the netbase package.

As of writing, the latest stable version of Debian (buster) has 5.6. And the latest Ubuntu LTS (18.04, bionic) netbase package is based on Debian 5.4 and you can see its changelog also mentions the removal of udp/22.

share|improve this answer

edited 7 hours ago

Stéphane Chazelas

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answered 7 hours ago

derobertderobert

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• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I may be mistaken, but also has to do with firewalls often blocking UDP
  
  – Sergiy Kolodyazhnyy
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @SergiyKolodyazhnyy I'd think not, as this practice predates firewalls.
  
  – derobert
  5 hours ago
  

  
  
  
  

add a comment
 | 

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4

Basically, it's because that was the tradition from way back when port numbers started being assigned through approximately 2011. See, for example, §7.1 “Past Principles” of RFC 6335. It's possible they will be un-alloced someday, of course, as they're a limited resource (only 1023 available, total).

And, by the way, HTTP/3 runs over UDP. Though it can use any UDP port, not just 80/443. So really those are still unused.

As far as Debian is concerned, its /etc/services already had 22/udp in 1.0 (buzz 1996).

It was however removed in this commit in 2016, first released in version 5.4 of the netbase package.

As of writing, the latest stable version of Debian (buster) has 5.6. And the latest Ubuntu LTS (18.04, bionic) netbase package is based on Debian 5.4 and you can see its changelog also mentions the removal of udp/22.

share|improve this answer

edited 7 hours ago

Stéphane Chazelas

336k5858 gold badges655655 silver badges10341034 bronze badges

answered 7 hours ago

derobertderobert

80.2k99 gold badges179179 silver badges236236 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I may be mistaken, but also has to do with firewalls often blocking UDP
  
  – Sergiy Kolodyazhnyy
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @SergiyKolodyazhnyy I'd think not, as this practice predates firewalls.
  
  – derobert
  5 hours ago
  

  
  
  
  

add a comment
 | 

4

Basically, it's because that was the tradition from way back when port numbers started being assigned through approximately 2011. See, for example, §7.1 “Past Principles” of RFC 6335. It's possible they will be un-alloced someday, of course, as they're a limited resource (only 1023 available, total).

And, by the way, HTTP/3 runs over UDP. Though it can use any UDP port, not just 80/443. So really those are still unused.

As far as Debian is concerned, its /etc/services already had 22/udp in 1.0 (buzz 1996).

It was however removed in this commit in 2016, first released in version 5.4 of the netbase package.

As of writing, the latest stable version of Debian (buster) has 5.6. And the latest Ubuntu LTS (18.04, bionic) netbase package is based on Debian 5.4 and you can see its changelog also mentions the removal of udp/22.

share|improve this answer

edited 7 hours ago

Stéphane Chazelas

336k5858 gold badges655655 silver badges10341034 bronze badges

answered 7 hours ago

derobertderobert

80.2k99 gold badges179179 silver badges236236 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I may be mistaken, but also has to do with firewalls often blocking UDP
  
  – Sergiy Kolodyazhnyy
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @SergiyKolodyazhnyy I'd think not, as this practice predates firewalls.
  
  – derobert
  5 hours ago
  

  
  
  
  

add a comment
 | 

Basically, it's because that was the tradition from way back when port numbers started being assigned through approximately 2011. See, for example, §7.1 “Past Principles” of RFC 6335. It's possible they will be un-alloced someday, of course, as they're a limited resource (only 1023 available, total).

And, by the way, HTTP/3 runs over UDP. Though it can use any UDP port, not just 80/443. So really those are still unused.

As far as Debian is concerned, its /etc/services already had 22/udp in 1.0 (buzz 1996).

It was however removed in this commit in 2016, first released in version 5.4 of the netbase package.

As of writing, the latest stable version of Debian (buster) has 5.6. And the latest Ubuntu LTS (18.04, bionic) netbase package is based on Debian 5.4 and you can see its changelog also mentions the removal of udp/22.

share|improve this answer

edited 7 hours ago

Stéphane Chazelas

336k5858 gold badges655655 silver badges10341034 bronze badges

answered 7 hours ago

derobertderobert

80.2k99 gold badges179179 silver badges236236 bronze badges

share|improve this answer

edited 7 hours ago

Stéphane Chazelas

336k5858 gold badges655655 silver badges10341034 bronze badges

answered 7 hours ago

derobertderobert

80.2k99 gold badges179179 silver badges236236 bronze badges

edited 7 hours ago

Stéphane Chazelas

336k5858 gold badges655655 silver badges10341034 bronze badges

edited 7 hours ago

Stéphane Chazelas

336k5858 gold badges655655 silver badges10341034 bronze badges

answered 7 hours ago

derobertderobert

80.2k99 gold badges179179 silver badges236236 bronze badges

answered 7 hours ago

derobertderobert

80.2k99 gold badges179179 silver badges236236 bronze badges